An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean)

Lombardo, Andrea; Marletta, Giuliana

doi:10.3390/oceans5030040

Open AccessArticle

An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean)

by

Andrea Lombardo

¹

and

Giuliana Marletta

^2,*

¹

Department of Biological, Geological and Environmental Sciences, University of Catania, 95124 Catania, Italy

²

Independent Researcher, 95124 Catania, Italy

^*

Author to whom correspondence should be addressed.

Oceans 2024, 5(3), 695-719; https://doi.org/10.3390/oceans5030040

Submission received: 16 July 2024 / Revised: 21 August 2024 / Accepted: 4 September 2024 / Published: 10 September 2024

(This article belongs to the Special Issue Feature Papers of Oceans 2024)

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Abstract

:

The Magnisi peninsula is a small portion of land located near the largest Italian petrol-chemical pole of Augusta–Priolo–Melilli (40 km²), which, since the 1950s, devastated the local environment and landscape and unloaded directly into the sea an impressive quantity of pollutants. Unlike the terrestrial part of the area, where a Natural Oriented Reserve (NOR) called “Saline di Priolo” was established in the 2000s, no concrete legislative action has been implemented or proposed for the marine environment. At the same time, the fauna of the marine environment has not been studied in the same way as that of the terrestrial environment. As concerns the molluscan fauna, most of the information dates back to the 1800s. These studies were exclusively focused on the shells of some mollusks in the area. Instead, no study related to this area has ever been carried out on the group of sea slugs. This study conducted between 2022 and 2023, through snorkeling activities, allowed to provide the first faunistic list of the sea slugs of this area, together with information on the biology and ecology of these gastropods, highlighting the potential biodiversity present in this small stretch of coastline affected by high industrial pollution.

Keywords:

industrial pollution; marine heterobranchia; Sicily; Mediterranean sea

1. Introduction

The Magnisi peninsula (also called Thapso, Thapsos, or Thapsus in ancient times [1,2]) is a small tear-shaped land portion located between the Augusta and Priolo–Melilli gulfs along the southern–eastern coast of Sicily (Italy), connected to the mainland by a thin and low sandy isthmus [1,3] (Figure 1A–C). The name Magnisi derives from the denomination “Magna Insula” (i.e., large island), a term formerly given to this peninsula by the Norsemen [1,2]. This calcareous peninsula, presenting a length of 2000–2300 m and a maximum width of 700–800 m in its central axis, is characterized by a generally flat surface (its maximum height is 16–20 m above the sea level) and steep coasts [1,4]. These latter are generally high along almost the entire peninsula border, except on the southeast coasts [1].

This small peninsula is located close to the largest Italian petrol–chemical pole of Augusta–Priolo–Melilli (40 km²), which since the 1950s has not only devastated and heavily disfigured the local environment and landscape (as well as people’s health) but has also dumped an impressive amount of pollutants such as benzene, chlorobenzenes, and mercury directly into the sea [3,5,6]. In the area that connects the peninsula with the sandy isthmus, there was an industrial area (nowadays abandoned) where bromide was synthesized through the acquisition and subsequent discharge of seawater, causing extensive damage to the marine environment [3].

These remarkable anthropogenic impacts have caused several damages to the local marine environment, such as a high quantity of non-living particles suspended in the water column, seawater turbidity, sedimentation, contamination of sediments and marine organisms, the presence of alien species, degradation of natural phytobenthic communities, and almost total destruction of Posidonia oceanica (Linnaeus) Delile meadows located in the area [7].

Although the anthropogenic impacts caused by the petrochemical hub have not yet disappeared, since the 2000s several environmental actions in the area have led to the restoration of some important wetlands and the associated birdlife through the establishment of a Natural Oriented Reserve (NOR) called “Saline di Priolo” and the homonymous Natura 2000 site (ITA 090013), into which the Magnisi peninsula was incorporated in 2013, due to its remarkable importance as both an ornithological and archaeological site [8].

However, these important environmental achievements only concerned the terrestrial domain of the area, almost completely neglecting the marine realm. Indeed, the only restoration action of the local marine environment involved the replanting of P. oceanica meadows with considerable success [7,9]. Nevertheless, unlike the terrestrial part of the area, no concrete legislative action has been implemented or proposed for the marine environment. At the same time, unlike the terrestrial part of the Magnisi peninsula, whose fauna is very well known, the marine fauna of this area is generally unstudied and almost completely lacking in specific studies.

As concerns the marine fauna, some studies regarding the phylum Mollusca were carried out in this area during the 1800s [10,11,12,13,14,15,16]. As highlighted by De Martino [17], the Magnisi peninsula (especially its south side) represents one of the most shell-rich coastal stretches of the entire Sicily. However, although these conchological studies provide an important knowledge base on the mollusks of the Magnisi peninsula, these are not specific to the area and only contain a list of mollusks with shells (a few with brief shell descriptions), without biological or ecological data of the living organisms. To date, the only description of the molluscan fauna of the Magnisi peninsula is that reported by De Martino [17], albeit very concise and concerning only stranded shells.

Within the Gastropod Mollusca, the subclass Heterobranchia includes the sea slugs (formerly belonging to the subclass ”Opisthobranchia” and no longer in use) in the broad sense, with the taxonomic groups Rhodopoidea, Acteonoidea, Cephalaspidea, Runcinida, Aplysiida, Pteropoda, Umbraculida, Ringiculimorpha, Pleurobranchida, Nudibranchia, Acochlidiimorpha, and Sacoglossa [18,19]. Thanks to their showy and colorful body shapes, these marine critters have become one of the most favorite subjects of underwater photographers worldwide. This can be easily noted by the large number of books and websites on them, e.g., [20,21,22,23,24].

For the Magnisi peninsula area, there are no data on the sea slugs. The only information about this group of mollusks is regarded as isolated or old findings of a few stranded sea slug shells (Table 1). Consequently, given the presumed malacological importance of this area and the almost total absence of data on this group of gastropods, this study aims to provide the first faunistic list (together with biological and ecological information) of the sea slugs of this Sicilian coastal stretch.

2. Materials and Methods

This study was carried out from January 2022 to December 2023 along a small coastal stretch of the Magnisi peninsula located between the dismissed ESPESI industrial complex and the left side of Cala Magnisi (Figure 1C–E). The examined coastal stretch runs from 37°09′09.1″ N 15°13′45.7″ E to 37°09′05.0″ N 15°13′51.2″ E (Figure 1D). This area is very shallow and characterized by two different types of bottoms. The first one is present in the corresponding area of the thin white pebble beach that delimits the north and northeast parts of the shoreline and is characterized by a large number of scattered pebbles, boulders, and rocks, which almost cover a sandy gravel bed (Figure 1F,G). The second one, which characterizes the area at the base of the small southeast cliff, has very shallow, large submarine rock terraces interspersed with large oblique rock blocks (Figure 1H).

Data collection, performed through snorkeling (within 0.50–0.70 m of depth), was carried out once a month during the study period, following the same underwater path (Figure 1) from 09:30 to 11:30 a.m. in the morning. During each snorkeling session, all the sea slug specimens found during the path were photographed by an Olympus TG-4 underwater camera and counted. Moreover, the seawater temperature (in Celsius degrees) was measured and recorded for each snorkeling session with a SUUNTO D6i dive computer and included in a line chart (Figure 2). In the case of two or more temperatures recorded in the same session, only their average value was entered in the chart.

Through subsequent examination of the photographs, it was possible to make the specific identification of each sea slug and to note the different substrates on which the specimens were found. The sources consulted for the identification of the sea slugs were Prkić et al. [19], Rudman [20], Trainito and Doneddu [21], Valdés et al. [25], Galià-Camps et al. [26], and OPK Opistobranquis [24]. The identification of zoo- and phytobenthic components was performed following Trainito and Baldacconi [27] and Rodríguez-Prieto et al. [28], respectively. The used nomenclature is that reported on WoRMS [29].

For each found sea slug species, the following information was reported: abundance, temperature range, seasonality, substrates/habitats, and any remarks. The abundance data of the different species during the months of the two years of study can be seen in Table 2.

The following graphs were created from the collected data: a bar chart on the number of species for each taxonomic group, a line chart on the variation in the number of nudibranch species per month, a bar chart on the variation in the number of nudibranch species per season, and a bar chart on the total abundance for each species. In this case, the seasons were considered as follows: winter (January + February + March), spring (April + May + June), summer (July + August + September), and autumn (October + November + December). Monthly water temperature values can be seen in Figure 2.

3. Results

3.1. General Section

Through the present study, a total of 37 sea slugs were found, divided into five taxonomic groups. The most species-rich group was the order Nudibranchia, with 28 species. The two suborders of this group were similarly represented in the examined area: 15 Cladobranchia and 13 Doridina. The other four taxonomic groups were represented by a lower number of species compared to the abovementioned order: Sacoglossa (5 species), Pleurobranchida (2), Cephalaspidea (1), and Aplysiida (1).

Considering the temporal variation in species number of the group Nudibranchia, several differences during months, years, and seasons were observed (Figure 3).

In 2022, after a rapid rise in species number between January (5 species) and February (9), there was a total decline in nudibranch species number in March (0). After this month, the nudibranch species number rose in April (5), then it fell slightly in the subsequent months until the stable lowering of October, November, and December (2) (Figure 3). In 2023, the nudibranch species number remained more or less constant from January (5) to April (4) and then decreased in May (2). After this month, there was an increase in the species number until September (5). Later, October (2) and November (1) recorded the lowest nudibranch species number of 2023. After this decline, there was a sharp increase in December (7).

Considering the variation in nudibranch species number per season (Figure 4), it can be seen that in 2022 there was an almost constant decline in species number across seasons [Winter (14 species), Spring (11), Summer (10), and Autumn (6)]. In 2023, on the other hand, the trend was more constant between seasons [Winter (14), Spring (9), Summer (13) and Autumn (10).

3.2. Faunistic Section

The list of the species found and their abundances throughout the months of study can be seen in Table 2.

Order Pleurobranchida

Family Pleurobranchidae Gray, 1827

Berthellina cf. edwardsii (Vayssière, 1897) (about 25–30 mm in length) (Figure 5A–G).

Abundance: 26 specimens.

Temperature range: from 15 to 28 °C.

Seasonality: this species was found in the area from July to November. However, it could also be found in December, January, and April.

Substrates/habitats: B. cf. edwardsii specimens were always found on the underside of rocks. These latter could be almost totally bare, colonized with scattered calcareous red algae, serpulids, and sponges, or covered with detritus.

Remarks: When found under the rocks, B. cf. edwardsii specimens had a roughly spherical body shape in which the rhinophores and oral veil were retracted under the notum (Figure 5A). As soon as they were exposed to light (natural or produced by the underwater camera flashes), the animals started to slightly expose the distal part of the rhinophores (Figure 5B) and, if they were still disturbed by the light, began to crawl toward a darker area with fully extended rhinophores, oral veil, and body (Figure 5C). Almost all of the specimens had a general orange body coloration; however, a few animals were pale yellow (Figure 5E).

During this study, a small teratological specimen of B. cf. edwardsii (about 4–5 mm in length) was found with a peculiar separation of the posterior part of the notum in two lobes, which, rejoining posterior-dorsally, formed a strange cavity from which a grey tegumentary structure (with white tips) leaked out (Figure 5F).

Another small specimen (4–5 mm in length) had part of the notum edge missing (Figure 5G). Both the strange posterior teratology and the lack of part of the mantle margin were probably caused by some hypothetical predator (such as a small crustacean) or by the movement of the rocks under which these two specimens were found.

It is important to highlight that Berthellina edwardsii is indistinguishable from Berthella aurantiaca (Risso, 1818) by examining only the external morphology of these animals [20,21]. Consequently, the information reported here for the examined area is associated with an entity whose determination was not possible with absolute certainty. However, the specimens we found were identified as B. cf. edwardsii because, in the Mediterranean, B. aurantiaca is not a common species, unlike B. edwardsii [20]. As proof of this, Cervera [20] pointed out that it is likely that most Mediterranean finds of B. aurantiaca are actually to be considered B. edwardsii.

Berthella stellata (Risso, 1826) (about 10–12 mm in length) (Figure 5H–J).

Abundance: 2 specimens.

Temperature range: the animals were found at 16.5 °C, while the egg masses of this species were documented between 21 and 26 °C.

Seasonality: B. stellata specimens were documented in April. The egg masses (Figure 5J) were found in May, June, September, and October.

Substrates/habitats: this species was found on the underside of rocks covered by calcareous red algae, serpulids, remains of Posidonia oceanica rhizomes, and detritus. The egg masses were documented on the underside of rocks covered with a very low substrate of filamentous algae.

Remarks: in contrast to previous species, B. stellata individuals were always found with a very flattened body strongly compressed against the substrate (Figure 5I). The animals stretched their bodies and only began to crawl away after being touched several times (Figure 5H).

Order Nudibranchia Cuvier, 1817

Suborder Cladobranchia

Family Aeolidiidae Gray, 1827

Berghia verrucicornis (A. Costa, 1867) (about 15 mm in length) (Figure 6A–C).

Abundance: 4 specimens.

Temperature range: from 18 to 21 °C.

Seasonality: April and May. Almost all B. verrucicornis specimens were found in April, a month in which two individuals were observed mating.

Substrates/habitats: the animals were found on the underside of rocks with calcareous red algae, tufts of filamentous red algae, the sponge Cliona viridis (Schmidt, 1862), serpulids, and detritus.

Spurilla neapolitana (Delle Chiaje, 1841) (about 15–20 mm in length) (Figure 6D–H).

Abundance: 9 specimens.

Temperature range: from 15 to 27.5 °C.

Seasonality: the individuals were documented in February, March, April, July, September, October, and December.

Substrates/habitats: S. neapolitana specimens were found on turfs of filamentous brown algae with Jania sp. and Dictyota dichotoma (Hudson) J. V. Lamouroux; on Halopteris scoparia (Linnaeus) Sauvageau; on turfs of Ceramium sp.; on the underside of bare rocks or covered with calcareous red algae, serpulids, detritus, and gravel. The individuals documented under rocks were almost always within rocks’ small concavities.

Remarks: during this study, it was noted that all the S. neapolitana specimens found under rocks had larger and stubbier bodies (Figure 6F–H) than those previously documented on algae in open environments (Figure 6D,E). Moreover, the former individuals always had a higher number of bright dots on the body surface than the latter.

On one occasion, when disturbed by one of the authors, a specimen peeled off several of its cerata, which became strongly adhesive on the author’s glove.

Family Facelinidae Bergh, 1889

Cratena peregrina (Gmelin, 1791) (about 8–25 mm in length) (Figure 7A–C).

Abundance: 16 specimens.

Temperature range: from 15 to 17 °C.

Seasonality: most C. peregrina specimens were documented in January. However, this species was also found in February, March and December).

Substrates/habitats: this species was found on turfs of Jania sp. with detritus; turfs of filamentous red and brown algae; calcareous red algae; Peyssonnelia spp.; Dictyota dichotoma; Caulerpa taxifolia (M. Vahl) C. Agardh; and under bare rocks.

Facelina annulicornis (Chamisso and Eysenhardt, 1821) (about 15–18 mm in length) (Figure 7D,E).

Abundance: 2 specimens.

Temperature range: 16.5 °C.

Seasonality: only in April.

Substrates/habitats: F. annulicornis individuals were found on the underside of rocks covered with calcareous red algae, Peyssonnelia spp., filamentous algae, and serpulids.

Remarks: the smallest of the two found specimens lacked the posterior part of the body, and some of its cerata were missing or shortened (Figure 7E). Its wounds were probably caused by the clipping action of a small crustacean predator.

Facelina rubrovittata (A. Costa, 1866) (8–15 mm in length) (Figure 7F–J).

Abundance: 29 specimens.

Temperature range: from 15 to 26 °C.

Seasonality: most F. rubrovittata specimens were found in January, February, and April. However, this species was also documented in May, September, and December.

Substrates/habitats: this facelinid was found on turfs of filamentous green, red, and brown algae; Jania sp.; Ellisolandia elongata (J. Ellis and Solander) K. R. Hind and G. W. Saunders; Peyssonnelia spp.; red laminar algae; Halopteris scoparia; Dictyota dichotoma; Taonia sp.; Dictyopteris sp.; and the sponge Cliona viridis.

Favorinus branchialis (Rathke, 1806) (about 4 mm in length) (Figure 7K).

Abundance: 1 specimen.

Temperature range: 16 °C.

Seasonality: this species was only documented in December .

Substrates/habitats: the individual was found on an egg mass of the dendrodorid Dendrodoris limbata (Cuvier, 1804) on the underside of a rock.

Family Flabellinidae Bergh, 1889

Calmella cavolini (Vérany, 1846) (about 15–18 mm in length) (Figure 8A–E).

Abundance: 8 specimens.

Temperature range: from 16 to 28 °C.

Seasonality: this species was documented in April, July, August, September, November, and December.

Substrates/habitats: C. cavolini was found on turfs of filamentous red and brown algae with Jania sp., Cladophora sp., and detritus; Caulerpa taxifolia; rocks covered by algal turfs; and Taonia sp.; on the underside of rocks covered by algal turfs, sponges, and detritus.

Calmella gaditana (Cervera, García-Gómez and García, 1987) (about 15–18 mm in length) (Figure 8F).

Abundance: 5 specimens.

Temperature range: from 15 to 21 °C.

Seasonality: this species was found in February, March and May. During this latter, the mating activity was documented.

Substrates/habitats: C. gaditana specimens were found only on turfs of filamentous red, brown, and green algae.

Remarks: externally, this species is virtually identical to Calmella cavolini. The only difference between these two flabellinids is the presence of bright dots on the distal part of the cerata of C. gaditana.

Edmundsella pedata (Montagu, 1816) (about 10–18 mm in length) (Figure 8G–J).

Abundance: 55 specimens.

Temperature range: the animals were present from 14.5 to 16 °C. The egg masses were found in the range of 14.5–15 °C.

Seasonality: E. pedata specimens were found from December to March. The egg masses (Figure 8J) were documented in January and February.

Substrates/habitats: this species was found on turfs of Jania sp. and filamentous algae; Peyssonnelia spp. (also the egg masses); Halopteris scoparia; Dictyota dichotoma; Taonia sp. (also the egg masses); Cladophora sp.; Caulerpa taxifolia; on and beneath rocks with calcareous red algae, serpulids, Cliona viridis and detritus.

Remarks: almost all the E. pedata specimens found during this study presented the typical pink-violet body and orange-reddish digestive gland’s ramifications of the species (Figure 8H,I). However, one individual, found in December 2023, had a bluish body coloration and very dark-colored digestive gland ramifications (Figure 8G). This difference in body coloration could probably be attributed to genetic causes or different food sources.

Family Dotidae Gray, 1853

Doto paulinae Trinchese, 1881 (about 4–5 mm in length) (Figure 9A–C).

Abundance: 2 specimens.

Temperature range: the individuals were found both at 15 °C. The egg masses (Figure 9C) were documented from 15 to 16 °C.

Seasonality: Doto sp. specimens were found in January and February. The egg masses were found in January, February, and December.

Substrates/habitats: this species and its egg masses were exclusively found on the hydrozoan of the genus Aglaophenia Lamouroux, 1812 (Figure 9A).

Family Trinchesiidae F. Nordsieck, 1972

Trinchesia foliata (Forbes and Goodsir, 1839) (about 4–5 mm in length) (Figure 9D,E).

Abundance: 2 specimens.

Temperature range: 15 °C.

Seasonality: this species was only found in February.

Substrates/habitats: T. foliata was found on the algae Halopteris scoparia and Dictyota dichotoma.

Trinchesia morrowae Korshunova, Picton, Furfaro, et al., 2019 (about 5–8 mm in length) (Figure 9F).

Abundance: 1 specimen.

Temperature range: 18 °C.

Seasonality: this species was only documented in April.

Substrates/habitats: T. morrowae was found on an entanglement of Cladophora sp.

Trinchesia sp. (about 5 mm in length) (Figure 9G).

Abundance: 1 specimen.

Temperature range: 15 °C.

Seasonality: the individual was only documented in January.

Substrates/habitats: Trinchesia sp. was found on the turf of filamentous red algae.

Remarks: the found specimen had a transparent-gray body coloration with scattered bright cream-azure blotches. On the dorsal area of the head, there was a rhomboidal-shaped blotch, which was white on its anterior part (located between the front edge of the head and the space between rhinophores) and faded grey with scattered bright dots on the posterior part (placed from the space behind the rhinophores and the first group of cerata). Each side of the white area of the head was bordered by an orange stripe. In addition, the rhinophores were transparent grey on their proximal half and stained blue–white in the distal half. The contact area between these two parts was colored orange. Despite a slight external similarity of this specimen with those identified here as Trinchesia foliata, the differences in the coloration of the body and rhinophores justify their separation.

Aeolid sp. (about 5 mm in length) (Figure 9H).

Abundance: 1 specimen.

Temperature range: 18 °C.

Seasonality: Aeolid sp. was documented in April.

Substrates/habitats: the individual was found on the turf of filamentous brown algae.

Remarks: the specimen characterized by a grey and white body probably belongs to the family Trinchesiidae.

Family Tritoniidae Lamarck, 1809

Candiella manicata (Deshayes, 1853) (about 8–10 mm in length) (Figure 9I–K).

Abundance: 1 specimen.

Temperature range: 26 °C.

Seasonality: the individual was documented in September.

Substrates/habitats: C. manicata was found on the turf of filamentous red algae covered with detritus.

Remarks: the animal was very cryptic on its substrate (Figure 9K).

Suborder Doridina

Family Chromodorididae Bergh, 1891

Felimare gasconi (Ortea, 1996) (about 5–15 mm in length) (Figure 10A–J).

Abundance: 30 specimens.

Temperature range: from 14.5 to 28 °C.

Seasonality: F. gasconi was documented in February, March, April, and from August to December. Most of the individuals were found in February and from August to December.

Substrates/habitats: the favorite substrate of this chromodoridid was the lower face of the rocks colonized by small Dysidea spp., Peyssonnelia spp., serpulids, and detritus. However, F. gasconi was also found on turfs of filamentous red algae; Jania sp. intermingled with detritus; Peyssonnelia spp.; rocks (bare or with algal turfs); gravel, and detritus.

Remarks: during this study, it was possible to observe the development of the chromatic pattern as F. gasconi grew. On the dorsal part of the notum, the smallest individuals showed a white stripe running from the space between the rhinophores to the posterior part of the notum. In this section, the white stripe encircled the gill tuft. Moreover, these juvenile specimens had the edge of the notum differently colored: the frontal mantle border (the one delimiting the notal space from the anterior part of the notum to the level of the rhinophores) was white, the middle edge of the mantle (from the level of the rhinophores to the level of the gill tuft) was yellow, and the posterior edge (from the level of the gill tuft to the posterior notal edge) was white (Figure 10A–C). Subsequently, along one side (or even both) of the central dorsal white stripe, a small, thin white line started to develop in several ways depending on the individual (Figure 10D–F). At the same time, the anterior and posterior white-colored mantle edges turn partially or totally yellow in coloration. The final step was the yellowing of the central white dorsal stripe (Figure 10G–J).

Felimare picta (R. A. Philippi, 1836) (about 70–80 mm in length) (Figure 10K,L).

Abundance: 5 specimens.

Temperature range: from 15 to 27.5 °C.

Seasonality: F. picta was documented in January, April, August, and October.

Substrates/habitats: this species was found on turfs of filamentous algae and Jania sp. with detritus, among Padina spp. and Dictyota dichotoma thalli, within a crevice.

Felimare villafranca (Risso, 1818) (about 5–15 mm in length) (Figure 10M–P).

Abundance: 7 specimens.

Temperature range: from 14.5 to 27.5 °C.

Seasonality: F. villafranca was documented in January, February, May, June, August, and November.

Substrates/habitats: this species was mainly found on the underside of rocks with Dysidea spp. and calcareous encrusting red algae. Moreover, it was also found on Peyssonnelia spp. and turfs of filamentous red and brown algae.

Remarks: despite the scarcity of F. villafranca specimens found in this study, some conclusions can be drawn about the development of the chromatic pattern of this species. It was noted that the smallest specimens showed the anterior and posterior parts of the mantle edge white, while the central part of the latter was orange (Figure 10M,N). In the largest individuals, the entire notum edge turned orange. Moreover, in small individuals, the general body coloration was bluish, and the pattern of the dorsal stripes was not very chaotic. In the largest animals, the general color of the body was almost black, and there was an evident complication in the pattern of stripes (Figure 10O,P).

Family Discodorididae Bergh, 1891

Aporodoris cf. millegrana (Alder and Hancock, 1854) (about 6 mm in length) (Figure 11A).

Abundance: 1 specimen.

Temperature range: 27.5 °C.

Seasonality: A. cf. millegrana was documented only in August.

Substrates/habitats: the individual was found on the underside of a small rock covered by serpulids.

Baptodoris cinnabarina Bergh, 1884 (about 6–8 mm in length) (Figure 11B–D).

Abundance: 1 specimen.

Temperature range: 23 °C.

Seasonality: B. cinnabarina was only documented in June.

Substrates/habitats: the animal was found on the underside of a rock.

Remarks: the found specimen lacked some portions of the mantle edge (Figure 11C,D).

Jorunna tomentosa (Cuvier, 1804) (about 15 mm in length) (Figure 11E,F).

Abundance: 1 specimen.

Temperature range: the individual and its egg mass were found at 16.5 °C.

Seasonality: J. tomentosa was only documented in April (also one egg mass).

Substrates/habitats: the animal and its egg mass (Figure 11F) were found on the underside of a rock covered by serpulids.

Taringa sp. (about 20 mm in length) (Figure 11G).

Abundance: 2 specimens.

Temperature range: 25 °C.

Seasonality: two Taringa sp. individuals were documented in June during mating.

Substrates/habitats: the animals were found on the underside of a rock covered with calcareous red algae.

Tayuva lilacina (A. Gould, 1852) (about 25–35 mm in length) (Figure 12A–D).

Abundance: 3 specimens.

Temperature range: from 26 to 27.5 °C.

Seasonality: T. lilacina was found in July and September. The mating (Figure 12C) was documented in July.

Substrates/habitats: the animals were found on the underside of rocks with calcareous red algae, serpulids, and tunicates.

Remarks: on one occasion, after exposing a specimen to the light while standing on the underside of a rock, the animal started to wrinkle its mantle margin (Figure 12D).

Compared to the other discodoridid species found during this study, T. lilacina responds quickly to disturbances by not contracting its body much and moving away quickly in search of darker areas.

Doridina sp. (about 10 mm in length) (Figure 12E,F).

Abundance: 1 specimen.

Temperature range: 28 °C.

Seasonality: Doridina sp. was only documented in August.

Substrates/habitats: this species was found on a yellow–orange sponge on the underside of a rock covered by calcareous red algae and serpulids.

Family Dendrodorididae O’Donoghue, 1924 (1864)

Dendrodoris grandiflora (Rapp, 1827) (about 40 mm in length) (Figure 12G,H).

Abundance: 1 specimen.

Temperature range: 28 °C.

Seasonality: D. grandiflora was only documented in July.

Substrates/habitats: this species was found on the underside of a rock covered with calcareous red algae and tufts of filamentous algae.

Dendrodoris limbata (Cuvier, 1804) (about 8–40 mm in length) (Figure 13A–G).

Abundance: 13 specimens.

Temperature range: the individuals were found from 15 to 28 °C. Instead, the egg masses (Figure 13G) were found from 16 to 20.5 °C.

Seasonality: this species was documented all year round except in November. The egg masses were only found in November and December.

Substrates/habitats: D. limbata was always found on the underside of rocks, which could be totally bare or presented Cliona viridis, serpulids, and detritus; calcareous encrusting red algae (also the egg masses); Peyssonnelia spp. and remnants of shells.

Remarks: During this study, it was observed that all small D. limbata specimens had a general black-brown mottled body coloration. However, the latter was only evident when these animals were placed on a black substrate, which made the mottled appearance very obvious (Figure 13A,B). In contrast, when these small individuals were on their natural substrate, they appeared almost completely black. The larger specimens showed several different chromatic patterns, with the lighter side predominating over the darker one (Figure 13D–F). Consequently, in D. limbata specimens, it can be seen that the development of the chromatic pattern moves toward a progressive lightening of the body coloration (dark → light) combined with the separation and narrowing of the dark component (large, clustered group of spots → small, scattered spots). This is easily visible through the finding of individuals with an intermediate chromatic pattern (Figure 13C).

Moreover, egg masses of D. limbata were documented to be preyed upon by the facelinid Favorinus branchialis.

Dendrodoris temarana Pruvot-Fol, 1953 (about 3–45 mm in length) (Figure 13H–K).

Abundance: 5 specimens.

Temperature range: from 23 to 28 °C.

Seasonality: this species was only documented in June, July, and September.

Substrates/habitats: D. temarana was found on the underside of rocks.

Remarks: The specimens of D. temarana documented here represent the first occurrence of this species in the central Mediterranean. To date, this species (which has a complex taxonomic history) has been found (under different names) in several areas of the western Mediterranean and eastern Atlantic [24]: in the Western Mediterranean, D. temarana has been found along the coasts of France (Thau lagoon), Spain (Catalan coast, El Portil, Cubelles, Ebro Delta, Balearic Islands, and La Línea de la Concepción); in the eastern Atlantic, this dendrodorid has been documented in Spain (Gulf of Cadiz), Portugal (Sado estuary), Morocco (Temara), the Azores, and the Canary Islands.

As reported by Ballesteros [24], this species exhibits considerable morphological variability during both growth and adulthood. Juvenile specimens show a completely red body coloration that changes during development, giving adults different color patterns (red, pink, yellowish, dark brown, and almost black). The color pattern of adults is usually associated with reddish, orange, or brownish spots. However, adults may have uniform black or brown coloration. During this study, two juvenile-colored (completely red) juveniles (Figure 13H), two adult-colored specimens (reddish-orange bodies with dark spots on the back) (Figure 13J,K), and one completely orange specimen (probably a specimen between juvenile and adult) (Figure 13I) were documented. The reddish juveniles were both found in July (one in 2022 and one in 2023), the adults both in June (one in 2022 and one in 2023), and the one with uniformly orange coloration during September 2022.

Family Polyceridae Alder and Hancock, 1845

Polycera quadrilineata (O. F. Müller, 1776) (about 5–6 mm in length) (Figure 13L) Abundance: 1 specimen.

Temperature range: 14.5 °C.

Seasonality: P. quadrilineata was documented in February.

Substrates/habitats: the individual was found on encrusting bryozoans on a Peyssonnelia sp. thallus.

Order Cephalaspidea P. Fischer, 1883

Family Bullidae Gray, 1827

Bulla striata Bruguière, 1792 (only the shell) (about 20–25 mm in length) (Figure 14A–F).

Abundance: 7 specimens.

Temperature range: from 16 to 28 °C.

Seasonality: the shells were found in May, June, August, October, and December.

Substrates/habitats: under and among rocks.

Order Aplysiida

Family Aplysiidae Lamarck, 1809

Aplysia depilans Gmelin, 1791 (about 30 mm in length) (Figure 14G–I).

Abundance: 2 specimens.

Temperature range: the individuals were found from 18 to 20 °C. Instead, the egg masses (Figure 14I) were found at 23 °C.

Seasonality: the animals were documented in April and May. The egg masses were only found in June.

Substrates/habitats: A. depilans was only found on the underside of rocks, always together with sea urchins Arbacia lixula (Linnaeus, 1758) and Paracentrotus lividus (Lamarck, 1816). The egg masses were found on Peyssonnelia spp. Thalli.

Superorder Sacoglossa

Family Plakobranchidae Gray, 1840

Elysia gordanae T. E. Thompson and Jaklin, 1988 (about 5 mm in length) (Figure 14J).

Abundance: 2 specimens.

Temperature range: from 18 to 21 °C.

Seasonality: E. gordanae was documented in April and May.

Substrates/habitats: this species was found on Caulerpa taxifolia and Padina spp.

Elysia timida (Risso, 1818) (about 5–15 mm in length) (Figure 14K–P).

Abundance: 1781 specimens.

Temperature range: E. timida was documented from 14.5 to 28 °C.

Seasonality: this plakobranchid was found all year round. However, the periods with the highest abundance in specimens were the late-end spring, summer, and beginning of winter (Figure 15). The mating (Figure 14N) was all year round except in October.

Substrates/habitats: the favorite substrates of this species were the rocky surfaces covered by very thin (nearly visible) or low turfs of filamentous algae. Often, these algal turfs were mixed with detritus (Figure 14L). Exclusively in April and May, several specimens were seen on the green algae Acetabularia acetabulum (Linnaeus) P.C. Silva (Figure 14M). Although this seaweed was present in the study area also in June and July, it was completely disregarded by E. timida specimens, probably due to its complete calcification.

Besides the abovementioned substrates, E. timida specimens were also found on Jania sp; calcareous encrusting red algae; Peyssonnelia spp.; Halopteris scoparia; Dictyota dichotoma; Taonia sp.; Padina spp.; Flabellia petiolata; Penicillus capitatus Lamarck; Caulerpa cylindracea Sonder; Posidonia oceanica’s rhizomes entanglements; bare rocks and coralligenous gravel/sand.

It was also seen that numerous specimens of E. timida were stationed (always resting on their substrate) at the air-water interface, often remaining exposed to the air for very short periods of time. This did not bother the animals.

Remarks: although Acetabularia acetabulum appears to be the favorite food of E. timida [30,31], only a small portion of the population in the study area was found on thisalgae. On the contrary, most E. timida specimens were found on rocky substrates covered by low turfs of filamentous algae. Consequently, as already hypothesized by Lombardo and Marletta [32] for E. timida populations of the central-eastern coast of Sicily, this species would base much of its nutrition on other algal material. However, it is important to point out that this species has the ability to maintain and retain functioning chloroplasts of A. acetabulum within its digestive gland for at least 45 days by not feeding for long periods of time [31]. Consequently, it is possible that the animals not found in this study on A. acetabulum fed on this alga previously while maintaining functioning chloroplasts (kleptoplasts).

During this study, two teratological E. timida specimens were found. The first animal was devoid of almost the entire body, presenting only the head and a small portion of the anterior part of the pericardial area and parapodia (Figure 14O). This teratology was almost certainly caused by an attack of a hypothetical predator [33].

The second teratological individual had an additional and smallest parapodial-tail portion behind the tail (Figure 14P). This teratology made the animal twice as long as a normal specimen. Given that this individual had no wounds or scars, it is likely that this strange teratology was genetic in origin.

Finally, it was documented an “attack” suffered from an E. timida specimen by a small fish of the genus Parablennius, who swallowed the first one whole and then immediately spat it out and ignored it.

Elysia viridis (Montagu, 1804) (about 5–10 mm in length) (Figure 16A,B).

Abundance: 2 specimens.

Temperature range: 15 °C.

Seasonality: E. viridis was documented in January and March.

Substrates/habitats: this sacoglossan was found on turfs of filamentous red algae and Flabellia petiolata.

Thuridilla hopei (Vérany, 1853) (about 5–10 mm in length) (Figure 16C).

Abundance: 2 specimens.

Temperature range: from 15 °C to 26 °C.

Seasonality: T. hopei was documented in January and September.

Substrates/habitats: this species was found on Halopterisscoparia and Jania sp.

Family Hermaeidae H. Adams et A. Adams, 1854

Cyerce cristallina (Trinchese, 1881) (about 25–30 mm in length) (Figure 16D–H).

Abundance: 2 specimens.

Temperature range: 28 °C.

Seasonality: this species was found only in July.

Substrates/habitats: the two specimens were found together on the underside of a rock covered by calcareous encrusting red algae Figure 16D.

Remarks: once exposed to light, the two animals began to rapidly crawl toward the shaded areas. Moreover, one of the animals had three malformations on its body. Specifically, this individual had several cerata of the anterior part of the body that were much smaller than normal, the right oral tentacle bent medially with an additional small tentacle originating from the bent point, and the right eye displaced towards the midline of the body (giving the feeling of being lacking) (Figure 16G,H). All these malformations were probably caused by a severe attack by a putative predator. This was easily detectable by the evident disproportion of the right anterior cerata compared to those of the rest of the body and by the fact that the other malformations were on the same side of the body. The oral tentacle malformation can be explained as a poor regrowth of the previously detached right oral tentacle. However, the movement of the right eye towards the reddish central part of the head is difficult to explain well.

4. Discussion

The sea slug fauna of the Magnisi peninsula comprised 37 species, which were divided into five taxonomic groups: Pleurobranchida (2 species), Nudibranchia (28), Cephalaspidea (1), Aplysiida (1), and Sacoglossa (5). This fauna was dominated by a large population of the sacoglossan Elysia timida, which represented the most numerous sea slug population of the area with its 1781 found specimens (829 in 2022 and 952 in 2023). It is notable that none of the other species’ populations reached this level of abundance. The other most abundant species of the area were Edmundsella pedata (55 specimens), Felimare gasconi (30), Facelina rubrovittata (29), Berthellina cf. edwardsii (26), Cratena peregrina (16), Dendrodoris limbata (13), Spurilla neapolitana (9), Calmella cavolini (8), Felimare villafranca (7), and Bulla striata (only the shell) (7). All remaining species were observed on only a few occasions (from one to five times) during the study period (Figure 17).

This fauna was dominated by a diverse community of sea slug species, indicative of brackish-water/euryhaline habitats and shallow waters (whose favorite habitat was the underside of rocks), which overall co-occurred with some more or less common/widely distributed species. Among the species found, those belonging to the first category were in order of abundance: Elusia timida, Dendrodoris limbata, Felimare villafranca, Bulla striata, Polycera quadrilineata, and Elysia viridis. In particular, Elusia timida is known to be a very common sacoglossan of brackish lagoons [34]; the dendrodorid Dendrodoris limbata is not only a very common nudibranch in lagoons [34,35], but it also exhibits invasive tendencies in such a type of habitat [36]; Felimare villafranca, along with other chromodoridid species, appears to be a common inhabitant of lagoonal habitats [34,35,37] as well as Bulla striata [34], Polycera quadrilineata, and Elysia viridis [35]. In contrast, Berthella stellata [38,39], Calmella gaditana [40], Aporodoris cf. millegrana [40], Baptodoris cinnabarina [37], Taringa sp. (as genus Taringa in general) [40], Doridina sp. [38], Dendrodoris grandiflora [40], Dendrodoris temarana [24], and Cyerce cristallina [41,42] are shallow water species that typically inhabit habitat beneath rocks. It is notable that several species documented during this study fall under both of these two habitat categories, namely brackish-water/euryhaline and shallow-water species that live under rocks. Examples of this include Berghia verrucicornis [35,37], Spurilla neapolitana [37,43], Favorinus branchialis [34,43] Calmella cavolini [34,35,37], Jorunna tomentosa [35,37], Tayuva lilacina [36,40,44] and Aplysia depilans [35,43]. In terms of the category of those species that can be considered more or less common/widely distributed species, the following species were found during this study: Cratena peregrina [37], Facelina rubrovittata [45], Edmundsella pedata [37], Trinchesia foliata [45], and Elysia viridis [21].

This faunistic composition would reflect the distinctive combination of environmental conditions that characterize the examined area. The coastal stretch under investigation is located in the northernmost part of the gulf, which is formed between the Magnisi peninsula (to the north) and Punta Cannone (to the south). The examined area is situated in the most secluded part of the gulf, which is predominantly characterized by very shallow and extensive sandy bottoms with a depth of less than one meter.

The combination of the concealed location of this site and the sandy bottoms, which almost completely surround the shallow stony studied area, could probably result in a reduction in the seawater flow from and to the open area of the gulf. Furthermore, this coastal stretch is subject to freshwater inflows, which may be attributed to industrial discharges [46]. Consequently, although the examined coastal stretch is not a real brackish-water/lagoon/estuarine area, its environmental conditions are similar to those of brackish environments due to the abovementioned reasons. This is evidenced by the presence in the study area of the green algae Penicillus capitatus, which is considered a common algal species of estuarine-marine/brackish habitats [47]. It can thus be argued that the specific environmental conditions present in this area, characterized by a shallow rocky seabed with a considerable amount of scattered pebbles, boulders, and rocks, provide an adequate rationale for the occurrence of both brackish-water/euryhaline species, along with those typical of shallow waters.

The presence of the common/widely distributed species, along with occasional ones, may indicate an enduring connection with the external area, exemplified by the coastal stretch examined at the foot of the southeast small cliff, where the surrounding sandy bottoms deepen by a few meters. This may nullify the stagnation effect observed in the neighboring coastal stretch located northward.

This study revealed that the most representative sea slug group in the area was the order Nudibranchia (28 species). The temporal variations in the number of species within this group can provide valuable insights into the seasonal dynamics of the marine waters of a given coastal area [48]. Similarly, some inferences can be drawn regarding this coastal area.

Given the very shallow nature and the general ecological conditions of the study area, the documented temporal differences in the number of nudibranch species in the study area may be the result and consequence of two environmental parameters: temperature and wave disturbance.

The occurrence of nudibranchs is affected by temperature in both direct and indirect ways. Directly, temperature affects the specific thermal sensitivity of each species (e.g., cold water/northern species vs. warm water/southern species [49,50]) [51]. Indirectly, temperature changes affect the presence of nudibranchs’ prey, seawater stratification, phytoplankton blooms, and the arrival of allochthonous nudibranchs’ larvae, among other factors [48].

The physical disturbance caused by waves on rocky substrates plays a significant role in determining the level of biodiversity in a given area [52]. In shallow, stony marine environments, wave action results in the displacement of rocks. The frequency of disturbance to stones, rocks, and pebbles determines the time available for sessile benthic suspension feeders (e.g., bryozoans, cnidarians, tunicates, and sponges [53], the favorite preys of nudibranchs [48]) to colonize these specific surfaces [52]. It is notable that the underside of rocks and spaces between them are among the most highly favored habitats for such mollusks, e.g., [23,54]. Therefore, the displacement, overturning, and impacts caused by wave motion play a pivotal role in determining the numbers of species and specimens of these gastropods in very shallow coastal areas.

The wave disturbance observed in this study area, when generated by sea storms, has the potential to be very impactful. In addition to the violent shifting and overturning of rocks, these events can result in the movement and suspension of a considerable amount of sediment.

The present study revealed that the highest number of nudibranch species was observed in both 2022 and 2023 during the winter season, when the lowest temperature values were recorded (Figure 4). This winter species abundance is in accordance with two recent findings concerning the seasonality of Mediterranean nudibranchs, which highlighted that the cooling of marine waters is always associated with an increase in the number of species [48,50]. With the exception of this similarity, the other seasons displayed a different trend over the two years. This discrepancy is likely attributable to the differential wave disturbance observed in 2022 and 2023. In 2022, a single sea storm event occurred in the area during March. Therefore, the species trend observed in 2022 was likely influenced by the seasonal changes in temperature, whose gradual increase determined the constant decline in species number across seasons. In contrast, in 2023, the coastal area under study was subjected to three sea storms, one in winter [February (very strong)] and two in spring (April and May). Consequently, during this year, the typical seasonal trend in the number of nudibranch species was likely offset by the prolonged and intense wave disturbance. Additionally, the impact of this disturbance may have altered the usual temperature trend, delaying the expected trend for a few months (Figure 2). Consequently, the spring of 2023, which was characterized by a near-continuous wave disturbance, exhibited a lower number of species compared to that of 2022. Conversely, the summer of 2023, and subsequently also the autumn of 2023, were distinguished by an exceptionally high number of species.

The temporal variation in nudibranch species number may, therefore, serve as an indicator of the seasonal changes that occur along the water column, including the temporal phytoplanktonic variations and seawater stratification, as recently proposed by Lombardo and Marletta [48]. However, it may also reflect the spatio-temporal variations in temperature and wave disturbance in very shallow environments.

The potential for making comparisons between the sea slug fauna of the Magnisi peninsula and faunas of other Mediterranean coastal areas is constrained by the lack of prior studies (to our knowledge) employing the methods outlined herein. Nevertheless, between 1984 and 1986, a noteworthy study into the sea slug fauna of the Marsala Stagnone, the largest Sicilian lagoon, was conducted using similar methods [34]. In this study, the authors found a total of 23 sea slug species, subdivided into five taxonomic groups: Cephalaspidea (6 species), Sacoglossa (1), Aplysiida (1), Pleurobranchida (2), and Nudibranchia (13). It should be noted that the present study differs from that of Cattaneo-Vietti and Chemello [34] in that the present study examined a single site (approximately 200 m in length), whereas the previous one [34] investigated ten sites distributed across approximately 4 km of the lagoon. From this comparison, it can be highlighted that despite the relatively limited coastal area of just 200 m investigated here, there is almost twice the number of sea slug species found in the examined 4 km of coastline of the Marsala Stagnone. Therefore, given its concealed and shallow location, coupled with the local environmental status, which is affected by industrial pollution, this coastal stretch of eastern Sicily could be considered a small biodiversity hotspot for the sea slug fauna.

5. Conclusions

In conclusion, after 53 years from the latter report [17], the data reported in this study demonstrated that the Magnisi peninsula exhibits markedly high marine mollusk biodiversity compared to similar shallow areas. Notwithstanding the considerable damage inflicted in the past on the marine environment by local industries, the presence of a terrestrial Natural Oriented Reserve (NOR) and Natura 2000 site could serve as a foundation for the conservation and protection of this area, which hosts significant marine habitats and species. As evidence, this study presented findings of species considered rare, such as Aporodoris cf. millegrana [55]. Tayuva lilacina [44], Cyerce cristallina [41], and unidentified species may represent new species, such as Trinchesia sp., Aeolid sp., and Doridina sp. Moreover, the area examined here represents the only known occurrence where Dendrodoris temerana was found outside its documented range of distribution (see above) with a stable population.

It will become increasingly important in the future to focus attention on the naturalistic significance of this small stretch of coastline and emphasize the enhancement of the natural marine environment of this small gulf, which represents a hotspot of diversity for marine sea slugs.

Author Contributions

Conceptualization, A.L. and G.M.; methodology, A.L.; validation, G.M.; data curation, A.L.; writing—original draft preparation, A.L.; writing—review and editing, G.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors wish to thank three anonymous reviewers and the journal editor for their constructive comments and suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Study area (A) Sicily and the location of the gulfs of Augusta and Priolo–Melilli; these latter are indicated by the white box in the lower right corner (the white box in the top right corner shows the position of Sicily (Italy) in the Mediterranean Sea); (B) Gulfs of Augusta and Priolo–Melilli, the Magnisi peninsula is indicated by the white box; (C) the Magnisi peninsula, the study area is indicated by the white box; (D) in the examined coastal stretch, the location of the two different bottom types is indicated by the different colorations. The arrow indicates the start point and the followed direction of the snorkeling path; (E) a general view of the examined coastal tract; (F,G) the appearance of the bottom of the first area, edge colored in green; (H) aspect of the bottom of the second area, edge colored in yellow.

Figure 2. Monthly temperature values in the study area.

Figure 3. Monthly variation in observed nudibranch species number.

Figure 4. Seasonal variation in observed nudibranch species number.

Figure 5. Berthellina cf. edwardsii (A) two specimens with the resting/contracted spherical body shape; (B) a specimen with its rhinophores slightly exposed; (C) a fully extended individual; (D) ventral view; (E) a yellowish specimen; (F) the teratological individual with the peculiar separation of the posterior part of the notum; (G) the specimen that lacked some part of the mantle margin. Berthella stellata (H) a specimen partially extended; (I) an individual with a very flattened appearance; (J) egg mass.

Figure 6. Berghia verrucicornis (A) left antero-lateral view ; (B) dorsal view; (C) two specimens during mating. Spurilla neapolitana; (D,E) two specimens found on algae; (F–H) two individuals found beneath rocks.

Figure 7. Cratena peregrina (A–C) three specimens. Facelina annulicornis (D) a specimen in the right dorsolateral; (E) the wounded individual. Facelina rubrovittata (F,G) two small specimens. (H) left view of a specimen; (I) left dorsolateral view; (J) dorsal view. Favorinus branchialis (K) a specimen feeding on the egg masses of Dendrodoris limbata.

Figure 8. Calmella cavolini (A–E) Some specimens. Calmella gaditana (F) dorsal view. Edmundsella pedata (G) the bluish individual; (H,I) normal specimens; (J) an egg mass.

Figure 9. Doto paulinae (A) a specimen with its egg mass on Aglaophenia sp.; (B) a small specimen; (C) an egg mass. Trinchesia foliata (D) left lateral view; (E) dorsal view. Trinchesia morrowae (F) dorsal view. Trinchesia sp. (G) dorsal view. Aeolid sp. (H) right dorsolateral view. Candiella manicata (I) right lateral view; (J) the same in anterior-left view; (K) a specimen on its substrate.

Figure 10. Felimare gasconi (A–C) specimens with the juvenile chromatic pattern; (D–F) individuals with the intermediate chromatic pattern; (G–J) specimens with the final chromatic pattern. Felimare picta (K) right dorsolateral view; (L) dorsal view. Felimare villafranca (M,N) specimens with anterior and posterior mantle edges white colored; (O,P) specimens with darker body color and complicated chromatic pattern.

Figure 11. Aporodoris cf. millegrana (A) dorsal view. Baptodoris cinnabarina (B) dorsal view; (C,D) detail of the lacking mantle’s portions; Jorunna tomentosa (E) dorsal view; (F) egg mass. Taringa sp. (G) two specimens during mating.

Figure 12. Tayuva lilacina (A,B) specimens in dorsal view; (C) two individuals during mating; (D) a specimen with wrinkled mantle margin. Doridina sp. (E,F) dorsal view. Dendrodoris grandiflora (G) dorsal view; (H) the same in a semi-resting/contracted body shape.

Figure 13. Dendrodoris limbata (A,B) small specimens with the black-brown mottled chromatic pattern; (C) an individual with the intermediate chromatic pattern; (D–F) large specimens with the light-colored chromatic pattern; (G) an egg mass. Dendrodoris temarana (H) a red juvenile; (I) an orange intermediate specimen; (J,K) two adult specimens. Polycera quadrilineata (L) right dorsolateral view.

Figure 14. Bulla striata (A–F) has several empty shells. Aplysia depilans (G) right dorsolateral view; (H) left dorsolateral view; (I) an egg mass. Elysia gordanae (J) dorsal view. Elysia timida (K) right dorsolateral view; (L) some specimens on their preferred substrate; (M) a specimen on the green algae Acetabularia acetabulum; (N) two individuals during mating; (O) the specimen that lacked almost all the body; (P) the individual with an additional and smallest parapodial-tail portion behind the tail.

Figure 15. Variation in abundance of Elysia timida over the study period.

Figure 16. Elysia viridis (A) dorsal view; (B) a specimen on Flabellia petiolata. Thuridilla hopei (C) dorsal view. Cyerce cristallina (D) the two specimens were found together on the underside of a rock; (E) the two individuals on an artificial substrate; (F) the normal individual; (G,H) the teratological one.

Figure 17. Abundance of the found sea slug species without Elysia timida.

Table 1. Sea slug species documented in the literature of the Magnisi peninsula.

Taxa	References
Superorder Ringiculimorpha
Family Ringiculidae R. A. Philippi, 1853
Ringicula auriculata (Ménard de la Groye, 1811)	[10,11]
Ringicula gianninii F. Nordsieck, 1974	[12,14]
Order Pleurobranchida
Family Pleurobranchidae Gray, 1827
Berthella aurantiaca (Risso, 1818)	[16]
Order Cephalaspidea
Family Bullidae Gray, 1827
Bulla striata Bruguière, 1792	[17]
Family Retusidae Thiele, 1925
Retusa laevisculpta (Granata Grillo, 1877)	[16]
Retusa mammillata (R. A. Philippi, 1836)	[10,11]
Retusa umbilicata (Montagu, 1803)	[12]
Family Rhizoridae Dell, 1952
Volvulella acuminata (Bruguière, 1792)	[10,11,12]
Family Haminoeidae Pilsbry, 1895
Haminoea navicula (da Costa, 1778)	[12,17]
Roxaniella jeffreysi (Weinkauff, 1866)	[16]
Weinkauffia turgidula (Forbes, 1844)	[16]
Family Philinidae Gray, 1850 (1815)
Philine quadripartite Ascanius, 1772	[17]

Table 2. List of all sea slug species found during this study. The numbers indicate the number of specimens documented for each month. The symbols + and ‡ indicate the finding of egg masses and the observation of mating, respectively.

Taxa	Year	January	February	March	April	May	June	July	August	September	October	November	December
Order Pleurobranchida
Family Pleurobranchidae Gray, 1827
Berthellina cf. edwardsii (Vayssière, 1897)	2022	2	0	0	1	0	0	0	2	2	1	0	1
	2023	0	0	0	0	0	0	2	4	6	3	2	0
Berthella stellata (Risso, 1826)	2022	0	0	0	0	0 +	0	0	0	0 +	0 +	0	0
	2023	0	0	0	2	0	0 +	0	0	0	0	0	0
Order Nudibranchia
Suborder Cladobranchia
Family Aeolidiidae Gray, 1827
Berghia verrucicornis (A. Costa, 1867)	2022	0	0	0	3 ‡	1	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Spurilla neapolitana (Delle Chiaje, 1841)	2022	0	2	0	0	0	0	0	0	0	0	0	0
	2023	0	0	1	1	0	0	2	0	1	1	0	1
Family Facelinidae Bergh, 1889
Cratena peregrina (Gmelin, 1791)	2022	8	1	0	0	0	0	0	0	0	0	0	1
	2023	3	0	1	0	0	0	0	0	0	0	0	2
Facelina annulicornis (Chamisso and Eysenhardt, 1821)	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	2	0	0	0	0	0	0	0	0
Facelina rubrovittata (A. Costa, 1866)	2022	6	13	0	6	2	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	1	0	0	1
Favorinus branchialis (Rathke, 1806)	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	1
Family Flabellinidae Bergh, 1889
Calmella cavolini (Vérany, 1846)	2022	0	0	0	3	0	0	1	0	0	0	1	0
	2023	0	0	0	0	0	0	0	1	1	0	0	1
Calmella gaditana (Cervera, Arcega-Gómez and F. J. García, 1987)	2022	0	1	0	0	2 ‡	0	0	0	0	0	0	0
	2023	0	0	2	0	0	0	0	0	0	0	0	0
Edmundsella pedata (Montagu, 1816)	2022	15 +	1	0	0	0	0	0	0	0	0	0	0
	2023	6	8 +	3	0	0	0	0	0	0	0	0	22
Family Dotidae Gray, 1853
Doto paulinae Trinchese, 1881	2022	0	1 +	0	0	0	0	0	0	0	0	0	0
	2023	1 +	0	0	0	0	0	0	0	0	0	0	0 +
Family Trinchesiidae F. Nordsieck, 1972
Trinchesia foliata (Forbes and Goodsir, 1839)	2022	0	2	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Trinchesia morrowae Korshunova, Picton, Furfaro et al., 2019	2022	0	0	0	1	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Trinchesia sp.	2022	1	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Aeolid sp.	2022	0	0	0	1	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Family Tritoniidae Lamarck, 1809
Candiella manicata (Deshayes, 1853)	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	1	0	0	0
Suborder Doridina
Family Chromodorididae Bergh, 1891
Felimare gasconi (Ortea, 1996)	2022	0	2	0	0	0	0	0	0	0	0	0	0
	2023	0	5	1	0	1	0	0	6	4	4	2	5
Felimare picta (R. A. Philippi, 1836)	2022	1	0	0	0	0	0	0	1	0	2	0	0
	2023	0	0	0	1	0	0	0	0	0	0	0	0
Felimare villafranca (Risso, 1818)	2022	0	0	0	0	0	0	0	1	0	0	1	0
	2023	2	1	0	0	1	1	0	0	0	0	0	0
Family Discodorididae Bergh, 1891
Aporodoris cf. millegrana (Alder and Hancock, 1854)	2022	0	0	0	0	0	0	0	1	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Baptodoris cinnabarina Bergh, 1884	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	1	0	0	0	0	0	0
Jorunna tomentosa (Cuvier, 1804)	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	1 +	0	0	0	0	0	0	0	0
Taringa sp.	2022	0	0	0	0	0	2 ‡	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Tayuva lilacina (A. Gould, 1852)	2022	0	0	0	0	0	0	0	0	1	0	0	0
	2023	0	0	0	0	0	0	2 ‡	0	0	0	0	0
Doridina sp.	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	1	0	0	0	0
Family Dendrodorididae O’Donoghue, 1924 (1864)
Dendrodoris grandiflora (Rapp, 1827)	2022	0	0	0	0	0	0	1	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Dendrodoris limbata (Cuvier, 1804)	2022	0	1	0	0	0	1	2	1	1	1	0 +	2
	2023	1	0	0	0	0	0	2	1	0	0	0	0 +
Dendrodoris temarana Pruvot-Fol, 1953	2022	0	0	0	0	0	1	1	0	1	0	0	0
	2023	0	0	0	0	0	1	1	0	0	0	0	0
Family Polyceridae Alder and Hancock, 1845
Polycera quadrilineata (O. F. Müller, 1776)	2022	0	0	0	0	0	0	0	0	0	0	0	0
	2023	0	1	0	0	0	0	0	0	0	0	0	0
Order Cephalaspidea
Family Bullidae Gray, 1827
Bulla striata Bruguière, 1792 (shell only)	2022	0	0	0	0	0	1	0	0	0	0	0	1
	2023	0	0	0	0	1	0	0	2	0	1	0	1
Order Aplysiida
Family Aplysiidae Lamarck, 1809
Aplysia depilans Gmelin, 1791	2022	0	0	0	1	0	0	0	0	0	0	0	0
	2023	0	0	0	0	1	0 +	0	0	0	0	0	0
Superorder Sacoglossa
Family Plakobranchidae Gray, 1840
Elysia gordanae T. E. Thompson and Jaklin, 1988	2022	0	0	0	1	1	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Elysia timida (Risso, 1818)	2022	53 ‡	75 ‡	10	70 ‡	108 ‡	184 ‡	88 ‡	94 ‡	42 ‡	17	37	51
	2023	96	38 ‡	65 ‡	15	18	109	98 ‡	146	43	83	73 ‡	168 ‡
Elysia viridis (Montagu, 1804)	2022	1	0	1	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0
Thuridilla hopei (Vérany, 1853)	2022	1	0	0	0	0	0	0	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	1	0	0	0
Family Hermaeidae H. Adams et A. Adams, 1854 Cyerce cristallina (Trinchese, 1881)	2022	0	0	0	0	0	0	2	0	0	0	0	0
	2023	0	0	0	0	0	0	0	0	0	0	0	0

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Lombardo, A.; Marletta, G. An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean). Oceans 2024, 5, 695-719. https://doi.org/10.3390/oceans5030040

AMA Style

Lombardo A, Marletta G. An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean). Oceans. 2024; 5(3):695-719. https://doi.org/10.3390/oceans5030040

Chicago/Turabian Style

Lombardo, Andrea, and Giuliana Marletta. 2024. "An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean)" Oceans 5, no. 3: 695-719. https://doi.org/10.3390/oceans5030040

APA Style

Lombardo, A., & Marletta, G. (2024). An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean). Oceans, 5(3), 695-719. https://doi.org/10.3390/oceans5030040

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An Unexpected Small Biodiversity Oasis of Sea Slugs (Mollusca, Gastropoda, Heterobranchia) in the Largest Petrochemical Hub of Italy (Central Mediterranean)

Abstract

1. Introduction

2. Materials and Methods

3. Results

3.1. General Section

3.2. Faunistic Section

4. Discussion

5. Conclusions

Author Contributions

Funding

Institutional Review Board Statement

Informed Consent Statement

Data Availability Statement

Acknowledgments

Conflicts of Interest

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