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Article

A New Species of Anthocotyle (Polyopisthocotyla: Discocotylidae) from the Gills of the European Hake Merluccius merluccius (Teleostei, Merlucciidae) with a Revision of the Composition of the Genus

by
Chahinez Bouguerche
1,2
1
Department of Zoology, Swedish Museum of Natural History, P.O. Box 50007, SE-104 05 Stockholm, Sweden
2
Institut Systématique Évolution Biodiversité (ISYEB), Muséum National d’Histoire Naturelle, 43 Rue Cuvier, CP 51, 75231 Paris Cedex 05, France
Parasitologia 2025, 5(1), 2; https://doi.org/10.3390/parasitologia5010002
Submission received: 21 October 2024 / Revised: 22 December 2024 / Accepted: 23 December 2024 / Published: 8 January 2025
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Abstract

:
This study revisits the taxonomy of Anthocotyle merluccii, originally described from the European hake Merluccius merluccius in the northeast Atlantic, addressing discrepancies in clamp morphology across populations. The original description from Belgium noted near-equal anterior clamp sizes, contrasting with populations from Plymouth (Atlantic) and the Mediterranean, which show marked size differences, questioning their conspecificity. We describe A. radkeaminorum n. sp. from M. merluccius in the western Mediterranean (off Algeria), distinguished from A. merluccii (Belgium) by differing anterior clamp size, genital atrium spine number, and overall anterior clamp dimensions. Populations from Plymouth, previously attributed to A. merluccii, are herein assigned to A. aff. merluccii based on differences in morphometrical traits pending further investigations. Additionally, A. radkeaminorum n. sp. differs from A. americanus in body and clamp size, atrial spine count, and hosts. Based on analysis of morphological and molecular data, we refute the synonymy of A. merluccii and A. americanus, and we reinstate the latter as a valid species. The distinction between A. merluccii and A. americanus was further supported by divergence in cox1 gene sequences analyzed from GenBank (10–11%). Finally, inconsistencies in terminal lappet hook morphology are discussed, cautioning against its use in species delineation. This work highlights the need for continued research to resolve species relationships within this genus.

1. Introduction

Fishes of the genus Merluccius constitute an important demersal resources, both worldwide and in the Mediterranean [1]. The European hake M. merluccius is a widely distributed merlucciid species found in the Mediterranean Sea and the northeastern Atlantic Ocean, ranging from the coasts of Norway and Iceland to Mauritania. It also occurs in the Black Sea, along the southern coasts. This species is denoted as a substantial component of demersal fish assemblages [1] and represents about 2% of the average total landings in the Mediterranean Sea [2]. To date, of the twelve known species of hake, only M. merluccius is found throughout the eastern North Atlantic and Mediterranean waters [1] with two subspecies, one occurring in the Atlantic and the other one occurring in the Mediterranean [3]. However, there is also the possibility a plurality of stocks for the Mediterranean subspecies [1] and other Merluccius species in oceanic waters [4]. Different endoparasites and one ectoparasite were proposed as biological tags for future monitoring of ecological stocks for Merluccius species [4]. Moreover, significant differences in the distributions of a various parasitic taxa suggest the existence of distinct stocks [5], notably for the Mediterranean species M. merluccius. For the latter species, Mattiucci et al. [6] analyzed metazoan parasite communities from 13 stations in the Mediterranean and northeast Atlantic using a multivariate analysis and found significant differences between areas, highlighting the importance of exploring the parasite fauna of these species.
Overall, metazoan parasites of hakes have been extensively studied in both Mediterranean and Atlantic populations. Gibson et al. [7] compiled the Host–Parasite Database at the Natural History Museum (London, UK), in which over fifty references detailing helminth parasites—monogeneans, digeneans, cestodes, nematodes, and acanthocephalans—are associated with hake [8]. However, to date, only two “monogenean” species are known from hakes: the gyrodactylid Gyrocerviceanseris passamaquoddyensis Cone, Abbott, Gilmore & Burt, 2010, first described from the Silver Hake, M. Bilinearis, in the Bay of Fundy, Canada, Northwest Atlantic [9]; and the discocotylid Anthocotyle merluccii Van Beneden & Hesse, 1863, first described from the European hake M. merluccius off Belgium, Northeast Atlantic (Van Beneden & Hesse, 1863). The latter species, which is the type species of the genus [10], was originally described from specimens found on the gills of M. vulgaris (a junior synonym of M. merluccius [11]) collected off the coast of Belgium [12]. Anthocotyle merluccii has since been recorded on various Merluccius species in both the eastern and western North Atlantic [13] and in Mediterranean waters [14,15]. MacCallum [16] proposed a subspecies, A. merluccii americanus MacCallum, 1916, which was later elevated to species rank as A. americanus (MacCallum, 1916) by Price [17]. However, Sproston [18], Dawes [19], and Brinkmann [20] considered A. americanus to be synonymous with A. merluccii [13].
In recent years, several studies have shown that the biodiversity of polyopisthocotylans of marine fish in the southern part of the Mediterranean is far from being completely known [21]. During a parasitological study of polyopisthocotylans of economically important fishes off the Algerian coast, we collected representatives of an undescribed species of Anthocotyle on the gills of M. merluccius, which is described here. The description of this new species is based on a combination of distinct morphological and morphometrical characteristics that set it apart from known species within the same genus, revealing significant differences from closely related species, particularly in the size and measurements of clamps, the count number of atrial spines supporting its classification as a new species. This finding is particularly significant given the limited knowledge of similar species in the region, contributing to a deeper understanding of the biodiversity of Polyopisthocotyla in northern Africa. We also discuss differences in Atlantic populations of A. merluccii and reinstate A. americanus as a valid species based on available morphological and molecular evidence.

2. Results

2.1. Morphology

Class Polyopisthocotyla Odhner, 1912
Family Discocotylidae Price, 1936
Genus Anthocotyle Van Beneden & Hesse, 1863
Anthocotyle radkeaminorum n. sp.
ZooBank: To comply with the regulations set out in article 8.5 of the amended 2012 version of the International Code of Zoological Nomenclature (ICZN), details of this species have been submitted to ZooBank with the Life Science Identifier (LSID) zoobank.org: urn:lsid:zoobank.org:pub:AEC7E6EC-9C86-4849-B87E-DE2FFA973EC8.
Type host: European hake Merluccius merluccius (Gadiformes, Merlucciidae).
Type locality: Off Bouharoun, Algeria, western Mediterranean.
Site on host: Gills lamellae.
Deposited material: Holotype (Type-9923), 14 paratypes (Type 9924–9937) deposited in type collections of the Swedish Museum of Natural History (SMNH).
Etymology: The specific epithet “radkeaminorum” is a heartfelt tribute to Branko Radujković from the University of Montenegro and Nadia Kechemir-Issad, Faouzi Amine, and Faiza Amine from the Université des Sciences et de la Technologie Houari Boumediene of Algeria. The name combines parts of their surnames to honor their valuable contributions as dedicated professors, researchers, and lecturers, whose work has greatly enriched the fields of parasitology and biodiversity research in the western Mediterranean.

2.1.1. Description

Body slender anteriorly, massive at level of ovary, narrowing and tapering at level of haptor (Figure 1A).
Haptor wide, orientated symmetrically to the body axis and visibly separated from body proper, bearing four pairs of clamps and a terminal lappet.
Clamps dissimilar in size and in type, with a variable disparity in the development of the right and left clamps of the anterior pair, giving a clear asymmetrical appearance to the posterior end of the body.
First pair of clamps sessile, highly developed, discocotylid-type (Figure 1B). Remaining clamps of the other three pairs distinctly smaller of microcotylid type (Figure 1C).
Anterior discocotylid-type clamps consisting of two jaws: anterior (Figure 2A) and posterior jaw (Figure 2B). Anterior jaw with an inverted U-shaped sclerotized median sclerite a1; a1 giving rise to an expansion in each jaw. In anterior jaw, a1 ending distally in T shape with relatively short branches. Proximally, a1 folding and giving rise to a2; a2 passing into posterior jaw. Margins of anterior jaw supported by two arched sclerites b. Distally, b ending parallel to a’. Proximally, each arched sclerites b folding back to widen the posterior jaw. Posterior jaw marked at its center by a median sclerite a2, which continues to a3. Distal end of a3 forming an inverted V with long branches. This end is hollowed out by a V-shaped notch with pointed edges. Below a3, two small crescent-shaped sclerotized a’ pieces are articulated (Figure 2C). Margins of posterior jaw supported by two arched lateral sclerites c. On proximal side, a muscle connecting a2 to b.
Small microcotylid-type clamps, consisting of two jaws: anterior (Figure 3A) and posterior jaw (Figure 3B). Ventral arm of median spring a1 T-shaped, long, distal part of a1 with short branches of equal size. Dorsal arm of median spring a2 slightly shorter than its ventral arm, a1, T-shaped, distally broad. Ventral arm of ventral jaw consisting of two lateral sclerites b1; dorsal arm b2, shorter and curved inwards; b3 not reaching dorsal arm of median spring. Dorsal jaw sclerites c not reaching midline on distal side. Muscles connecting a and b2 present on proximal side (Figure 3C).
Terminal lappet present, with two pairs of hooks (Figure 1D): posterior pair with two large hooks with a pronounced base; inner pair with two hooks with a curved, tapering base. Mouth sub-terminal (Figure 4). Oral suckers muscular, paired, oval, opening laterally; oral suckers divided by a longitudinal septum into two subequal chambers. Pharynx muscular, voluminous. Esophagus bifurcating into two caeca of unequal length. Caeca with numerous lateral diverticula, not joining posteriorly; only right caeca extending into haptor.
Testes post-ovarian, consisting of numerous relatively small follicles, located in intercaecal space of median quarter of body and largely obscured by intestinal diverticula and vitellaria. Vas deferens sinuous, extending anteriorly and opening into genital atrium. Genital atrium (Figure 1E) armed with a circle of slender hooks with bifid bases (Figure 1F). Atrial hooks inserted at different levels. Ovary pre-testicular, sinuous, located in middle third of body (Figure 5). Ovary nesting posteriorly, rising anteriorly and passing over to left side; it then descends, forming an inverted V shape. Oviduct detaching from ovary and receiving genito-intestinal canal. Genito-intestinal canal opening into left intestinal branch. Oötype followed by uterus. Uterus narrow, ascending dorsally along midline to genital opening. Vitelline follicles large, forming two lateral bands around the caeca. Transverse vitelloducts uniting ventrally on midline, forming a short odd vitelline reservoir. Vitelline reservoir opening into oviduct. Vaginae paired, submarginal, opening ventro-laterally near right and left body margins. Eggs oval with short filaments at each pole (Figure 1G).
Remark: The subsequent European records of A. merluccii, mainly that of Sproston [18] from Plymouth, reveal significant differences when compared to A. merluccii (sensu stricto), originally described from Belgium. Specifically, the Plymouth specimens exhibit a noticeable dissimilarity in the size of the clamps of the first pair, while those of A. merluccii (sensu stricto) are of a similar size. Furthermore, the number of atrial hooks in the Plymouth population exceeds 50, compared to 40 in the Belgian specimens. These morphological differences should be investigated further to support the recognition of the Plymouth population as a distinct species, and we refer to it herein as A. aff. merluccii Van Beneden & Hesse, 1863.

2.1.2. Differential Diagnosis

Morphometric data of A. radkeaminorum n. sp. are presented in Table 1, alongside data for other Anthocotyle spp. Herein, the newly collected discocotylids from M. merluccius off Algeria can be easily differentiated from A. merlucii off Belgium by the clamps of the first pair being dissimilar in size in A. radkeaminorum n. sp. vs. of equal size in A. merlucii [12,22]. Additionally, A. radkeaminorum n. sp. has less spines at level of the genital atrium (28–36 in A. radkeaminorum n. sp. vs. 40 in A. merlucii). The type localities are distinct (Algeria, western Mediterranean for A. radkeaminorum n. sp. vs. Belgium, northeast Atlantic for A. merlucii). Anthocotyle radkeaminorum n. sp. can be distinguished from A. aff. merluccii [18] by having a smaller anterior right clamp (450–870 (720) vs. 665, 625 in width) and a smaller anterior left clamp (610–855 (710) vs. 1100, 1250 in width). Moreover, A. radkeaminorum n. sp. has fewer spines in the genital atrium (28–36 for A. radkeaminorum n. sp. vs. over 50 for A. aff. merluccii), and the type localities are distinct (Algeria, western Mediterranean for A. radkeaminorum n. sp. vs. Plymouth, northeast Atlantic for A. aff. merluccii). Anthocotyle radkeaminorum n. sp. is differentiated from A. americanus [16] by having a smaller body (6000–8000 × 600–1100 vs. 9000–12,000 × 1200–2000), smaller anterior clamps (148 × 720 vs. 1200), smaller posterior clamps (38 × 60 vs. 50), and less spines in genital atrium (28–36 (35) vs. 35–38). Additionally, type hosts (M. merluccius vs. M. bilinearis) and type localities (Algeria, western Mediterranean vs. Massachusetts, USA, northwest Atlantic) are widely separated.

2.2. Molecular Characterization

Available partial cox1 sequences of A. merluccii ex M. merluccius, off France, and of A. americanus ex M. gayi, off Peru and Chile, southeast Pacific, and ex M. hubbsi, off Argentina, southwest Atlantic, were analyzed together with all available sequences for Discocotylidae. The plectanocotylid Plectanocotyle gurnardi (Van Beneden & Hesse, 1863) [21] and the microcotylid Ktarius patrickbrueli Hamdi, Benmansour, Ahmed, Gulsher, Bouguerche 2024 were used as the outgroup. The tree resulting from the ML analysis is shown in Figure 6 together with the statistical support from the NJ analysis.
All discocotylid genera included in the analysis, Anthocotyle, Pseudodiscocotyla Yamaguti, 1965, and Discocotyle Diesing, 1850 clustered in the same clade, distinct from the outgroup. The available sequences of Anthocotyle spp. from merlucciid hosts clustered in a highly supported clade. The available sequences of A. merluccii ex M. merluccius, off France, an of A. americanus ex M. gayi, off Peru and Chile, southeast Pacific, and ex M. hubbsi, off Argentina, southwest Atlantic clustered in highly supported clades, thus supporting A. merluccii and A. Americanus as distinct species. Within the A. americanus subclade, the two sequences of A. merluccii ex M. gayi from two different localities, Chile and Peru (both in the southeast Pacific) clustered in supported groupings.
The sequence of P. mikiae Kamio, Inoue & Nitta, 2023, ex the crimson jobfish Pristipomoides filamentosus had a basal position in the Anthocotyle clade. Similarly, the Discocotyle clade included was highly supported, and it included two subclades: D. ciray Ogawa, Shirakashi, Sata, Itoh, Ito, Lewisch, Bornstein, 2024 subclade, with isolates ex the Japanese huchen Parahucho perryi; and the D. sagittata (Leuckart, 1842), with isolates ex the sea trout Salmo trutta, the Vendace Coregonus Albula, and an unidentified salmonid fish. Within the D. sagittata subclade, the sequence of D. sagittata from Coregonus albula occupied a distinct lineage, whereas the isolates from Salmo trutta and an unidentified salmonid formed a well-supported monophyletic clade.
Genetic p-distances are presented in Table 2. The two available sequences of A. americanus from M. gayi in Peru and Chile differed by 1%, indicating the presence of a single species in southeast Pacific waters. These sequences differed by 6–7% from the isolate of A. americanus from M. hubbsi off Argentina (southwest Atlantic), raising questions about their conspecificity. Similarly, the genetic divergence between A. merluccii from M. merluccius and A. americanus from the aforementioned hosts ranged from 10–11%, confirming that they represent distinct species. The isolates of D. sagittata from salmonids (Salmo trutta and an unidentified salmonid) in Germany and Lower Austria were identical, suggesting conspecificity. In contrast, the isolates of D. sagittata from salmonids differed by 11% from those obtained from Coregonus albula (Russia), calling into question their conspecificity.

3. Discussion

The type species of the genus A. merluccii was first described from gills of M. merluccius from the Belgian coast, northeast Atlantic, and was figured with equal anterior clamps [12]. It was redescribed by Cerfontaine [22] based on newly collected specimens from the type locality (Belgian waters, off Ostend), who once more demonstrated that the anterior pair of clamps are nearly equal in size.
The subsequent European records of A. merluccii, mainly those of Sproston [18] from Plymouth, reveal significant differences from A. merluccii originally described from Belgium. The Plymouth specimens show visible dissimilarity in size between the clamps of the first pair, whereas those of A. merluccii are similar in size. Additionally, the number of atrial hooks exceeds 50 in the Plymouth population, compared to 40 in the Belgian specimens. These morphological differences justified the recognition of the Plymouth population as a potential distinct species, A. aff. merluccii.
Stafford [24] was the first to report A. merluccii on the gills of M. bilinearis in Canada. Subsequently, MacCallum [16] described a new subspecies, A. merluccii americanus MacCallum, 1916, from the same host off Massachusetts, USA, northwest Atlantic. Price [17] elevated A. merluccii americanus subspecies to a species, A. americanus MacCallum, 1916. The author speculated that the parasites reported by Stafford [24] correspond to A. americanus and not to the European species A. merluccii [17]. However, based on the asymmetry of the clamps, a criterion on which the distinction between the two species is made and which could not be verified on the Plymouth population of “A. merluccii”, Sproston [18], considers the separation of the two species doubtful and that the genus is monospecific. However, specimens of both Van Beneden and Hesse [12] and Cerfontaine [22] mentioned and illustrated anterior clamps of an equal size in A. merluccii from the type locality (Belgian waters, off Ostend). Thus, we refute Sproston’s [18] synonymy and reinstate A. americanus as a valid species (WoRMS [19] lists Anthocotyle americanus (MacCallum, 1916) Price, 1943, as “unaccepted (malformed suffix)” (despite such information not being available in the literature we searched) and lists A. americana as “accepted name”. MacCallum [14] originally described it as a subspecies, A. merluccii americanus MacCallum, 1916, which was later elevated to species rank as A. americanus (MacCallum, 1916) by Price [15], and thus, A. americanus was never reported in the literature under the combination A. americana. Note also that Dactylocotyle americanus MacCallum, 1916, listed under the synonymized name “unaccepted, superseded combination”, is placed under Diclidophoridae [19], while A. americanus is a Discocotylidae and was never reported in the literature under the combination D. americanus).
This suggestion is supported by molecular data, as a phylogeny study based on 28S rDNA sequences showed A. merluccii and A. americanus to be distinct species [25]. Herein, the available cox1 sequences of A. merluccii ex M. merluccius, off France [26], and of A. americana ex M. gayi, off Peru and Chile, southeast Pacific, and ex M. hubbsi, off Argentina, southwest Atlantic [27,28,29] clustered in highly supported clades, thus supporting A. merluccii and A. americanus as distinct species.
Within the A. americanus subclade, the two sequences of A. merluccii ex M. gayi from two different localities, Chile, clustered in a supported grouping and differed only by 1%, indicating the presence of a single species in southeast Pacific waters. This level of divergence corresponds to intraspecific variations previously depicted within polyopisthocotylan “Monogenea” [30,31,32,33,34,35,36,37,38].
On the other hand, they differed by 6–7% from the isolate of A. americanus from M. hubbsi, off Argentina (southwest Atlantic). Bouguerche et al. [30] conducted a comprehensive analysis of the intraspecific and interspecific variation in cox1 sequences among polyopisthocotylans. Their findings revealed that the intraspecific differences in cox1 sequences ranged from 0.2% to 5.6%. Herein, the two populations identified as A. americanus, collected from two different hosts, M. hubbsi and M. gayi, are probably not conspecific. The genetic divergence between these populations was notably high (6–7%), exceeding the typical interspecific divergences previously reported in the literature. Such a level of divergence suggests thus that these populations identified as “A. americanus” may represent distinct species rather than variations within the same species. These findings underscore the need for further investigations into these populations, including morphological and molecular studies, to clarify their taxonomic status.
Similarly, the genetic divergence between A. merluccii, collected from M. merluccius [26], and A. americanus, obtained from M. hubbsi and M. gayi [27,28,29], ranged from 10% to 11%. This level of genetic difference is clearly also far greater than the intraspecific variations generally reported for polyopisthocotylans [30], which are typically much lower [39]. Such a high degree of divergence provides robust molecular evidence supporting the conclusion that A. merluccii and A. americanus (both populations, ex. hubbsi and M. gayi) are distinct species rather than variations. Interestingly, these genetic divergences agree with the findings of Suriano and Martorelli [40], who reported three populations of the genus Anthocotyle: the population from M. merluccius from the European Atlantic coast and the Mediterranean, the population from M. bilinearis from the Atlantic coast of the United States, and the population from M. hubbsi from the South Atlantic. These findings emphasize the importance of integrating genetic data with morphological and ecological observations to ensure accurate species delimitation, particularly in groups with closely related taxa where phenotypic plasticity or cryptic speciation may obscure taxonomic boundaries [41].
Similarly, we also observed that for another discocotylid analyzed in this study, isolates of D. sagittata obtained from salmonids Ogawa, Shirakashi et al., 2024, exhibited an 11% genetic divergence compared to those collected from C. albula in Russia [42]. This substantial level of divergence raises significant questions about their conspecificity and suggests the possibility that these populations may also represent distinct species rather than a single, broadly distributed taxon. These findings also highlight the need for further integrative studies to clarify the taxonomic status of these populations and assess whether the observed genetic differences correspond to reproductive isolation and other key species-defining criteria.
The voluminous discocotylid clamps of the first anterior pair are characteristic of the genus Anthocotyle [12,16,17,18,22]. In this paper, we provide, for the first time, a description of the sclerites and the organization of these clamps, characterized by a large size, a median inverted U-shaped sclerotized piece with equal branches, and two small crescent-shaped sclerotized pieces that are articulated. The remaining clamps are smaller and of a Microcotyle type. Despite the discocotylids of the genus Anthocotyle sharing the possession of Microcotyle-type clamps with microcotylids, an apparent difference is a continuous addition of clamps on the anterior margin of the haptor in post-larval life in microcotylids vs. the constant number of a total of four pairs in Anthocotyle spp. [12,16,17,18,22]. Microcotyle-type clamps are also smaller in size, as their large number in microcotylids shares the burden of attachment, which in falls to eight in discocotylids of the genus Anthocotyle [18].
In Anthocotyle spp., the haptor also bears a terminal lappet with hooks. The number of hooks in the terminal lappet appears to vary more among species than within populations of the same species. In A. aff. merluccii [18] and A. merluccii, as redescribed by Cerfontaine [22] from the type locality, there are only two pairs of clamps, similar to A. radkeaminorum n. sp. In contrast, A. americanus has two pairs of hooks in its terminal lappet [17]. We refrained from using this character for species delineation, as Cerfontaine [22] noted that the terminal lappet is often altered when parasites are detached from the gills, particularly if the host has been dead for several days or remains bound in the gill mucosa. In this study, several specimens were collected alive and then heat-killed. Nevertheless, we recommend avoiding the use of this character for species delineation until a reexamination of different populations of A. merluccii can be conducted based on newly collected material.

4. Materials and Methods

4.1. Morphological Methods

A total of 22 specimens of M. merluccius were examined for gill parasites. Fish were collected by local fishermen from off the coast of Bouharoun, Algeria (Division 37.1.1, western Mediterranean Sea), during February of 2017 and June of 2024. Fish were stored on ice and transferred to the laboratory shortly after capture. Fish were identified using keys [43,44].
Fish were dissected fresh on the day of purchase, and the gills were investigated for parasites under a stereomicroscope. The gills were removed, placed in individual Petri dishes, and examined. Newly collected Polyopisthocotyla were heat-killed, fixed in near-boiling saline, and preserved immediately in 70% ethanol. Polyopisthocotylans were stained with acetic carmine, dehydrated in a graded series of alcohol for 15 min each (70, 96, and 100%), cleared in clove oil, and mounted in Canada balsam [14]. Polyopisthocotylans were identified on stained whole mounts. Drawings were made through a Nikon Eclipse i80 microscope (Nikon Eclipse Ni, Nikon, Tokyo, Japan) with DIC (differential interference contrast) and a drawing tube. Drawings were scanned and redrawn on a computer with Adobe Illustrator 2023 (Adobe Inc., San Jose, CA, USA). Measurements of whole mounts are in micrometers and indicated as the range followed by the mean in parentheses.
For high-level parasite terminology, we followed the systematics proposed by Brabec et al. [45], which elevated the former subclasses of Monogenea to the rank of classes. Accordingly, we used the classes Monopisthocotyla and Polyopisthocotyla. For the designation of the ventral and dorsal arms of microcotylid clamp sclerites, we followed the terminology of Euzet and Marc [46]. The designation of the ventral and dorsal arms of discocotylid clamp sclerites followed the terminology of Bouguerche [14].

4.2. Molecular Methods

Available partial cox1 sequences of A. merluccii ex M. merluccius, off France [26], and of A. americanus ex M. gayi, off Peru and Chile, southeast Pacific, and ex M. hubbsi, off Argentina, southwest Atlantic [27], were analyzed together with all existing sequences for Discocotylidae (Table 3). The plectanocotylid Plectanocotyle gurnardi and the microcotylid Ktarius patrickbrueli were used as the outgroup. The alignment was constructed in AliView [47] and trimmed to the shortest sequence. Nucleotide substitution models for phylogenetic analyses using the maximum likelihood method were selected using MEGA11 [48]. The Tamura–Nei 93 (TN93) model [49] with a discrete gamma distribution (HKY + G) was selected. The neighbor-joining (NJ) method [50] was also used for comparison in MEGA11 from the same dataset. P-distances [51] were computed from the same datasets with MEGA11. Trees were constructed in MEGA11 with 500 and 2000 replications for ML and NJ, respectively.

Funding

Chahinez Bouguerche was supported individually by framework agreement projects: 1. the DeepBlue Project: Distance Crossborder Traineeship Programme (MNHN-SJ N°176/20), co-financed by “The European Maritime and Fisheries Fund (EMFF)”, for the analysis, interpretation of data, and writing of the manuscript; 2. “The Ocean Fellowship” (edition 2021), offered by the TBA21 Academy, Ocean Fellowship 2021 held at Ocean Space in Venice; and 3. the Swedish Taxonomy Initiative, Artdatabanken, Swedish University of Agricultural Sciences, within the scope of the project “Systematics and integrative taxonomy of Monogenea parasitizing fishes of Sweden” (SLU.dha 2023.4.3-248) during the writing of the manuscript.

Institutional Review Board Statement

Not appliable.

Informed Consent Statement

Not appliable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We thank the local fishermen from Bouharoune, Algeria, for helping with fish acquisition.

Conflicts of Interest

The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. (A) Whole body; (B) discocotylid-type clamp, ventral view; (C) microcotylid-type clamp, ventral view; (D) hooks of terminal lappet; (E) genital atrium; (F) atrial spine; (G) egg.
Figure 1. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. (A) Whole body; (B) discocotylid-type clamp, ventral view; (C) microcotylid-type clamp, ventral view; (D) hooks of terminal lappet; (E) genital atrium; (F) atrial spine; (G) egg.
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Figure 2. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Disposition of clamp sclerites in discocotylid-type clamps. (A) Ventral jaw; (B) dorsal jaw; (C) clamp, dorsal view.
Figure 2. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Disposition of clamp sclerites in discocotylid-type clamps. (A) Ventral jaw; (B) dorsal jaw; (C) clamp, dorsal view.
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Figure 3. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Disposition of clamp sclerites in microcotylid-type clamps. (A) Ventral jaw; (B) dorsal jaw; (C) clamp, dorsal view.
Figure 3. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Disposition of clamp sclerites in microcotylid-type clamps. (A) Ventral jaw; (B) dorsal jaw; (C) clamp, dorsal view.
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Figure 4. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Anterior end showing relative position of prohaptoral suckers, vaginae, and genital atrium.
Figure 4. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Anterior end showing relative position of prohaptoral suckers, vaginae, and genital atrium.
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Figure 5. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Detail of the reproductive organs in the region of the ovary, ventral view.
Figure 5. Anthocotyle radkeaminorum n. sp. ex Merluccius merluccius off Algeria, western Mediterranean. Detail of the reproductive organs in the region of the ovary, ventral view.
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Figure 6. Tree inferred using the ML method based on the available cox1 sequence data; only bootstrap values higher than 70 are indicated.
Figure 6. Tree inferred using the ML method based on the available cox1 sequence data; only bootstrap values higher than 70 are indicated.
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Table 1. Measurements of Anthocotyle radkeaminorum n. sp. and other Anthocotyle spp. All measurements are in micrometers and indicated as the range followed by the mean in parentheses (when available). Abbreviations: NWA, northwest Atlantic. NEA, northeast Atlantic. WM, western Mediterranean. N: number. N.a.: not available.
Table 1. Measurements of Anthocotyle radkeaminorum n. sp. and other Anthocotyle spp. All measurements are in micrometers and indicated as the range followed by the mean in parentheses (when available). Abbreviations: NWA, northwest Atlantic. NEA, northeast Atlantic. WM, western Mediterranean. N: number. N.a.: not available.
A. radkeaminorum n. sp.A. aff. merlucciiA. americanus
HostMerluccius merlucciusM. merlucciusM. bilinearis
LocalityAlgeria, WM.Plymouth, NEA.Massachusetts, NWA.Scotian Shelf, Bay of Fundy, NWA.
N.153N.a. 4N.a. 5254, 256
SourcePresent study[18][16][17][23]
Body length6000–8000 9000–12006000–69001710–11 170 (6400)
Body width600–1100 1200–20001100110–1820 (980)
Oral suckers30–45 (40) × 35–40 (38) 79–95 *
Pharynx55–60 (58) × 50–55 (52) 120 × 76
Anterior right clamp140–150 (148) × 450–870 (720)665, 625, 8751200 1 *935–1200
Anterior left clamp172–215 (205) × 610–855 (710)1100, 1250680–850
Posterior clamps30–40 (38) × 62–70 (60) 50 2 *68–85
Outer pair of anchors55–68 (52) 60
Inner pair24–30 (28) 45
Genital atrium35–30 (32) × 32–35 (32)
N. of atrial spines28–36 (35)Over 50 3 35–38
* Diameter. 1 As largest clamps. 2 Smaller ones. 3 Forty spines were reported for A. merluccii sensu stricto off Belgium [11]. 4 The authors mention only that the worm was found in sparse number [16]. 5 The authors only specify that at their disposal, they consisted of a few specimens collected by the late Dr. G. A. MacCallum on three dates [17].
Table 2. Genetic p-distances between cox1 sequences of Polyopisthocotylans. Distances within species are in italics; intraspecific variations are low, ranging between 0 and 1%. Distances between species (interspecific variations for congeneric species) are higher.
Table 2. Genetic p-distances between cox1 sequences of Polyopisthocotylans. Distances within species are in italics; intraspecific variations are low, ranging between 0 and 1%. Distances between species (interspecific variations for congeneric species) are higher.
p-DistancesA. americanus
ex M. gayi		A. americanus
ex M. hubbsi		A. merluccii ex M. merlucciusP. mikiaeD. cirayD. sagittata
ex Salmonids
Anthocotyle americanus ex Merluccius gayi 1%
Anthocotyle americanus ex Merluccius hubbsi6–7%
Anthocotyle merluccii ex Merluccius merluccius10–11%10%
Pseudodiscocotyla mikiae22%24%24%
Discocotyle ciray27%26%26%24%
Discocotyle sagittata ex salmonids26%26%27%29%16%
Discocotyle sagittata ex C. albula29%28%27%27%14–15%11%
Table 3. Collection data for cox1 sequences analyzed in this study.
Table 3. Collection data for cox1 sequences analyzed in this study.
PolyopisthocotylaHostLocalityGenBankSource
Anthocotyle americanus (MacCallum, 1916)Merluccius gayiPeru, southeast PacificMT890376[27]
Anthocotyle americanusMerluccius gayiChile, southeast PacificMT890378[28]
Anthocotyle americanusMerluccius hubbsiArgentina, southwest AtlanticMT890377[29]
Anthocotyle merluccii Van Beneden & Hesse, 1863Merluccius merlucciusFranceMT890379[26]
Pseudodiscocotyla mikiae Kamio, Inoue & Nitta, 2023Pristipomoides filamentosusJapan, northwest PacificLC732580[25]
Discocotyle ciray Ogawa, Shirakashi, Sata, Itoh, Ito, Lewisch & Bornstein, 2024Parahucho perryiJapan, northwest PacificLC776943[52]
Discocotyle cirayParahucho perryiJapan, northwest PacificLC776941[52]
Discocotyle cirayParahucho perryiJapan, northwest PacificLC776942[52]
Discocotyle cirayParahucho perryiJapan, northwest PacificLC776940[52]
Discocotyle sagittata (Leuckart, 1842)Coregonus albulaRussiaAY009164[42]
Discocotyle sagittataSalmonids fishLower AustriaLC776946[52]
Discocotyle sagittataSalmo truttaGermanyLC776950[52]
Discocotyle sagittataSalmo truttaGermanyLC776949[52]
Discocotyle sagittataSalmo truttaGermanyLC776948[52]
Discocotyle sagittataSalmonids fishLower AustriaLC776945[52]
Discocotyle sagittataSalmonids fish Lower AustriaLC776944[52]
Discocotylidae gen. sp. Trachinotus carolinusBrazil, southwest AtlanticMT890368[53]
Plectanocotyle gurnardiEutrigla gurnardusSweden, northeast AtlanticPP297665[21]
Ktarius patrickbrueliDentex gibbosusTunisia, western MediterraneanQ319852[39]
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Bouguerche, C. A New Species of Anthocotyle (Polyopisthocotyla: Discocotylidae) from the Gills of the European Hake Merluccius merluccius (Teleostei, Merlucciidae) with a Revision of the Composition of the Genus. Parasitologia 2025, 5, 2. https://doi.org/10.3390/parasitologia5010002

AMA Style

Bouguerche C. A New Species of Anthocotyle (Polyopisthocotyla: Discocotylidae) from the Gills of the European Hake Merluccius merluccius (Teleostei, Merlucciidae) with a Revision of the Composition of the Genus. Parasitologia. 2025; 5(1):2. https://doi.org/10.3390/parasitologia5010002

Chicago/Turabian Style

Bouguerche, Chahinez. 2025. "A New Species of Anthocotyle (Polyopisthocotyla: Discocotylidae) from the Gills of the European Hake Merluccius merluccius (Teleostei, Merlucciidae) with a Revision of the Composition of the Genus" Parasitologia 5, no. 1: 2. https://doi.org/10.3390/parasitologia5010002

APA Style

Bouguerche, C. (2025). A New Species of Anthocotyle (Polyopisthocotyla: Discocotylidae) from the Gills of the European Hake Merluccius merluccius (Teleostei, Merlucciidae) with a Revision of the Composition of the Genus. Parasitologia, 5(1), 2. https://doi.org/10.3390/parasitologia5010002

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