Finding a Pied-à-Terre: Harbour Infrastructure Facilitates the Settlement of Non-Native Corals (Tubastraea spp.) in the Southern Caribbean

Abstract

Semi-submersible platforms are used in the offshore oil and gas industry. They are specialised marine vessels that float on submersed drafts, which are composed of pontoons and columns and can serve as habitats for biofouling marine benthic communities. When these vessels sail from one place to another, either by using their own propellers or being towed, they can act as vectors for introducing non-native marine species. To establish themselves in new areas, these exotic species require suitable benthic habitats. Artificial substrates, such as harbour infrastructure where such vessels are moored, appear to be highly suitable for this purpose. In the present study, a mooring buoy and a harbour piling at Curaçao (southern Caribbean), frequently used by semi-submersible platforms, were found to be colonised by the sun corals Tubastraea coccinea and T. tagusensis at shallow depths. This report presents the first record of T. tagusensis as an introduced non-native species in the southern Caribbean, highlighting the potential role of harbour infrastructure in facilitating coral settlement at depths shallower than those typically observed. These findings underscore the ecological impact of artificial substrates in supporting invasive species and emphasise the need for monitoring programs and defouling facilities.

Maritime ports and marinas are recognised as gateways for the introduction of exotic marine species transported via ballast water and hull biofouling [1,2,3,4,5,6,7]. Non-native and cryptogenic marine species have also been reported from the Caribbean and the Gulf of Mexico, although only a few of these cases involve species observed on shipping and harbour infrastructure [8,9,10,11,12,13,14,15]. Others are introduced through different pathways, such as aquarium animals released into the wild, including fishes and octocorals [10,16,17,18,19,20,21,22], mushroom corals that were left over from a scientific experiment [23,24], and biofoulers on shipwrecks and oil rigs [25,26,27,28,29,30,31,32,33,34].

Curaçao is an island nation in the southern Caribbean, where extensive research on coral reef biodiversity has been conducted [35]. It is the second locality from which the invasive coral Tubastraea coccinea Lesson, 1930, has been reported, the first being Puerto Rico [8,36,37]. St. Michiel Bay and Caracas Bay (Figure 1) are among several sites at Curaçao, where this species was reported as Tubastraea tenuilamellosa (Milne Edwards & Haime, 1848) [38], based on collections made by the Dutch naturalist Dr. Pieter Wagenenaar Hummelinck in the 1940s and 1950s [39,40]. In 1955, Wagenaar Hummelinck himself mentioned the presence of corals on a mooring buoy anchored in Caracas Bay since 1942, but he did not mention which species [40].

Both sites have a history of moored semisubmersible platforms for the offshore oil and gas industry, which use these locations for maintenance or parking during temporary or final decommission [14]. One of these platforms was examined in 2017 for the presence of non-native species, resulting in new records of some non-indigenous species in the southern Caribbean, among which was Tubastraea tagusensis Wells, 1982, co-occurring with T. coccinea [14,15].

Tubastraea tagusensis had not been reported before as an introduced species in the southern Caribbean [14]. Previous records in the West Atlantic are from Brazil, where it was first observed in 2000 [37,41,42] and the northern Gulf of Mexico, where it was recorded in 2019 [37,43]. The observation of T. tagusensis on a semisubmersible platform that was moored for one year near Boca Sami Beach off Curaçao [14] does not really constitute a record for a new introduced species. This is because it was found on a floating structure that was only temporarily stationed there after operating in the southern Gulf of Mexico for two decades [14]. Two monitoring dives on the mooring buoy at Boca Sami in March 2019 only revealed high densities of T. coccinea but no presence of T. tagusensis (B.W.H., unpublished observation). The latter species was also not found during extensive reef surveys at these localities in October–December 2021 and April 2022 [44,45].

In March 2023, new coral diversity surveys were performed, which included Boca Sami Beach in St. Michiel Bay and Tugboat Beach in Caracas Bay (Figure 2 and Figure 3). At both sites, T. tagusensis was found to co-occur with T. coccinea. The mooring buoy at Boca Sami (12°08′49.8″ N, 69°00′02.1″ W) showed a high cover of T. coccinea at its underside (Figure 4a) and on the 40 cm wide anchor chain (ranging from 0.5 to 16 m depth) underneath the buoy (Figure 4b). A cluster of T. tagusensis was found at 7–8 m depths. Other chains, attached to concrete blocks on the reef flat, were either partly covered by native reef corals or had no coral cover (Figure 4c,d).

The pink, orange T. coccinea with relatively short calyces (Figure 5a) and the yellow-orange T. tagusensis with longer calyces (Figure 5b,c) on the buoy could easily be distinguished based on their morphology and colouration [14,37,41]. Another colony of T. tagusensis was observed at 1 m depth on a harbour piling at Tugboat Beach in Caracas Bay (12°04′05.9″ N, 68°51′43.3″ W), next to a colony of T. coccinea (Figure 5d).

These observations represent the first records of T. tagusensis settling in a coral reef area in the southern Caribbean, but it has to be noted that the corals were only found on man-made substrates. Without artificial substrates, such as mooring buoys, harbour pilings and jetties, this species may not be able to colonise the coastline of Curaçao. Another sun coral, the endemic deep-water species Cladopsammia manuelensis (Chevalier, 1966), was recently discovered on Caribbean reefs and can also be found on artificial substrates [28,46]. It can be distinguished from both invasive Tubastraea species by the presence of stolons, fused septa (following the so-called Pourtalès plan), and its unique colouration, which ranges from grey to brick-red or dark orange [28]. Furthermore, there is a smaller endemic cup coral in the Caribbean, Rhizopsammia goesi (Lindstrom, 1877), usually found in deeper, mesophotic waters [47,48,49] but also reported from less than 30 m deep [50].

It is noteworthy that the T. tagusensis colonies in Curaçao were observed at shallow depths (7–8 m and 1 m). In its native Indo-Pacific range, this species has been recorded from depths between 3 and 43 m [51]. In Brazil, it has been reported at various depths, such as 2 m [52], 5 m [53], 3–8 m [54], 4–7 m [55], and 15–30 m [56]. In the Northern Gulf of Mexico, it has been reported from a depth range of 20–40 m [43]. Depth records from some other non-native localities are ambiguous, as they refer to a mix of T. tagusensis and T. coccinea. The shallow records from Curaçao could be related to the availability of artificial substrates, such as harbour infrastructure that extends up to and above the water surface. Maximum depths are less relevant to this study because the species is azooxanthellate and does not need sunlight [36]. Minimum depth records of other Caribbean corals are also linked to man-made substrates. For example, the deep-water species C. manuelensis was observed at 3 m depth underneath a sunken pontoon in Piscadera Bay and at 4 m depth inside a shipwreck in Caracas Bay (both Curaçao) [28], despite its typical depth range of 70–366 m [36].

Similarly, the cryptogenic Phyllangia pequegnatae Cairns, 2000, was recorded at 0.3 m depth on a concrete substrate under docks in a marina at Puerto Velero, Colombia [57], rather than its previously known depth range of 48–112 m [36]. These findings suggest that artificial substrates in harbour environments may enable certain coral species to establish at much shallower depths than those typically observed in natural habitats.

The present observations support earlier findings that semi-submersible platforms can act as vectors for introducing non-native species [14,15,37,58,59], facilitated by harbour infrastructure that serves as a settlement substrate (present study). These findings should motivate port authorities to (a) monitor semi-submersible platforms and mooring infrastructure for the presence of non-native marine biota and (b) offer defouling facilities [58,59].