Neutral interactions among three nonindigenous coral species in a tropical marine fouling community

The introduction and range expansions of alien species can impact the populations of native species. Among marine benthic invasive species, there are numerous instances of alien species competing with native ones, but examples of co-existing invasives that interact or compete with each other are harder to find (e.g., Kuebbing & Nuñez, 2015). If invasive species have functionally equivalent roles in their new communities, their interactions can lead to competitive displacement when resources, such as space, become limiting (Rauschert & Shea, 2017; Russell et al., 2014). A well studied case of congeneric marine invasives in the Atlantic is that of three sun coral species of the genus Tubastraea (Scleractinia: Dendrophylliidae), which are originally from the tropical Indo-Pacific. They show overlapping distribution ranges in the southwestern Atlantic, the Caribbean, and the Gulf of Mexico, where they can negatively impact native benthic communities (Creed, 2006; Lages et al., 2012; Luz & Kitahara, 2017; Miranda et al., 2016). Tubastraea coccinea Lesson, 1830 was the first species that invaded the Atlantic region, dated to the 1930s (Cairns, 2000), followed by T. tagusensis Wells, 1982, in 2000, and T. micranthus (Ehrenberg, 1834), in 2006 (Creed et al., 2017a). These species show an opportunistic preference for a variety of substrates and can settle on natural and man-made materials, including mussel shells, plastic nets, and flotsam (Ballesteros et al., 2018; Creed & De Paula, 2007; Faria & Kitahara, 2020; Hoeksema & Hermanto, 2018; Mantelatto et al., 2020; Mantelatto & Creed, 2015). They can rapidly overtake open spaces through asexual reproduction (Capel et al., 2017) and function as habitats for a variety of native and nonnative coral-associated organisms, some of which are parasitic and have adopted these newly introduced corals as their new hosts (Alves et al., 2020; De Paula et al., 2017; Hoeksema & ten Hove, 2017; Vinagre et al., 2018). In addition to these Tubastraea species, various introduced or cryptogenic octocoral species have recently been discovered in the southwestern Atlantic and the Caribbean, including fouling corals of the genera Stragulum (Hoeksema et al., 2023a; Samimi-Namin et al., 2022) and Chromonephthea sp. (Hoeksema et al., 2023b). Given the expected range expansions and potential invasiveness of nonnative coral species, the probability that, in addition to native fauna, they also face competition from one another increases, depending on their co-occurrences. Although many studies have found that invasive coral species have impacted native populations (e.g., through the displacement of native species), few have examined the mechanisms underlying these competitive interactions (e.g., Guilhem et al., 2020; Sammarco et al., 2015). Neighboring individuals belonging to different Tubastraea species apparently show no signs of visible interaction in laboratory settings (T. coccinea and T. micranthus; Hennessey & Sammarco, 2014) or in the field, where they have jointly invaded certain areas (T. coccinea and T. tagusensis; Barbosa et al., 2023). The generality of this apparent lack of competition among co-invading coral species is currently not well known. Recently, four nonnative coral species were reported from a fouling community on a semisubmersible platform that had operated in the southern Gulf of Mexico for the duration of 20 years and was then moored near a coral reef in Curaçao in the southern Caribbean (Hoeksema et al., 2023b). The nonnative coral fauna consisted of two scleractinians, T. coccinea and T. tagusensis, and two octocorals, Chromonephthea sp. and another unidentified nephtheid. Among these species, T. coccinea was already known from Curaçao at depths ranging from 0.2 m down to 55 m (Cairns, 2000; Hoeksema et al., 2019; Hoeksema & ten Hove, 2017) and T. tagusensis was known only from the northern Gulf of Mexico at 20–40 m depth (Figueroa et al., 2019). Tissue contact occurred among all invasive corals and no harmful consequences of such interactions were observed when Chromonephthea sp. occurred adjacent to a colony of T. coccinea (Figure 1a). Despite their contact, neither showed tissue necrosis nor any other kind of damage in the contact zone. Interspecific interactions between the two Tubastraea species occurred frequently and involved direct tentacle contact (Figure 1b). This interaction was similar to nonaggressive intraspecific contacts among colonies of the same species, that is, without any signs of tissue damage in the contact zone as is commonly observed in colonies that actively compete (Lang, 1973). When space for lateral growth was not available anymore, colonies of both Tubastraea species usually grew upward, forming large, globular colonies (Figure 1b; Hoeksema et al., 2023b), a growth pattern also seen in Tubastraea populations with dense cover on concrete breakwaters (Ho et al., 2017), rocky reef walls (Mantelatto et al., 2011), and gas production platforms (Creed et al., 2017b). Some neighboring colonies belonging to different coral species had fused, creating a chimeric mosaic of long T. tagusensis polyps surrounded by short T. coccinea polyps (Figure 2), further confirming earlier findings that coral colonies of different Tubastraea species do not harm each other (Barbosa et al., 2023; Hennessey & Sammarco, 2014). When and how this fusion could have happened in the life history of both species is unclear. Fusion of larvae is known to occur in Tubastraea corals (Luz et al., 2020; Mizrahi et al., 2014), which may explain how the present multispecies chimeric colonies could have developed. To our knowledge, illustrations of chimeric colonies with intermixed polyps belonging to different scleractinian species have not been published from other localities, although "mixed colonies" of T. tagusensis and T. coccinea are mentioned in a report on the removal of Tubastraea corals in Brazil (Creed et al., 2017b). This indicates that our case is not unique and could be more widespread but perhaps only in a nonnative setting. The colony mixing of two Tubastraea species differs from coral fusion involving two hydrocoral species, in which corals of the genus Stylaster (family Stylasteridae) settle on corals of the genus Millepora (family Milleporidae) (Fourreau et al., 2024; Montano et al., 2020). The introduction and colonization of Caribbean reefs by new nonnative reef fauna seem inevitable as long as semisubmersible platforms are allowed to moor close to coral reefs, rocky shores, and breakwaters. The nearshore presence of semisubmersible platforms not only offers the opportunity to observe and study the development of nonnative marine communities and the interactions between nonnative and native organisms, but also to observe and study nonnatives themselves in their newly occupied habitats. The ability of invasive Tubastraea species to coexist in dense mixed assemblages, partially consisting of chimeric colonies, is in stark contrast with the competitive hierarchy that typifies native Caribbean corals (Lang, 1973) and could contribute to their success as invaders of a variety of benthic landscapes in their nonnative range. Bert W. Hoeksema identified the project, contributed to the field sampling and the species identification, and produced the first manuscript draft. Kaveh Samimi-Namin contributed to the species identification and to the writing. Mark J. A. Vermeij contributed to the field sampling and to the writing. We are very grateful to Jos Bruijninckx for the use of his photographs in Figure 1. We thank two anonymous reviewers for their constructive comments. Kaveh Samimi-Namin was supported by the Richard Lounsbery Foundation. The authors declare no conflicts of interest.