Abstract
Epibionts are organisms that utilize the exterior of other organisms as a living substratum. Many affiliate opportunistically with hosts of different species, but others specialize on particular hosts as obligate associates. We investigated a case of apparent host specificity between two barnacles that are epizoites of sea turtles and illuminate some ecological considerations that may shape their host relationships. The barnacles Chelonibia testudinaria and Chelonibia caretta, though roughly similar in appearance, are separable by distinctions in morphology, genotype, and lifestyle. However, though each is known to colonize both green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) sea turtles, C. testudinaria is >5 times more common on greens, while C. caretta is >300 times more common on hawksbills. Two competing explanations for this asymmetry in barnacle incidence are either that the species’ larvae are spatially segregated in mutually exclusive host-encounter zones or their distributions overlap and the larvae behaviorally select their hosts from a common pool. We indirectly tested the latter by documenting the occurrence of adults of both barnacle species in two locations (SE Florida and Nose Be, Madagascar) where both turtle species co-mingle. For green and hawksbill turtles in both locations (Florida: n = 32 and n = 275, respectively; Madagascar: n = 32 and n = 125, respectively), we found that C. testudinaria occurred on green turtles only (percent occurrence – FL: 38.1%; MD: 6.3%), whereas the barnacle C. caretta was exclusively found on hawksbill turtles (FL: 82.2%; MD: 27.5%). These results support the hypothesis that the larvae of these barnacles differentially select host species from a shared supply. Physio-biochemical differences in host shell material, conspecific chemical cues, external microbial biofilms, and other surface signals may be salient factors in larval selectivity. Alternatively, barnacle presence may vary by host micro-environment. Dissimilarities in scute structure and shell growth between hawksbill and green turtles may promote critical differences in attachment modes observed between these barnacles. In understanding the co-evolution of barnacles and hosts it is key to consider the ecologies of both hosts and epibionts in interpreting associations of chance, choice, and dependence. Further studies are necessary to investigate the population status and settlement spectrum of barnacles inhabiting sea turtles.
Highlights
Barnacles in the family Chelonibiidae are common epibionts of sea turtles (Zardus, 2021)
Between 2007 and 2020, juvenile and subadult green and hawksbill turtles were captured from co-occurring populations in the nearshore waters of SE Florida United States (Palm Beach through Monroe Counties) (Figure 1), and the islands of Nosy Sakatia, Nosy Tanikely, and Nosy Komba, which are part of the Nosy Be Island complex located in the northwest region of Madagascar (Figure 1)
Discriminating Chelonibia testudinaria from C. caretta was possible from photographs because in the former, wall sutures widen upward and the parieties become splayed at their tips with radii extended in between; whereas, in the latter species the seams between the parieties remain pressed close together with no radii visible but with alae visibly underlapping the parities at their apex (Figure 3)
Summary
Barnacles in the family Chelonibiidae (superfamily Coronuloidea) are common epibionts of sea turtles (Zardus, 2021). Their highly mobile hosts provide them with a substratum that protects them from predators (Foster, 1987) while aiding their dispersal (Rawson et al, 2003) and supporting their reliance on passive feeding (Lane et al, 2021). While predominantly associating with marine turtles, chelonibiids have been observed on other aquatic reptiles including the American alligator, diamondback terrapins, and saltwater crocodilians (Monroe and Garrett, 1979; Seigel, 1983; Nifong and Frick, 2011), as well as manatees (Zardus et al, 2014) and various crabs and other arthropods (Ortiz et al, 2004; Cheang et al, 2013; Ewers-Saucedo et al, 2017). Along with several morphological differences between the two (Monroe, 1981) are distinctions in their attachment modes that leave diagnostic marks on their hosts
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