Eretmochelys imbricata shells present a dynamic substrate for a facilitative epibiont relationship between macrofauna richness and nematode diversity, structure and function

Although nematodes are the most abundant metazoans in marine environments and present an important biological and ecological model organism to assess marine ecosystem processes and functions, there are in fact very few studies that use nematodes to investigate ecological communities and relationships on “mobile” ecosystems. Arguably one of the most mobile or dynamic marine ecosystems is a sea turtle carapace, hosting tens to hundreds or even thousands of epibiotic organisms; and as the turtle breeds, feeds and migrates, provides an ecosystem that is continuously exposed to changes and potential colonizers. In this study we investigated the nematode communities associated with 19 Hawksbill sea turtle carapaces (Eretmochelys imbricata), and compared nematode structural (composition, richness and diversity) and functional (trophic types and gender/life stages) community parameters with those of other comparable epibiotic substrates (macrophytes, natural and artificial hard substrates) to see whether turtle carapaces are hotspots of nematode diversity and function among substrates suitable for epifauna. We also addressed potential epibiotic macrofauna-nematode interactions by looking at the relationships between macrofauna richness and nematode richness, diversity and community composition. Results suggest that the macrofauna play a bioconstructing role, creating several microenvironments, and thereby enhancing the richness and diversity of the associated nematode assemblages. This was supported by a direct and positive relationship between macrofauna and nematode richness, and implies a genera enrichment process across size classes and phyla. All heterotrophic nematode feeding guilds were recovered from the carapaces, with dominance of predators/omnivores and epistrate feeders. Nematode juveniles dominated in terms of abundance, and a female/male ratio of 1.11 was observed. Nematode richness and diversity were higher than found on other substrates, but feeding guild, gender and life stage structure did not differ compared to nematode communities from all other epibiotic substrates. As a result, we argue that turtle carapaces can be seen as hotspots for nematode biodiversity compared to other epibiotic substrates, but this is not reflected in the function of the nematode community. This study is the first to investigate in detail sea turtle carapace nematode communities, their richness, diversity, trophic and life cycle structure, and potential interactions with their co-epibionts, the macrofauna.