Zoantharian abundance and species identification of host and symbionts on rocky benthic habitat in the southernmost part of the Caribbean

Zoantharians are colonial cnidarians commonly distributed throughout Caribbean marine ecosystems. They are found along the northeastern coast of Trinidad where they cover rocky marine habitat close to the reef crest and are exposed to periods of desiccation during extreme low tides. Both Palythoa and Zoanthus spp. cover rocks; however, it is unknown what specific zoantharian species cover exposed rocks and how abundant these species are in these benthic habitats. Additionally, the algal endosymbionts of the family Symbiodiniaceae are currently unknown to zoantharians covering exposed rocks during low tides in this southernmost region of the Caribbean. The line and point transect method was used to record benthic species abundance at six sites along the northeastern coast. For molecular analyses, mitochondrial Cytochrome c Oxidase subunit I (mt COI) and 16S (mt 16S) genes were amplified using a polymerase chain reaction, then sequenced to determine zoantharian species identity. The second internal transcribed spacer (ITS2) region was used to determine symbiont identification. Results showed that zoantharians covered between 10 and 80% of benthic habitat (N = 6 sites) with mt COI and 16S analyses identifying five zoantharians: Palythoa caribaoerum, Palythoa grandiflora, Zoanthus pulchellus, Zoanthus sociatus, and Zoanthus aff. pulchellus. Additionally, species covering rocks (N = 10 per site) only identified three zoantharians: the brown color morphotype of P. grandiflora, and green color morphotype species Z. aff. pulchellus and Z. sociatus. For symbionts, Symbiodiniaceae genera identified Cladocopium C1 in P. grandiflora, but not in Zoanthus spp. Endosymbiont biodiversity and phylogeny have not been well studied in this southernmost part of the Caribbean region, hence more studies need to focus on zoantharian symbiont identification using more specific molecular analyses, specifically for those covering exposed rocks, to determine if symbiont shuffling occurs in these dynamically changing environments.