Ribosomal RNA Reveals Phylogenetic Conservatism in The Evolution Of Host Associations in Caribbean Symbiotic Zoanthids

Symbiotic zoanthids (Cnidaria, Anthozoa, Hexacorallia, Zoanthidea, Macrocnemina) form associations with a diverse group of invertebrates (representing at least 5 phyla) in most major benthic habitats with a bathymetric range from mangrove roots to the deep sea. The currently accepted morphology-based taxonomy arranges zoanthid species with dissimilar host associations into the same higher taxa, apparently challenging patterns of phylogenetic conservatism often observed in the evolution of symbiotic associations. Shallow-water Caribbean symbiotic zoanthids (10 species representing 2 families and 3 genera) form associations with a single hydroid species and approximately 100 species of coral reef sponges (representing 6 orders, 21 families, and 33 genera). The taxonomy of Caribbean symbiotic zoanthids results in congeneric zoanthid species forming associations dissimilar hosts and heterogeneric zoanthid species forming associations similar hosts. The complete internal transcribed spacer region of the ribosomal RNA nuclear gene was PCR amplified from multiple individuals representing each symbiotic zoanthid species collected from 11 locations spanning the entire greater Caribbean region. Phylogenetic analyses resulted in topologies that are paraphyletic for all zoanthid genera and families included in the analysis. The clades of zoanthid species recovered by the phylogenies have similar host associations, suggesting phylogenetic conservatism in zoanthid-host association evolution. A single example of a zoanthid species switching hosts was detected within a clade where all other members shared the same sponge hosts. The host switch was away from symbioses with sponges that generally host photo-endosymbionts to symbioses with sponges that generally do not host photo-endosymbionts and was accompanied by a loss of photosynthetic dinoflagellate symbionts (Symbiodinium) of the zoanthid. The loss of Symbiodinium coinciding with a host switch on the phylogeny maintains the match in photo-endosymbionts first detected by examining the specificity of sponge hosts and zoanthid symbionts