Hexacoral-crinoid associations from the modern mesophotic zone: Ecological analogues for Palaeozoic associations

Coral-crinoid associations, where a coral overgrew a crinoid's stem, were among the very common Palaeozoic benthic associations, lasting until the end of the Palaeozoic. Many skeletal overgrowths described so far document syn vivo relationships. This type of interaction is unknown from later, Meso- and Cenozoic deposits, and to date has been unknown from recent seas. Here we analysed two individuals of the crinoid Metacrinus rotundus collected from mesophotic depths off the Japanese Pacific coast, overgrown by a single zoantharian polyp of Abyssoanthus sp. (Anthozoa: Hexacorallia: Zoantharia: Abyssoanthidae), and by some sea anemones identified as Metridioidea sp. indet. (Hexacorallia: Actiniaria). These azooxanthellate hexacorals do not possess skeletons, and were located below the host's feeding fan. Our microtomography examinations showed that the anemones did not modify the host's columnals. These specimens offer a good ecological analogue to similar associations of rugose, Cladochonus-like and tabulate corals known from the Palaeozoic. While in our specimens competition for food between hexacorals and crinoid likely does not occur, such interactions are possible. Both zoantharians and sea anemones show similar corallite/oral disc diameters to rugose corals and Cladochonus-like cnidarians that overgrew crinoids in the Palaeozoic, and therefore they probably obtained similar sizes of food particles. In environments with low relief seafloors these hexacorals benefited from their elevated position, and therefore stronger feeding currents. As both Actiniaria and Zoantharia have their phylogenetic roots deep in the Palaeozoic, and coral-crinoid associations are common among Palaeozoic tabulate and rugose corals, we speculate that also Palaeozoic non-skeletal corals, inferred from molecular studies, may have also developed this strategy of settling on crinoids, and therefore occupying similar ecological niches to these hexacorals described here. This report documents that coral-crinoid associations, characteristic of Palaeozoic benthic communities, and thought to have disappeared by the end of Permian, exist in modern seas.