Ontogenetic development of the crinoid Poliometra prolixa in the Arctic deep sea

Poliometra prolixa is a common species of comatulid crinoid in the Arctic deep sea. In this study, we characterize the ontogenetic development through the cystidean and pentacrinoid stages, using specimens from the LTER (Long-Term Ecological Research) observatory HAUSGARTEN in the Fram Strait, Arctic Ocean. While embryos and early larval stages (e.g., the doliolaria) were not observed, both post-settlement stages and adults of P. prolixa were observed on the same moored experimental platform at 2,500 m water depth, suggesting that larvae of P. prolixa do not disperse far from their mothers. This indicates that doliolariae may have an abbreviated pelagic duration period or may be brooded in this species. The cystidean has a short, translucent stalk with a star-shaped attachment disc and a diamond-shaped translucent head. Metamorphosis from the cystidean to the pentacrinoid is characterized by the formation of brachial ossicles from oral ossicles and by fusion of the basal and radial ossicles to form the calyx. The pentacrinoid stalk is opaque and first develops synarthrial joints at the distal end. Late pentacrinoids have a xenomorphic stalk, bifurcated arms with pinnules, and cirri. We discuss the reproductive and ecological niche of P. prolixa and also consider the question of whether cystidean and pentacrinoid stages undergo metamorphosis.