Brooding and paedomorphosis in the deep-water feather star Comatilia iridometriformis (Echinodermata: Crinoidea)

Abstract

Adults of Comatilia iridometriformis, A. H. Clark, 1909, a deep-water feather star, retain the following attributes limited chiefly to juveniles of other species in its family, the Comasteridae: small size (maximum arm length about 30 mm), large radial ossicles, incomplete pinnulation, slender cirral ossicles and coarse stereom. This is the only known instance of paedomorphosis (retention of juvenile characters in the adult) among extant crinoids. Observations from the submersible “Alvin” in about 580 m indicate that C. iridometriformis lives associated with ahermatypic, bankforming corals (Lophelia prolifera and Enallopsammia profunda). The crinoids cling to dead coral and other solid substrates, but not to living coral. The crinoids appear to prefer slow, locally variable and multidirectional water movements. Although some other crinoids brood early developmental stages externally or within brood pouches, C. iridometriformis is the only crinoid that appears to brood giant embryos and larvae within the ovarian lumen (although poor histological fixation left some doubt about the exact location). Larvae approaching the cystidean stage were the most advanced brooded forms observed; release from the ovary probably occurs at about this stage. Stalked pentacrinoid larvae are sometimes found on the cirri of the adult crinoids. C. iridometriformis is also peculiar in being the only brooding member of the family Comasteridae and the only internally brooding feather star not found in middle or high southern latitudes. It is suggested that: paedomorphosis in this feather star results from accelerated sexual maturation (progenesis), the potentially unstable habitat may contribute to selection for rapid growth; and neither food nor space may be limiting. Moreover, brooding may be both a design constraint (due to small size) and a way to prevent loss of progeny from a discontinuous habitat where current flow tends to be unidirectional.