Larval development of a molpadiid holothuroid, Molpadia intermedia (Ludwig, 1894) (Echinodermata)

Large adults of the caudate molpadiid Molpadia intermedia were collected from soft mud at depths of 26–28 m in East Sound, Orcas Island, Washington. In early December, both sexes spawned in the laboratory: males released occasional puffs of sperm and one female forcefully jetted eggs into the water column. The egg is 267 ± 12 μm in diameter and negatively buoyant with an orangish pink yolky cap at the animal pole which, after fertilization, develops into a coeloblastula through equal, holoblastic cleavages. The doliolaria larva bears two posterior ciliary rings and is uniformly ciliated on the anterior third of its body. The larva takes up a benthic existence soon after formation of the ciliary rings and becomes a pentactula with the protrusion of five primary tentacles. Addition of fine mud to cultures induced metamorphosis, at which time the collar of pink yolk is seen to be transferred to the region of the digestive tract. The larvae can delay metamorphosis for at least 5 d in the absence of mud. The early juvenile is transparent, and spired triradiate ossicles proliferate in the body wall. From this study and from a review of the literature, we suggest that the reproduction of the approximately 85 species of sea cucumbers in this cosmopolitan order is likely to be consistent with what we have described for M. intermedia. It is also suggested that the pink colored yolk can be used as a marker in experimental studies of development.