Photographic observations of the swimming behavior of the deep-sea pelagothuriid holothurianEnypniastes (Elasipoda,Holothurioidea)

A peculiar swimming holothurian referable toEnypniastes eximia Theel, of the family Pelagothuriidae was photographed frequently on the continental slopes within Suruga Bay and Sagami Bay. The holothurians of this family are gelatinous, medusa-like creatures with a specific gravity nearly equal to that of seawater, and they have been thought to live a pelagic life. Of the 35 individuals in the bottom photographs taken near the bottom, 26 individuals (74.3 %) were swimming, 1 (2.9 %) was just landing on the bottom and 8 (22.8 %) were walking on the sea floor. Correction for the pyramidal shape of the camera's field of view suggests that 90 % of the population was swimming within several meters of the bottom, and 10 % was on the sea floor. When swimming, 12 antero-dorsal appendages, inter-connected by webe to form a velum as in the elpidiid holothurianPeniagone, are either folded anteriorly in a circle or opened posteriorly like an umbrella. Propulsion seems to be accomplished by metachronal fanning of the posterior webbed podia and by undulatory movement of the antero-dorsal veil-like structure. The latter might also function as a parachute or stabilizer in descending. When on the sea floor, the holothurian always faces downstream with the velum stretched forward, while the aspidochirote tentacles, which are folded in swimming, are pressed on the sediment surface. In addition, this organism leaves trails and constricted fecal strings on the sediment. These observations suggest thatE. eximia does not ingest food in midwater, but, rather feeds selectively on the surface deposits of the sea floor. Although the holothurian has lost the ability to walk effectively on the sea floor by podia, it may utilize the swifter bottom current above the turbulent Ekman layer for locomotion and dispersal.