Physiological Ecology of the Deposit-Feeding Sea Star Ctenodiscus crispatus: Ciliated Surfaces and Animal-Sediment lnteractions

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Abstract
C. crispatus is a non-selective deposit feeder on organically-rich (9.7% of sediment dry weight) oxidized clayey-silt in the Gulf of Maine, USA. The mud star''s lack of selectivity is attributed to the thorough mixing and very fine particle size of the habitat sediment, in which 90% of the particles are < 4 .PHI. (62.5 .mu.m) in diameter. These sediments are easily disturbed and present problems in maintaining unimpaired gas-exchange surfaces. C. crispatus adapted to this by establishing a temporary burrow and using its extensible epiproctal cone to maintain connection with the water column. The burrow is irrigated by ciliary currents which, based on laboratory observation of burrowed mud stars, provide a continuous respiratory flow with a minimum of disturbance to the flocculent surface sediment. Particles incidentally drawn into the burrow are trapped in cleansing mucus which is ingested, likely accounting for the enrichment in organic content of stomach contents relative to habitat sediment. The low convection requirement (1.69 l of water pumped per ml of O2 removed from the current reflects the respiratory, as opposed to nutritional, nature of the current. The cribriform organs create the driving force for the respiratory current, provide a large surface area (11 cm2 in a 1.5 g individual), and are richly supplied with microvilli. Both the extensive ciliation and anatomical arrangement of the cribriform lamellae eliminate unstirred layers at their surface and account for low Kt values for dissolved amino acid uptake, which.sbd.despite a low concentration (average 4.4 .mu.M) in interstitial water in the habitat.sbd.provide a net income in the animal''s energy budget. The biology of the eurybathic C. crispatus is discussed with reference to that of the related Porcellanasteridae, a group of truly deep-sea deposit feeding asteroids.

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