Animal-sediment relations in Cape Cod Bay,Massachusetts II. Reworking by Molpadia oolitica (Holothuroidea)

Populations of Molpadia oolitica (Holothuroidea) were sampled over an area of 440 km² in Cape Cod Bay, Massachusetts, USA. This species occurs in silt-clay muds in densities of 2 to 6/m², and is limited to depths greater than 22 m where the highest annual temperature does not exceed 10°C. M. oolitica lives within the mud, with its anterior end located approximately 20 cm below the surface. This species ingests only fine-grained particles at depth in the sediment, and deposits uncompacted feces at the surface, producing a fecal mound around the anal opening. This reworking produces vertical sediment sorting, high sediment-water content, and topographic relief of the sea floor. The fecal cones of M. oolitica provide a relatively stable surface for settlement and growth of the suspension feeders Euchone incolor (polychaete), Aeginina longicornis (amphipod), and Thyasira gouldi (bivalve) Uncompacted feces, accumulated in depressions between the mounds, form an unstable substratum frequently resuspended by tidal flow. Suspension feeders are absent from intercone areas.Contribution No. 236 of the Systematics-Ecology Program, Marine Biological Laboratory, Woods Hole, Massachusetts, USA.