Suspension feeding in the brittle-star Ophiothrix fragilis : ef®ciency of particle retention and implications for the use of encounter-rate models

Dense beds of the suspension-feeding brittlestar Ophiothrix fragilis are common in European waters. Their potential importance in benthic±pelagic coupling has been highlighted, but little is known about the feeding dynamics of this species. Encounter-rate models provide a potential mechanism for the estimation of feeding rates on suspended material of varying sizes. This work investigates factors essential to the application of such models. Particle-retention eciency (RE) converts encounter rate into capture, or clearance rate. Laboratory studies demonstrated that RE varied with the interactive e€ects of ̄ow velocity and particle size. RE was lowest for large particles, particularly at high ̄ow velocity where RE as low as 59% was observed. This indicates that if RE is not accounted for in encounter-rate models, signi®cant overestimates of feeding rates on large particles may occur. Flow around feeding arms and tube feet was characterised by intermediate Reynolds numbers, precluding application of the most simple encounter-rate models. Complex secondarȳow patterns were observed, which carried particles along the downstream side of the feeding arms, but these did not appear to increase the area available for particle capture. Previously reported particle capture by arm spines was not observed. Evidence of active rejection of large particles by tube feet was recorded. Diculties in the application of encounter-rate models for prediction of seston-removal rates are highlighted by these results. Predicted encounter rate may deviate from actual clearance rate due to the e€ects of retention dynamics, localised ̄ow patterns and di€erential particle handling. Other methods of estimation of seston-removal rates are equally problematic however, so that encounter-rate models are likely to remain a useful tool for such estimates.