Suspension feeding in the brittle-star Ophiothrix fragilis : efficiency of particle retention and implications for the use of encounter-rate models |
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Authors: | J R Allen |
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Institution: | (1) Port Erin Marine Laboratory, University of Liverpool, Port Erin IM9 6JA, Isle of Man, British Isles Fax: +1624 831001 e-mail: janallen@liverpool.ac.uk, XX |
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Abstract: | Dense beds of the suspension-feeding brittle-star 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 efficiency (RE) converts encounter rate into capture, or clearance rate. Laboratory studies demonstrated
that RE varied with the interactive effects of flow velocity and particle size. RE was lowest for large particles, particularly
at high flow velocity where RE as low as 59% was observed. This indicates that if RE is not accounted for in encounter-rate
models, significant 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 secondary-flow
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. Difficulties 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 effects of retention dynamics, localised flow patterns and differential 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.
Received: 23 January 1998 / Accepted: 24 June 1998 |
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