Behavior of a passive suspension-feeder ( Spiochaetopterus oculatus (Webster)) under oscillatory flow

Abstract

The chaetopterid polychaete Spiochaetopterus oculatus (Webster) was observed under oscillatory flow in a water tunnel with a natural sediment bed. Observations were made at maximum oscillatory flow speeds of 5–40 cm·s −1, with amplitudes of 8–32 cm and periods of 4–25 s. Two feeding behaviors were detected: deposit-feeding with palps on the sediment surface, and suspension-feeding with palps held into the flow and coiled helically. Deposit-feeding predominated at maximum flow speeds of <10cm·s −1 but was utilized at flow speeds as high as 30 cm·s −1, though with decreasing frequency. S. oculatus ceased deposit-feeding when shear stress on the bed was >0.9 dyne · cm −2. This is lower than the critical shear stress for bulk sediment (1.02 dyne·cm −2) and appears to be related to the worm's inability to hold the palp tip on the sediment surface. S. oculatus began suspension-feeding with coiled palps immediately upon initiation of oscillatory flow. With each flow reversal, palps were rotated to keep the food groove in the upstream direction. The amount of time that worms spent suspension-feeding was positively correlated with maximum speed, and suspension-feeding predominated at maximum flow speeds of > 15 cm·s −1. Maximum flow velocity and estimated bed shear stress proved to be better predictors of feeding mode than either oscillation period or orbital excursion amplitude alone. At flows typical of protected wave-dominated environments, S. oculatus spends more time suspension-feeding than deposit-feeding. The ability to switch rapidly from deposit- to suspension-feeding may be an adaptation to capture material suspended near the bed which is widely regarded as a superior food resource. We argue that the palp-coiling particle capture mechanism works well in both steady and oscillatory flow regimes, but that it is tied to a particular feeding appendage functional morphology.