|Influence of flow speed on the functional response of a passive suspension feeder, the spionid polychaete Polydora cornuta|Shimeta, J. (2009). Influence of flow speed on the functional response of a passive suspension feeder, the spionid polychaete Polydora cornuta. Mar. Biol. (Berl.) 156(12): 2451-2460. hdl.handle.net/10.1007/s00227-009-1270-6
In: Marine Biology. Springer: Heidelberg; Berlin. ISSN 0025-3162, more
Passive suspension feeders rely on surrounding flow to deliver food particles to them. Therefore, the classic conception of functional response (feeding rate vs. food concentration) may require modification to account for flow speed as a second independent variable. I compared the functional response of Polydora cornuta at different velocities and determined whether food capture was proportional to particle flux (concentration × velocity). To understand feeding responses at a mechanistic level, I measured the functional responses in terms of contact and capture rates and determined particle retention efficiency. Experiments were run separately with two sizes of food particles, and with juvenile or adult worms. For both worm sizes and both particle sizes, capture rate in weak flow was directly related to concentration, but in strong flow it was constant. Worms were therefore unable to benefit from abundant food when in strong flow. The critical velocity at which the capture rate became constant was lower for adult worms than for juvenile worms, and it was lower for small particles than for large particles. Retention efficiency was constant among all treatments, and the results for contact rate were essentially the same as for capture rate. Therefore, the mechanics of particle contact must explain the effects of velocity on the functional response. Contact rate was not a constant proportion of particle flux; treatments with similar fluxes yielded different contact rates depending on the strength of flow. The results appeared to be caused by a velocity-induced behavioral change in appendage posture that affects contact rates: in moderate flow, worms form their feeding palps into helical coils, which they tighten as the velocity increases. I suggest this behavior constrains suspension feeding rates and the mechanical selection between particle sizes when worms are in strong flow, and that the effect changes with ontogeny. Because the results are consistent with patterns in measured growth rates of P. cornuta, I hypothesize that this influence of velocity on the functional response can constrain growth and population dynamics in this species.