Influence of flow speed on the functional response of a passive suspension feeder, the spionid polychaete Polydora cornu
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ORIGINAL PAPER
InXuence of Xow speed on the functional response of a passive suspension feeder, the spionid polychaete Polydora cornuta JeV Shimeta
Received: 23 December 2008 / Accepted: 23 July 2009 / Published online: 9 August 2009 © Springer-Verlag 2009
Abstract Passive suspension feeders rely on surrounding Xow to deliver food particles to them. Therefore, the classic conception of functional response (feeding rate vs. food concentration) may require modiWcation to account for Xow speed as a second independent variable. I compared the functional response of Polydora cornuta at diVerent velocities and determined whether food capture was proportional to particle Xux (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 eYciency. 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 Xow was directly related to concentration, but in strong Xow it was constant. Worms were therefore unable to beneWt from abundant food when in strong Xow. 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 eYciency 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 eVects of velocity on the functional response. Contact rate was not a constant proportion of particle Xux; treatments with similar Xuxes
Communicated by U. Sommer. J. Shimeta Biology Department, Franklin & Marshall College, Lancaster, PA 17603, USA J. Shimeta (&) School of Applied Sciences, RMIT University, Building 223 Plenty Road, Bundoora, VIC 3083, Australia e-mail: [email protected]
yielded diVerent contact rates depending on the strength of Xow. The results appeared to be caused by a velocityinduced behavioral change in appendage posture that aVects contact rates: in moderate Xow, 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 Xow, and that the eVect changes with ontogeny. Because the results are consistent with patterns in measured growth rates of P. cornuta, I hypothesize that this inXuence of velocity on the functional response can constrain growth and population dynamics in this species.
Introduction The functional response is the relationship between an animal’s feeding rate and food concentration. It is important for understanding the constraints on feeding and growth rates. It inXuences the strength of trophic interactions and is critical for models of population dynamics (Riisgård and Randløv 1981; Leising et al. 2003). Holling (1959) described three types of funct
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