Effects of animal density and supply of deposited and suspended food particles on feeding, growth and small-scale distributions of two spionid polychaetes

Abstract

Many soft-sediment taxa can switch between deposit feeding and suspension feeding, depending on near-bottom flow conditions. Because the foraging spaces of individuals are typically greater when deposit feeding than when suspension feeding, density-dependent interactions should be more pronounced when deposit feeding. To test this hypothesis, laboratory experiments were conducted with two species of spionid polychaetes, Boccardia pugettensis (Blake) and Pseudopolydora kempi japonica (Imajima & Hartman). Densities of the two species were manipulated singly and together to examine intraspecific and interspecific effects. Food particles were supplied by episodic bedload transport, under which conditions both species deposit feed, or by continuous suspended load transport, under which conditions both species facultatively suspension feed. Contrary to the hypothesis, feeding and growth rates in the intraspecific experiments showed no density dependence when deposit feeding or suspension feeding, even when densities of conspecifics were as high as 220% (for Boccardia) and 940% (for Pseudopolydora), the average densities in the field. Supporting the hypothesis, there was weak evidence from the interspecific experiments that a high density of Boccardia exerted a negative effect on Pseudopolydora's growth rate when animals were deposit feeding but not when suspension feeding. Other, more clear, density-dependent effects were detected. Emigration of Boccardia from high-density treatments was pronounced when deposit feeding, but no emigration occurred when suspension feeding. Pseudopolydora emigrated at high density whether deposit feeding or suspension feeding. When deposit feeding, individuals of both species maintained a constant distance to their nearest neighbor, regardless of density of animals. When suspension feeding, however, distances between neighbors significantly decreased at high densities.