The body size–burial depth relationship in the infaunal clam Mya arenaria

Abstract

Larger infaunal organisms tend to be buried more deeply than smaller conspecifics, but the ultimate (natural selection) reasons for this have received little attention. A model to explain this ubiquitous pattern is based on several key assumptions: (1) deeper burial increases survival, but (2) slows the feeding rate, and (3) large individuals are able to feed more rapidly than small individuals at any depth. Under these assumptions, the fitness-maximizing burial depth for a large individual is greater than for a small individual. Larger individual soft-shell clams Mya arenaria (Linnaeus) were buried more deeply than smaller conspecifics on each of five beaches. Field observations and experiments confirmed that shallowly buried individuals had a lower survival rate than those buried more deeply, due to predation by the red rock crab Cancer productus (Randall). Laboratory measurements of the clearance rates of experimentally buried clams confirmed assumptions (2) and (3). Field measurements of the survival of marked and experimentally buried clams showed that clams were at greater risk of excavation by red rock crabs at low than at high beach elevations, and showed that crab excavation activity decreased with beach elevation. Thus, the risk of mortality for soft-shell clams is higher at low beach elevations. As the model predicts, the burial depth–body size relationship is shifted downwards at low beach elevations. Laboratory experiments discounted the possibility that substratum differences could account for this difference, and furthermore showed that clam burial behavior could be altered by treatments intended to simulate cues about crab activity. We conclude that the positive relation between size and burial depth observed in Mya arenaria arises from trade offs between feeding opportunity and mortality risk.