Abstract

Arrival and settlement of successive cohorts of Dungeness crab, Cancer magister, megalopae were observed in Grays Harbor, Washington, in 1991. The first cohort of megalopae entered the estuary between May 15 and 20 and settled in (''occupied'') previously constructed artificial, intertidal oyster shell habitats at densities ranging from 155 to 196 first instar juvenile (J1) crabs.m(-2). Subsequently, a second set of shell habitats was constructed that did not contain crabs of the first cohort (''unoccupied''). Between lune 15 and 18, when crabs of the first cohort were a mix of second and third instars, a second cohort of megalopae settled in both occupied and unoccupied shell habitat at respective J1 crab densities of 9-37 and 168-298 crabs.m(-2) The possible roles of cannibalism, competition, and conspecific avoidance are proposed as alternative hypotheses to explain significantly lower density of the second cohort in shell habitats previously occupied by larger conspecifics of the first cohort. Based on laboratory experiments, we suggest that cannibalism is a plausible explanation of our field observations.