Date of Award


Degree Type


Degree Name

Master of Science in Zoology



First Advisor

J. S. Cobb


Interspecific interactions of four decapod (Crustacea) species were assessed in Narragansett Bay, Rhode Island using aspects of current community theory. Microhabitat distribution was analyzed for three brachyuran species, Carcinus maenas, Cancer irroratus and Cancer borealis and for the American lobster, Homarus americanus. Various microhabitats differed in spatial heterogeneity and ultimately in available shelter sites. Shelter may constitute a limiting resource and competition for this resource may be important in the structuring of the decapod community.

Habitat selection studies for two of the species, Cancer irroratus and Cancer borealis indicated s reduction in the fundamental niche of Cancer irroratus as a result of interactions with its congener. Specifically, C. irroratus was excluded from the heterogeneous rock habitat, the preferred substratum for both species. Avoidance reactions were of greater importance than direct aggression in the allocation of-the shelter resource for these species. When in the presence of C. borealis, C. irroratus actively chose the sand substrate. reducing competitive stress. This species was particularly adept at rapid burying "behavior in soft substrates, an effective predator avoidance mechanism. This adaptation may offset the disadvantage of being excluded from the preferred rock habitat to some extent.

Diel activity patterns differed for the two species of Cancer. Locomotor activity was completely suppressed during the photophase for C. borealis while C. irroratus exhibited a less pronounced photonegative response. Flexibility in daily activity periods for C. irroratus may allow temporal partitioning of foraging sites in rock areas surrounded by sand where both species might occur in close proximity.

Laboratory data were integrated with field distribution data to yield an overall analysis of competitive interactions. between the two species of Cancer and the effect of these interactions on the community as a whole. Laboratory data were used to estimate potential levels of niche overlap and the stability of the system was examined under varying levels of competition.



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