Date Awarded


Document Type


Degree Name

Master of Arts (M.A.)


Virginia Institute of Marine Science


Benthic communities on the outer continental shelf of the Middle Atlantic Bight are characterized by abundant populations of amphipods in the families Ampeliscidae and Corophiidae. Members of these families occupy tubes at or near the sediment surface and, thus, potentially compete with each other for spatial and trophic resources. This study examines the spatial, temporal and trophic resource use of six numerically dominant species in these families as a means of assessing the relative importance of competition as a structuring force in outer shelf benthic communities.

Analysis of abundance data and the use of discriminant analysis to relate abundance to measured environmental parameters indicated that habitat partitioning was of major importance within the ampeliscid group. Spatial partitioning appeared to be of little importance within the corophiid group. Between group patterns suggest that only one species, the corophiid Unciola irrorata, was able to coexist in deep swales with the ampeliscid Ampelisca agassizi.

Seasonal abundance patterns were exhibited by all corophiids, but the high degree of temporal overlap in abundance suggests that this was not an important partitioning mechanism. Similarly, persistent abundances over· time (!. agassizi) or variation in a non-seasonal manner suggests that temporal partitioning of resources was not important within the ampeliscid group.

Differences in body size which might facilitate differential spatial or trophic resource use among species were observed. These differences were most strongly pronounced in closely related species which frequently co-occured.

The use of trophic resources was indirectly assessed by comparing mouthpart morphologies and known feeding behaviors of each species. Quantitative morphological differences among species were evident which relate well to observed habitat preferences.

A complex set of physical and biological factors were found to govern the distribution and abundance of these species within the outer shelf zone. In outer shelf swale habitats the ampeliscid, A. agassizi, effectively excludes the other members of this family. This may be the result of the species superior abilities to utilize spatial or trophic resources. Outside . of swale habitats the abundances of Ampelisca vadorum and Byblis serrata may be limited by the availability of trophic resources. The corophiids show little evidence of resource partitioning, although some differences in microhabitat distribution may facilitate coexistence among ·the species in this family, as well as between families. Populations · of these species may be held below the levels at which competitive interactions become important by benthic predators.

The corophiids are known to comprise a major portion of the diet of benthic fishes on the outer shelf.



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