Preliminary investigations of shelter competition among the Asian shore crab and native mud crabs

This study examined the potential impact of the recently introduced Asian shore crab, Hemigrapsus sanguineus, on shelter utilization by two native species of mud crabs, Eurypanopeus depressus and Panopeus herbstii, using laboratory experiments and field sampling at two sites in western Long Island Sound (Black Rock Harbor, BRH; Milford Harbor, MH). Abundance and distribution patterns of these species differed at the two sites. Similar numbers of mud and Asian crabs were found under rocks at BRH, but Asian crabs outnumbered mud crabs 15:1 at MH. Asian crabs were most abundant at mid-tide level, whereas 90% of the mud crabs occurred low in the intertidal. This is likely due to the low tolerance for desiccation exhibited by xanthid crabs. At low tidal elevation, where most of the overlap occurred, between-site differences in under-rock microhabitat utilization were present. Only mud crabs were found beneath 75% of the rocks sampled at BRH, but at MH, mud crab species alone were found under only 5% of the rocks. Relative crab densities likely affect competitive outcomes and ultimately space utilization patterns. Results of shelter competition experiments conducted in the laboratory did not support the hypothesis that H. sanguineus affects shelter utilization by native mud crabs. The percentage of mud crabs occupying shell shelters remained unchanged when Asian crabs were present, but the percentage of Asian crabs occupying shell shelters decreased relative to controls in trials where mud crabs were present. These findings suggest that E. depressus and P. herbstii may affect patterns of habitat use by H. sanguineus, especially in the lower intertidal, where these species occur together. However. direct experimental manipulations in the field coupled with long-term monitoring are needed to fully understand the role of competitive interactions in determining the local distribution of these species.