Space partitioning and interactions in an intertidal sand-burrowing amphipod guild

A nine-year study at a moderately exposed, fine sand intertidal beach at Long Sands, Maine, USA identified marked seasonal changes in the foci of sand-burrowing amphipod activities. These included seasonal population movements, degree of interspecific horizontal overlap, life stage segregation within and between species, and sand depth stratification. Behavioral interactions related to competition for available space are considered important in affecting the distribution and abundance of amphipod species, and community structure. Significant negative correlations were shown between the abundance and upper distributional limits of the smaller, lower shore Acanthohaustorius millsi and the abundance of the larger, higher shore species Haustorius canadensis. Tolerance limits to a variety of prevailing physical factors alone, were not considered a serious obstacle to occupation of higher intertidal level sands by A. millsi. Summer brooding females of both species occupied similar 5 to 10 cm sand depths, but were strongly segregated horizontally. Laboratory coexistence experiments during the reproductive period showed increased mortality of adults and negligible reproductive output of A. millsi when in combination with H. canadensis compared to controls where the former species was alone. A third species, Amphiporeia virginiana, occupied primarily very shallow sand (0 to 2.5 cm), and performed seasonal movements opposite in direction to A. millsi and H. canadensis. The presence, amount, and refinement of biological interactions across the wave exposure gradient requires further study. Until then, we consider as inappropriate the blanket designation of all types of intertidal sand communities as physically controlled.