Vertical Distribution in the Sediment in the Co-Occurring Benthic Amphipods Pontoporeia affinis and P. femorata

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Abstract
The depth distributions in the sediment of the two deposit-feeding amphipods Pontoporeia affinis and P. femorata were investigated in the field and in laboratory experiments. At night in the field, P. femorata occurred significantly deeper than P. affinis. In aquaria with homogenized sediment, we found depth distributions similar to those in the field, with P. femorata occurring significantly deeper than P. affinis, both day and night, not only in sympatry but also in allopatry. The partial vertical separation may thus be at least partly due to different innate preferences of the two species. The difference in depth distribution is, however, enhanced in sympatric populations by interactive segregation. In sympatry, P. affinis was found significantly closer to the sediment surface at night and P. femorata significantly deeper in the sediment (when day and night samples were pooled) than in allopatry. We argue that the interactive segregation demonstrated does not, by itself, imply that interspecific competition occurs. Other types of data are needed to show the presence of real competition between the two species. By burrowing deeper in the sediment, P. femorata may experience reduced predation pressure, and thus be able to coexist with the otherwise similar, and more fecund P. affinis.