Feeding depths and organic absorption for the deposit feeding benthic amphipods Pontoporera affinis and Pontoporeia femorata

Abstract

Using fluorescent particles as sediment markers, we show that the congeneric, deposit‐feeding amphipods Pontoporeia affinis and Pontoporeia femorata feed almost exclusively on the surface layer (upper centimeter) of sediment, even though they may spend much of their time deeper. Pontoporeia affinis feeds more rapidly than P. femorata. There was some evidence that food intake decreased at night for P. affinis , but not for P. femorata. The two species were equally good at absorbing sedimentary organic carbon, both absorbing ~ 40% from surficial and 28% from deeper sediment. Neither species was very selective in ingesting the organic fraction of sediment. Pontoporeia femorata resides deeper in the sediment than its morphologically similar congener P. affinis , and this difference is enhanced in sympatric populations. Yet, there is no evidence of food partitioning between the two species. An innate tendency of P. affinis to move up, and P. femorata down, upon disturbance from burrowing activities of neighbors may be the primary cause of the enhanced vertical stratification in sympatric populations. It may result in differential susceptibility to predation. In addition, P. affinis swims more actively than P. femorata and should more easily find new patches of high‐quality, surficial sediment. Thus predation and microhabitat partitioning may contribute to the coexistence commonly observed in the two species, in spite of suspected exploitative food competition.