Competition and sediment-related responses explaining segregated zonation of two closely related, co-occurring key species on sandy beaches?

To address their role in promoting and/or maintaining observed cross-shore segregation of the co-occurring amphipods Bathyporeia pilosa and B. sarsi, sediment preferences and tolerances were investigated through lab experiments. Preferences were investigated in both allotopic and syntopic conditions, in order to investigate possible avoidance (as a proxy for interspecific competition). Selection of grain size showed no significant differences between the species, nor did the presence of the other species affect the selection. Both species avoided the finest (63-125 μm) and coarsest (>500 μm) grain sizes. Potential niche width with regard to grain size, as established from our experiments, was larger than the occupied width, as observed from field data. Selection of level of fines did not show any difference between species or condition (allo- vs. syntopy) either, while the highest abundance of B. pilosa occurred in 5% fines, whereas that of B. sarsi occurred in the absence of fines. Mortality of animals forced to survive in a single given sediment, was significantly higher in B. sarsi and both species had low mortality levels at a grain size best resembling natural beach sediment (255-350 μm). Thus, whereas preferences of both species are largely comparable, tolerances are not. Conclusively, no clear evidence for interspecific competition could be found, disallowing full explanation of observed segregation. Other possibly determining (abiotic and biotic) factors should be investigated to understand cross-shore segregation, as observed in these closely related yet coexisting species of sandy beach macrofauna