In situ study of the autecology of the closely related, co-occurring sandy beach amphipods Bathyporeia pilosa and Bathyporeia sarsi

Population dynamics and zonation of the amphipods Bathyporeia pilosa and B. sarsi, co-occurring on some beaches, were studied through monthly sampling of eight cross-shore transects along the Belgian coast (October 2003–October 2004). Their biomass and production were assessed for the first time. Abundance and biomass of B. pilosa were ten times higher along western ultra-dissipative transects than along slightly more reflective, eastern transects. For B. sarsi (less prominent), differences between the two westernmost transects (2–5× higher) and all others were observed, whereas P/B ratio was comparable for all. B. pilosa could reach two times higher abundance and biomass and higher levels of production (max B. sarsi = 7,580 g m⁻² y⁻¹; max B. pilosa = 16,040 g m⁻² y⁻¹), while the species was nearly absent from the eastern transects. Continuous reproduction and recruitment with three relative peaks of the latter (February, July, October) were observed. Fecundity showed parallel temporal variation for both species, peaking in February and September–October. Interestingly, the July relative “recruitment” peak could not be explained by relative abundance of gravid females or fecundity, but was probably caused by adult mortality. Both species displayed comparable gonad production (B. pilosa: P _g  = 0.73 mg/ind year; B. sarsi: P _g  = 0.71 mg/ind year), but B. pilosa produced fewer yet larger embryos. Peak abundances were found at 436 ± 25 SD cm (B. pilosa) and 357 ± 40 SD cm (B. sarsi) above MLLWS, corresponding to a 40–62 m cross-shore distance between the peaks of both species. The occupied cross-shore range was larger for B. sarsi than for B. pilosa (35–54 m), for females than for males (15–23 m), and for adults than for juveniles of B. pilosa (5–8 m). Both species displayed many comparable life history features. Differences in abundance and biomass may be related to beach morphodynamics and zonation.