Sediment processing by two estuarine crabs in Bangladesh: small efficient consumers alongside big inefficient ones

Abstract

Abstract Due to their foraging and burrowing behaviors, many crabs in the brachyuran superfamily Ocypodoidea are considered important bioturbators that enhance sediment turnover, energy and matter flow, and nutrient cycling in sandy beaches, mangrove forests, and estuaries. There is surprisingly little comparative work assessing the relative effects of sympatric species, even though many co-occur over broad ranges. We compared uptake of sediment water, organic matter, and carbonate by two sympatric deposit-feeding crabs from Chittagong, Bangladesh, the small sand-bubbler Dotilla intermedia De Man, 1888 and the much larger ghost crab Ocypode macrocera H. Milne Edwards, 1837. Comparisons between feeding pellets and surrounding surface sediments showed that D. intermedia reduced sediment water content by 66% and organic matter content by 67%. In contrast, feeding by O. macrocera had no significant effects on these two parameters. Although sediments excavated by O. macrocera were twice as rich in organic matter compared to surface sediments, variance was high, rendering the difference not statistically significant. Inorganic carbonate distribution was affected by both crabs in different ways. Dotilla feeding pellets had significantly less carbonate than unprocessed surface sediments. In contrast, Ocypode feeding pellets had similar carbonate concentrations to surface sediments, but sediments excavated by those crabs were significantly lower in carbonates (~28%) than those at the surface. While both crab species have the potential to affect sediment properties, they do so through different mechanisms.