Spatio-temporal variation of sediment properties reveals missing trophic links for deposit-feeding crabs in sandy shore food webs

Quantifying trophic relationships among basal food sources and primary consumers is essential to our understanding of food web structure and dynamics, but is challenging in soft shore systems where the variety of potential food sources often masks the actual pathways of energy flow. To elucidate trophic relationships among basal food sources and primary consumers, we assessed the diet composition of a deposit-feeding crab, Scopimera intermedia, an abundant primary consumer inhabiting tropical shores, using direct enumeration and stable isotope techniques. Stable isotope analyses revealed that meiofauna accounted for the majority of the crab's diet (nematodes reaching 43% in median proportion) compared to, at most, 11% contributed by microphytobenthos (MPB). Although the abundances of MPB and meiofauna varied, meiofauna dominated over MPB in the crab's diet at different sites and seasons. Such an important role of meiofauna in consumer diets contrasts with previous studies on sediment food webs, where MPB has often been proposed as the most plausible primary food source. The relative roles of meiofauna and MPB as food sources are, however, likely to be mediated by local physical environments. Sediment particle size, for example, is more coarse higher on the shore, where the larger interstitial spaces enable colonization of meiofauna but will limit large, motile MPB due to increased photostress. The trophic importance of commonly assumed primary food sources may thus be modified by environmental physical stress and, as a result, the role of intermediate trophic links in modulating food web dynamics should not be neglected.