Leaf-litter consumption slows crab growth but transforms mangrove food chains

Abstract

Leaf litter consumption has implications for growth and nutrient balances of mangrove crabs and mangrove trophic pathways. This study (1) addressed the impact of leaf litter quality and quantity on crab life history and growth, and (2) proposed keystone roles of crabs in mangrove material fluxes. Estimates of crab standing stocks, growth, and carbon and nitrogen mass balances were obtained for a field population of Parasesarma messa, a primary consumer of leaf litter in Queensland mangroves. P. messa exhibited slow growth and individuals took >>4 yr to reach maximum size. Females appeared to breed once per year with high reproductive effort relative to other crabs. An apparent trade-off between growth and reproduction in the dry-wet transition season was caused by a shortage of available leaf litter. A nitrogen mass balance indicated that the population ingested twice the amount of nitrogen that could be supplied by leaf litter. Assuming mortality was mainly due to predation, the estimated loss to predators (28 kg ha-1 yr-1) supported a hypothesis that crab consumption by fish short-circuits mangrove food chains. Faecal production by crabs (0.48 Mg C ha-1 yr-1 and 0.03 Mg N ha-1 yr-1) represents a substantial contribution to deposit-feeding food chains. P. messa is a keystone species whose impact on ecosystem processes is large relative to its abundance and biomass.