Short-circuit in the mangrove food chain

Crabs of the subfamily Sesarminae are important components of mangrove ecosystems in the Indo-west Pacific, Africa, the Caribbean and South America. By retaining a large proportion of mangrove leaf-litter within mangrove forests, they profoundly influence the functioning of mangrove ecosystems. Despite obvious importance to ecosystem functioning, little is known about predation on sesarmid crabs. Three large, predatory fishes of tropical Indo-Pacific estuaries, the groupers Epinephelus coioides and E. malabaricus and the snapper Lutjanus argentimaculatus are known to feed on brachyuran crabs. However, the contribution of sesarmids to the brachyuran component of the diets of these fishes is unknown. To determine the extent to which these fishes prey on sesarmid crabs, the gut contents and stable isotope values (δ13C and δ15N) of E. coioides, E. malabaricus, and L. argentimaculatus from 3 mangrove estuary systems on the northeast coast of tropical Australia were investigated. All 3 species fed extensively on sesarmid crabs. Sesarmid crabs were the dominant food items for E. malabaricus and L. argentimaculatus, occurring in 50% of the stomachs that contained prey, and being the most common prey in terms of overall numbers. Although still the dominant prey, sesarmids occurred in only 30% of E. coioides stomachs. As well as being numerically dominant, sesarmids were large relative to other prey types. The 3 species also had stable isotope values enriched by about +0.75 to +2 δ13C and +1.5 to +2.5 δ15N, which were also consistent with extensive feeding on these crabs. Most other sympatric species had quite different diets and stable isotope profiles. Extensive feeding on sesarmid crabs by these fishes has a range of implications for the ecology of tropical mangrove ecosystems. Food webs are apparently more complex, and food chains leading from mangroves to top predators may be shorter than previously thought. Furthermore, a substantial part of the mangrove productivity sequestered by sesarmid crabs may be exported from mangrove ecosystems as a result of offshore migration by these fishes. The low incidence of piscivory in these fishes adds support to theories that reduced predation pressure may enhance the nursery ground value of tropical mangrove systems for fishes