Short-circuit in the mangrove food chain

Abstract

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 lndo-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 (613C and 615N) 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 6I3C and +1.5 to +2.5 6I5N, which were also consistent with extensive feeding on these crabs. Most other sympatric species had quite u f e r e n t 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.Published June 26

Contribution to the feeding ecology of the banded puffer fish Colomesus psittacus (Tetraodontidae) in north Brazilian mangrove creeks

“…Therefore, one of the dominant fish species of tidal mangrove channels along the north Brazilian coast lives on prey of low trophic levels. This emphasizes the conclusion of Sheaves and Molony (2000) that food chains leading from mangroves to top predators are likely to be shorter than previously thought. C. psittacus have a key function in a short intertidal mangrove food web.…”

“…The mangrove productivity is, however, not exported due to offshore migration by the fish as Sheaves and Molony (2000) assumed for a system in tropical East Australia. South of the Amazon delta, where mangroves cover more than 400 km of continuous coastline (Lacerda et al, 2000, p. 9), other complex shallow water environments such as coral reefs or sea grass beds that could motivate offshore movements of the fish are absent.…”

Fate of mangrove organic matter along a subtropical estuary: small-scale exportation and contribution to the food of crab communities

“…Indeed, Guest et al (2004) showed, using stable (δ 13 C) isotope signature of crabs as an indicator of carbon source, that carbon movement between 2 adjacent habitats (mangroves and salt marshes) in a subtropical estuary was spatially restricted to a scale of several metres (< 30 m). However, in the Indo-Pacific region, many organisms migrate into inundated mangroves at high tide to feed (Nagelkerken & Van der velde 2004) and, therefore, a substantial part of the mangrove productivity sequestered by herbivorous crabs might be exported from mangrove ecosystems as a result of tidal migration of predatory fish feeding on these crabs (Sheaves & Molony 2000). Nevertheless, the scale of this transfer of mangrove production cannot be estimated without a full understanding of the diet of the invertebrates living in the mangrove ecosystem.…”

Feeding Habits of the Mangrove Sesarmid Crab <i>Neosarmatium trispinosum</i> (Crustacea) in the Ryukyu Islands, Japan

“…Although most grapsoids are opportunistic scavengers, many are essentially detritivorous as they feed on dead leaves that have fallen off the mangrove, particularly members of the family Sesarmidae, which are important components of mangrove ecosystems in the Indo-West Pacific, the African, the Caribbean and the South American coasts (Sheaves and Molony, 2000). About fifty species of sesarmid crabs have been reported to be predominantly associated with mangrove ecosystems (Islam et al, 2002).…”