Potential trophic importance of the faecal material of the mangrove sesarmine crab Sesarma masse

Abstract

The potential trophic importance of the sesarmine crab Sesarma masse Campbell in a northeastern Australian mangrove dominated by Rhizophora stylose Griff, was evaluated by quantifying the rates of mangrove leaf litter consumption, the corresponding faecal production and assimilation efficiency of mangrove organic matter by the crab. S. masse seems to remove primarily carbon rather than nitrogen from the mangrove litter, as is reflected by the decrease in C but slight increase in N content of the faeces over mangrove litter. Assimilation efficiency for mangrove organic matter was only at about 50%. The potential significance of the crab faecal material in initiating a coprophagous food chain involving mangrove invertebrates was assessed by a series of growth experiments using the hyalid amphipod Parhyalella sp., which is commonly (mean density of 204 ind. 1-1 of detritus) found amongst accumulated detritus near the mangrove forest. Parhyalella sp. individuals fed with crab faecal material or a mixture of faecal material and mangrove detritus attained significantly higher moulting frequency but lower mortality rates than their counterparts in the mangrove detritus only treatment and the control (no food) over a period of 26 d, although % gut fullness was not significantly different amongst the 3 diet treatments. Assimilation of crab faecal material by Parhyalella sp. was almost complete, while individuals fed with R. stylosa litter could assimilate only about 51% of the organic matter in their diet. Decomposition of crab faecal material resulted in a significant increase in N but decrease in C content over 24 d. The C/N ratio of the faecal material was significantly lower than that of the mangrove litter from which it was generated and decreased linearly with time of decomposition. S. messa therefore are not only important as a significant agent effecting fast recycling of mangrove leaf litter, but their faeces, from leaf litter consumption, may also form the basis of a coprophagous food chain contributing to mangrove secondary production by small invertebrates such as Parhyalella sp.