MEPS 293:165-176 (2005)  -  doi:10.3354/meps293165

ABSTRACT: The fate of crab-processed mangrove Avicennia marina leaf litter was compared with decomposition of whole leaf litter of A. marina and Rhizophora stylosa. Shredding by the grapsid crab Parasesarma erythodactyla resulted in reduction of mangrove leaf litter to fragments ~200 µm in the faecal material. The C/N ratio decreased linearly with time, attaining ~20 after 28 d. While the δ¹³C signature remained constant, the δ¹⁵N value steadily changed from ~4‰ to 1.8‰ over 4 wk. Decomposing whole leaf litter, however, showed only modest increases in %N content with time, attaining a C/N ratio of ~20 only after 24 wk, with no significant temporal trend in either δ¹³C or δ¹⁵N. δ¹⁵N of decomposing crab faecal material was significantly negatively correlated with surface bacteria density. Demonstrating a sigmoid pattern of increase, surface bacteria density on crab-processed leaf fragments was ~70 times higher than that on whole leaf litter after 28 d of decomposition. Analysis of the C/N ratio, δ¹³C and δ¹⁵N signatures of micro-POM collected from 5 mangrove waterways in southern Moreton Bay, Australia, failed to detect significant differences in these parameters at 3 distances from mangrove forests, but location of the waterways had a strong effect. In laboratory feeding experiments involving the copepods Temora turbinata and Oithona rigidis, a diet of faecal material from crabs significantly improved copepod survivorship compared with a diet of the alga Nannochloropsis sp. or the no-food treatment for both copepods. Stable isotope analysis of the copepods suggest that the animals were able to utilise both crab faecal material and algal carbon, with greater assimilation of the former food source, in the mixed diet (Nannochloropsis + crab faeces) treatment. By acting as shredders, grapsid crabs process large quantities and effect rapid enrichment of mangrove organic production, potentially benefiting both benthic and pelagic estuarine consumers.

KEY WORDS: Mangroves · Grapsid crabs · Copepods · Outwelling · Decomposition · Micro-POM · Estuarine food chains