Feeding ecology of the semi-terrestrial crab Ucides cordatus cordatus (Decapoda: Brachyura) in a mangrove forest in northern Brazil

The objective of this thesis was to investigate the feeding ecology of the intensively exploited semi-terrestrial crab Ucides cordatus cordatus, and to contribute to the understanding of its influence on the flow of organic matter, nutrients, and energy in a mangrove ecosystem in northern Brazil. The mangrove crab U. cordatus is the most conspicuous species of the benthos, contributing to about 84% of its biomass. Stomach content analyses showed that the crabs´ diet is mainly composed of mangrove leaves, unidentified plant material, roots, and sediment. The gut evacuation rates were used in conjunction with the mean daily gastrointestinal contents to calculate the daily food intake of U. cordatus. The daily food intake was 1.0 g dw in small males (cw 3.0-3.5 cm), corresponding to 19.8 % of the crabs´ body dry weight. Large males (cw 7.0-7.5 cm) consumed 3.3 g dw daily, corresponding to 6.0 % of their body dry weight. The overall daily food intake of the U. cordatus population at an Rhizophora mangle dominated forest stand was estimated as 4.1 g dw m-2, corresponding to 81.3 % of the daily litter and propagule production. This indicates that litter processing by U. cordatus highly influences the flux of organic matter, leading to the retention of nutrients and energy inside the mangrove forest. High litter removal rates of the crabs, a low quantity of litter material in most investigated burrows, and high consumption rates during field experiments indicate that the U. cordatus population is food-limited in the investigated areas.Crabs maintained on a pure R. mangle diet showed higher assimilation efficiencies than those fed on Avicennia germinans leaves. Energy assimilation by the U. cordatus population was 10291 and 2870 kJ m-2 y-1 in an R. mangle and A. germinans dominated forest, respectively. The role of microorganisms for the nutrition of U. cordatus was investigated by using fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes.