The feeding behavior of two of the most abundant species of leaf-eating mangrove crabs occurring in North Queensland (Sesarma messa and Sesarma smithii), was studied during the dry season of 1989. Laboratory feeding choice experiments showed that S. messa did not exhibit significant preference for any of the four species of newly fallen mangrove leaves offered, while consuming significantly more decayed than senescent leaves, irrespective of leaf species. S. smithii chose Rhizophora stylosa mangrove leaves over all other species (Avicennia marina, Bruguiera exaristata, Ceriops tagal). The choice of leaves by both crabs was not influenced by any of the leaf properties measured: tannin and water content, % of organic matter; C:N ratio; and leaf toughness. Crabs displayed no clear pattern of preferential consumption of different species of mangrove leaves in field experiments carried out in five separate forests. However, at all sites significantly greater amounts of C. tagal were found shredded and accumulated inside crab burrows compared to the other species of mangroves. Most of the litter accumulated by the crabs in their burrows was consumed within two weeks, before significant changes in the leaf C:N ratio occurred. The lesser selectivity observed in the field than in the laboratory might be the result of a shortage of food in the field during the dry season, when litter fall is minimum. The proportion of leaves shredded and accumulated in the burrows was significantly greater where S. smithii was present than at sites where only S. messa was found. S. smithii is rarely seen foraging on the forest floor and is active only at night. S. messa, in contrast, is frequently seen at the surface during day and night. I suggest that litter storage is a consequence of a time constraint to above ground activity for the nocturnal species rather than a mechanism for improving the nutritional characteristics of the food. The fresh mangrove litter most commonly consumed by crabs is characterised by a high C:N ratio and is not likely to meet the crabs' nitrogen requirements. Observations of the foraging activity of S. messa in the field, indicated that this species spends most of its feeding time scraping material from the mud surface. Bacteria and microalgae living in the upper layer of the sediment may be the principal source of nitrogen for mangrove crabs. Crabs spent a minor proportion of their time grazing on mangrove leaves and on the benthic algae covering the mangrove prop roots, with no significant preference for either one of these two substrates.
