Changes in properties of mangrove sediment due to foraging on Kandelia obovata leaves by crabs Parasesarmea plicatum (Grapsidae: Sesarminae)

Sesarmid crabs are important ecosystem engineers in mangroves because they remove a large proportion of mangrove leaf litter. However, the effects on mangrove sediments of sesarmid crab foraging on mangrove leaves has been poorly quantified. Through a laboratory experiment, the present study aimed to quantify changes in nutrient concentrations and enzyme activities in mangrove sediment due to foraging on Kandelia obovata leaves by Parasesarmea plicatum crabs. Nonleaf foraging activity (i.e. the crabs were not fed with leaves) of P. plicatum had insignificant effects on the concentrations of sediment organic carbon (OC), total nitrogen (TN) and phosphate, but significantly increased sediment NO3-N concentration. Sediment urease activity was inhibited while invertase activity was enhanced by crab activity. Compared with non-leaf foraging, crabs fed with K. obovata leaves showed different effects on sediment properties. Leaf foraging by crabs significantly increased sediment OC and TN concentrations, and activities of enzymes except for invertase and urease. Significant correlations were found among sediment nutrient concentrations and enzyme activities. Foraging on 1.00 g K. obovata leaves by P. plicatum resulted in accumulations of 154.94 mg OC and 10.26 mg TN in sediment. Removing crab faecal material from the sediment reduced these effects of leaf foraging by crabs on sediment properties, but there was still significant accumulation of OC and TN in the sediment compared with the control. The present study demonstrated that foraging on mangrove leaves by P. plicatum plays an important role on nutrient retention in mangrove sediment and that crab faecal material mediates this ecological function.