Consumption of mangrove litter by the crabs Sesarma messa and Sesarma smithii in north eastern Australia

Leaf characteristics, which may be relevant to food preferences, were determined using 10 senescent leaves from each of the species to be used in experiments (Avicennia marina, Rhizophora stylosa, Bruguiera exaristata and Ceriops tagal). Leaves were weighed and oven dried for 5 days at 65°C and reweighed to calculate dry weight/wet weight regressions, which would be used in later experiments. Three dried leaves from each species were then ground and analysed for percent organic matter, percent carbon and nitrogen and tannin content. An index of relative toughness of leaves was determined from 18 leaves of each species by estimating the force required to pierce the leaves with a metal point. Carbon and nitrogen content and leaf toughness were also determined for decayed leaves of Rhizophora stylosa, Bruguiera exaristata and Ceriops tagal, produced by placing senescent leaves in mesh bags and leaving them in the field for 17 days.The feeding preferences of 47 Sesarma messa and 10 Sesarma smithii, captured at Cape Ferguson, were tested in the laboratory using senescent leaves of Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal and Avicennia marina from the same area. Leaves of each species were cut in half and after weighing, each crab was offered half a leaf of each species. The other half was retained as a control for natural weight loss. Leaf remains from experimental and control containers were removed after 24 hours, dried and weighed. Ingestion rates were calculated as mg dry weight of leaf/g weight of crab /24 hours. A second similar experiment was carried out with 24 Sesarma messa, which were offered both senescent and decayed leaves of Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal simultaneously.Field experiments were carried out during the dry season, April to November 1989, at 4 sites at Cape Ferguson and 1 site at Coral Creek, Missionary Bay, Hinchinbrook Island.Leaf choice experiments were carried out at all field sites using leaves from Rhizophora stylosa, Bruguiera exaristata, Ceriops tagal and Avicennia marina. Twenty senescent leaves of each species were weighed, marked and tied by the petiole to lengths of twine, which were then tied to a prop root or a tree. Leaves were deployed haphazardly, retrieved after 24 hours and the number taken down burrows counted. Burrows were excavated and all litter from the burrow walls collected. All uneaten leaf material was rinsed, dried and weighed. Leaf consumption was calculated as percent dry weight loss.An experiment was carried out at three sites at Cape Ferguson, to test if crabs allowed leaves to decompose in their burrows before consumption. The sites were located along an intertidal gradient. Sixty leaves of the most abundant tree species at each site were weighed, marked, tied and deployed. Twenty leaves were retrieved 24 hours, 8 days and 17 days after deployment and burrows excavated to retrieve leaf fragments. Leaf remains were dried and weighed. Controls, inside mesh bags, were deployed on the mud surface and down burrows and retrieved on the same dates. These leaves were dried, reweighed to determine weight loss, ground to a powder and analysed for carbon and nitrogen content.The feeding behaviour of 30 Sesarma messa was monitored at Cape Ferguson during October and November 1989. Each 30 minute observation commenced when the crab left the burrow and the type and duration of feeding and other activities recorded.
 Field and laboratory experiments were undertaken to determine:1. whether sesarmid crabs show preferences for particular ages or species of leaves2. whether the two species of crabs chosen for the study have different food preferences3. whether crabs store litter in their burrows for prolonged periods before consumption4. whether food items other than leaf litter play an important part in the diet of mangrove crabs

Organization: Australian Ocean Data Network