Adults of the mangrove crab, Sesarma messa were captured using pit traps in a mid-intertidal Ceriops tagal stand located at Cape Ferguson, near Townsville. The crabs and sediment from the capture site were transferred to 9 plastic aquaria after sieving the sediments to remove macroinvertebrates. Three randomly selected males and females were assigned to each aquaria after being measured across the carapace, weighed and marked with a plastic tag. The aquaria were maintained under ambient conditions in an outside shadehouse. Seawater was connected to each aquarium and the supply was regulated to simulate a one hour tide every day, which covered the sediment surface with about 10 cm of water.
The first rearing experiment was designed to test the effects of litter quantity, using senescent Ceriops tagal leaves. Three replicate aquaria were subjected to one of three treatments: no leaf litter added, limited leaf litter added and excess leaf litter added. At the end of each week leaf litter remaining in each aquaria were removed and new leaves added. The aquaria were checked every 2-3 days for ovigerous and moulting individuals over a period of 10 weeks. Ovigerous females were removed at the end of the incubation period and placed in jars containing seawater until hatching occurred. Three subsamples of 200 larvae were collected from each female and the subsamples, as well as the remaining larvae were dried at 65 °C for 3 days and the total number of larvae calculated.
In order to assess the availability of microbial organisms on the mud surface as an alternative food source, bacterial densities were determined from three replicate 3.14 ml samples of sediment collected with a syringe from each aquaria and from the forest where the sediment was collected.
The second rearing experiment, using the same aquaria as above, was designed to test the effects of leaf species on the performance of Sesarma messa. Crabs were given excess amounts of senescent leaves from one of three species: Ceriops tagal; Rhizophora stylosa and Bruguiera exaristata. Three replicate aquaria were assigned to each treatment. Leaves were removed weekly and new leaves provided. In this experiment, only two males and two females were assigned to each aquaria to reduce the possibility of overcrowding. The number of live, moulting and ovigerous individuals was recorded weekly for 9 weeks.
The absorption efficiency of Sesarma messa was assessed by placing each of 10 crabs in individual bowls for one week and then adding to each container half a senescent leaf of Ceriops tagal, Rhizophora stylosa, Bruguiera exaristata and Avicennia marina. The other half of each leaf was placed in a separate container as a control for leaf weigh loss not due to crab feeding. Leaves were weighed at the beginning of the experiment and wet weights converted to dry weights. After 24 hours, leaf remains and control leaves were collected, oven dried at 65°C for 5 days and reweighed. Leaves were then ashed at 500°C for 24 hours to determine ash free dry weight. The total amount of organic matter ingested by each crab was calculated by summing the ingestion rate for each leaf species. After a further 24 hours, crabs were removed from each container and all faecal matter collected on GFC filters. The filters were oven dried to a constant weight at 65°C. Faecal material was then weighed to the nearest 0.001g, ashed at 500°C for 24 hours and reweighed.
