Factors affecting the distribution and abundance of Grapsid crabs in the Murray River estuary, north Queensland

Pitfall traps were used to sample crab populations at five sites along a 17km reach of the Murray River, Queensland in May 1987. Sites were located along a gradient in salinity and in the dominant mangrove vegetation. Two 2x2 m trapping grids were set up at each site with a trap in each corner and one in the centre and were checked daily for 4 days. Each captured crab was identified to species (where possible), sexed, measured (carapace width), marked and released. Crabs with carapace widths of At each site three sediment cores were collected. Porewater salinity was measured on water that percolated into the core holes. Porewater pH and Eh were measured at 5, 25 and 45 cm depths along each core. Subsamples of cores were also taken at these depths for determination of organic matter content and grain size analysis (percent sand, silt and clay) in the laboratory.In the laboratory, individuals of the four dominant grapsid species (Sesarma semperi-longicristatum, Sesarma messa, Sesarma brevicristatum and Sesarma brevipes) were totally immersed in separate 200 ml containers of water at one of the following salinities: 0%, 2.5%, 5%, 10%, 20%, 30%, 40%, 50% or 60%. Survival was monitored for 7 days. Two 5 µl blood samples were taken from each surviving crab and one 5 µl sample from the water in its container was collected. These samples were analysed for sodium content. Field studies were undertaken to investigate the effects of intertidal location, sediment characteristics and salinity on the distribution and abundance of grapsid crabs. Laboratory studies were used to investigate salinity tolerances and the osmoregulatory ability of grapsid crabs.