Burrowing by crabs is an important component of their functional role in mangrove and saltmarsh habitats. The grapsid crab Helograpsus haswellianus (Whitelegge, 1889) is one of the more conspicuous burrowing organisms in the saltmarshes of southern Australia. To evaluate intraspecific differences in burrowing behaviour among saltmarshes on a regional scale, we compared vegetation cover, sediment composition and burrow morphology at four sites using resin casts. Six burrow morphology characters were measured (burrow depth, number, lengths and diameter of the shafts, ratio of the shafts, number and diameter of the burrow openings), and the overall 3D burrow complexity was described using a single parameter, the fractal dimension D. Apart from the number of shafts, all morphological characters of the burrows differed significantly among sites. Analyses of the fractal dimensions lead to the identification of three groups of burrows based on D: a group of highly complex burrows (one site), a group of burrows of intermediate complexity (two sites) and a group of less complex burrows (one site). Burrow morphology variation was correlated with non-dominant vegetation, plant matter in the soil and very coarse sand in the sediment. Site-specific differences in burrows caution against generalising the functional role of crabs across sites.

The effect of bioturbation by the ghost shrimp Trypaea australiensis and semaphore crab Heloecious cordiformis was compared in sediment-filled tanks in the laboratory. Effect of bioturbator density was also investigated with high- and low-density treatments. It was hypothesised that the two species would influence the sediment profile in different ways owing to their contrasting burrowing and feeding habits. Both species increased porosity of surface sediments relative to control tanks. Crab activity did not alter redox potential, but low densities of shrimp created more oxidising conditions and high densities of shrimp created more reducing conditions than controls. Burial of tracer particles by crabs was restricted to the top 5 cm, whereas shrimp mixed particles to depths of 25 cm. Bioturbator density had little effect on the extent of particle mixing. The presence of both shrimp and crabs increased benthic microalgae in the sediments relative to the controls. Again, crabs had the greatest effect at the sediment surface, whereas shrimp also enhanced concentrations at 25 cm depth. High densities of shrimp had the greatest effect. Overall, shrimp bioturbation influenced deeper sediments than crab bioturbation, but there was no clear density-dependent effect.

An important aspect in determining the ecology of a species is an understanding of its abundance and distribution. The population structure and distribution of Helograpsus haswellianus (Brachyura, Grapsidae) were evaluated over 12 months at four intertidal wetland sites that differed in vegetation and inundation. Surface activity of H. haswellianus varied over time with no consistent seasonal pattern. Sex ratios were always significantly skewed towards males, and crabs of both sexes were distributed evenly across habitats at all sites. Recruitment patterns and size frequency distributions varied significantly between sites, independent of habitats. Crabs were smaller in wetter, more sheltered areas, but size ranges overlapped between habitats. The site-specific difference in population structure and distribution of H. haswellianus indicates that the functional role of this burrowing crab may vary across wetlands.