Scientific name: Helograpsus haswellianus
These crabs are one of the most common crustaceans found in the saltmarsh habitats of South East Queensland including Hays Inlet, Moreton Bay . The crab are distributed widely, ranging from southern Queensland, through New South Wales, Victoria, Tasmania to the Spencer Gulf in South Australia. They are cryptic in nature, nocturnal and they often form burrows around marine couch and other saltmarsh vegetation. They are also found above the high tide line, hiding under vegetation and debris associated with the saltmarsh. This behaviour is likely to reduce predation and desiccation as well as provide a safe place for moulting and breeding. The crabs on the right were found under a piece of an old bumper bar adjacent to a saltwater drainage channel. The photograph underneath is the cast of a crab washed up onto the saltmarsh vegetation they are often associated with.
They are members of the Grapsid family of crabs and are characterised by a wide front, short eyes and an almost square-shaped body. They are a small to medium sized crab, measuring up to 30 mm across their carapace. Grapsid crabs are generally herbivores and feed on mangrove litter and their feeding behaviour is important in maintaining carbon within the habitat. Haswell's Shore Crabs are generally brown in colour with traces of a pale honey colour particularly with their claws which can almost appear to be translucent. Mature males generally have slightly larger claws (also known as chelae) than the females. Research has indicated that thes shore crabs feed on microphytobenthos (unicellular algae and diatoms) and to a lesser degree organic matter from marine couch (Sporobolus virginicus).
The breeding behaviour is influenced by the tidal cycle. The Haswell’s Shore Crab will release it’s larvae at the peak of the king tide (maximal inundation of the saltmarsh area) so that larvae are carried away from the saltmarsh into adjacent creeks and channels. Interestingly a number of researchers have found that the crabs wait until the second or third day of the spring tide before releasing their larvae. Fish also take advantage of this release with local residents such as the Estuary Perchlet Ambassis marianus moving into the saltmarsh area and consuming significant numbers of crab larvae. This concentrated "flooding behaviour" may be a strategy to overwhelm predators. Adult crabs are also preyed on by fish and wading birds associated with the saltmarsh habitat. They are clearly an important link in the saltmarsh and mangrove habitats food chains.
Apart from their impact on the food chain (and carbon distribution) their burrowing habit has an impact on the physicochemical properties of the saltmarsh. Refer to the section on bioturbation.