The Haswell's Shore Crab:A key species in the Saltmarsh of Hays Inlet

Reflections

 

The first time I came across a Haswell’s Shore Crab was during a “Clean Up Australia Day” at a reserve area in Hays Inlet which included mangrove and saltmarsh habitats. There was a number of car wrecks in the wetland area (which were all removed at a later date by the local Council) as well as domestic debris including a large piece of carpet overlay which was almost welded to the ground surface. As we pulled up the offending piece of carpet we were surprised by a number of brown and orange crabs who immediately lifted their claws and let us know of their displeasure!

Since then, and many clean up days later, I have always rushed to any piece of flat rubbish or debris in order to look for these marvelous creatures. They are one of the dominant species in the saltmarsh habitat which can prove to be a tough area to make a living in. Over the years we have seen the salinity in these habitats vary greatly from below ocean levels (approximately 35 parts of salt per thousand) to more than triple the salinity. During extended dry periods, the saltmarsh soil dries out, cracks and is often covered with a salt encrustation. There is no shade, except from the saltmarsh succulents and the marine couch so it can get very hot as well.

The crabs are pretty hardy and dig burrows in the marine couch area in order to escape the sun and avoid desiccation. They are generally nocturnal feeders which makes sense in terms of avoiding the harsh conditions as well as reducing the risk of predation. With a number of wetland birds scouring the area during daylight hours it is a good strategy to avoid becoming an avian meal. Of course another idea is to find a piece of debris such as carpet and make their way underneath in order to take advantage of a moist retreat from the harsh conditions of the saltmarsh.

Bob Crudgington

Bioturbation

 

Bioturbation

 

 

 

Haswell’s shore crabs dig burrows up to 40 cm deep in the saltmarsh sediment (up to 30 m away from the mangrove saltmarsh interface) and this behaviour has been recognised as an important factor in terms of physicochemistry. The maintenance of these burrows means that deeper soil sediments are brought to the surface and the holes themselves have an impact on tidal inundation by increasing the surface area and thus impacting on the hydrology of the saltmarsh. Abandoned burrows are utilised by gastropods(between inundation periods).

An occupied burrow is characterised by fresh sediment around it and a sharp edge. Abandoned burrows don’t have the sediment and appear eroded around the edge. These burrows are important as they form significant pathways for chemical exchange between sediments and seawater as well as providing structures for reproduction, ecdysis (moulting), and protection from predation and desiccation.

Resources

Reading List:

Australian Saltmarsh Ecology (2008) edited by Neil Saintilan CSIRO Publishing

Wild Guide to Moreton Bay edited byMichelle Ryan (1998) Queensland Museum

A Field Guide to Crustaceans of Australian Waters (1994) Diana Jones & Gary Morgan Western Australian Museum   

       
         
Haswell's Shore Crab

Scientific name: Helograpsus haswellianus

(Family Grapsidae)

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.

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