Crustaceans
Although there has been a significant amount of research conducted in mangrove habitats on crabs there has been less work on neighbouring saltmarsh species. Like their mangrove “cousins” many of these crabs are conspicuous by their burrows. It is believed that this behaviour may have an impact on the saltmarsh environment, as they burrow over significant areas and this activity would increase aeration, moisture retainment and assist with the redistribution of detritus and organic matter. This activity would also help maintain vegetative structure in the saltmarsh. Their burrows are also utilised by other species including gastropods associated with the saltmarsh substrate.
Shore Crabs
Saltmarsh crabs or Shore Crabs are from two families; the Grapsidae and Ocypodidae with both types common on the saltmarsh and adjacent channel and creek banks. Grapsids are characterised by their square shaped body and short eyes, whereas Ocypodids have long eye stalks and often have one claw much larger than the other. Occasionally Portunid crabs (swimming crabs) such as the mud crab (Scylla serrata) will also utilise the saltmarsh habitat with some individuals even burrowing into the saltmarsh substrate. Portunid crabs can be easily identified as their last set of legs are flattened like oars, enabling the crab to swim.
Diet of Shore Crabs

The diet of crabs in the saltmarsh is still not fully understood, although some species utilise the saltmarsh vegetation and other herbivores feed on the associated microalgae. Some studies have also found that detritus and other benthos such as diatoms are a key source of nutrients for shore crabs.

Impact of Shore Crabs in Saltmarsh
Shore crabs also release larvae into the saltmarsh during tidal inundation and these releases are utilised by zooplankton feeding fish. Adult shore crabs are also preyed upon by fish including the common toadfish and wading birds. They are a significant contributor to the saltmarsh ecosystem through their burrowing behaviour and their contribution to the food web. Monitoring crab numbers through burrow counting and observation is an important tool in understanding the health and viability of a saltmarsh habitat.
Bioturbation

Impact on sediments

Burrowing – sediment mixing, redistribution of organic matter, change in irrigation patterns and increase of oxygen supply to deeper sediments.

Feeding – intensive surface grazing decreases the populations of meiofauna, microphytobenthos and organic matter, all of which influence the biochemistry of the saltmarsh and the adjacent mangrove and estuarine areas.

Because shore crabs can occur in large numbers they can have a significant impact on the ecology and geochemistry of the saltmarsh they inhabit.

 

Mud Crab Scylla serrata (Portunid)

Haswell's Shore Crab (Grapsid) Note the short eyes

Fiddler Crab (Ocypodid) Note the long eye stalks

Crab burrows in Saltmarsh Marine Couch

Video of Haswell's Shore Crab

Video of Mud Crab