Across an estuarine shore there are a range of habitats. In the photo above, at the far right is a band of trees where the highest tide never reaches. In front of the trees are saltmarsh flats, which may be quite large. Saltmarsh flats may only be covered with water during king tides. Haswell's Shore Crabs live here.

Here two girls are looking for Haswell's Shore Crabs under blown-in seagrass now lying on the saltmarsh flat.

A low mound of sand divides the saltmarsh flat from the beach. Both Ghost Crabs dig their burrows in this low sandy ridge and on the high sandy beach in the middle of the photo above.

The dead tree trunk lies on the edge between the pure sand and the muddy sand where most high tides reach. Both the Smooth Shore Crab and Spotted Smooth Shore Crab live under debris at this level.

Both Smooth Shore Crabs are very common under stones, boulders and oyster clumps as in the scene to the left. The Lemon-yellow Clawed Fiddler Crab may also be seen here.

The most densely occupied habitat is under Grey Mangrove trees, Avicennia marina. Underneath mangroves the mud is usually firm.

Here we will commonly find Semaphore Crabs, Red-fingered Marsh Crabs and some Spotted Smooth Shore Crabs under debris, and occasional Leach's Shore Crabs.

Surrounding and in front of the mangroves large flat sandy areas are the home of the soldier crabs. In the north we are more likely to find the Light-blue Soldier Crab, while in the south the Dark-blue Soldier Crab. Both Soldier Crabs form large feeding armies moving in unison across the sandflats.

On the far side of the mangroves, in some areas, the mud begins to become quite sloppy. On low broad islands in the mud, shy Orange-clawed Fiddler Crabs, waving their chelae may be seen.

On low but large clumps of oysters embedded in the sloppy mud, White-faced Crabs live.

If the estuary has seagrass flats, a common crab is the Seagrass Sentinal Crab.

In the deeper mud the most common crab is the Mudflat Sentinal Crab. They seem to be everywhere. Migratory Whimbrels and Curlews find these crabs to be quite tasty.

References:

Davey, K. 1998. A Photographic Guide to Seashore Life of Australia.
New Holland press, Sydney.