Surely one of the most prolific species of marine creatures, and one that we all have come across in one or other of its many different forms. Crabs belong to a group known as the crustaceans. These are marine organisms having a hard outer skeleton and include species such as crabs, prawns, shrimps, rock lobsters and many, many more. Even our old friend the barnacle is technically member of that large and very diverse group.
We doubt that anyone who has ever spent time on the seashore – either a sandy beach or or rocky coast – has not seen a crab. On the beaches the most commonly occurring crab along our KZN shoreline is the so-called ghost crab. There are three different species of ghost crabs all of which are notable burrowers, excavating deep burrows going down to the water table. One species lives on the high beach, another in mid-beach and the third along the wash zone. Seen in daylight the ghost crab is a delicate pink, whereas the same crab seen at night by torchlight is a pale, off-white – hence its name. The ghost crab survives away from water by using a set of brush-like hairs situated at the base of the hindmost pair of legs. These soak up moisture, passing it to the gills enabling an oxygen exchange.
These rather speedy creatures perform a valuable clean-up function by scavenging along the shoreline and consuming remains of all manner of other sea creatures that might have washed up. For this purpose these crabs have powerful pincers designed to tear and cut. They are also a primary predator of sea-turtle hatchlings that have to run the gauntlet of all three ghost crab species on their perilous trip from their nest to the sea. When one seas the myriad ghost crabs on those northern Zululand beaches at night as we have so often done, it seems a miracle that any turtle hatchlings survive.
The rocky areas are where one finds the greatest variety of crabs species – for it is here that nature presents a veritable feast for different crabs. What is fascinating is to see how different species have adapted to living in different zones of the rocky shores, and how they have adapted to a wide variety of different foods. Perhaps the most visible of the rocky shore crabs are the Natal rock crabs that are found right in the surf zone and can be seen clinging to the rocks in spite of the surge and foam of incoming waves. The crabs are designed to cope with this environment, having broad, flat bodies and fairly long legs. As a wave surges in, crashing and splashing, the crabs simply flatten themselves onto the rocks and grip the rock tightly – and lo! – when the water recedes there are the crabs. These crabs are extremely quick and agile; they live off the various forms of marine algae which they scrape off the rocks with specially adapted spoon-like pincer tips. They are very shy and will scuttle away from you and disappear into the nearest crack. Relatives of these crabs can be seen scuttling in and out of rock pools. They have a mottled green/brown colour.
The brown rock crab can often be spotted hidden in a secure hole in a corner of even the most tiny rock pool. It’s a slow-moving, quite ponderous crab. With is distinctive red eyes and massive pincers this is nevertheless, a crab to be left alone.
None of these crabs will deliberately attack a person but it’s very much a case of “leave me alone and I’ll leave you alone.”
In the rock pools themselves one can often see beautifully coloured swimming crabs too. These have the tips of the back pair of legs flattened into “swimming paddles.” These chaps also have quite powerful but sharp pincers, and have spiky carapaces. Related to these is the Knysna mud crab which is the biggest of all our crabs and is a denizen of our tidal estuaries. It is a dark green colour and favours the muddy bottoms of estuaries where they excavate a burrow. Their pincers are massive and very powerful, and a big specimen can have a body the size of a saucer and pincers the size of a human hand. It is definitely a crab not be be trifled with.
Then too, one can see the the many hermit crabs, busily scuttle around in the pools, dragging their houses of an abandoned shell around with it. As the crab grows it has to find a bigger shell – unlike its more regular cousins who simply shed the outer, hard exo-skeleton and form another, bigger one within hours.
In the estuarine mudflats and mangrove swamps of Zululand one can the tiny fiddler crabs. The females are very small, drab-coloured creatures, but the males have an enlarged, brightly coloured pincer which is waved around to attract a female. There is also the brightly coloured, red sesarmid crab that eats the fallen leaves of the mangrove trees, emerging from its burrow in response to the faint impact of a falling leaf nearby.
These are but a few of a great many fascinating crabs to be found along our shores. A literature study will make a visit to the coast that much more rewarding – and again we recommend George and Margo Branch’s “Living Shores of Southern Africa.”