What is a semaphore crab?
Semaphore crabs are small marine crustaceans. They live in the intertidal zone along the edge of creeks and rivers on mudflats, river beaches and in mangrove forests and saltmarsh communities.
They are named ‘semaphore’ as they are able to communicate with other crabs by waving their chelae. The scientific name for a crab’s pincers or claws is chelae. The word ‘semaphore’ means to send messages using signals or flags.
Their scientific name is Heloecius cordiformis.
What do semaphore crabs look like?
Semaphore crabs like all crabs have 10 legs. The first pair of legs are called chelipeds and have the chelae. The chelae are used for feeding, fighting and communicating. The other four pairs of legs are used for walking.
Semaphore crabs have two long eyestalks with eyes on the tips. They can be held upright or can lie along their mottled purple to black carapace. Adults are the size of a large grape, with a carapace up to 2.5 centimetres wide. Adult males have purple claws with a white tip. Females and juveniles have orange claws with a white tip.
Where do semaphore crabs live?
Semaphore crabs live on the ground and shelter in burrows within the intertidal zone of estuaries. These areas experience high and low tides and are a mix of saltwater and freshwater with muddy, sandy soils. They can be found on mudflats in mangrove forests and on the sndy foreshores of creeks and rivers.
Semaphore crabs dig into the ground to make their burrows. Their burrow entrance is round in shape and between 25 and 40 centimetres deep. During the day and during a low tide, when the ground is exposed to the air, the crabs emerge from their burrows to feed. They will stay close to the burrow and defend it from intruders. They return to their burrow at dusk and at the end of the low tide, plugging the entrance with a large mud ball. They remain in their burrows during winter, feeding on material in their burrows and slowing their metabolism.
What do semaphore crabs eat?
Semaphore crabs collect their food by making mud balls. They scoop up mud with their claws and lift the mud to their mouthparts. They feed on algae, micro-organisms and dead organic matter such as fallen mangrove leaves and branches found on the forest floor. The leftover material is dropped, leaving behind a small ball of mud. These mud balls are characteristic of mangrove forests, river foreshores or areas where crabs feed.
Semaphore crabs will also make mud balls to take into their burrows. They can make up to 30 balls during the low tide. The balls can be as large as the crabs themselves.
What eats semaphore crabs?
Like many species of crabs in the intertidal zone, the semaphore crab is an important part of the wetland ecosystem. They are food for shore birds during low tide and fish during high tide.
What are the life stages of a semaphore crab?
Crabs lay eggs. The eggs develop on the female abdomen attached by a glue like material. When the eggs hatch they are called zoea larvae. They are released into the water during an outgoing tide. Like all plankton, the zoea drift in the water unable to swim against tides or currents. During this life stage they grow larger and moult with each instar stage. In the final instar stage the larva returns to the estuary where it settles as a juvenile crab.
How are semaphore crabs adapted to their environment?
Semaphore crabs are invertebrates and have a hard exoskeleton for protection. The mottled coloured carapace provides camouflage and the bright chelae are used to protect themselves from predators. Their chelae are spoon shaped allowing them to easily lift and manipulate mud.
The compound eyes on stalks help semaphore crabs to see what is happening on the outside of their burrow. It is similar to looking through a periscope and having a panoramic view.
In water, semaphore crabs breathe using gills. When exposed to the air at low tide, they lower themselves onto water on the muddy surface. They take in air by keeping their gills wet.
A semaphore crab burrow provides protection from predators. They can quickly move into the burrow during low tide and a deep burrow will protect them from fish, especially toadfish, when the burrow is submerged during high tide.
