Semaphore crab fact sheet | Field of Mars EEC

What is a semaphore crab?

The semaphore crab is a crustacean that lives on intertidal mudflats and around mangroves in sheltered estuaries. The intertidal zone is the area between high and low tides.

Semaphore crabs get their name from the way males wave their claws to signal to other crabs from the opening of their burrow. They are from the order Decapoda which means they have 10 legs. The 2 legs at the front of the crab have been modified into claws used for feeding, defence and communication.

Their scientific name is Heloecius cordiformis.

A semaphore crab

Juvenile semaphore crab with orange-red claws.

What do semaphore crabs look like?

The body of a semaphore crab is up to about 3 centimetres wide when fully grown. Adults are usually a mottled, dark purple colour. The claws curl at the tips and can look slightly twisted.

Semaphore crabs have long eye stalks that lift their eyes up above the shell like periscopes. This helps scientists (and students) distinguish them from the red-fingered marsh crab, which has short eye stalks.

Close-up of a juvenile semaphore crab showing long eye stalks.

Adult semaphore crabs have purple claws.

Where do semaphore crabs live?

Semaphore crabs live on mudflats at high tide level in sheltered waters, including estuaries and mangrove-lined bays, creeks and rivers.

They dig vertical burrows with a chamber at the bottom. Semaphore crabs often live close together in groups, but each crab will defend its own burrow.

Semaphore crabs create 25 to 40cm deep burrows in the mud of mangrove forests.

Semaphore crabs live in protected mangrove forests along the banks of the Lane Cove River.

What do semaphore crabs eat?

Semaphore crabs help clean mangrove mud by feeding on tiny bits of decaying leaves and other organic material mixed through the sediment. As the tide drops, they come out of their burrows and shovel mud into their mouthparts to filter out edible material.

In places like Buffalo Creek Reserve, semaphore crabs play an important role in mangrove food webs by processing large amounts of leaf litter.

Semaphore crabs eat algae, micro-organisms and decaying plant material found in mud.

What eats semaphore crabs?

Semaphore crabs are part of the estuary food web. Predators include fish, especially at high tide and shorebirds, especially at low tide.

What is the life cycle of a semaphore crab?

Semaphore crabs lay eggs, and females carry developing eggs on the underside of their abdomen. When the eggs hatch, the young are tiny drifting larvae called zoea that move with tides and currents as plankton. After several moults and growth stages, the larvae return to estuaries and settle as juvenile crabs.

How are semaphore crabs adapted to their environment?

Like other crustaceans, semaphore crabs have a tough outer exoskeleton or carapace that protects their body. The mottled coloured carapace provides camouflage. Semaphore crabs breathe using gills in water, but they can also survive out of water as long as they keep the gill chamber moist.

Semaphore crabs use burrows as shelter from predators and harsh conditions. When the tide comes in, they can quickly retreat into the burrow and plug the entrance with mud. Their compound eyes on stalks help them to see what is happening on the outside of their burrow. It is similar to looking through a periscope and having a panoramic view.

Males use their claws as bright signals, waving them to warn off other males and attract females.

Semaphore crabs use their claws to defend their burrows.

During high tide semaphore crabs hide in their burrows.

Related fact sheets

More invertebrate and environment fact sheets

• Mangrove forest fact sheet – habitat overview where semaphore crabs live.
• Crustaceans fact sheet – broader crustacean group including crabs, slaters and other isopods.
• Red-fingered marsh crab fact sheet – another common mangrove crab found in Sydney.

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