Semaphore crab fact sheet | Field of Mars EEC

What are semaphore crabs?

Semaphore crabs are small native Australian crabs that live on intertidal mudflats and around mangroves in sheltered estuaries. The intertidal zone is the area between high and low tides.

Their scientific name is Heloecius cordiformis. Semaphore crabs get their name from the way males stand near their burrows and wave their claws to signal to other crabs.

Like other crabs, semaphore crabs are crustaceans. They have a hard outer shell called a carapace and ten legs. The front pair of legs has been modified into claws used for feeding, defence and communication.

A semaphore crab.

Juvenile semaphore crab with orange-red claws.

Where do semaphore crabs live?

Semaphore crabs live in sheltered coastal environments where mudflats are covered and uncovered by the tide. They are often found in mangrove forests, saltmarsh edges, estuaries and quiet riverbanks.

They dig burrows in soft mud. These burrows provide shelter from predators, drying out and changing tide conditions. Semaphore crabs may live close together in large groups, but each crab defends its own burrow.

Around Sydney, semaphore crabs can be found in mangrove habitats such as those along the Lane Cove River and Buffalo Creek Reserve.

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

Adult semaphore crabs have purple claws.

Semaphore crabs create 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 feed by scooping up mud and sorting out tiny pieces of food. They eat algae, micro-organisms and decaying plant material mixed through the sediment.

This feeding helps recycle nutrients in mangrove mud. By processing leaf litter and organic material, semaphore crabs help keep energy moving through the estuary food web.

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

What is the life cycle of a semaphore crab?

Semaphore crabs begin life as eggs carried by the female under her abdomen. When the eggs hatch, the young are tiny larvae called zoea.

These larvae drift in the water as plankton and grow through several stages. After moulting and changing shape, they return to suitable estuary habitat and settle as juvenile crabs.

Juvenile semaphore crabs grow by moulting. This means they shed their hard outer shell and form a new, larger one. Over time, they develop the stronger claws and darker colours seen in adult crabs.

Crab larvae hatch as tiny plankton called zoea before developing into juvenile crabs.

What adaptations do semaphore crabs have to help them survive?

Semaphore crabs have a hard carapace that protects their body. Their mottled colours help them blend in with wet mud, shadows and mangrove roots.

Their long eye stalks lift their eyes above the carapace like periscopes. This helps them watch for predators while staying close to the safety of their burrow.

Semaphore crabs breathe with gills. They can survive out of water while their gill chamber stays moist, which helps them feed on exposed mudflats at low tide.

Their burrows are important shelters. At high tide, semaphore crabs can retreat underground and plug the burrow entrance with mud. This helps protect them from fish and other predators that move across the mudflat when the water returns.

Male semaphore crabs use their colourful claws for signalling. Waving their claws may help them warn other males away from their territory or attract females.

Semaphore crabs use their claws to defend their burrows.

During high tide, semaphore crabs shelter in their burrows.

Why are semaphore crabs important?

Semaphore crabs are important members of mangrove and estuary ecosystems. They help break down decaying plant material and recycle nutrients through the mud.

Their burrows also disturb and mix the sediment. This can help move water and air through the mud, creating tiny spaces used by other small organisms.

Semaphore crabs are also food for other animals. Fish may eat them when the tide is high, while shorebirds may hunt them when the tide is low. This makes semaphore crabs an important link between mangrove plants, mudflat invertebrates, fish and birds.

Semaphore crabs help recycle nutrients as they feed and burrow through estuary mud.

How can you help semaphore crabs?

You can help semaphore crabs by protecting mangroves, mudflats and estuaries. These habitats may look muddy, but they are full of life and provide food, shelter and nursery areas for many animals.

When visiting mangroves or estuary edges, stay on tracks, paths or boardwalks where possible. Avoid trampling mudflats, poking sticks into burrows or collecting crabs.

Keep litter, chemicals and garden waste out of gutters, drains, creeks and rivers. Pollution can wash into estuaries and harm the animals that live there.

If you see semaphore crabs, watch quietly from a distance. They are sensitive to movement and will quickly disappear into their burrows if disturbed.

Boardwalks help people explore mangrove habitats without trampling mudflats and crab burrows.

Related fact sheets

More invertebrate fact sheets

• Invertebrate fact sheets – overview
• Crustaceans fact sheet – explains the animal group that includes crabs, prawns, slaters and other crustaceans.
• Red-fingered marsh crab fact sheet – another common crab found in Sydney mangrove habitats.

Habitats and ecosystems

• Mangrove forest fact sheet – explains the habitat where semaphore crabs live.
• Field of Mars Reserve fact sheet – explains the reserve habitats that support native plants and animals.

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