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.

A semaphore crab showing long eye stalks above its shell.

Adult semaphore crabs can have purple claws with pale tips.

Semaphore crabs create burrows in soft mud near mangrove roots.

Semaphore crabs can live in protected mangrove forests along tidal rivers such as 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 sift mud for algae, micro-organisms and decaying plant material.

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.

A crab zoea larva shows one stage in the life cycle before juvenile crabs settle in estuary habitat.

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, helping them watch for predators while staying close to the safety of their burrow.

Semaphore crabs breathe with gills, but they can feed out of water while their gill chambers stay moist. Their burrows are important shelters from drying out, heat and predators. At high tide, semaphore crabs can retreat underground and plug the burrow entrance with mud.

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

Semaphore crabs raise their claws near burrow entrances during territorial displays.

At high tide, semaphore crabs can shelter in burrows beneath flooded mangrove mud.

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.

Raised boardwalks help people explore mangroves without trampling mudflats or crab burrows.

Related fact sheets

More invertebrate fact sheets

• Invertebrate fact sheets – Explore insects, spiders, crustaceans, molluscs and other small animals found in Sydney and NSW.
• Crustaceans fact sheet – Learn about the animal group that includes crabs, prawns, slaters, pill bugs and land hoppers.
• Red-fingered marsh crab fact sheet – Compare another mangrove crab that burrows in intertidal mud and recycles plant material.

Habitats and ecosystems

• Mangrove forest fact sheet – Learn about the intertidal forest habitat where semaphore crabs live, feed and burrow.
• Coastal salt marsh fact sheet – Explore another tidal wetland habitat found beside estuaries and mangrove areas.
• Water habitats fact sheet – Learn how water habitats support animals, food webs and life cycles.
• Field of Mars Reserve fact sheet – Find out about local bushland, mangroves, salt marsh, creeks and wetlands in the Lane Cove River catchment.

Plants and animals that share this habitat

• Grey mangrove fact sheet – Learn about a mangrove plant that provides habitat for crabs, shellfish, fish and waterbirds.
• Australian white ibis fact sheet – Explore a native waterbird that feeds in wetlands, parks and muddy places.
• White-bellied sea eagle fact sheet – Learn about a waterway predator that depends on healthy waterways and food webs.

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