Red-fingered marsh crab fact sheet | Field of Mars EEC

What are red-fingered marsh crabs?

Red-fingered marsh crabs are small native crustaceans that live in mangrove forests, saltmarshes and muddy intertidal areas. The intertidal zone is the area between high and low tide.

They are also called red-fingered shore crabs. Adult crabs are named for the red-orange tips on their claws. Their scientific name is Parasesarma erythodactylum. Some references use the spelling Parasesarma erythodactyla.

Red-fingered marsh crabs are decapods, which means they have ten legs. The two front legs are modified into claws that help them feed, dig, defend themselves and move food towards their mouth.

A red-fingered marsh crab.

Red-fingered marsh crabs use their large claws for feeding and defence.

Crabs have ten legs, including the two front claws.

What do red-fingered marsh crabs look like?

Red-fingered marsh crabs have a square to rectangular body called a carapace. Adult males can have a green, blue-green or dark shell, while younger crabs may look darker and less colourful.

Their most noticeable feature is the red-orange colour on the tips of their claws. These claws look like small red fingers, which gives the crab its common name.

Red-fingered marsh crabs have short eye stalks, so their eyes sit close to the top of the shell. This can help distinguish them from semaphore crabs, which have much longer eye stalks and are also found in mangrove habitats.

The claws of red-fingered marsh crabs have bright red-orange tips.

Red-fingered marsh crabs have short eye stalks.

Where do red-fingered marsh crabs live?

Red-fingered marsh crabs live on the muddy floor of mangrove forests and saltmarshes. They are often seen at low tide, when the mud is exposed and they can leave their burrows to feed.

Their burrows help them stay safe from predators and protect them from drying out. When disturbed, red-fingered marsh crabs usually move quickly sideways and retreat into the nearest burrow, crevice or shelter.

At places such as Lane Cove National Park and other Sydney estuaries, mangrove forests provide the muddy habitat, fallen leaves and shelter that red-fingered marsh crabs need.

Red-fingered marsh crabs live in the mud of mangrove forests.

Red-fingered marsh crabs live in mangrove forests like this one in Lane Cove National Park.

What do red-fingered marsh crabs eat?

Red-fingered marsh crabs are important recyclers in mangrove forests. They feed on fallen mangrove leaves and help break them into smaller pieces.

This feeding helps return nutrients to the mud and supports the wider mangrove food web. In this way, red-fingered marsh crabs do a similar job to decomposers and detritus feeders in other habitats.

They may also eat algae, small animals and decaying organic matter. Their claws help them hold and break up food before they eat it.

Red-fingered marsh crabs eat mangrove leaves and other organic matter.

What is the life cycle of a red-fingered marsh crab?

Red-fingered marsh crabs hatch from eggs. Like other crabs, females carry developing eggs under the abdomen before the young hatch.

The young do not look like adult crabs at first. They begin life as tiny larvae called zoea, which drift with tides and currents as part of the plankton. After several moults, they develop into a later larval stage called a megalopa.

The young crabs eventually settle into suitable estuary, mangrove or saltmarsh habitat. As they grow, they moult their hard outer skeleton and develop into juvenile and then adult crabs.

Crabs begin life as tiny drifting larvae before developing into juvenile crabs.

What adaptations do red-fingered marsh crabs have to help them survive?

Red-fingered marsh crabs have a hard exoskeleton that protects their soft body and helps reduce water loss when they are out of the water at low tide.

Their gills need to stay moist so they can breathe. Sheltering in damp burrows helps them avoid drying out while the tide is low.

Their claws help them feed, dig and defend themselves. Their sideways movement helps them escape quickly, while their flat body allows them to squeeze into burrows, crevices and other small spaces.

Their green or dark body colour can help them blend in with mangrove mud, leaves and seedlings. This camouflage makes them harder for predators to see.

Red-fingered marsh crabs retreat to burrows when threatened.

Flat bodies allow red-fingered marsh crabs to hide in crevices.

Why are red-fingered marsh crabs important?

Red-fingered marsh crabs are important animals in mangrove ecosystems. By eating fallen leaves, they help break down plant material and recycle nutrients through the mud.

They are also part of the food web. Fish may feed on crabs when the tide is high, while birds and other animals may hunt them when the mud is exposed at low tide.

Their burrowing also helps move and mix the mud. This can change the tiny spaces in the sediment and influence how water, air and nutrients move through the mangrove floor.

Red-fingered marsh crabs help recycle nutrients as they feed and move through mangrove mud.

How can you help red-fingered marsh crabs?

You can help red-fingered marsh crabs by protecting mangrove forests, saltmarshes and estuary edges. These habitats are important nurseries and feeding areas for many animals, not just crabs.

When visiting mangroves, stay on tracks, boardwalks or firm edges where possible. Avoid trampling burrows, seedlings and soft mud.

Keep rubbish, chemicals and garden waste out of drains and waterways. Stormwater can carry pollution into creeks, rivers and estuaries where mangrove animals live.

If you see red-fingered marsh crabs, watch quietly from a distance. They are best observed at low tide when they leave their burrows to feed.

Staying on boardwalks and tracks helps protect mangrove mud, seedlings and crab burrows.

Related fact sheets

More invertebrate fact sheets

• Invertebrate fact sheets – overview
• Crustaceans fact sheet – explains the animal group that includes crabs, slaters, prawns and other crustaceans.
• Semaphore crab fact sheet – another crab that lives in mangrove forests and mudflats.
• Pill bugs fact sheet – land-living crustaceans that help recycle organic matter in damp habitats.

Habitats and ecosystems

• Mangrove forest fact sheet – explains the habitat where red-fingered marsh crabs live.
• Saltmarsh fact sheet – another intertidal habitat found near estuaries.
• Field of Mars Reserve fact sheet – explains the local habitats that support native plants and animals.

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