Cyclograpsus lavauxi facts for kids

The smooth shore crab (Cyclograpsus lavauxi) is a fascinating marine crab with large eyes. It belongs to the Grapsidae family and can be found in New Zealand and the Juan Fernández Islands in Chile.

Quick facts for kids Cyclograpsus lavauxi
In a tidal rock pool
Scientific classification

About the Smooth Shore Crab

The smooth shore crab, known scientifically as Cyclograpsus lavauxi, is a small crab that lives in mild climates. Adult males are usually about 28 millimeters (just over an inch) wide. Females are a little smaller, around 26 millimeters.

What Does it Look Like?

This crab has a smooth, shiny shell called a carapace. Its shell is a bit wider than it is long. On average, the shell is about 9 millimeters long and 11 millimeters wide. The front edge of its shell is straight. It has two curved eye sockets, one in each front corner.

The smooth shore crab has well-developed antennas. These help it sense things like texture, food, and even sounds. It has eight legs for walking and two large pincers at the front. Its legs are slender and long, helping it move very quickly. They also have tiny hairs along their length. This crab is known for being very agile and alert.

Crab Colors

Smooth shore crabs come in many different colors. Their shells can be slate blue, bluish-grey, or yellowish-brown. You might also see them in shades of grey, green, or brown. They often have dark brown and red speckles. The underside of the crab is usually a lighter, plain color. Their legs match the color of their shell but might have brighter speckles.

Where Do Smooth Shore Crabs Live?

Smooth shore crabs live in two main parts of the world. They are found in New Zealand and the Juan Fernández Islands off the coast of Chile.

Smooth Shore Crabs in New Zealand

In New Zealand, C. lavauxi has been found in many places. These include the Hohoura Harbour and areas down to Westland. Large groups of these crabs live in the North Island.

Where They Like to Live

This crab is very adaptable and can live in many different places. It can even handle tough weather conditions. You might find them under stones on beaches. They live on both open, exposed coasts and sheltered bays. They are quite common on New Zealand shores. Often, you'll see more smooth shore crabs than the larger common rock crab.

They often live near the edge of the sea. This is where the water might only touch them a little, or not at all. They are also found in intertidal levels (areas covered and uncovered by tides), mudflats, estuaries, and rocky shores. Older crabs tend to live higher up on the shore or mudflats. Younger crabs usually stay closer to the water's edge. In summer, the whole population moves down to the lower parts of the shore.

Life Cycle of the Smooth Shore Crab

Reproduction

Smooth shore crabs usually mate during the summer months. This happens from mid-October to late December. Most crabs in a group mate at the same time. These crabs do not mate with just one partner. Both males and females will mate many times during the breeding season.

During this time, male crabs often compete with each other. Bigger males might attack other males that are mating. This can sometimes give the female a chance to escape. To avoid this, mating pairs often hide under rocks.

Studies have shown that males will hold onto a female's shell for about an hour. After that, they mate for about two hours. The mating then ends suddenly. Females sometimes try to resist the males and get away.

Females carry eggs only between November and February. They can receive sperm for less than 24 hours in the wild. In captivity, this time can be almost a week. They lay thousands of eggs in batches. All the females in a group lay their eggs around the same four-week period.

Interestingly, females can store sperm from a previous mating season. This means they can lay fertile eggs even if they don't mate during the current season. They only lay eggs once per mating season.

When first laid, the eggs are dark purple. As they grow, they become paler. You can often see tiny black eyespots on them. The eggs are very small, about 0.25-0.3 millimeters wide. They hatch in about two months. Once hatched, the baby crabs are just over 1 millimeter long.

Moulting

Smooth shore crabs usually shed their old shells, a process called moulting, after the breeding season ends. This happens in late summer to early autumn.

What Smooth Shore Crabs Eat

The smooth shore crab is a scavenger. This means it eats dead animals and plants. However, it mostly eats plants, making it a herbivore. Its diet includes algae, drift, and seaweed. These crabs only feed when the tide is high. They can change their feeding patterns to make sure they eat when conditions are best. This helps them get the most food and nutrition.

Predators and Defenses

Several animals like to eat smooth shore crabs. Since these crabs live where the tide doesn't often reach, their main predators are usually not fish. Instead, birds like seagulls and Kingfishers are common predators.

Some fish that eat them include the Smooth Hound Fish, Spined Dogfish, Terakihi, Sea Perch, Moki, and Red Gurnard. One way the crab protects itself is by living on beaches where fish can't easily reach them. They can also use their quick reactions and speed to escape predators.

Other Cool Facts About Smooth Shore Crabs

Smooth shore crabs have some other interesting traits and behaviors. They can live successfully even when there's a lot of sand or mud under them. One study found that C. lavauxi can handle up to 63% sand and silt under rocks. This is much more than some other crab species.

Crab Postures and Movement

These crabs can be quite aggressive when they feel trapped. This is unusual for a crab. They will raise their pincers to make themselves look bigger. They also show the pale color of their pincers.

They have several main body positions:

• Flat: They lie on the ground with their pincers folded and legs close together.
• Raised: They lift their body at a 45-degree angle. Their back part stays on the ground, and their pincers rest flat or halfway up.
• Tiptoe: Their body is angled and lifted completely off the ground. Their pincers are folded but held up, and their walking legs are fully stretched out.

Digging and Clicking

Another fascinating thing about C. lavauxi is how they use stones. They are often found under rocks, grasping them with their back legs. This pushes the back of the crab up onto the stone. They use their other legs to support their weight. This likely helps them move better and stay stable against the tide. They also use their pincers and claws to dig burrows and push stones away. These burrows can be 20 to 40 millimeters deep under rocks.

One of the most surprising behaviors of C. lavauxi is its ability to make clicking noises. This sound can be quite loud, like distant artillery. It is one of the only crabs in New Zealand that can do this.

C. lavauxi has become a very strong and adaptable species. It can survive in extreme conditions that many other New Zealand crabs cannot. This has led to a large and successful population, making them one of New Zealand's most common crabs. More studies are needed to fully understand all their amazing abilities and why they are so successful.