Velvet swimming crab

The velvet swimming crab (Macropipus depurator) is a crustacean belonging to the family Polybiidae in the order Decapoda.

It is easily identifiable by its reddish-brown carapace, which is covered in a dense layer of short, fine, velvet-like hairs.

Its specialized morphology includes flattened, paddle-shaped hind legs, which enable efficient swimming in the water column.

Adult crabs typically have a carapace width of about 5-6 centimeters and possess strong claws for seizing prey.

The pattern on the shell often looks like a fine network, which is a common diagnostic feature for this specific species.

In aquaculture environments, the velvet swimming crab acts as a significant predator on bivalve mollusks and small crustaceans.

By consuming juvenile shellfish, they cause direct economic loss and reduce the stock density in protected sea cages.

They can crush the shells of cultured mollusks, rendering the product unmarketable and susceptible to subsequent infections.

The presence of these crabs can lead to increased stress levels in cultured populations, leading to slower growth rates.

Their invasive-like behavior in aquaculture systems requires constant vigilance to prevent destruction of the cultured stock.

The activity of the velvet swimming crab is largely temperature-dependent, peaking during the spring and summer months.

During these warmer periods, adult crabs migrate to shallower coastal waters, increasing their interaction with human activities.

The life cycle involves planktonic larval stages that can be widely dispersed by currents before settling on the seabed.

Once settled, juvenile crabs grow rapidly, feeding on whatever prey is available in their local habitat.

Seasonal environmental factors, such as salinity and nutrient availability, play a key role in regulating local population numbers.

Signs of an infestation include an abundance of broken or predated mollusk shells located in or near the farming enclosures.

A rapid decline in the stock of juvenile shellfish that cannot be attributed to disease or poor water quality is a primary indicator.

Night-time monitoring of cages often reveals large numbers of these active crabs hunting for food near the structure.

The presence of discarded molted exoskeletons is a clear sign that the population has established itself in the vicinity.

Direct damage to the edges of shells caused by crab chelae is a definitive diagnostic sign of their predatory activity.

The most reliable form of control is the installation of fine-mesh netting that physically blocks the crabs from entering cultivation areas.

Active trapping using baited crab pots is recommended to manage and reduce the density of the local crab population effectively.

Regular maintenance of the farming infrastructure helps to keep the area clear of debris that could serve as shelter for the crabs.

Monitoring programs should be implemented to track the arrival of seasonal migrations and initiate control measures promptly.

An integrated approach, combining physical barriers with systematic trapping, provides the best protection for aquaculture ventures.