Velvet swimming crab

Taxonomic position: Macropipus depurator belongs to the order Decapoda and the family Portunidae. It is a common marine crab known for its distinctive swimming paddles. While it plays a functional role in natural marine food webs, its presence near commercial aquaculture sites is often problematic for operators.

Biology and life cycle: The lifecycle of this crab involves several larval stages, including zoea and megalopa, which drift in the water column before settling on the seabed. As they mature, they inhabit sandy or muddy substrates. They are opportunistic feeders, showing high activity levels during warmer months when metabolic rates increase significantly.

Crops affected: The primary concern relates to the shellfish aquaculture sector. Farms cultivating mussels, oysters, and other bivalves are the most vulnerable. Macropipus depurator is attracted to the high concentration of prey within cages and hanging lines, leading to potential significant depletion of the crop stock.

Nature of damage: The species uses its powerful chelae to crush the shells of juvenile bivalves. Beyond direct predation, their presence disrupts the feeding behavior of the shellfish. Furthermore, they can damage aquaculture infrastructure, specifically netting, which might lead to the loss of commercial biomass and additional maintenance costs for farmers.

Protection measures: Effective management strategies are crucial for minimizing losses. These include the implementation of high-strength, fine-mesh screens that prevent crab entry into culture zones. Additionally, physical trapping methods are used to manage populations in areas surrounding the farms. Careful site selection, keeping structures away from known high-density crab habitats, is also a recommended preventative strategy.