Measuring mud crabs at Saimon’s Market. (image: Andrew Halford, SPC)
The mud crab or mangrove crab, Scylla serrata, is a highly desirable crustacean species that is widespread throughout the Indo-west Pacific region, where it lives in close association with mangrove ecosystems . Genetic analysis has demonstrated that there are at least three distinct stocks of mud crabs in the region: western Indian Ocean, eastern Australia and the tropical Pacific Ocean, and northwestern Australia (Fratini et al. 2010). The larvae of S. serrata can drift at sea as part of the plankton for up to 75 days, enabling them to colonise habitats far from the home of their parents, resulting in genetically well-mixed populations.
The life cycle of these crabs is complex, involving an extended period in the planktonic phase where they go through five developmental stages before settling into habitat close to mangroves. As they grow, individuals move incrementally farther into the mangrove system. Sexually mature females undergo long-distance migrations of up to 90 km, to offshore waters to spawn and renew the cycle. Crabs can reach maturity in as little as 12–18 months in tropical latitudes but can take as long as 24 months in more temperate locations. Sexual maturity for S. serrata also occurs at different sizes throughout their geographic range. For example, male crabs in South Africa reach sexual maturity at a carapace width (CW) of 11–12 cm, compared with Pohnpei, which is quite close to the equator, where they are a full 1 cm larger at 12–13 cm CW (see Table 2 in Alberts-Hubatsch et al. 2015). Mud crabs typically only live for a maximum of three to four years, such that constant recruitment is necessary to keep the population replenished and stable.
Andrew R. Halford, Senior Coastal Fisheries Scientist, Coastal Fisheries Programme, Pacific Community. Email: [email protected]
Pauline Bosserelle, Coastal Fisheries Science Officer, Coastal Fisheries Programme, Pacific Community. Email: [email protected]