Expert topic: Gazami crab

by Rebecca Sherratt, Features Editor, International Aquafeed

The gazami crab (Portunus trituberculatus) is the most widely fished species of crab on Earth, commonly found off the coasts of Korea, Japan, Taiwan, China and Pakistan. Many readers might recognise it by its various other names, including the horse crab or the Japanese blue crab.

This species of crustacean can be distinguished by its three remarkably triangular teeth, the central of which is slightly forwards projecting compared to the lateral ones. The merus, or legs, also possess four spines on their inner margins and are laterally flattened to varying degrees. Their last two legs at the back are paddle-like and with bluish tints. The carapace colour of the gazami crab can vary anywhere in colour between a dull green through to brown. Males can grow to a decent size, with carapaces of up to 15cm in length.
 

P. trituberculatus make their habitats in sandy-muddy depths in shallow waters of up to 50 metres deep and prefer tropical waters of around 24C. Gazami crabs began to make a splash in the aquaculture sector in the 1960s and since then significantly increased in terms of global capture production. In 2000, the number of gazami crabs captured was 303,431 tonnes which, in just six short years, increased to 557, 728t. The top exporter of this crustacean is China, the top importer being the US.

Reproductive methods
Gazami crabs have been noted for their unique precopulatory courtship rituals, through tactile and olfactory queues. Spawning season takes place in early spring, between March and April. Females migrate to the spawning grounds before males, often resulting in over 70 percent of catches at this time being female gazami crabs when trawling takes place.

A 100g female produces approximately around 350,000 eggs at first mating, whilst larger gazami crab females may produce as many as 800,000 eggs.

The mating period in China for these crustaceans takes place from July through to November, with most occurrences of fertilisation occurring between September and October. Mated females store male sperm and reach sexual maturity in the spring, once fertilisation and incubation begin to occur. Two broods may be incubated annually. Should a second incubation occur, it takes place between 10-20 days after the first incubation.

Larvae are planktonic and have six larval stages which occur over 30 days. Gazami crabs grow the most rapidly during the summer. One key advantage with this species, that no doubt contributes towards the gazami crab as being the most fished crustacean available, is its rapid growth rate. Crabs which are hatched in spring can reach commercial size by late-summer or autumn and can spawn again by the following spring.

Read more, HERE.

The Aquaculturists

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