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Wednesday, Feb. 22, 2006


Signal crayfish

* Japanese name: Uchida zarigani
* Scientific name: Pacifastacus leniusculus
* Description: Crayfish look like small lobsters. Despite their name, they are not fish, but freshwater crustaceans. They have a segmented body, up to 20-cm long, with 10 legs and a hard exoskeleton that they must molt as they grow. The "tail" part of the abdomen is flat but strongly muscled, and this is where most of the edible flesh comes from. The shell is a dark brown color. The front two legs bear huge pincers. The eyes are large and the head carries two long sensory antennae.
* Where to find them: In cool waters, in rivers and lakes from Honshu to Okinawa. But the Signal crayfish found in Japan is not indigenous to the country: It was introduced from the western United States (where they are also known as crawfish) for cultivation as a food source.
* Food: Small fish and other crustaceans. They will also scavenge dead animal and plant material from the river bed.
* Special features: If you see a crayfish in the wild, don't feel too bad about catching and eating it. The Signal crayfish carries "crayfish plague," which is caused by a fungus and has wiped out native crayfish in many countries where is has been introduced, especially Britain and Japan.

When they have the urge to mate, males locate a female by sight and smell. They then catch the female and hold her tight with their pincers and transfer sperm to her. The female then hides away on her own under rocks, and her eggs develop within her. A few weeks later, she lays a clutch of large eggs. Any eggs that were not fertilized, the female discards. She keeps the rest safe under her abdomen until they hatch.

Juvenile crayfish stay with their mother for several weeks, depending on the temperature. If it is warm they grow more quickly and leave sooner. As they grow, they need to shed their exoskeleton and expand. Very young animals can do this daily, but as they age, molts take place less frequently. A sluggish, slow-moving crayfish is a sure sign that a molt is about to take place. Calcium is extracted from the shell and kept internally. The animal then fans its legs and the carapace splits. For a day or two after molting, the soft animal is vulnerable to predators.


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