Invertebrates, inhabiting Russian north-eastern seas, are in real danger – the situation reminds of a standard Hollywood thriller. However, human beings are drama characters too, and they are even capable of changing the situation.

Scientists from the Institute of Marine Biology of Far Eastern branch of Russian Academy of Sciences are really worried about health of commercial crabs, dwelling in the Sea of Okhotsk. Crabs are infested with rhizocephalan crustaceans (Sacculina sp. and others), and human beings do nothing but making the situation worse.

A rhizocephalan parasite is very hard to be noticed and even harder to be recognized as a crustacean for a non-professional eye. Part of a parasite’s body, which is hidden inside a host crab, is colored emerald-green and consist of numerous dendritic shoots, known as “roots”. Roots grow between crab’s muscle fibres, causing their atrophy, and go further to crab’s internal organs, including sexual glands, and degenerate them – that is why infested crabs are unable to propagate.

One parasite produces about 300-400 thousand larvae – naupliuses. When time to propagate comes, a parasite grows so-called “externa” on host’s abdominal side – it is a node, connected with parasite’s inner part with a thin stem. Externa stores female reproductive products and is located in the place, where healthy crabs usually carry their spawn. This node, which initial colour is orange, late turning to red due to hemoglobin, slowly grows, until becoming 10-12 cm long and 2-2.5 cm in diameter, sometimes sticking out a crab shell.

When parasite’s naupliuses mature, they leave externa, spend some time in water and then infest other crabs (a notable fact is that only female naupliuses enter crabs, while male ones exist only for delivering male germinal cells to externa). Infested crab remains alive for several years allowing its parasite eating on account of its host, without letting it propagate, and weakening crab’s organism. Parasites even affect behavioural patterns of its hosts – infested crabs nurse hazardous node as if it is their own spawn without trying to get rid of it. Starveling crabs are often infested with other parasites – one-celled animals and worms. Such secondary infections can kill crabs and could have eliminated crab population if not infesting rhizocephalans too.

What is the role of human being on this terrifying drama? When fishermen perform commercial fishing of crabs, they pick up only the best invertebrates, throwing sick ones back to the sea, because their look is not that good. When a crab carries a parasite, it’s smaller than healthy one, his legs are thin, and claws are undersized. By throwing infestes crab back to waters fishermen perform a kind of reverse natural selection – they withdraw healthy amnimals, leaving sick ones, which continue spreading parasite larvae and infesting healthy crabs. The more intensive commercial fishing is, the more sick crabs appear in the sea. Scientists of Russian Far East have following data on infested crabs: in the Sea of Okhotsk only golden king crabs (Lithodes aeguispina) are infested, red king crabs (Paralithodes camtschatica) showed nearly no signs of infection, and as for snow crab (Chionoecetes opilio) population – it appears to be absolutely healthy. Situation at the Pacific coast of North America is not that optimistic – rhizocephalan parasites are found in golden king crabs, red king crabs, blue king crabs (Paralithodes platypus) and other commercial crab species.

In order to keep crab infestation on the existing level, scientists recommend destroying infested animals, not throwing them back to the sea. Infested crab can be easily told either by externa, or by traces it leaves on shell bottom. Such activities allow eliminating anthropogenic effect on crab population infestation.

Sources:
    Russian Science News
    Wikipedia
     Amazon.com

Kizilova Anna