A rhizocephalan parasite is very hard to notice and even harder to recognize as a crustacean with a non-professional eye. Part of a parasite’s body, which is hidden inside a host crab, consists of numerous dendritic shoots, known as “roots."
Roots grow between a crab’s muscle fibers, causing their atrophy, and go further into the crab’s internal organs, including sexual glands – rendering the infested crab unable to propagate.
One parasite produces about 300-400 thousand larvae – naupliuses. When the time to propagate comes, a parasite grows so-called “externa” on the host’s abdominal side – in its' node, connected to the parasite’s inner part with a thin stem. Externa stores female reproductive products and is located where healthy crabs usually carry their own spawn.
When parasite’s naupliuses mature, they leave the 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 crabs remain alive for several years, remaining a host to the original parasite. The crab is ultimately weakened and unable to propagate.
When fishermen perform commercial fishing of crabs, they pick up only the best invertebrates, throwing sick ones back to the sea.
By throwing infested crab back to waters, fishermen perform a kind of reverse natural selection – they withdraw healthy animals, 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 been following data on infested crabs: in the
The situation on the Pacific coast of
In order to keep crab infestation on the existing level, scientists recommend destroying infested animals, not throwing them back to the sea.
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2 comments:
Not only should they be worried, they better start taking action against it by destroying the infected crab. There should be some type of safeguard in place that would allow them to do so without penalty. Instead, it sounds like the only thing they can do is help to proliferate it. Our newest threat (invasive specie) here in the Great Lakes is Viral Hemorrhagic Septicemia (VHS) it is the Ebola of the fish world, a true killer. While it poses no risk to humans, it causes internal and external hemorrhaging in fish. Attack organs fail, especially the liver and causes unsightly bleeding just under the skin. It kills fish relatively quick and has never been found in fresh water to be causing moralities like we are seeing here. This virus causes massive fish die off and is a major threat to the sport industry that is worth over 1 billion dollars a year to our state alone. This was once thought to be only a concern to commercial aquaculture in Europe but has now been found in Lake Ontario, Erie, Huron, Michigan and Lake St. Claire and was just recently found in one of the inland lakes in northern Michigan. Better hope this one never makes it to the Berring Sea. And while steps are being taken to prevent the spread I am afraid the Ginnie is out of the bottle. One footnote one the Russian King Crab, in the sixties when the Russians planted king crab it was called the creation of a valuable resource, today it would be called the release of an invasive specie.
Thanks Wishn, I like it when I learn from my readers. I had no idea about the disease in the Lakes. You are right about it being an invasive species, and perhaps this parasite is the reason that the king crab were not successful there before?
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