Sometimes a new scientific discovery comes about while one is doing the most mundane things, and it might not even be a scientist who happens to be doing it. Last year, an unusually high number of tanner crabs started showing up in the waters off the southern coast of Hokkaido. While these crabs are a bane for flounder fishermen as they have a habit of tearing up their nets, tanner crabs are easy to catch and they taste good, so numerous crabs have ended up in markets all over Japan, being sold for a relatively low price.
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| Top right: a cooked tanner crab with a coiled-up Nectonema worm inside of it. Top left: another cooked crab with a smaller Nectonema worm inside of it. Bottom: A Nectonema worm extracted and unraveled from the first tanner crab (scale bar = 2 cm). Photos from Figure 1 of the paper, taken by Rieko Yamamoto |
So what does this have to do with parasitology? Well, earlier this year a woman named Yuta Yamamoto had bought and boiled up some tanner crabs for a meal, but upon opening her would-be crab dinner, she discovered that one of the crabs came with an extra helping of worm, all coiled up like a bundle of cables. This worm was about 82 centimetres long took up a lot of space in the crab's body. But instead of doing what some people might do, which is to toss the crab out the window in disgust, she calmly placed the parasitised crab in the freezer and contacted Dr. Keiichi Kakui, an invertebrate zoologist at Hokkaido university, who was able to identify the worm as Nectonema.
Nectonema is a genus of horsehair worm, and while horsehair worms are more commonly known from land-dwelling arthropods such as crickets and praying mantis, there is one offshoot lineage of horsehair worms that have taken up life within the denizens of the seas. Nectonema has previously been reported in many types of crustaceans, including rock crabs, shrimps, lobsters, squat lobsters, and even marine isopods, but this is the first time that it has been found in a tanner crab.
A week after this discovery, Ms Yamamoto bought another eight crabs and found one of those crabs also came with a worm, which means this parasite might not be all that uncommon among tanner crabs. Fortunately, Nectonema doesn't cause any harm to humans, so there are no public health issues here, though it might be an alarming sight to those who are unfamiliar with these worms.
The life cycle of these marine horsehair worms is a mystery, though if their more well-studied relatives is anything to go by, it might involve the larva infecting a smaller invertebrate first, before being eaten by the final host where it can grow to its full adult size. While horsehair worms in land-dwelling hosts are known for altering the behaviour of their hosts, such behavioural manipulation is due to the worm needing to move its terrestrial host into a water body to complete its life cycle. This is not necessary for Nectonema since it is already surrounded by water in the sea.
Nature is full of surprises, but if you are prepared and observant, you might come across a scientific discovery while having your next meal. So if you ever find a worm in your dinner - don't panic! It might turn out to be an important scientific discovery.
Reference:
Kakui, K. (2024). Nectonema horsehair worms (Nematomorpha) parasitic in the Tanner crab Chionoecetes bairdi, with a note on the relationship between host and parasite phylogeny. Diseases of Aquatic Organisms 159: 153-157.
