Extremely rare parasitic crustacean discovered in museum shark collection

November 17, 2020

Scientists have discovered an extremely rare species of cymothoid from the mouth of a museum specimen of a deep-sea shark caught from the East China Sea, suggesting its wide distribution around the globe.

Cymothoids are a family of isopods (a type of crustacean) that are ectoparasites of fish. Some species in this family are also known as tongue-biter or tongue-eating louse (e.g., Cymothoa exigua).

Assistant Professor Ryota Kawanishi and Dr. Shinpei Ohashi from Hokkaido University have reported the discovery of an extremely rare species of cymothoid, Elthusa splendida, from the East China Sea. Their paper, published in the journal Species Diversity, expands the range of this species to almost the opposite sides of the Earth.

Cymothoids are a diverse family of more than 300 species of parasites, and parasitize a wide variety of fish, from freshwater to the deep sea. A recent study into the genetics of the family has shown that it is highly likely that they evolved in the deep sea and diversified. A number of deep sea cymothoids, however, are poorly studied, primarily due to the difficulty of deep sea sampling.

Elthusa splendida is the least studied of all deep sea cymothoids. Only five specimens have ever been cataloged and described, in 1981. Those specimens were recovered from a Cuban dogfish, a deep-sea shark, which was captured off southern Brazil in the western South Atlantic. No additional specimens have been reported since then.

In the current study, the scientists discovered the specimen of Elthusa splendida while processing fish specimens at the Fisheries Science Center, Hokkaido University Museum (HUMZ), Hakodate. The specimen was found in a Japanese spurdog, also a deep-sea shark, that had been collected from the East China Sea in June 2003 and preserved in formalin. The scientists confirmed the identification of the specimen by comparing the morphological features of the specimen with the original description of the species. The unique feature that defines Elthusa splendida is the presence of four pits on the first pereonite (first segment behind the head); this feature was examined using a computerized 3D measurement system. DNA sequencing was not used for identification as the sequence of the original specimen is unknown.

This discovery is important as it shows the distribution of Elthusa splendida extends from two locations that are almost antipodal to each other -- almost as far as it is physically possible to be on the planet. The scientists suggest that other species of deep sea cartilaginous fish in the genus Squalus (to which both the Cuban dogfish and the Japanese spurdog belong) can potentially serve as hosts for this parasite. They have also confirmed that Elthusa splendida is rare among other Elthusa species in parasitizing the mouth of the host, rather than the gills; furthermore, even among those cymothoids that parasitize the mouth, Elthusa splendida is one of the rare species that attach to the palate.

This study indicates that there is much that remains to be investigated when it comes to deep sea cymothoids. The scientists also propose using existing specimens of fish in museums across the world to reveal the distribution of cymothoids; from a broader perspective, this work suggests the hidden value of museum natural history collections in studying parasites.
-end-
Ryota Kawanishi is a zoologist and ecologist at the Faculty of Environmental Earth Science, Hokkaido University. He is broadly interested in aquatic ecology, evolution, and conservation, including the evolutionary history and ecology of parasitic isopods.

Hokkaido University

Related Fish Articles from Brightsurf:

Fish banks
Society will require more food in the coming years to feed a growing population, and seafood will likely make up a significant portion of it.

More than 'just a fish' story
For recreational fishing enthusiasts, the thrill of snagging their next catch comes with discovering what's hooked on the end of the line.

Fish evolution in action: Land fish forced to adapt after leap out of water
Many blennies - a remarkable family of fishes - evolved from an aquatic 'jack of all trades' to a 'master of one' upon the invasion of land, a new study led by UNSW scientists has shown.

How fish got onto land, and stayed there
Research on blennies, a family of fish that have repeatedly left the sea for land, suggests that being a 'jack of all trades' allows species to make the dramatic transition onto land but adapting into a 'master of one' allows them to stay there.

Fish feed foresight
As the world increasingly turns to aqua farming to feed its growing population, there's no better time than now to design an aquaculture system that is sustainable and efficient.

Robo-turtles in fish farms reduce fish stress
Robotic turtles used for salmon farm surveillance could help prevent fish escapes.

Heatwaves risky for fish
A world-first study using sophisticated genetic analysis techniques have found that some fish are better than others at coping with heatwaves.

A new use for museum fish specimens
This paper suggests using museum specimens to estimate the length-weight relationships of fish that are hard to find alive in their natural environment.

Reef fish caring for their young are taken advantage of by other fish
Among birds, the practice of laying eggs in other birds' nests is surprisingly common.

Anemones are friends to fish
Any port in a storm, any anemone for a small fish trying to avoid being a predator's dinner.

Read More: Fish News and Fish Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.