Greenland sharks roam the waters of the Arctic and North Atlantic for centuries. In the process, they also encounter some unwelcome contemporaries, such as parasitic barnacles, that attach themselves to their posteriors
Greenland sharks are certainly among the most extraordinary vertebrates. They are especially known for the fact that they can live for 500 years or more, according to experts That’s a very long time, but it may not be at all pleasant for the large cartilaginous fish. This is because they, like most other fish, are plagued by numerous parasites, both external and internal.
The copepod Ommatokoita elongata, which attaches to the cornea of the eyes and likely leads to partial vision loss, is one of the well-documented parasites that plague Greenland sharks. But it gets even more unpleasant: a Canadian-Norwegian research team has identified it suffereing from a parasite that has never been observed in cold-water sharks before.
While the team was tagging Greenland sharks in the Canadian Arctic in 2018 and studying their metabolic rate, they noticed the barnacle Anelasma squalicola, which was previously known mostly from lantern sharks, in a female Greenland shark.
“On the 14th of August 2018 […], we observed the burgundy-coloured mantle of an Anelasma squalicola specimen protruding from the cloaca of a 275-cm-long female Greenland shark,” the scientists write in their paper, which appeared in Fish Biology.
Externally, the barnacle resembles a goose barnacle, due to its muscular stalk. However, there is an important difference, Dr Robert van Syoc, a barnacle expert at the California Academy of Sciences, said: “The stalk has evolved various modifications that serve to function as a feeding appendage rather than simply a site of attachment. The stalk of Anelasma is embedded in the host shark and absorbs nutrients from the host tissues.[er verankert sich] The thoracic limbs, cirri, have atrophied to the point where they can no longer function in filter-feeding, as they do in other stalked barnacles. In addition, Anelasma squalicola is found only as a parasite of relatively rare deep-water sharks. This indicates that Anelasma squalicola is a relict species, perhaps a sort of living fossil if you will.”
Eric Ste-Marie, a shark biologist at the University of Windsor, Canada and the lead author of the study, and his team had freed the female Greenland shark from the barnacle and preserved it for later identification and molecular analysis. Because this is the first observation of A. squalicola in Greenland sharks, the scientists are not sure if the barnacle regularly selects Greenland sharks as hosts.
The scientists involved in the study can only speculate about the effect the barnacle has on Greenland sharks. However, it is known from lantern sharks that infested animals have smaller testes, claspers and ova, suggesting that A. squalicola impedes the development of reproductive organs in some of its hosts.
According to the study, transmission of the barnacle to Greenland sharks may be facilitated by the overlap in the geographic ranges of Greenland sharks and other sharks it affects. The authors hope that other scientists and fisheries observers interacting with Greenland sharks monitor them for parasitisation by this barnacle to determine how frequently this happens and if it affects the species.
Julia Hager, PolarJournal
Source Ste-Marie, E., Glenner, H., Rees, D.J. and Hussey, N.E. (2023), First Recorded Occurrence of the Parasitic Barnacle (Anelasma squalicola) on a Greenland Shark (Somniosus microcephalus) in the Canadian Arctic. J Fish Biol. Accepted Author Manuscript. https://doi.org/10.1111/jfb.15421
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