''Gyrodactylus salaris'', commonly known as salmon fluke, is a tiny

monogenea Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female repr ...

ectoparasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

which lives on the body surface of freshwater fish. This leech-like parasite has been implicated in the reduction of

Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlantic salmon are ...

populations in the

Norwegian fjords This list of Norwegian fjords shows many of the fjords in Norway. In total, there are about 1,190 fjords in Norway and the Svalbard islands. The sortable list includes the lengths and locations of those fjords. Fjords See also * List of gla ...

. It also parasitises other species, including

rainbow trout The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coast ...

. ''G. salaris'' requires

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...

, but can survive in

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

for up to 18 hours. The parasite is long, and cannot be seen with the naked eye, but it can be seen with a

magnifying glass A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to c ...

. On its posterior end is a haptor, a specialized organ for attaching to the host fish, which has sixteen hooks around its edge. The parasite is

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

, that is, it produces live offspring. The parasites give birth to live young nearly as big as themselves and at this time, a further generation is already growing inside the neonates.

Interactions with host fish

When feeding, the parasite attaches its anterior end to the fish with cephalic glands. It everts its

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

through the mouth and releases a digestive solution with proteolytic enzymes which dissolves the salmon skin.

Mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...

and dissolved skin are then sucked into the gut. Attachment of many parasites can cause large wounds, damaging the epidermis of the host fish, which allows secondary infections.

History

''G. salaris'' was first described in 1952, after being removed from a Baltic strain of Atlantic salmon kept at the Hölle Laboratory in Sweden, near to the river Indalsälv. At the time, it was not thought to cause disease in the host fish. The presence of ''G. salaris'' on fish became a World Organisation for Animal Health notifiable disease in 1983. Catastrophic losses of Atlantic salmon occurred in Norway in the 1970s following the introduction of ''G. salaris''. By 2001, the salmon populations of 41 Norwegian rivers had been virtually wiped out in this way. Historically, ''Gyrodactylus''-infected rivers have been treated with the indiscriminate pesticide/piscicide rotenone. A newer method of treatment employs dosing small volumes of aqueous

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

into the river. A huge advantage of this method is its ability to kill the parasites without harming the hosts. This new method has shown promising results in

Batnfjordelva Batnfjordelva is a river in Gjemnes Municipality in Møre og Romsdal county, Norway. It originates from the lake Botnvatnet and it flows to the east and later northeast until it empties into the Batnfjorden at the village of Batnfjordsøra. T ...

and Lærdalselva, two rivers in

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of ...

References

{{Taxonbar, from=Q292947 salaris Ectoparasites Animals described in 1957