The carp louse ''Argulus coregoni'' is an obligate ectoparasitic branciuran species on fish. It occurs in China and Japan, and Scandinavia. ''Argulus coregoni'' can pose a threat to fish kept in fish farms.

Life cycle

''Argulus coregoni'' has a direct life cycle, meaning that the parasite is transmitted between individuals of the same host species without an

intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...

. At Northern latitudes, adults do not survive winter, but only the eggs attached to rocks survive throughout the winter. Egg hatching occurs in May once water temperatures exceed 10 °C. From the eggs,

metanauplii Metanauplius is an early larval stage of some crustaceans such as krill. It follows the nauplius stage. In sac-spawning krill, there is an intermediary phase called pseudometanauplius, a newly hatched form distinguished from older metanauplii by ...

emerge which actively search for a fish host to feed on. These can survive up to two days without finding a host. One or two generations can be completed each year, depending on environmental conditions. This means that infection rates of host typically reach their peak during the summer months. Females start laying eggs from July until the end of summer. Although some differences are seen in the development between males and females, the adults generally have the same size . This is in contrast with other

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

crustacean species The

sex ratio The sex ratio (or gender ratio) is usually defined as the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species devia ...

— the ratio between males and females in the population — is generally close to 1:1, as expected for sexually reproducing species under

Fisher's principle Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females. A. W. F. Edwards has remarked that it is "probably the mo ...

. Both sexes can detach from the host and find alternative hosts. However, this behaviour is more common in males as they can also switch between hosts to search for mates.

Host species

''Argulus coregoni'' mainly infect salmonids, such as

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 coasta ...

and

brook trout The brook trout (''Salvelinus fontinalis'') is a species of freshwater fish in the char genus ''Salvelinus'' of the salmon family Salmonidae. It is native to Eastern North America in the United States and Canada, but has been introduced elsewhere ...

. It is, however, also able to infect other species of fish, such sweet fish. This is supported by evidence that larvae can successfully develop on both salmonids and cyprinids, but developmental rate and reproductive output on the latter host group is reduced. In choice experiments it shows increased host preference with maturation, this likely reflects the better development on its normal salmonid hosts.

Distribution

In Japan ''A. coregoni'' has shown a geographical expansion over recent years.

On the host

''Argulus coregoni'' aggregate on specific parts of their hosts. The prefer attaching near the fins, in particular the pectoral and

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

s. Larger individuals are found towards the head of the host, but not on the head. To move from one host to another it can swim freely in water column. Adults can survive up to two weeks without feeding on a fish host. Argulus coregoni on Oncorhynchus

''Argulus coregoni'' attaches itself to the host with suckers (maxillae) and hooks. The latter are abundant on the underside of the body. The suckers are used to tear the host skin. ''A. coregoni'' feed on the blood and mucus from the resulting wound. They bury their bodies into the scale pockets and their flattened body shape allows them to sit closely against the body of the host, minimising drag and the risk of detachment. The eggs of gravid females protrude from the fish scales.

In the host population

Like many ectoparasites, ''A. coregoni'' shows aggregation within the host population. This means that some host individuals carry many parasites, but that most only carry a few. This appears to be due to differences in exposure rather than in susceptibility. Indeed, it appears that host behaviour plays an important role in exposure and that certain hos behaviours make them more susceptible to infections.

Symptoms

Infected fish can show a range of symptoms, especially at heavier infections. The symptoms include scales peeling off, red spots at the site of infection, and reduced growth rates. Fish can become weak, but ''Argulus coregoni'' does normally not cause host mortality. In

coinfection Coinfection is the simultaneous infection of a host by multiple pathogen species. In virology, coinfection includes simultaneous infection of a single cell by two or more virus particles. An example is the coinfection of liver cells with hepatiti ...

s it can, however, increase host mortality in some host species. ''Argulus coregoni'' infections do not lead to threats to natural fish populations. In fish farms the infections may case problems due to the lack of natural predators of the parasites, and the high density of fish.

References

{{Authority control Crustaceans Crustaceans described in 1865 Freshwater crustaceans of North America Parasitic crustaceans Animal parasites of fish Parasites of fish