Sea louse
Sea lice are copepods of the family Caligidae within the order Siphonostomatoida. They are marine ectoparasites that feed on the mucus, epidermal tissue, and blood of host fish. The roughly 559 species in 37 genera include around 162 Lepeophtheirus and 268 Caligus species.
The genera Lepeophtheirus and Caligus parasitize marine fish. Lepeophtheirus salmonis and various Caligus species are adapted to salt water and are major ectoparasites of farmed and wild Atlantic salmon. Several antiparasitic drugs have been developed for control purposes. L. salmonis is the best understood in the areas of its biology and interactions with its salmon host.
Caligus rogercresseyi has become a major parasite of concern on salmon farms in countries including Chile and Scotland. Studies are under way to gain a better understanding of the parasite and the host-parasite interactions. Recent evidence is also emerging that L. salmonis in the Atlantic has sufficient genetic differences from L. salmonis from the Pacific to suggest that Atlantic and Pacific L. salmonis may have independently co-evolved with Atlantic and Pacific salmonids respectively.
Diversity
The family Caligidae is estimated to contain around 559 species in 37 genera. The largest of these are Caligus, with around 268 species, and Lepeophtheirus with around 162 species.Wild fish
Most understanding of the biology of sea lice, other than the early morphological studies, is based on laboratory studies designed to understand issues associated with sea lice infecting fish on salmon farms. Information on sea lice biology and interactions with wild fish is sparse in most areas with a long-term history of open net-cage development, since understanding background levels of sea lice and transfer mechanisms has rarely been a condition of tenure license for farm operators.Many sea louse species are specific with regard to host genera, for example L. salmonis, which has high specificity for anadromous fish including sticklebacks and salmonids including the widely farmed Atlantic salmon. Lepeophtheirus salmonis can parasitize other salmonids to varying degrees, including brown trout, Arctic char, and all species of Pacific salmon. In the case of Pacific salmon, coho, chum, and pink salmon mount strong tissue responses to attaching L. salmonis, which lead to rejection within the first week of infection. Pacific L. salmonis can also develop, but not complete, its full lifecycle on the three-spined stickleback. This has not been observed with Atlantic L. salmonis.
How planktonic stages of sea lice disperse and find new hosts is still not completely known. Temperature, light, and currents are major factors and survival depends on salinity above 25 ‰. L. salmonis copepodids migrating upwards towards light and salmon smolt moving downwards at daybreak have been hypothesized to facilitate finding a host. Several field and modeling studies on L. salmonis have examined copepodid populations and have shown that planktonic stages can be transported tens of kilometres from their source, including how their behaviour results in their being moved towards the coastline and mouth of estuaries.
The source of L. salmonis infections when salmon return from fresh water has always been a mystery. Sea lice die and fall off anadromous fish such as salmonids when they return to fresh water. Atlantic salmon return and travel upstream in the fall to reproduce, while the smolts do not return to salt water until the next spring. Pacific salmon return to the marine nearshore starting in June, and finish as late as December, dependent upon species and run timing, whereas the smolts typically outmigrate starting in April, and ending in late August, dependent upon species and run timing.
Sea lice possibly survive on fish that remain in the estuaries or they transfer to an as yet unknown alternate host to spend the winter. Smolt get infected with sea lice larvae, or even possibly adults, when they enter the estuaries in the spring. How sea lice distribute between fish in the wild also is not known. Adult stages of Lepeophtheirus spp. can transfer under laboratory conditions, but the frequency is low. Caligus spp. transfer quite readily and between different species of fish, and are regularly found in the plankton.
Morphology
L. salmonis tends to be about twice the size of most Caligus spp.. The body consists of four regions: cephalothorax, fourth segment, genital complex, and abdomen. The cephalothorax forms a broad shield that includes all of the body segments up to the third leg-bearing segment. It acts like a suction cup in holding the louse on the fish. All species have mouth parts shaped as a siphon or oral cone. The second antennae and oral appendages are modified to assist in holding the parasite on the fish. The second pair of antennae is also used by males to grasp the female during copulation. The adult females are always significantly larger than males and develop a very large genital complex, which in many species makes up the majority of the body mass. Two egg strings of 500 to 1000 eggs, which darken with maturation, are roughly the same length as the female's body. One female can produce 6–11 pairs of egg strings in a lifetime around seven months.Development
Sea lice have both free-swimming and parasitic life stages, all separated by moults. The development rate for L. salmonis from egg to adult varies from 17 to 72 days depending on temperature. The lifecycle of L. salmonis is shown in the figure; the sketches of the stages are from Schram.Eggs hatch into nauplii I, which moult to a second naupliar stage; neither naupliar stage feeds, depending on yolk reserves for energy, and both are adapted for swimming. The copepodid stage is the infectious stage and it searches for an appropriate host, likely by chemo- and mechanosensory clues. Currents, salinity, light, and other factors also assist copepodids in finding a host. Preferred settlement on the fish occurs in areas with the least hydrodynamic disturbance, particularly the fins and other protected areas. Once attached to a suitable host, copepods feed for a period of time prior to moulting to the chalimus I stage. Sea lice continue their development through three additional chalimus stages each separated by a moult. A characteristic feature of all four chalimus stages is that they are physically attached to the host by a structure referred to as the frontal filament. Differences in the timing, method of production, and the physical structure of the frontal filament are seen between different species of sea lice. With exception of a short period during the moult, the preadult and adult stages are mobile on the fish, and in some cases, can move between host fish. Adult females, being larger, occupy relatively flat body surfaces on the posterior ventral and dorsal midlines and may actually outcompete preadults and males at these sites.
Feeding habits
Until they locate a host, the naupliar and copepodid stages are non-feeding and live on endogenous food stores. Once attached to the host, the copepodid stage begins feeding and begins to develop into the first chalimus stage. Copepods and chalimus stages have a developed gastrointestinal tract and feed on host mucus and tissues within range of their attachment. Pre-adult and adult sea lice, especially pregnant females, are aggressive feeders, in some cases feeding on blood in addition to tissue and mucus. Blood is often seen in the digestive tract, especially of adult females. L. salmonis is known to secrete large amounts of trypsin into its host's mucus, which may assist in feeding and digestion. Other compounds such as, prostaglandin E2, have also been identified in L. salmonis secretions and may assist in feeding and/or serve the parasite in avoiding the immune response of the host by regulating it at the feeding site. Whether sea lice are vectors of disease is unknown, but they can be carriers of bacteria and viruses likely obtained from their attachment to and feeding on tissues of contaminated fish.Disease
Pathology
Sea lice cause physical and enzymatic damage at their sites of attachment and feeding, which results in abrasion-like lesions that vary in their nature and severity depending upon a number of factors, including host species, age, and general health of the fish. Whether stressed fish are particularly prone to infestation is unclear. Sea-lice infection causes a generalized chronic stress response in fish since feeding and attachment cause changes in the mucus consistency and damage the epithelium resulting in loss of blood and fluids, electrolyte changes, and cortisol release. This can decrease salmon immune responses and make them susceptible to other diseases and reduce growth and performance.The degree of damage is also dependent on the species of sea lice, the developmental stages that are present, and the number of sea lice on a fish. Little evidence exists of host tissue responses in Atlantic salmon at the sites of feeding and attachment, regardless of the development stage. In contrast, coho and pink salmon show strong tissue responses to L. salmonis characterized by epithelial hyperplasia and inflammation. This results in rejection of the parasite within the first week of infection in these species of salmonids. Heavy infections of farmed Atlantic salmon and wild sockeye salmon by L. salmonis can lead to deep lesions, particularly on the head region, even exposing the skull.
Interactions between wild and farmed fish
Some evidence indicates that sea lice flourishing on salmon farms can spread to nearby wild juvenile salmon and devastate these populations. Sea lice, particularly L. salmonis and various Caligus species, including C. clemensi and C. rogercresseyi, can cause deadly infestations of both farm-grown and wild salmon. Sea lice migrate and latch onto the skin of wild salmon during free-swimming, planktonic nauplii and copepodid larval stages, which can persist for several days. Large numbers of highly populated, open-net salmon farms can create exceptionally large concentrations of sea lice. When exposed in river estuaries containing large numbers of open-net farms, mathematical models have suggested that many young wild salmon may be infected Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned juvenile salmon migrating to sea are highly vulnerable. Sea trout populations in recent years may have seriously declined due to infestation by sea lice, and Krkosek et al. have claimed that on the Pacific coast of Canada the louse-induced mortality of pink salmon in some regions is over 80%. A few studies indicated no long-term damage to fish stocks in some locations, and a population decline in wild salmon that occurred in 2002 was caused by "something other than sea lice". However, the repeated epizootics of lice on wild fish have only occurred in areas with salmon farms in Ireland, Britain, Norway, Canada, and Chile. Field sampling of copepodids, and hydrographic and population models, show how L. salmonis from farms can cause mass infestations of seaward-migrating salmonids, and this effect can occur up to from the farms.Several scientific studies have suggested that caged, farmed salmon harbour lice to a degree that can destroy surrounding wild salmon populations. Other studies have shown that lice from farmed fish have relatively no effect on wild fish if good husbandry and adequate control measures are carried out. Further studies to establish wild-farmed fish interactions are ongoing, particularly in Canada, Britain, Ireland, and Norway. A reference manual with protocol and guidelines for studying wild/cultured fish interactions with sea lice has been published.