Argulus foliaceus: a well-adapted, destructive freshwater fish louse

by Brad Gorzek
BIOL 490, Fall 2014

Key taxa: Arthropoda, Maxillopoda, Branchiura, Argulus

There are many destructive forces in this world: erosion, hurricanes, earthquakes, the development of mankind, and I could go on with this list for a page. There is one such destructive force in freshwater habitats that is much smaller than a hurricane, or even a man, but proves to be a devastating pest nonetheless. Argulus foliaceus is a species of fish lice that has been shown to be a well-adapted parasite, exhibiting unique hunting and breeding strategies that enable it to live in the harsh and variable climates of Europe, East Asia and Siberia, wreaking havoc on the profitability of any freshwater fishery it inhabits and infests. To understand how this louse has become such an effective predator, we must understand what tools it utilizes to prey upon its host.

Fish lice are crustaceans of the subclass Branchiura. They are found on every continent in freshwater lakes and streams and are known to live in some estuaries, but none have been discovered in the open sea. They are ectoparasites that feed on fish and occasionally amphibian hosts, consuming flesh and blood as a food source. They are flat, ovular crustaceans segmented thrice with a head, thorax, and abdomen. Fish lice have very prominent compound eyes on the head containing several lenses. They have antennae with claws to grab onto fish and use mandibles to tear into the flesh of their host so they can bring their food into their mouths. They generally latch onto fish in the region of the operculum where they are protected by the gill cover from being scraped off of the fish. All four of their branched limbs on the thorax are used for swimming, and males have an extra third and fourth pair of limbs to transfer sperm to a female. The abdomen has no appendages and ends in a doubly-lobed tail. The anatomy varies slightly between populations and species of branchiurans, but these general characteristics enable the fish lice to effectively parasitize a wide variety of hosts and potentially reduce fish populations under extreme conditions.

Each fish louse exhibits its own life history traits, and one of those facets is how the organism approaches breeding. According to studies done by Pasternak et al. (2000), A. foliaceus has a predictable breeding schedule that coincides with seasonal changes in temperature. It is thought that the fish lice have a hard time surviving the winter cold, since less of them are found on fish hosts during the winter months, so it is imperative that A. foliaceus is successful in reproducing before this time. It lays its eggs in the autumn which rest during the winter months (5-7 months) before hatching in the spring over an extended period of time when the water temperature ranges from 8-20° C (25 to 240 days). This extended period of hatching is thought to be adaptive when fish populations are lower in early spring and there is a scarcity in hosts. This lengthy period of hatching differs from other species of Argulus with shorter periods of hatching, and is thought to have evolved because of the difference in their habitats and climates. A. foliaceus also differs in its egg laying strategy. Many of the females do not deposit all of their eggs at once, like most other branchiurans do. Instead, they lay half of their eggs or less and then attach to a host for a period of time (sometimes lasting 3 weeks) before detaching again and depositing the rest. This strategy may increase the chances of the future hatchlings to find a host when host populations are sparse and scattered since the eggs will mirror that distribution. The aforementioned breeding facets make A. foliaceus a well-adapted breeder under variable environmental and food conditions.

There is also evidence to suggest that A. foliaceus targets a particular host species to further ensure reproductive success. Though A. foliaceus feeds on a wide range of hosts, as I will elaborate on in the next section, Pasternak et al. (2000) found that their Finnish populations exhibited a host preference of perch (Perca fluviatilis) over roach (Rutilis rutilis), especially in the breeding months. This host preference would make it easier for this louse to find mates and, therefore, create a better scenario for effective reproduction. The potential downside to this behavior is that a decline in the population of perch would lead directly to a decline in natality of A. foliaceus and would be counterproductive. That being said, this kind of a selective population decline in perch is unlikely to occur under natural circumstances.

The breeding season is not the only thing, however, that influences the prevalence of the host that A. foliaceus will choose. There are several feeding strategies that this fish louse exhibits depending on time of day, point in life cycle, and time of day. Many aquatic invertebrates participate in either active or passive hunting. In nematodes they are known as cruising species (those who are very mobile and search for their prey) and ambush species (those who don’t move much and feed on passersby) (Campbell & Gaugler 1993). The unique thing about A. foliaceus is that they change their hunting strategies between active and passive techniques depending on the time of day. Mikheev et al. (2000) uncovered this strategy and tried to explain the phenomenon using a series of life history traits and their newly-collected data. They found that when the night grows dark, A. foliaceus begins to move 3 to 4 time faster and seeks out prey producing higher infection rates than in the daytime. Their hosts cannot see them and, thus, cannot avoid them. The lice can smell the fish and feel the fish’s movements in the water. This gives the lice an advantage and provides them with a window of time to be more effective hunters. They are especially effective in infecting perch (Perca fluviatilis) during this time, since perch slow their swimming speed in the evening hours and make themselves easier targets. In the daytime, A. foliaceus adopts a new technique. Their hosts can see them, so they choose instead to float motionless and ambush their hosts rather than attempt futile active hunting techniques. They use their vision and total stillness to be more effective hunters under light conditions, as well as reduce their energy expenditure. Knowing when their two hunting strategies are most effective is a very efficient way to use energy and is extremely adaptive. Once they have latched onto a host, they will feed on that fish until it is time to breed or die, and often times the effects are evident in the wounds that remain on the fish.

A study done by Walker et al. (2008a) showed that these fish lice also show different preferences in host size over time. In their larval stage, A. foliaceus seem to prefer smaller fish like the three-spined stickleback, Gasterosteus aculeatus, to infect. This is most likely because they are slower swimmers and don’t produce a forceful enough water current to pull the lice off as they swim. The adult lice, however, infected the largest fish (Scardinius erythrophthalmus, Abramis brama and Cyprinus carpio) in their environment most commonly. In another one of Walker et al.'s studies (2008b) they found that even within these larger fish species, individuals had increasing infestation magnitudes of adult lice with increasing body weights. This may be because these large fish are so much easier to see and sense in the water that they are easier for A. foliaceus to locate. Another possibility is that their operculum is much bigger and provides better shelter for the lice, and their large bodies provide more nutrition for a longer period of time which leads to the accumulation of more lice over time. There is more research to be done to prove why A. foliaceus chooses the hosts it does. No matter the reason, it is clear that these lice are effective hunters throughout their lifecycles and do well to adapt to a variety of fish hosts.

One may think that it is not so important to document the infection of species like carp and three-spined sticklebacks since we do not often use those fish species for food resources or anything of great value, but unfortunately these are not the only fish species preyed upon by A. foliaceus. Northcott et al. (1997) did eradication research in the Scottish Stillwater Fishery, where outbreaks of A. foliaceus led to a huge loss in revenue for the angling hotspot. A year or two after spotting the first fish lice in the loch, or lake, the couple pests turned into an extreme infestation. With plenty of fish hosts abound, due to stocking of trout by the fishery, the A. foliaceus flourished and reproduced in monumental numbers. The fish lice did so much damage that they wounded the fish to the extent that anglers didn’t want them and even killed many of the trout and native fishes, found with large infected wounds near their gills. The angler’s paradise turned into a nightmare and the business had to shut down for subsequent years to try and rid the waters of the parasites. The fishery found it impossible to remove all of the lice from the loch. They were able to decrease the populations by netting out the deposited eggs each winter before the loch began to ice over, but total eradication was out of the question. This is just another example of how adaptive and effective A. foliaceus is as a fish parasite. Not even humans can stop A. foliaceus once it has found a place to breed, feed, and flourish.

There are many reasons why A. foliaceus is a well-adapted, and destructive parasite, and their life history traits of feeding and breeding tell the whole tale. By infecting specific hosts during the breeding season, they are more likely to find another louse of the opposite sex to produce offspring with. Once females are inseminated, they make sure to spread their eggs around the waters by using their fish hosts as a mode of transportation. These eggs effectively overwinter at the bottom of the lake or stream and hatch when spring comes and brings warmth and food. Larval lice feed on smaller fish hosts, because they contain proper nutrients and are easier to latch on to. As the lice grow, they choose larger hosts so they can sustain themselves with a larger supply of food and, since they too are growing, remain protected by a larger gill cover. This enables them to accumulate on their hosts and severely damage the fish. A. foliaceus is such an effective parasite that they are nearly impossible to eliminate once they have infested a water body. In commercial fisheries this becomes a serious problem that results in loss of revenue due to declines in fish populations and fish quality, since adult lice can severely wound their hosts and have been known to kill them. These qualities make A. foliaceus a well-adapted, destructive menace. It has the ability to feed on many hosts, proliferate quickly, infest temperature variant freshwater environments, and devastate fishing economies. The only thing fishery managers can hope to do is keep Argulus foliaceus out of their waters through boat washings and continuous monitoring for the presence of this fish louse on the fish they retrieve.

References Cited

• Campbell, J.F. and Gaugler, R. 1993. Nictation behaviour and its ecological implications in the host search strategies of entomopathogenic nematodes (Heterorhabditidae and Steinernematidae). Behaviour 126: 155-169.
• Mikheev V.N., Mikheev A.V., Pasternak A.F. and Valtonen E.T. 2000. Light-mediated host searching strategies in a fish ectoparasite, Argulus foliaceus L. (Crustacea: Branchiura). Parasitology 120: 409-416.
• Northcott S.J., Lyndon A.R., and Campbell A.D. 1997. An outbreak of freshwater fish lice, Argulus foliaceus L., seriously affecting a Scottish Stillwater fishery. Fisheries Management and Ecology 4: 73-75
• Pasternak A.F., Mikheev V.N., and Tellervo Valtonen E. 2000. Life history characteristics of Argulus foliaceus L. (Crustacea: Branchiura) populations in Central Finland. Ann. Zoo. Fennici 37: 35-35.
• Walker P.D., Harris J.E., van der Velde G., and Wendelaar Bonga S.E. 2008a. Differential host utilisation by different life history stages of the fish ectoparasite Argulus foliaceus (Crustacea: Branchiura) Folia Parasitologica 55: 141-149.
• Walker P.D., Harris J.E., van der Velde G., and Wendelaar Bonga S.E. 2008b. Effect of host weight on the distribution of Argulus foliaceus L. (Crustacea, Branchiura) within a fish community. Acta Parasitologica 53: 165-172.

Also, Fish Lice: Branchiura - Physical Characteristics, Behavior And Reproduction, Fish Louse (Argulus foliaceus): Species Account - GEOGRAPHIC RANGE, HABITAT, DIET, FISH LICE AND PEOPLE, CONSERVATION STATUS