Sea lice are small, parasitic marine crustaceans that attach to the skin, fins, and gills of fish. Found naturally in marine environments worldwide, they feed on the mucus, epidermal tissue, and blood of their aquatic hosts. While a natural part of marine ecosystems, their presence can lead to significant issues, especially in concentrated fish populations. This article explores their biology, life cycle, and impact on marine life.
Biology and Characteristics
Sea lice are classified as copepods, a subclass of small crustaceans within the phylum Arthropoda. Specifically, they belong to the family Caligidae, which includes hundreds of species across numerous genera. The most well-known species, such as Lepeophtheirus salmonis and various Caligus species, commonly parasitize marine fish, especially salmonids. These parasites are ectoparasites, meaning they live on the exterior of their host.
Adult sea lice typically measure a few millimeters, with females being larger than males. Their flattened, oval-shaped bodies often blend with the host’s skin color, ranging from translucent to brown or gray. They possess specialized appendages, like hooked antennae and frontal filaments, for firm attachment. Once attached, they use mouthparts to feed on mucus, skin, and blood, sometimes secreting digestive enzymes.
Habitat and Life Cycle
Sea lice inhabit marine environments globally, predominantly found on the external surfaces of fish hosts. While common on many fish species, some, like L. salmonis, are highly host-specific to salmon. Their complex life cycle involves both free-swimming and parasitic stages. Development rate depends on water temperature, with warmer temperatures accelerating the process.
The life cycle begins when eggs hatch into two free-swimming naupliar stages (nauplius I and II), which do not feed. These planktonic larvae disperse with ocean currents. The nauplius II stage then molts into the infectious copepodid stage, which actively seeks and attaches to a fish host. Once attached, it develops through four chalimus stages, remaining fixed by a frontal filament and feeding on host tissues. These are followed by two pre-adult stages and finally the adult stage, both more mobile on the host’s surface.
Impact on Marine Life
Sea lice infestations can have considerable effects on fish populations, particularly within aquaculture settings. In fish farms, the high density of fish provides an ideal environment for sea lice to reproduce rapidly and spread efficiently among hosts. Heavy infestations cause physical damage to fish, including skin lesions, ulcers, and open wounds, which can lead to secondary bacterial or fungal infections. This constant irritation and tissue damage induce stress in fish, compromising their immune systems and making them more susceptible to other diseases.
Infested fish often experience reduced appetite, slower growth, and poorer food conversion, impacting their health and market value. Severe parasite loads can lead to significant mortality, especially in vulnerable juvenile fish. Beyond direct harm to farmed fish, sea lice from aquaculture can spread to wild populations, threatening their health and survival. The economic burden on the aquaculture industry is substantial, with costs from lost biomass, reduced growth, and treatment efforts.
Sea Lice and Humans
Despite their name, the sea lice that parasitize fish do not pose a direct threat to humans. The term “sea lice” often refers to human skin irritation after swimming, a misnomer. This condition, formally known as seabather’s eruption, is caused by the larvae of jellyfish or sea anemones, not the copepod parasites affecting fish.
These tiny, transparent larvae can become trapped under swimwear, releasing stinging cells that cause an itchy, red rash when pressure or friction is applied. The rash usually appears in areas covered by a bathing suit and can include small, itchy bumps or hives. Symptoms may begin shortly after leaving the water or several hours later. While uncomfortable, these reactions are mild and temporary, resolving within a few days to two weeks. In rare instances, more severe symptoms like nausea, fever, or headaches can occur, particularly in children. This condition differs from “swimmer’s itch,” caused by freshwater parasites.