Copepods: Copepoda

Physical characteristics: Adult females measure 0.27 to 0.49 inches (7 to 12.5 millimeters) in length, while males are 0.17 to 0.26 inches (4.5 to 6.7 millimeters). Their bodies are flat from top to bottom. The antennules of both males and females are short. The clawed antennae and mouthparts are used to grab the skin of their hosts. The mouthparts form a cone-shaped structure. The third pair of thoracic limbs makes an apronlike structure that forms part of a sucker on the fore body. The fourth pair of limbs is uniramous. The abdomen is long and slender.

Geographic range: They are found in the North Atlantic and North Pacific oceans, including Arctic waters.

Habitat: Salmon louse live as external parasites on the bodies of salmon and their relatives living in the sea. They abandon their hosts when they enter freshwaters to reproduce.

Salmon louse live as external parasites on the bodies of salmon and their relatives. (Illustration by John Megahan. Reproduced by permission.)

Diet: Salmon louse eat the tissue and blood of their host.

Behavior and reproduction: They eat by scraping the salmon skin with the needlelike tips of their mandibles. After a while the wound begins to bleed and they feed on the blood.

The life cycle begins with two free-living nauplius larval stages that do not eat. Each nauplius stage lasts about a day. These are followed by a larval stage that resembles the adult. It is this stage that attacks the salmon host. It can survive up to 10 days without a host. However, its chances of survival are much better if it can find a host in the first 48 hours. Six additional larval stages take place on the host before adulthood is reached. The first four larval stages after the nauplius have a special threadlike structure that helps to keep them firmly attached to the host. The last two larval stages are able to crawl around on the host, just like the adults.

Males locate females while they are in their last larval stage and guard them. Mating only takes place after the female molts and reaches adulthood. Males deposit a pair of sperm packets on the female. Females store the sperm in special sacs inside their bodies until the eggs are laid. They produce several pairs of eggs in strings. Each string has as many as 700 disc-shaped eggs all stacked together.

Salmon louse and people: The feeding activities of salmon lice weakens fishes through the loss of blood and by leaving open wounds that become infected. They are a serious pest in salmon farms in northern Europe and North America, causing losses of up to $30 million per year in Europe alone.

Conservation status: Salmon louse are not considered threatened or endangered. ∎