Sea lice are a type of marine crustacean that live as external parasites on fish. They belong to the family Caligidae and are common in both wild and farmed fish populations worldwide. These tiny organisms feed on the mucus, skin, and blood of their host fish. Understanding their appearance helps in identifying these parasites in aquatic environments.
Key Physical Characteristics
Adult sea lice measure a few millimeters in length, ranging from 2 to 10 millimeters. Females of species like Lepeophtheirus salmonis can reach up to 12 millimeters, or even 29 millimeters when including their egg strings, while males are smaller, around 6 millimeters. Their bodies are flat, oval, or teardrop-shaped, and can appear translucent or semi-transparent. Sea lice bodies display segmentation.
The color of sea lice can vary, appearing whitish, grayish, brown, or greenish, which helps them blend into their host or surroundings. Some individuals may take on a reddish hue, particularly if they have recently fed and are engorged with blood. Sea lice possess appendages for their parasitic lifestyle. These include mouthparts shaped like a siphon or oral cone for feeding, and clawed antennae that help them attach firmly to their host fish. They also have legs, which are tucked beneath their bodies.
Appearance Through Life Stages
Sea lice undergo several distinct life stages, each with a unique appearance. The life cycle begins with eggs that hatch into free-swimming larval stages. The first two larval stages, called nauplii, are microscopic and do not feed. These nauplii are about 0.5 to 0.6 millimeters long, with a single body division and three pairs of swimming appendages.
Following the nauplius stages, sea lice develop into the copepodid stage, which is the infective stage. At 0.7 millimeters in length, these larvae are more streamlined and possess developed swimming legs, enabling them to actively seek out a host. Once a host is found, the copepodid attaches using hooked antennae and a frontal filament, then develops through several chalimus stages. Chalimus stages are attached to the host by this frontal filament and resemble smaller versions of the adult, growing in size from about 1.1 to 2.3 millimeters.
The final developmental phases are the pre-adult and adult stages. In these stages, sea lice become mobile on the fish. Adult females are noticeably larger than males and develop a prominent genital complex, trailing long egg strings that can be several centimeters in length. These egg strings darken as the eggs mature.
Observing and Distinguishing Sea Lice
Sea lice are found on the external surfaces of marine fish, including the head, back, fins, and gills. Due to their small size and translucent bodies, observing them may require a close examination of the fish. They are marine organisms and do not affect humans directly; skin irritations sometimes referred to as “sea lice” are caused by jellyfish larvae.
Distinguishing sea lice from other small marine organisms or debris is important for accurate identification. Freshwater fish lice, such as Argulus, differ from sea lice. Argulus have a more disc-shaped, jelly-like appearance and possess conspicuous black eye spots, which sea lice do not. Unlike sea lice, Argulus also lay their eggs on underwater surfaces rather than carrying them in strings.
Other small marine invertebrates lack the flattened, disc-like form and attachment to fish that characterize sea lice. Fish scales or general debris would lack the distinct body parts and movement of a living sea louse.