chalimus

Chalimus is a larval stage in certain parasitic crustaceans, most commonly copepods such as the sea lice that infest fish. In these species, the chalimus represents an early, sessile juvenile phase during which the parasite remains attached to its host by a frontal filament and does not swim freely.

Following the free-swimming nauplius and copepodid stages, an infective copepodid settles on the host and molts into chalimus stages (often two instars, chalimus I and II). During these stages the body is anchored to the host surface by the frontal filament, while the appendages are used to feed on host tissue and mucus. Molting to the next stage yields mobile preadults, which eventually become adults.

Chalimus-stage sea lice commonly infest marine fish, including farmed salmon and other commercially important species. They can cause skin irritation, tissue damage, impaired osmoregulation, and increased susceptibility to secondary infections. Heavy infestations are a concern in aquaculture, affecting animal welfare and growth, and necessitating management measures such as chemical therapies, environmental controls, and welfare-oriented husbandry practices.

Chalimus stages are found in several parasitic copepod families, particularly Caligidae (sea lice) and related taxa. They occur in marine environments where their fish hosts are present, with outbreaks documented in aquaculture regions worldwide. The term chalimus denotes a specific, relatively immobile juvenile phase within the copepod life cycle, distinct from later free-swimming stages.