It is an external parasite of wild and cultured marine fish. Among the 200 species of this group some parasitise brackish-water fish in cage or pond culture systems. Lepeophtherius , Caligus and Pseudocaligus are the major genera of interest, as they can cause high mortalities. Recently, mullet fingerling from shrimp ponds in Rayong Province were found to be heavily infested with Caligus sp. Adult Caligus sp are light yellow or creamy in colour and are sexually dimorphic.
Females are usually larger than the males.
Adult females of C. elongatus are 6-8 mm long and males about 5 mm long. The cephalothorax is flat and round, with frontal lunules and paired eyespots. Paired egg sacs extend from the posterior of the body and each sac contains around 30 eggs. The main diet for Caligus sp are the mucus and epithelial cells of the host. The parasitic stage of Caligus sp.