Parasitism prevails in several groups of Crustacea — Copepoda, Branchiura, Cirripedia, Isopoda and Amphipoda.
Copepoda (Fish Lice): Majority of crustacean parasites. Exhibit diverse array of morphological modifications, from free living forms to most degenerated parasites. Some are even shaped almost like worms e.g. Ergasilus, Monstrilla, Caligus, Anthosoma.
Branchiura: Ectoparasitic on the skin or in gill chambers of freshwater and marine fishes and some amphibians. e.g. Argulus.
Cirripedia: Syngoga, Laura.
Isopoda: Gnathia.
Amphipoda: Cyamus.
Peculiarities of Parasitic Crustaceans:
Degeneration: Lost organs no longer used in the adult form. Copepoda and Cirripedia look similar due to degeneration.
Adhesive Organs: sucker and hook, for attachment externally or inside cavities.
Feeding: Feeding by suction or absorption through organs that do not always represent true organs.
Reproduction: Vast and wasteful to maximise survival of a few offspring.
Sexual Dimorphism: Remarkable correlation between degree of parasitism and sexual dimorphism.
Hermaphroditism: induced in some cases due to parasitism.
Life Cycle: Compared to other parasites, there is not much complexity (transfer between hosts is mostly non existent).