Which crustaceans live as parasites on other aquatic animals?

Various crustaceans have evolved to live as parasites on other aquatic animals, utilizing specialized adaptations to attach to or inhabit their hosts. These parasitic species are found across several groups within the subphylum Crustacea:12

• Crustacean lice: This group includes fish lice (Branchiura), copepod parasites, and certain amphipods and isopods. Many of these infest fish, residing on their skin, in gill chambers, or within the mouth cavity. The family Cymothoidae (isopods) is exclusively parasitic and possesses powerful thoracic claws for secure attachment to their hosts.1
• Copepods: Beyond fish lice, various copepod species parasitize a wide range of aquatic animals, including marine mammals like whales and even invertebrates such as sea anemones.2
• Rhizocephalans: These highly specialized barnacles exhibit a radical, degenerate form. A female cyprid larva attaches to a host—typically a crab—and sends rootlike absorption processes throughout the host's body to derive nutrients. This process often leads to the castration of the host and can even feminize male crabs.34
• Isopods: In addition to fish parasites, some isopods are known to dwell within the gill chambers of larger crustaceans, such as decapod prawns.2