Adaptations of pearlfish (Carapidae) to their life inside sea cucumbers

Carapidae (or pearlfish) are eel-like fishes that live inside different marine invertebrates, such as sea cucumbers, sea stars, and bivalves. Those in the genus Carapus are commensals and use their host as a shelter, while Encheliophis species are parasitic and eat the gonad(s) of their hosts. Life within the host requires different adaptations at the morphological and physiological levels. In this chapter, we have considered the different adaptations of symbiotic Carapus and Encheliophis species that have been investigated for living inside sea cucumbers and hope it can trigger new research in this field. Our discussion mainly focuses on the mode of penetration, diets, type of symbiotic relationships, morphological adaptations, breathing abilities, and resistance to host saponins in the fishes.