Defence against predators
Withdraw to a prepared retreat: many methods. The pearl-fish Carapus apis retreats backwards into the anus, and then into the body cavity of a sea cucumber (Holothuria).p138 It turns out that the fish, when a juvenile, is a parasite on the sea cucumber. Although it is free-living as an adult, it is still able to re-enter the holothurian when danger threatens. Better-known examples include the tortoise, which just withdraws its head and legs under its shell, and the hermit crab, which retreats into its shell when in danger.