The pearlfish, a long, slim fish belonging to the Carapidae family, has a peculiar way of life. These fish, found in shallow tropical waters around the world, don’t have scales or any means of self-defense. To survive, they seek shelter, but not in a seagrass meadow or the crevices of a rock. Instead, they choose an unusual refuge: the anuses of sea cucumbers.
Sea cucumbers breathe through their butts, offering pearlfish an easy way to sneak into their unwitting hosts. The pearlfish sniff out their host and wait for it to open its anus for a breath before swimming inside. This routine repeats every time the pearlfish needs to return home, leaving only to find food.
Even the Cuvierian tubules, special sticky threads that sea cucumbers eject from their anuses in self-defense, don’t affect pearlfish. And even sea cucumber species with anal teeth can’t deter these persistent butt-dwellers. A pearlfish researcher at the University of Liège, Belgium, Eric Parmentier, discovered a new, smaller species capable of entering even those with anal teeth.
These eel-like fish often, but not always, share a home with one pearlfish inside a host. While typically one pearlfish resides within a single sea cucumber, some species have been known to pair up in a single host. The number of pearlfish residing in a single sea cucumber can even reach double digits. In 1975, scientist Victor Benno Meyer-Rochow found a leopard sea cucumber with 15 pearlfish inside.
Some species of pearlfish can live inside sea cucumbers without harming them. They live in symbiosis, where neither the host nor the invader is harmed. However, there are also parasitic species that feed on the sea cucumbers’ gonads, according to Parmentier. These parasites do not kill their host, but they can disrupt its reproduction.
