The pearlfish makes its home inside a sea cucumber's anus to survive the ocean

Nature · Published 3 October 2026 Swipe more like this →

In short: Some fish seek shelter in the most unlikely of places: the rear end of a sea cucumber. These slender, eel-like creatures navigate a toxic environment by coating themselves in a thick layer of protective mucus, turning a biological necessity into a bizarre, permanent, and sometimes parasitic living arrangement.

The pearlfish, a member of the Carapidae family, is famous for its unconventional choice of habitat. While some species live independently, many adults seek refuge within invertebrates like starfish, shellfish, and sea cucumbers. The relationship varies from simple commensalism, where the fish uses the host merely for shelter, to full-blown parasitism. Species in the Encheliophis genus are particularly notorious, as they may consume their host's gonads and internal organs from the inside out.

Living inside a sea cucumber is a dangerous game. These hosts produce saponins, toxic chemicals that would typically kill most fish. Research by Igor Eeckhaut demonstrated that pearlfish can survive exposure to these toxins 45 times longer than other fish species. Unlike the sea cucumbers themselves, which possess specialized cell membranes to resist their own poisons, pearlfish rely on a physical barrier. They secrete copious amounts of mucus—six to ten times more than non-resident fish—which acts as a shield against the chemical environment.

Accessing this 'anal abode' requires perfect timing. Sea cucumbers breathe through their anuses, rhythmically expanding and contracting to pull water into a respiratory tree. This process creates currents that pearlfish use to track their hosts. When a sea cucumber exhales, it dilates its anus, allowing the fish to slip inside. Despite the host's ability to expel its own respiratory organs as a defense mechanism, they strangely never seem to use this power to evict their unwanted lodgers. In 1975, researcher Victor Benno Meyer-Rochow famously discovered 15 pearlfish emerging from a single leopard sea cucumber, suggesting that these fish may occasionally tolerate one another, perhaps to breed or due to a scarcity of available hosts.

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