Some Fish Hide Inside Sea Cucumbers: Scientists Say the Reason Is Surprisingly Practical

Underwater life may expose unusual survival patterns, but none are as bizarre and intriguing as the habits of certain fish that take refuge inside the bodies of sea cucumbers. In a predator-dense oceanic ecosystem, certain species of small fish use sea cucumbers as a refuge from predators by taking shelter inside the body of the slow-moving invertebrate.

This habit may seem extreme, but according to scientists, it is a survival pattern that has been shaped by the forces of evolution.

Scientists who have been studying coral reef ecosystems have long observed this phenomenon, particularly among certain species of pearlfish.


These fish, which are characterized by their slender bodies, enter the sea cucumber through its anus and take refuge inside the body of the invertebrate.

Fish Species That Use Sea Cucumbers as Shelter

The most common fish species participating in this unusual symbiotic relationship are pearlfish of the genus Encheliophis. The two species that exhibit this behavior are the silver pearlfish (Encheliophis homei) and the pinhead pearlfish (Encheliophis boraborensis). These fish have an eel-like body that is well-suited to living in small spaces, and they are able to easily squeeze through openings that are too small for other fish.

As stated on the species page for the silver pearlfish, these fish are known to live in the body cavities of sea cucumbers, where they are protected from predators and the environment. The sea cucumbers most frequently hosting these fish are species of Bohadschia argus, Thelenota ananas, and Stichopus chloronotus, which provide sufficient space for the fish to live comfortably.

How Pearlfish Enter Their Hosts

The pearlfish have a series of behaviors that enable them to gain access to sea cucumber hosts without harming them immediately. The fish are seen to first patrol the outside of a sea cucumber and seem to inspect it before trying to gain access.

Once the fish has located a potential host, it gains access by swimming headfirst through the anus and then into the coelom or respiratory tree. According to the summary of information on the pearlfish species page, the fish’s slender nature enables it to swim through the small space inside the host undetected.

Usually, scientists regard the relationship between pearlfish and sea cucumbers as commensalism. This is because in this relationship, one species benefits while the other is not harmed in any way.

According to an article by the marine science publication Shuncy, pearlfish mainly benefit from the sea cucumber by getting shelter and protection from predators while the sea cucumber continues with its normal activities of feeding and moving without being harmed. In most cases, the pearlfish feeds on invertebrates or organic matter found in the body cavity of the host.

Other Fish That Use Sea Cucumbers for Protection

Pearlfish are not the only marine species that employ sea cucumbers as a protective shelter. Juvenile fish have been known to employ the outside of such creatures as a shelter when they are in a juvenile stage of life.

A research paper published in the Diversity journal investigated the juvenile Senegal sea bream (Diplodus bellottii) in the Gulf of Cadiz and found that the juvenile fish were often hiding under the shelter of sea cucumbers. The authors of the paper concluded that sea cucumbers act as microhabitats that provide physical cover in areas where shelter is limited.

The paper states that benthic organisms, such as sea cucumbers can provide a protective zone for smaller marine organisms by modifying the seafloor topography to provide temporary shelter from predators.

A Window Into Marine Adaptation

One of the reasons why this survival mechanism is so effective is that sea cucumbers are in constant motion on the floor of the ocean. Unlike coral crevices or rock shelters, the shelter created by a sea cucumber is portable, and it follows the fish.

As per the observations of marine biologists, as mentioned in the research summaries on pearlfish ecology, this ability enables the fish to stay hidden while still being able to feed in different areas as the sea cucumber moves. The hard exterior of the sea cucumber also serves as a shield that deters many predators.

This ability to move and be protected serves as a living shelter system that minimizes the chances of encounters with predators.