The sea cucumber and the pearlfish
You might have heard of the commensal / parasitic relationship that pearlfish have with sea cucumbers, but how many of you have actually seen a pearlfish enter its host cucumber through the anus where it hangs out most of the time?
Pearlfish (family Carapidae) are scaleless, their bodies are somewhat translucent, and they look remotely like eels when they swim outside of their host. They can be found in the Atlantic, Pacific and Indian oceans and have been found down to depths of over 2,000 meters. Some of the species can reach upwards of 50 cm in length when full grown.
These fish are unique in that the adults normally live inside invertebrates like clams, sea squirts, and starfish in a commensal relationship (not harming their host). This trait is common throughout the Carapidae family.
Parasitic and commensal relationships
Some species of this family, however, are possibly parasitic. These live inside sea cucumbers where they will first eat the gonads of their host and then live inside the anal pore. The Pinhead pearlfish ( Carapus boraborensis ) is suspected to be parasitic whereas the Silver pearlfish ( Encheliophis homei ) is suspected to be non-parasitic based on stomach content analysis and where they are found within a cucumber when dissected.
Before entry, a pearlfish will normally spend several minutes inspecting the entire body length of a cucumber before proceeding to the back end. A knocking or tapping movement is performed near the anus as if “asking” to gain entry. Reports indicate that pearlfish normally back in tail first but have also been observed to enter head first. More than one pearlfish has been known to live inside a cucumber at any one time as well making for a somewhat cramped living environment. This trait has been observed with the Silver pearlfish where sexual pairing has been found within a cucumber.
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