Most people who encounter a lesser weever (Echiichthys vipera) keep as far away from it as possible; a pity (from a scientific point of view, at least), since they are occasionally host to an intriguing and macabre parasite. Ceratothoa steindachneri, known as the tongue-biter, is an isopod (an order of crustaceans which also includes woodlice) with a strange life-cycle. All individuals begin life as males, and find their home inside the mouth of their host fish. They attach themselves to the fish's tongue, and cut off the blood supply, so that the tongue withers away and falls off; the isopod remains in place, and from then on must fulfil at least some of the tongue's functions.
The parasite then changes sex, becoming female, and waits for another male to enter the fish's mouth, so that they can reproduce.
Until 2021, Ceratothoa steindachneri had only been observed in weevers on the Cornish coast. However, in April that year we caught 3 weevers in the Medway estuary in Kent, each of which had a pair of tongue-biters in their mouths.
It seems likely that they are much more commonly-occurring around our coast - however, until more people start looking in weevers' mouths, we might never find out.
More on Ceratothoa steindachneri here and here.
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