When Dr. Kevin Conway set out to describe a new species of fish, he didn’t know that he would make not one, but two discoveries ­– and that one of them would involve venom.

For the past several years, Conway (in collaboration with Smithsonian researcher Carole Baldwin) has been meticulously comparing several species of clingfish: tiny fishes (some only a few centimetres long) notorious for their remarkable ability to latch on to surfaces using large sucking discs on their bellies

Clingfish in the genera Acyrtus and Acros Image: Conway and Baldwin/PLOSONE

Most clingfishes have already been well described by science, but while working in Belize, Conway and his team noticed subtle differences in specimens of one species of Caribbean clingfish (Acyrtus artius) that were caught at different depths along the coast.

Conway, who works as an assistant professor and curator of fish at Texas A&M University, hypothesized that what they had found was a case of ‘cryptic diversity’ … two different species hiding behind the guise of one. "The first clue that we were on to something was divergence [difference] in the fish's DNA," he says. 

A closer look revealed differences in the fish's colour patterns, and the shapes of individual bones.

"We noticed that the deep-water group had an opaque patch of skin associated with a very sharp and spine-like subopercular bone [one of four bones that support the gill covers in fishes]. Conway explains. "The shallow water group did not have this trait." 

They had indeed found a new species of shallow-dwelling clingfish (Acyrtus lanthanum), but in the process they had uncovered something much stranger.

In order to describe a new animal, scientists have to compare their proposed new species against several closely related ones. "This allows us to ensure they aren’t rediscovering something already caught by another researcher," Conway explains.

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The cleared and stained suction discs of Acyrtus clingfish Image: Conway and Baldwin/PLOSONE

It was during this compare and contrast process that the team discovered venom glands ... not in the new species, but in its deep-dwelling kin, Acyrtus artius, and in Arcos nudus, a species first described 260 years ago by naturalist Carl Linnaeus.

"Sometimes we think we know everything about something only to find out we really don't," Conway says. “We discovered that several species of clingfish have a strange gland associated with [that] spine-like bone. The cells inside the gland are incredibly similar to those present inside the venom glands of scorpion fishes, lion fishes, and certain catfishes.”

Based on this familiar makeup, the team is confident that these clingfish are also producing some type of toxin. If they are right, Acyrtus artius will earn the title of the world’s smallest venomous vertebrate.

“We predict that [the venom] is most likely a defence mechanism (as is the case for most venomous fishes) based on the position of the gland … It’s unlikely that it would have any other function,” he says. 

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A venomous (left) and non-venomous (right) Caribbean clingfish. showing the much sharper subopercular (gill cover) bone in the venomous clingfish. Image: Conway and Baldwin

The properties of the substance inside the gland remain unknown, but Conway hopes this study will bring attention to it, and that scientists interested in venomous fishes will take a closer look.

“New groups of venomous fishes are not discovered very often, in fact the last such discovery happened back in the ‘60s … Our work shows that even in well-studied areas of the world’s oceans, new species can be discovered as can unknown traits in well-documented species.”

Top header image: Conway and Baldwin/PLOSONE