Venom in Fangblennies

A shorthead fangblenny looking innocent. Photo by Paddy Ryan. 

The fangblenny reveals its fangs! Photo by Serge Abourjeily.

Blenny is a general term that refers to six families of fish (the suborder Blenniodei), most of which are cute little elongated fishes with big eyes and mouths. The fangblenny (or sabre-toothed blenny), however, is another story! The fangblennies encompass many species in 5 genera, only one of which (Meiacanthus) has venom as well as fangs. This week's paper on the evolution of fangs and venom in blennies is in Current Biology, lead authored by Nicholas Casewell from the Liverpool School of Tropical Medicine. 

What they did

Casewell and his colleagues first created a phylogeny of the fangblennies using five molecular markers. They then conducted extensive morphological analyses of the tooth and venom gland structure in each fangblenny species. Next they analyzed the composition of the fangblenny venom. 

What they found

First off, they were able to confirm that in blennies the evolution of the fang structure proceeded the evolution of venom. The ability to bite appears to function in blenny defense, as fish that have eaten fangblennies have been seen to spit them out (unharmed). In accordance with this defense hypothesis, most fangblennies have bright, distinctive, aposomatic coloration which may have evolved because a distinctive looking fish that gets eaten, bites, and is spit back out is more memorable and less likely to get eaten again. 

They also showed that one of the components of fangblenny venom is an opiod hormone that functions by binding to opiod receptors in the the bitten individual causing numbness in that area. Another component of blenny venom reduces blood pressure. This is very different from the effects of the spine venom that is most common in fishes. Spine-venom is quite painful, and therefore appears to function as a defense. So why would fangblenny venom numb and relax the bitten animal? Apparently it allows the fangblenny to get away while the bitten animal is impaired by numbness and lethargy. It is interesting that venom has evolved along two very different pathways (pain versus numbness) that both function in defense. 

The takeaway

One of the coolest things about this group is their mimicry. The venomous blennies are only one genus, and blennies from other genera look and swim like these venomous blennies, presumably allowing them to be sheep in wolf's clothing. One genus of fangblennies (Plagiotremus), however, is what the authors call an "aggressive mimic". These fangblennies feed by swimming up to other larger fishes and taking little bites out of them (or micro-predation). Some of these blennies mimic cleaner wrasse, so the victim fish might think they are about to be cleaned, when instead they get nipped. Others of these blennies, however, look like the venomous blennies. This affords them protection against the fish they are biting, because the bitten fish does not want to mess with a venomous fangblenny.