It’s ugly as sin and poisonous as hell.
This is a stonefish (Synanceia verrucosa). There are several species in the genus, and they all look like algae covered rocks. They also have one of the most dangerous venoms known to science.
If you go look for stonefish in Google Scholar, you will find a bucketload of papers on stonefish venom. You find research on molecules that have been isolated from the venom, clinical cases of poisoning, development of antivenoms, and more molecules extracted from venom. Indeed, you’re hard pressed to find any papers that treat the fish as more than a venom-making machine.
I’m not going to delve into the venom itself here. Instead, I want to talk about how venom fits into the ecology and evolution of stonefish.
Stonefish are ambush predators that make their living by burying themselves in sand. And they are able to completely conceal themselves in only a few seconds.
Once hidden in sand, they wait for fish to come by. Stonefish have lost a couple of joints between jawbones that many other fish have, so they are able to extend their mouth further some of their relatives. This reach, plus suction generated by opening the mouth, is enough to draw many smaller unsuspecting fish to their doom. Stonefish are also clever enough to use movements of the exposed fins of their back to “herd” fish back into the strike zone.
The strike itself is over in about 50 milliseconds. For comparison, human reaction times are typically around 200 milliseconds.
Stonefish are relying on camouflage and surprise to make their living.
What do they need venom for?
Stonefish aren’t using their venom to catch their prey, like venomous snakes do. That suggests that the venom might be a defence.
The problem is that camouflage and venom is a bizarre combination. When you look throughout the animal kingdom at species that use toxic chemicals as defences, you tend to see bright colours! You don’t built an ultimate weapon and not tell anyone. You want to advertise that you have a defence. Poison dart frogs are a classic example.
Yet, weirdly, Cameron and Endean noted, “venomous fishes often adopt a sedentary or station-keeping mode of life.”
And I cannot find any references to fish that routinely eat stonefish.
An alternative hypothesis is that stonefish are venomous because their ancestors are venomous. But this doesn’t work, either. A massive tree of relationships by Smith (2006) puts sea robins and other non-venomous, but bottom-dwelling, fishes as the closest relatives to stonefish. Furthermore, prowfishes and velvet fishes, which are lineages of fishes that branch off from stonefishes, are not venomous.
There seems to be no clear functional reason that I can find for stonefish to have the feature that most compels our attention. I am sure there’s a great doctoral dissertation for a grad student who want to work on that problem!
If there’s a grad student who want to work on a species that is both incredibly dangerous and hard to see, that is.
This talk was inspired by one of my seminar students, Luis, who gave a talk on stonefish, and another student (Philip? Was that you?) who asked what ate them.
Camercon A, Endean R. 1973. Epidermal secretions and the evolution of venom glands in fishes Toxicon 11(5): 401-410. DOI: 10.1016/0041-0101(73)90115-3
Grobecker D. 1983. The ‘lie-in-wait’ feeding mode of a cryptic teleost,Synanceia verrucosa Environmental Biology of Fishes 8(3-4): 191-202. DOI: 10.1007/BF00001085
Smith W. 2006. Venom Evolution Widespread in Fishes: A Phylogenetic Road Map for the Bioprospecting of Piscine Venoms. Journal of Heredity 97(3): 206-217. DOI: 10.1093/jhered/esj034
Photo by Nemo's great uncle on Flickr; used under a Creative Commons license.