I often get confused between stonefish and scorpionfish and need to remind myself what the differences between them are. Both belong to the order Scorpaeniformes – mail-cheeked fishes - including 36 families and over 1,600 species such as lionfish, gurnards and even the cute (in my opinion) lumpsuckers.
While stonefish are highly venomous and considered the world’s most venomous fish with a killer sting, scorpionfish do not share this killer reputation, even though a sting with their less potent venom is not pleasant either. Both fish are hard to spot and avoid, making them dangerous to divers, swimmers and snorkelers, but they are a photographers dream when they are finally located as they do not scare easily.
There are several features that both fish share, but several that distinguish the two. Stonefish disguise themselves very well by burying themselves in the sand (looking like a rock, hence the name stonefish) while scorpionfish have fantastic camouflaging ability and are able to replicate the pattern of their surroundings. They are so confident of their camouflaging skills that they will not move when approached, only moving their eyes to inspect a threat. They have a “sit and wait” strategy, grabbing whatever comes along that looks tasty enough to eat, but scorpionfish are much more skittish while stonefish will only move away slowly when disturbed.
Both have characteristic spines on their dorsal fins that contain venom inside which causes excruciating pain and damage to an unsuspecting victim. Stonefish spines, however, are positioned much more prominently than on a scorpion fish dorsal fin, and they are thicker. Their tails also differ, with the stonefish tail curled to one side, whereas the scorpionfish has a straight tail. Both fish have large heads with armoured bony plates and spines and a bony ridge across their cheeks, but a stonefish does not look like a fish from above but more like a rock, while the scorpionfish still looks like a fish.
Stonefish eyes are smaller and hard to notice while scorpionfish eyes stick out as if on stalks, positioned a bit further down the body. Both have a very wide mouth that can swallow surprisingly large prey such as crustaceans and fish in one go. Neither of them is a particularly good looking fish, but the stonefish always looks as if it were in a bad mood, whereas scorpionfish seem in much better form, almost smiling. As they live on the seafloor, both have no swim bladder, and both reproduce by laying orange clusters of eggs among seaweed and rocks. Stonefish can grow up to 40cm long and are bigger than scorpionfish.
So, if you see a fish that looks like a stone, it’s probably a scorpionfish, as stonefish are really hard to spot - don't try to find out though, as even though they are not aggressive, stonefish venom can kill you. Scorpionfish venom is still more than unpleasant - either way, you’re at a loss and the injured party.
Text by: Bogna Griffin, BSc (Hons) Freshwater and Marine Biology, Ireland