If it looks like a fish.... it's a scorpion fish. They can be very nicely camouflaged and can change colour quickly but they retain their fish-like form. The adults 'usually' flare themselves when agitated, often turning a reddish colour. The babies (less than 10cm) will not move IME. This is a toxic camouflaged fish in the Scorpaenidae family with many different sub-species such as leafy, weedy, demon stingers, walkman, and 'false stonefish' as well as the different species of lionfish. Scorpionfish are usually bottom-dwellers except for the lionfish, but can swim surprisingly fast over short distances.
True stonefish are in the Synanceia family. They are apparently the 'dumberest' of all fish with the smallest brain to body-size ratio. the poison glands are much larger than scorpionfish and they have killed a few people- the neurotoxin plays havoc with the heart and long-term heart problems can result after a sting. Getting to a hospital is a very good idea as cardiac arrest sometimes occurs after stonefish encounters. A Japanese man died after getting hit in Okinawa a couple of years ago. Death is uncommon but can happen.
There are basically just two 'true' stonefish but a couple of new species are waiting for classification:
Synanceia verrucosa (Reef stonefish)
This is the most commonly seen stonefish on reefs. They are bulky squat fish with a pronounced up-turned mouth, sometimes buried in substrate with only eyes and mouth visible. They are horrible swimmers compared to scorpionfish.
Synanceia horrida (Hollow-cheeked stonefish)
Sometimes called the estuarine stonefish, seen from above it looks like a grinning skull. They prefer silty environments and are most commonly a shade of grey. They can be found in very very shallow water.
The easiest way to tell scorpionfish from stonefish is the shape. Scorpionfish retain their 'fish shape'; they can be extremely camouflaged but still decidedly fishy. Compare this to the stonefish which are squat blobs.
Stings from scorpion or stonefish are treated the same way- heat to try to break down the protein-based toxin, compression (not tourniquet) immobilisation, and elevation, and seeking the nearest hospital asap.
I believe DevonDiver recently had an encounter with a walkman or demon-stinger. It really took him out and he was suffering the effects for a good week with heart palpitations and inability to walk even a few hundred meters.
