A sponge. But look quickly and it may resemble a frog fish.
Photo by Graham Heiberg
A sponge with a frog fish at the bottom of it, well hidden.
Photo by Guido Zsilavecz
The same frog fish revealed.
The most amazing mimicry - in this case it nearly seems the sponge mimics the frog fish!
Photo by Andrew Taylor
More detail - the sponge even has an "eye" and the little "front feet", actually modified fins.
Photo by Andrew Taylor
Frog fishes are ambush predators. They rely on camouflage to pounce on unsuspecting prey. Their speciality is to look like sponges, and the sponge in Graham's photo does look at first glance that it could be a frog fish - there's an eye formed by a hole, a body, and even pectoral and tail fins. Of course, it is only a sponge - frog fishes do not occur in Cape Town waters, but the resemblance is quite impressive.
The resemblance of frog fishes to sponges is sometimes quite uncanny. In the photos from Guido a frog fish is well hidden at the bottom of the sponge, which it resembles in colour and even pattern. Only when viewed up close is the fish apparent.
In Andrew's amazing photo this resemblance is even more striking, to the extent that one wonders whether the sponge is not mimicking the fish! First of all the overall shape is similar, with a steep forehead , a down-turned mouth, the same prop-up "legs" the fish has, and a hole for an eye. If the fish had been the same colour it is likely Andrew might not even have seen it! By resembling the sponge this closely, down to the bumpy skin, the fish gains perfect camouflage, and the more it resembles sponges, of course, the more it is likely to survive, and allows evolution to do its wonderful thing.