Let's have a look inside a cave. Seaweeds, different coralls and variously coloured sponges. An urchin by the entrance.
At the lowest point, the passage splits in two, to end in a pair of latent cracks on an other side of a rock. That makes this cave an ideal shelter for a fish, allowing to escape a danger from the main entrance by sweeping through the backdoor. That is what they all, actually, did while I was prepairing to take a shot.
Only the urchin keeps still. He does not really care.