There are many brightly coloured marine animals, such as fish, corals, sea stars, and octopodes. And yet Wikipaedia says it is thought that those colours don't serve to deter predators, while they do serve that purpose in insects and frogs. Why should bright colours not deter marine predators? And what, then, is their purpose?
Wikipaedia gives a few arguments, but some don't seem terribly strong, while others I don't quite understand:
It has been proposed that aposematism and mimicry is less evident in marine invertebrates than terrestrial insects because predation is a more intense selective force for many insects, which also disperse as adults rather than as larvae and have much shorter generation times. Further, there is evidence that fish predators such as blueheads may adapt to visual cues more rapidly than do birds, making aposematism less effective.
I read what may be four different arguments, or perhaps some together constitute one argument:
- predation is a more intense selective force for many insects
Is this really true? If so, what causes this stronger pressure on insects?
- insects disperse as adults
By this I presume they compare insects, whose eggs are often found clumped together in large numbers, with marine animals whose eggs are fertilised while floating around dispersed; but why does that make aposematism less useful for those marine animals? And aren't there also many marine animals whose eggs are laid in clumps on solid sea floors? Is this related to the supposedly lower pressure of predation from 1?
- insects have much shorter generation times
That's very interesting: do they really have much shorter times between generations? At least a reference is made, but it is inaccessible. But how does that make aposematism more beneficial? Is this, too, related to 1?
- fish predators such as blueheads may adapt to visual cues more rapidly
This, too, refers to a scientific source. The experiment showed that fish can quickly overcome instinctive aversion from certain colours (mainly red) in food. At least I understand this argument (although I'm not sure I think it provides nearly enough evidence).
Some thoughts: marine animals often do exhibit camouflage and mimicry successfully, cf. angler fish and harmful parasitic fish that mimic cleaner fish; could it be that there is something different about the metabolism of marine animals?; or could it be that the three-dimensional environment makes other strategies more effective, such as flight—but then how about all the floor-bound animals, and the unmoving ones?