On the other hand, there are a lot of marine invertebrates and this is a nudibranch that has very bright color patterns
One of the things that we've been investigating is do these color patterns that are aposematic exist, because these organisms do in fact possess a great deal of toxicity and very unique biochemical compounds in their body walls that defend them against predators. How do we know this? Well, we've observed it directly. If you take a brightly colored nudibranch, toss it up into the water column, a fish will immediately come over, usually either look at it very closely or more commonly, engulf that nudibranch and immediately spit it out. The nudibranch crawls away unharmed. That fish has had one of those learning experiences that I referred to. And it generally, after one experience, will associate that color pattern with that unpleasant experience. And if you keep offering that same nudibranch to the same individual fish over a period of weeks, it will refuse in every case, to feed on that nudibranch based on that one experiment.
Well, one of the things that intrigued me when I was working on my Ph.D. was some of the immediate relatives of nudibranchs have a well-developed shell. As you can see here, there is a shell and it's fairly well calcified. There are other representatives of the group where the shell is relatively small compared to the rest of the body. There are other representatives where the shell has been reduced, is internal, and is no longer providing any protective function, which is the main reason that mollusks have shells. Those shells are merely supporting the respiratory cavity so that water movement can move across the gill and gas exchange can occur.