We know that there's a transition and that ontogenetically in the development of these organisms they still retain a shell as a juvenile. We can also, by looking at the outgroups of the Opisthobranchs, the nudibranchs and their relatives, see that all of them have shells. So therefore, using that basis we can safely assume that the ancestral condition in the Opisthobranchia was to have a shell and that the advanced condition, the one that is derived from that original condition is the loss of the shell in the adult.

Well, not all of the nudibranchs and their relatives are brightly colored. Many of them are very cryptic and every time I look at this photo in situ there are differing numbers of slugs that I see there, there are at least four. And it depends on how many glasses of wine you've had at lunch as to the number that you can actually see here. But others are also very cryptic such as this one right here that's found on this stinging hydroid. And you can see that it has these branches that correspond to the spacing of the branches of the colony of hydroids. So some of them are not just generally cryptic but some of them in fact are very specific and we call this a case of special adaptation to a particular substrate that's found in their environment and usually their food.

Here's another case where a species of nudibranchs feeds on a sponge. Here's a nudibranch here and here's another one here. See, in some cases this special resemblance to their food is really carried to extremes. Now the question that you immediately ask is, well, are some of them, are the ones that are cryptic in coloration and blending in with their background, are they also toxic or not? The fact of the matter is that most of them that have been tested biochemically are in fact distasteful and toxic and just avoid the whole problem. Rather than advertising their presence to potential predators, they try as a first initial strategy of avoiding detection. There's another species of sea hare that lives only on a particular sea grass. It blends in extremely well, having similar markings to the veins and the sea grass and is extremely well camouflaged in its natural habitat.

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