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In some cases, there can be real amalgamations of different organisms in the same habitat that have similar mimicry patterns and color patterns.

Here you have three different nudibranchs that are all within the same family but are in two different genera. This one is in a different genus. This is a sea cucumber which is an echinoderm related to sea stars and sea urchins. It is also distasteful. When it is a juvenile, the same size as the nudibranchs, it has the same color pattern as the nudibranchs. When it grows larger than the nudibranchs its color pattern changes completely and over a very short period of time. We've seen intermediates and found all of the transitional stages so that we know that's the case. So in this case, the mimicry of this sea cucumber with these three nudibranchs is one that only really works for a certain stage in the life history of that particular sea cucumber.

One of the things that phylogeny allows us to do, in addition to testing the direction of evolutionary change, is to make comparisons between phylogenetically equal units. What I mean here is if we look at this node on this tree and look at this branch compared to this branch, this is called the sister group of this particular branch. Since they both were derived from a common ancestor, we know that the splitting of those two lineages was simultaneous. That allows us to measure what goes on in one evolutionary lineage and compare it with an equivalent evolutionary lineage. And in this case, we are looking at two different families of nudibranchs. There's one called Actinocyclidae which is composed of these taxa and Chromodorididae which is composed of those taxa.

One of the things I want you to note, that one of the characteristics that unites Chromodorididae is that they have mantle glands. What are the consequences of having mantle glands and how do they compare in sister groups? Well, if you look at Actinocyclidae, this is what all of Actinocyclidae look like, they're all cryptic and they don't have any defensive mantle glands. They have some toxins that are generally distributed in their body walls but they don't have them concentrated in certain areas nor do they have mechanisms that can readily release those toxins but the toxins sort of generally leach out over a period of time.


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