Gelastocaris - Sclerodoris - who looks like whom?

And who came first, and why the resemblance? The questions can go on and on. This is a fascinating example of Mimicry. Mimicry is one of my favorite biological topics. There are dozens if not hundreds of examples, but often it is arguable which species is the mimic, and which is the model. This is important in understanding the relationship and why one species would evolve to look like the other.

In evolutionary biology, mimicry is the evolved resemblance between one organism and another object, often an organism or another species. In the simplest case, as in Batesian mimicry, a mimic resembles a model, so as to deceive wanton predators. In this case the model is either distasteful or poisonous. Predators learn to avoid the model through a series of unpleasant experiences,which they then remember when they confront a mimic. In the most-studied mimetic relationships, the advantage is one-sided, one species, the mimic, gaining advantage from a resemblance to the other, the poisonous model.

Mike's photo here shows a shrimp Gelastocaris paronai displaying a striking resemblance to one of the caryophyllidia-bearing dorid nudibranchs such as Sclerodoris. The pits on both the shrimp and several Sclerodorids resemble the oscula (pores) on the sponge which the critters live. Let's see now - a slug mimicking a sponge while being mimicked by a shrimp. Crazy story but one must remember that all these species are blind and did not wake up, look around and say - "Hey today I think I'll look like so and so." The resemblance has evolved over many thousand if not millions of years. We can discuss evolution and the natural selection that drives it another day.

In Mike's case, the way it works is that the sea slug likely secretes nasty tasting chemicals when disturbed, so the predators learn to avoid critters that look like this species. Since these predators are visual, like fish, they now avoid anything that looks like the sponge or the nasty slug, shrimp included. If you don't look like the model slug or the mimic shrimp, you are liable to be eaten.