Information on Mexichromis porterae

Dear Lacey,
You're lucky that Mexichromis porterae is one of the few Californian nudibranchs I have a picture of on the Forum. Like many nudibranchs, I don't think a lot has been published on its biology and natural history. One important point for you to know though is that for the last 20 years it has been placed in the genus Mexichromis, rather then Chromodoris, because parts of its anatomy (eg: radular teeth, reproductive system)are rather different from those found in species of Chromodoris.

Like all chromodorids, Mexichromis porterae is a sponge feeder, and has been reported by a number of authors to feed on the sponge Dysidea amblia DeLaubenfels. Chromodorids, like Mexichromis, have some very interesting defence mechanisms. Like many plants, sponges make and store in their tissues, a variety of distasteful chemicals to deter animals from eating them. Sponge-feeding chromodorids are not affected by these chemicals and remove them from the sponges they eat, and store them in glands around the edge of their mantle. In combination with their bright colour patterns, predators, such as fish, apparently learn that these nudibranchs taste terrible and aren't worth eating. Have a look at the pages on Defensive Glands, and Mexichromis macropus. Also have a look at the section on 'Warning Display' in the Defence - Colour Patterns Page. Mexichromis porterae is one of a number of species on the west coast of America with a blue and yellow colour pattern, suggesting a mimicry group like the red-spotted chromodorids of south-eastern Australia. Have a look at Mike Miller's messages illustrating some of these West Coast animals - though I think they are all yellow spotted rather than yellow-lined. I guess you are familar with Dave Behren's book Pacific Coast Nudibranchs which will also give you some background information. All I can find in the literature on other aspects of their natural history is that they are fairly uncommon and found from the intertidal to about 18m depth. Perhaps someone with further information will let us know. For example does anyone know if they have free-swimming veliger larvae or direct development?

About the last part of your essay topic What is it's place in the biosphere?. It's not possible to answer such a question. To me it suggests there is some plan to nature and the environment, which is wrong. Evolution is directionless and without a plan. That's how it works. Nudibranchs don't have a 'place' or a 'purpose' they just 'are'.

Closely related species have their own food requirements and possibly other slight differences in ecology. However, because at any one place, the actual species present, and their numbers, can change over time, the inter-relationships between species is continually changing. You can say that Mexichromis porterae has a direct predator-prey relationship with the sponge Dysidea amblia but every other relationship it has with the fauna and flora at that place is in a continual state of flux. Sorry there is no easy answer to life's big questions.

Good luck with your assignment,
Bill Rudman.