About Mimicry & flatworms

July 11, 1999
From: Christine

Dear Dr. Rudman,
I recently gave a little talk about nudibranchs to the scuba divers on board the boat 'Pro Diver' in Brisbane. I gave the example that the flatworm Pseudoceros imitatus mimics Phyllidia pustulosa. One of the divers wanted to know how the flatworm 'knew' what the nudibranch looked like, so that it could copy it, since flatworms only have primitive eyes?

My guess is that flatworms 'tried' a number of colours and patterns by chance, and that the Pseudoceros pattern was selected through increased survival because of its likeness to Phyllidia.

Is there a better explanation?

Also, are flatworm tentacles analogous to nudibranch rhinopores?



Christine, 1999 (Jul 11) About Mimicry & flatworms. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/1020

Dear Christine,
Mimicry is a good example of evolution at work. It is difficult not to give human attributes to animals when explaining how evolution works but it is important not to give the impression that the process is 'guided towards perfection'. Evolution is directionless. To say they 'tried' a number of patterns suggests they were aiming at a goal which is quite the wrong way to approach it.

If we go back to first principles - as first stated by Charles Darwin - to be 'fit' or 'survivable' species must occur in large numbers and show variability. If, or rather when, environmental conditions change, there is a good likelihood that some members of a very variable species will be able survive in the new conditions and so pass on their selection of the original genes to the next generation.

In the case of Phyllidia and the flatworm, let us presume that Phyllidia is the 'model'. Phyllidia has extremely toxic and distasteful secretions which deter fish from eating it. Fish learn to avoid the Phyllidia-shape and colour and so Phyllidia are somewhat protected from predation. In this case, any animal that looks like Phyllidia will gain some protection from predation by 'tricking' predatory fish into thinking they are a Phyllidia. If a flatworm species has a vaguely similar shape and colour to Phyllidia then those individuals that look most like a Phyllidia are most likely to escape fish attack. Gradually the surviving flatworms of each generation will become more and more like Phyllidia simply because only those with the genes to look like Phyllidia will survive. At no time will the flatworms 'know' they are improving their chances of survival, or even that there are lots of Phyllidia in their environment.

In more complicated situations there can be two or three mimic species and sometimes some or all will be distasteful. In other cases there can be many species, all of which are distasteful, as I describe for the red-spotted chromodorids of southeastern Australia.

Concerning flatworm 'tentacles'. Yes they are sensory and as with the nudibranch rhinophore can detect chemical 'smells' in the water.

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Jul 11). Comment on About Mimicry & flatworms by Christine. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/1020



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