Opisthobranch of the Week Data
I've previously featured Phyllidiella pustulosa on these pages, and for a short discussion of the species see the earlier presentation. The above image illustrates one of several kinds of mimicry recognized by biologists, Batesian mimicry, where a "mimic" adopts the color or shape of an unpalatable model (sometimes referred to as a model and a mimic system). The similarity of these two animals is quite apparent in the image. On the anterior end of the animals the polyclad flatworm has pseudotentacles and the nudibranch has true rhinophores. The polyclad flatworm of course is greatly dordo-ventrally compressed whereas the nudibranch is relatively thick in comparison. This body shape isn't normally apparent with animals in the normal crawling position from an upper view.
Phyllidiella pustulosa is considered to be very common in Okinawan waters and is seen on the majority of local dives; relatively few specimens have been collected but numerous individuals have been seen. Brunckhorst (1993) considers P. pustulosa to be one of the most common nudibranchs throughout the tropical Indo-West Pacific. The flatworm (Pseudoceros imitatus), on the other hand, is considered to be rare in the waters of Okinawa's main island in as much as I've personally seen only three or four individuals.
The above featured two animals were not found together but found on separate parts of the same live stony coral reef in six meters of water during the same dive; thee two similar appearing animals were posed together in order to get the above image.
The word mimicry implies a self-directed or conscious effort to copy something else in order to possibly gain some kind of benefit in doing so. Experimentally mimicry of course is at best difficult to prove (evolution takes place perhaps over multiple millions of years of time). Bill Rudman (1999) in commenting on a Sea Slug Forum post concerning mimicry makes the following comments:
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.
There are several photographs of this remarkable F/W available (Gosliner, et al, 1996 (p. 106); Newman & Cannon, 2003 (p. 40); plus several on-line sites: Seifarth, 2004; Litvaitis & Newman, 2002).