FAKE – FRAUDULENT – FALLACIOUS
Mimics of Sea Slugs
Other NudiNotes have discussed sea slugs that look like other sea slugs (see Specious Sea Slugs NudiNote), for a number of complex reasons, and all the confusion that it causes taxonomists, the publishers of identification guides and us lowly sea slug aficionados. Sneaky, sly sea slugs indeed. Now we turn our attention to other forms of marine life that take on the appearance of sea slugs, mimicking them to various degrees of similarity. Those of you who are dedicated Nudi hunters will have, no doubt, been temporarily deceived by them at some point. The term mimic is used here in the loose sense of similar appearance rather than the strictest scientific meaning.
Mimic – Model – Predator
We often speak of defensive mimicry in nature as if it is something that the animal is trying to achieve in order to protect itself. This is incorrect. Mimicry is actually an undirected, random, evolutionary process brought about by a combination of genetic changes (germline mutations) occurring during DNA replication, creating a different appearance, which are then passed onto successive generations, together with selection pressure within the habitat whereby those with a favourable mutation have a higher rate of survival. Selection pressure in the habitat related to mimicry is mostly derived from predator choice of prey. That selection process is influenced by prey mimicry confusing or deceiving the predator thus making that mimicry an anti-predator adaptation.
The evolved mimicry serves to either protect the animal from predation because it appears to be a form that a predator does not recognise/prefer or it appears to be a form that the predator thinks is noxious/distasteful. It is ironic that it is actually the predator, through that avoidance therefore, that eventually causes the creation of a mimic population. Mimic, model and predator are the three corners of the triangle driving this process. Of course mimicry can also be used by predators to deceive prey but this is not relevant in this discussion about sea slugs.
If we are talking about the appearance or resemblance of a mimic it is not just limited to colour and pattern but also shape, size and behaviour.
Phyllidiids as Models
An excellent example of models are the phyllidiid nudibranchs. They have proven to be a most successful model for mimics given their highly noxious and distasteful bodies, exposed presentation in the habitat and that the family contains many very similar looking species. Some polyclads (flatworms), polychaetes (scale worms), holothurians (sea cucumbers) and juvenile ovulids (allied cowries) utilise the colours and patterning and sometimes texture of the phyllidiids to their protective advantage. These examples encompass species from four diverse animal groups mimicking the one family of nudibranchs.
There are also several flatworms that have developed the black striped notum common to the Chromodoris nudibranchs. Often these are not a perfect match to any one nudibranch species but are of a generic presentation that could be considered similar to several of the Chromodoris species with noxious dermal glands, in the habitat – “playing” therefore, the percentage game.
As mentioned in previous mimicry articles, and it is worth repeating, the mimicry need not necessarily be perfect. Often a vague resemblance to a highly noxious species can bestow significant protection. It may be that the mimic is evolving towards the perfect look (evolutionary lag). Also, only certain traits may be necessary to deceive a predator and we, as humans, may perceive the mimicry different to how a predator perceives it. Other factors that might work against obtaining perfect mimicry include: the limit to the degree of change possible in their genomic structure and also the cost to the mimic, e.g. degree of change verses efficiency. Having an appearance that is generally similar to a whole group of models rather than absolutely matching a particular species can actually be helpful. This “universal” presentation serves to exhibit the salient features of the whole group of models without the distraction, clutter, limitation and economic cost of finer detail. The signal to the predator is still clear but cosmopolitan.
The above is merely a brief introduction with some known, oft quoted examples. As with most biological subjects mimicry is a complex phenomenon. In fact it is quite difficult to establish that two animals with a similar appearance are the consequence of a lengthy mutation/selection process. However this does not diminish the likelihood of the similar appearance conferring a degree of protection.
David A. Mullins – January 2024
– Gosliner, T. M., Behrens, D. W. & Williams, G. C. (1996). Coral Reef Animals of the Indo-Pacific. Sea Challengers; Monterey, California.
– Sherratt, T. N. (2002). The evolution of imperfect mimicry. Behavioral Ecology Vol. 13 No. 6: 821–826.
– Rudman, W. B. (2004 July 23). Mimicry. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/mimicry and associated messages.
– Behrens, D. W. (2005). Nudibranch Behaviour. New World Publications, Florida, USA.
– Coleman, N. (2008) Nudibranchs Encyclopedia. Neville Coleman’s Underwater Geographic Pty Ltd, Springwood, Qld.
– Mullins, D. A. (2013). A Flatworm Mimic – Pseudoceros imitatus. https://www.marineimages.info/the-mimic-2/
– Mullins, D. A. (2013). Duplicity – Ovula ovum. https://www.marineimages.info/duplicity/
– This NudiNote has been modified from a previously published article in Dive Log Australasia Magazine – NudiNotes Column, Issue: #403 (December 2023): pp. 18-20 by David A. Mullins.