Nudi Notes


Feb 2, 2024

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.


Above: Mimicry by a juvenile ovulid (allied cowrie), Ovula ovum, of the toxic phyllidiid nudibranch, Phyllidia madangensis. This is a very close likeness given the different body forms of model and mimic. Left: The juvenile Ovula ovum. Centre: The phyllidiid nudibranch Phyllidia madangensis. Right: A pair of adult Ovula ovum illustrating the change wrought with maturity from a colourful mimic to an almost plain black mantle with minute papillae carrying a white tip.


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. 


Above Left: The polyclad flatworm Pseudoceros imitatus exhibits an uncanny resemblance, by colour, pattern, texture and size, to the extremely toxic and common nudibranch, Phyllidiella pustulosa on the Right. To the casual observer the only clues are a flatter appearance and the more easily discernible movement of the flatworm compared to the nudibranch. There are no dorsal gills to simulate and the marginal pseudotentacles formed by upfoldings of the anterior margin do an excellent job of appearing like phyllidiid rhinophores. The dorsal surface of its body also has a lumpy texture like the sea slug it seeks to mimic. This is not to suggest that this flatworm does not have its own store of toxic chemicals within its body. Although not yet proved, this may in fact be the case because if the visual deterrence fails then by having distasteful or toxic flesh usually the worst damage is limited to a bite mark. Additionally not all potential predators have the faculty of sight. It is certainly a better outcome to be only just bitten rather than completely devoured.


Above Left: This Pseudoceros sp. is another flatworm with an appearance mimicking a phyllidiid nudibranch this time with an orange margin and also some tinting to some of the dorsal “pustules”. Right: Phyllidia exquisitia is one example of a probable model for this mimic.


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.


Above Upper: There are a number of Goniobranchus nudibranchs that present with a white mantle and variously coloured mantle margins. Above Lower: By sending the same generalised signal to predators this flatworm tunes into all the similar nudibranch anti-predator signals.


Imperfect Mimicry
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.


Above Left: This is the juvenile stage of the sea cucumber Pearsonothuria graeffei. As it grows into the adult its features change and gradually looks less and less like the toxic nudibranch it is mimicking during its most vulnerable stage. Right: The noxious nudibranch Phyllidia varicosa with blue to grey ridges carrying yellow-tipped pustules. The sea cucumber is certainly thinner and the “pustules” are more pointy but as in many cases of mimicry, presentation of certain features to a predator can be just as effective as a perfect match.


Above: A couple more mimics. Left: The polychaete scale worm Lepidonotus melanogrammus presenting a reasonable imitation of a phyllidiid nudibranch with the grey and white pattern and yellow-tipped swellings. The colour of this species can be variable depending on habitat/region. Right: Another polyclad flatworm – Pseudoceros sp. This flatworm has a more generalised mimic colour and patterning. It would seem to be “covering its bases” by mimicking several models belonging to the black-striped group of noxious Chromodoris nudibranchs. Chromodoris magnifica would be the closest model but as mentioned previously the mimic does not have to be a perfect match to deceive or confuse a predator. Exhibiting a few strong features = strong signals, to the predator, can often be sufficient.


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 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 MimicPseudoceros imitatus.

– Mullins, D. A. (2013). DuplicityOvula ovum.

– 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.