Defenses of nudibranchs and their relatives include aposematic (warning) coloration & Batesian mimicry, considered in this section, and

CAMOUFLAGE (CRYPSIS),
FAST CRAWLING & SWIMMING,
MUCOUS COATINGS,
CERATAL AUTOTOMY,
NUTRITIONAL CONTENT,
SPICULES,
NEMATOCYSTS,
VACUOLATED SKIN WITH PROTECTIVE SPINDLES,
ACID SECRETIONS,
INK & OPALINE SECRETIONS,
SECONDARY METABOLITES,
ALARM PHEROMONES, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

An individual aeolid Hermissenda crassicornis may have several hundred cerata.  Each, when mature, bears a cnidosac containing several thousand undischarged nematocysts. Brightly demarcated orange and white bands demarcate the location of the cnidosacs in Hermissenda, and orange and blue markings adorn the dorsal part of the body, with white piping on the rhinophores and oral tentacles. Similar colorful markings are exhibited by other aeolids.  It is generally accepted that these are examples of aposematic¹ coloration, that is, a warning to a nudibranch’s predators about the presence of something unpleasant.  An aeolid will often “bristle” or wave its cerata about when irritated, perhaps making itself appear bigger or, a propos of the defensive function being considered in this section, offering a warning display of something noxious.  In theory, a naive predator takes a mouthful of cerata, gets stung and, on later contact with an individual of the same species, remembers the experience and does not bother it again.  For warning or aposematic coloration to work, the predators have to be daytime visual hunters with good colour vision.  This restricts the field to fishes, crustaceans², and birds (the last in shallow subtidal and intertidal areas).  Although convincing experiments³ on aposematic coloration in nudibranchs are lacking, it is hard to ignore an apparent convergent evolution of possible warning coloration⁴ seen in many aeolids.  Drawing from Bürgin 1965 Veliger 7: 205.

NOTE¹ lit. “away signal” G., referring to bright, easily visible colours displayed by toxic animals (and some plants) that convey a “do not eat, for I am unpalatable, and will make you sick” message to predators with colour vision.  A familiar example of aposematic colouration is the black and yellow markings on bees and wasps

NOTE² learning ability in crustaceans is poorly developed, or at least is not evident in the kinds of experiments used to test for it

NOTE³ by feeding Hermissenda on non-nematocyst-bearing food, such as tunicates, it is possible to eliminate all nematocysts from the cerata through routine cycling of these in the cnidosacs. As a possible research project, couldn't these nematocyst-free specimens now be tested with experienced and naive predators, such as cottid fishes, to test the efficacy of the nematocyst defenses, learning in the predator, and putative aposematism? This has been tried elsewhere, with inconclusive results, but appears not to have been investigated in west-coast species

NOTE⁴  although this section deals with warning coloration associated with cnidosacs within the cerata of west-coast aeolids, studies on other aeolids, such as Eubranchus spp. in Europe, reveal that ceratal glands of unknown function may also be present (see drawing on Right).  For example, when a ceras of Eubranchus is is pinched, these glands release a white secretion.  It the secretion is defensive, then warning coloration on the cerata could be associated with it, with nematocysts, or with both. There are several west-coast species of Eubranchus to experiment on. Edmunds 1966 J Linn Soc Lond (Zoology) 46: 27

Colours in nudibranchs are created by pigment, both in the epidermis and in the digestive-gland diverticula, and by structure, where light is diffracted through granules contained in vacuoles in special cells of the epidermis.  Studies on colour patterns of Hermissenda crassicornis at Scripps Institution of Oceanography, La Jolla, California reveal that the bright orange coloration is from fat-soluble carotenoid pigments in epidermal cells. 

Colours of the digestive diverticula, easily seen through the transparent skin, may be brown, ochre, or reddish, partly owing to the type of food being eaten and partly to the presence of pigment-bearing vacuole cells among the digestive and glandular cells in the gastrodermis of the diverticula (see diagram on Left). These pigmented vacuole cells are usually brown, and their colour is less dependent upon the type of food being eaten. 

The bright structural colours of blue, white, and yellow in Hermissenda are caused by diffraction of light passing through granules within the epidermis followed by differential absorption of the wavelengths emitted.  Apparently, the granules in different parts of the body differ in chemical composition, and this produces the different shades of blue and yellow. Where the granule cells are mixed, as in the piping along the tail, the resulting colour is white.  The blue piping in Hermissenda is quite striking, especially where it forms the 2 unusual rhomboidal outlines, one just posterior to the rhinophores, and the other outlining the heart/pericardium and in the centre part of the tail.  Bürgin 1965 Veliger 7: 205.

NOTE colours in H. crassicornis vary geographically, especially with respect to the extent of white at the tips of the cerata and blue in the pipings

Discussion of aposematic coloration in opisthobranchs is hampered by not knowing their predators and, thus, not knowing the extent to which vision plays a role in hunting and capture. One confirmed predator of nudibranchs, the cephalaspidean Navanax inermis, hunts by chemotactile means and has only tiny eyes with no visual resolution. Also, potential visual predators, like fishes and crustaceans, may use a portion of the ultraviolet part of the light spectrum to identify their prey. In short, what looks brightly coloured to our eyes may appear quite different to a crab or fish on the hunt for prey. Aeolid nudibranchs appear to our eyes to be more brightly coloured than dorids, yet aeolids lack the repertoire of defenses (skin chemistry, spicules, acid) present in many dorids. It would be interesting to know the proportion of dorids that do NOT contain secondary metabolites, but unfortunately there are no statistics kept on such negative data. One author proposes 4 criteria that must be met to justify a certain organism being considered to be aposematic.  It must be brightly coloured, be unpalatable to some predators, be avoided by some predators because of its colours, and be better protected than comparable cryptic species.  Edmunds 1966 J Linn Soc Lond (Zoology) 46: 27; see also Edmunds 1987 Amer Malacol Bull 5: 185 and Edmunds 1991 Malacologia 32: 241.

NOTE of crustaceans, only tropical mantid shrimps are known to see in ultraviolet

NOTE this same author, after reviewing protective mechanisms in aeolid nudibranchs up to 1966, concludes that there is "no evidence" for the presence of warning coloration in aeolids. However, more recently, other "nudibranchologists" have adopted a "brighter" outlook on the subject

Given that the secondarily acquired nematocysts in aeolids are defensive, and assuming that the bright colours of the aeolids are aposematic, the next question to ask is are there any mimics? That is, are there animals that are otherwise palatable but have evolved colour, shape, and behaviour to mimic those of a toxic model? These are known as Batesian mimics and, while their existence is well documented in insects such as butterflies and moths, they are essentially unknown among marine invertebrates.  The mimic is recognised by a potential predator as something not good to eat, and thus gains a measure of protection.  The next “order” of mimicry is Mullerian mimicry, where several unrelated but toxic prey organisms evolve similar warning signals, thus sending a common message to potential predators.  The best example from the terrestrial world is the evolution of orange/black/yellow coloration in wasps, ants, and lepidopterans (both larvae and adults), which sends a common “don’t eat” message to potential predators.  In simple terms, it makes it easier for the predator to remember what not to eat.  Keep in mind that the predators involved in terrestrial examples of Batesian and Mullerian mimicry are thought to be mainly birds, with high-resolution, good colour-vision eyes, and good memory.  As noted in Research Study 3 above, the only known predators of opisthobranchs are other “sightless” opisthobranchs that hunt by chemotactile means.  Those who write about mimicry in opisthobranchs understandably tend to be cautious, though optimistic. The problem, once again, is in devising experiments to demonstrate that a certain colour or pattern is: 1) functioning as warning, and 2) being copied by another organism.  A good review of world literature on the topic can be found in Gosliner & Behrens 1990 p. 127 In, Adaptive Coloration in Invertebrates (Wicksten, compiler) Texas A&M Univ Press; see also Gosliner 2001 Bollettino Malacologico, Roma 37: 163 for field tests of aposematic coloration and mimicry in species of Flabellina and Chromodoris in Papua New Guinea.

NOTE the several references in the literature to marine animals resembling one another, usually one with known toxic or otherwise unpalatable properties and one without (or at least not known), are placed in this "unknown" category until suitable experiments are done. Some of these pairings are quite convincing, as shown by the examples in Research Study 5 below; others, less so

On the west coast there are 2 striking examples of convergent colour patterns in nudibranchs and amphipod crustaceans that could represent Batesian mimicry. The first involves Flabellina trilineata and its remarkable look-alike amphipod Podocerus cristatus (35mm and 5mm in length, respectively) collected from Cape Arago, Oregon (see photos on Left). In this area both purported mimic and model inhabit the same rock overhangs and cobble areas. The similarity of the 2 organisms is quite striking, with the white antennae of the amphipod resembling the white cephalic tentacles and rhinophores of the nudibranch, and the red-orange pigment spots on Podocerus resembling the cerata clusters of Flabellina.

A second example of aeolid-amphipod look-alikes concerns Flabellina iodinea and the same amphipod Podocerus cristatus, this time from San Luis Obispo, California (photos on Right). Given that the amphipod is truly one and the same species, it is remarkable that selection favours one mimicking pattern in one area, and another pattern not so far distant. Some colours in crustaceans are skin pigments and fixed, while others are contain in chromatophores and are adjustable, so it would be interesting to know how morphologically labile are the pigments in Podocerus.

As most amphipods are edible to fishes, the scientist who assembled these photos at the site listed below suggests that the resemblance may indeed be an example of Batesian mimicry, where a palatable organism (the amphipod) gains protection from its predators by mimicking an inedible or repugnant organism (the nudibranch). All that is needed now is to find enough of each species to do some experiments with naive and experienced fishes. Goddard 1984 Shells & Sea Life 16: 220. Photos courtesy Jeff Goddard, Santa Barbara, California, Todd Huspeni, Mike Behrens, and seaslugforum.