Nudibranchs, and opisthobranchs in general, have a repertoire of defenses unmatched in any other invertebrate group.  Our knowledge of predators of nudibranchs & their relatives is scanty, even though there are numerous isolated examples of predation.

Defenses include camouflage (crypsis), considered in this section, and
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,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

NOTE  one known predator of opisthobranchs is the cephalaspidean Navanax inermis

NOTE  lit. “away signal” G.  Brightly demarcated colour patterns, as the yellow and black markings on wasps, warn potential visual predators, such as birds (and humans), to stay away.  In toxic marine fishes, on which most research on marine warning colorations has been done, such aposematic markings are often blue or yellow – colours that transmit well through seawater.  Each colour pattern that warns of toxicity has to be learned anew by each naïve predator (juvenile?), and each encounter can result in a loss of an individual to the predator – but with the potential for long-term benefit to the species

There are many examples of crypsis, that is, camouflage mimicry, in opisthobranchs. One of the best examples among west-coast nudibranchs is that of Corambe steinbergae.  In colour, body form, and transparency it almost perfectly mimics its bryozoan prey Membranipora spp.  Another example of crypsis involves Phyllaplysia taylori.  On its usual habitat of blades of seagrasses and eelgrasses, the narrow body shape of Phyllaplysia and white striping blend in convincingly. No work has been done on the functional significance of this apparently defensive mimicry. Photograph courtesy Bill Rudman, Sea Slug Forum, Australia seaslugforum.

NOTE  this type of mimicry is also known, aptly, as “concealing imitation”

Egg ribbons of west-coast nudibranchs are frequently coloured.  For example, those of Doris montereyensis and Anisodoris nobilis are yellow; Rostanga pulchra, orange/red; Hopkinsia rosacea, rose; Diaulula sandiegensis and Triopha carpenteri, white; and Hermissenda crassicornis, white/pink.  Other than Rostanga, which often lays its eggs directly onto its prey sponges and which therefore may be camouflaging, the functions of other nudibranch egg-mass colours are not known. Costello 1938 J Morph 63: 319.

NOTE  derivation of food pigments and incorporation into egg masses is well known for anaspid sea hares Aplysia spp., but not so much for nudibranchs.  Egg masses in some aplysiids, most notably A. dactylomela in the Caribbean and Gulf of Mexico regions, and A. punctata in the north Atlantic/Mediterranean region take on subtle shades of colour from the algal pigments, and may function in camouflaging the eggs from predators

Rostanga pulchra mimics the colour of its prey sponge, such as Ophlitaspongia pennata, by taking up and sequestering carotenoid compounds in the same proportion as found in the sponge.  The carotenoid compounds are non-polar, that is, they are poorly soluble in water. For study they must be extracted with a solvent such as hexane.  The research question asked in an investigation at the Bamfield Marine Sciences Centre, British Columbia is whether the compounds that Rostanga sequesters are the same as those that attract them to their food.  The researchers use a Y-chamber to test the attractiveness of whole sponge O. pennata and 2 extracts¹, the first extracted in methanol only, and the second in methanol followed by hexane.  As noted, the second extract will have the non-polar fraction of the sponge extracts, including carotenoids.  Tests² of the non-polar extract in the Y-chamber result in 80% “correct” choices by Rostanga.   The authors are not certain if the attractant stimulus is a carotenoid compound, but consider it likely.  If so, it is the first report of a non-polar attractant³ for a carnivorous gastropod, and indicates extreme sensitivity to detect at a distance the non-soluble compound(s) it sequesters from its prey.  Ong et al. 2001 Veliger 44: 99.

NOTE¹  the extracts are set in agar blocks to allow slow diffusion

NOTE²  a test individual is considered to have made a choice if it enters an arm of the Y-chamber within 30min.  Some individuals are multiply used in the experiments, so the results should be interpreted with this in mind

NOTE³  attractants used in studies to date are polar compounds with high solubility in seawater; hence, diffusing over a distance