Defenses of nudibranchs and their relatives include Navanax: a special case study, 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
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, considered in other sections. 

The large (>20cm in length) cephalaspidean Navanax inermis in southern California is a specialist predator on other opisthobranchs and is catholic in its choice of prey.  In the field, Navanax appears to locate its prey by following their mucous trails, rather than using distance chemoreception.  In laboratory tests Navanax eats many species, but does reject a few.  As such, it presents a unique opportunity for assessing the effectiveness of nudibranch defenses.  The study is special in that it takes such a broad approach and provides so many interesting research ideas. The results of the study are presented below, along with possibly defenses of the nudibranch prey. Paine 1963 Veliger 6: 1. Photo courtesy Kevin Lee, Fullerton, California diverkevin.

A Navanax inermis seems to be about to contact
and catch a nudibranch Cadlina flavomaculata, but
itactually bypasses the Cadlina and moves on.
The inset shows a close-up of Navanax's head.

Potential defenses of nudibranchs examined in the study:

Nematocysts:  of 8 species of nematocyst-bearing aeolids offered in laboratory tests to Navanax, the predator eats all but Flabellina iodinea (photo on Right).  Since nematocysts in the other species seem to offer little defense against the predator, the author concludes that some other feature of Flabellina iodinea¹ is acting as a deterrent.

Spicules:  24 species of dorids are offered to Navanax, and whether they are eaten or not seems to depend upon their dry mass-ratio².  Thus, those species with about 90% or greater water content are eaten, while those spcies with 70-80% water content are not eaten.  Navanax swallows its prey whole and the higher body rigidity imparted by a greater load of spicules may interfere with this ingestive process.  However, Navanax seem to turn away before contacting some of these spicule-rich species, suggesting to the author that other aversive factors³ may be involved. 

Acid secretion: tests with litmus paper applied to the skin show that 3 species of potential opisthobranch prey (Pleurobranchaea sp., Rictaxis punctocaelatus, and one unidentified species) have acid secretions (pH 1-3).  On contact with them, Navanax withdraws and eats none of the three, suggesting that acid is an effective deterrent.  Paine 1963 Veliger 6: 1. Photo of Flabellina iodinea courtesy Todd Huspeni and Jeff Goddard, California; photo of Pleurobranchaea californica courtesy Kevin Lee, Fullerton, California diverkevin; photo of Rictaxis punctocaelatus courtesy Jeff Goddard, Santa Barbara, California.

NOTE¹  this same species appears to have a Batesian mimic: an amphipod.  For more information on this topic, go to DEFENSES: APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY

NOTE²  the ratio of dry to live mass gives a rough estimate of spicule content. These observations for Navanax differ somewhat from those reported for other non-opisthobranch predators. For information on this go to DEFENSES: NUTRITIONAL CONTENT

NOTE³  another possibility that may come to mind is that the predator has learned from previous experience to avoid a certain spicule-rich species and turns away when it perceives its scent; however, the author demonstrates in the same paper that Navanax recognises its prey only by contact, not by scent. For information on this go to FOODS, FEEDING, & GROWTH: PREFERRED FOODS, FEEDING ECOLOGY, & GROWTH: NAVANAX