Defenses of nudibranchs and their relatives include mucous coatings, considered in this section, and
CAMOUFLAGE (CRYPSIS),
FAST CRAWLING & SWIMMING,
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.  

How does Aeolidia papillosa and other cnidaria-eating nudibranchs avoid being stung by their prey’s nematocysts?  For over a century scientists have believed the snail’s own mucus to be an inhibitory agent, but the presence of other substances that prevent nematocyst discharge cannot be discounted, and it may also be that the nudibranchs are immune to the toxin in their prey’s nematocysts.  Preliminary evidence for the nematocyst-inhibition idea is provided in a study in Barkley Sound, British Columbia where mucus from the anemone-eating Aeolidia papillosa  is found to cause fewer nematocysts to discharge from the potential prey species Anthopleura elegantissima than mucus from other species of non-anemone-eating snails.  The technique involves touching clean glass cover-slips to each of 4 test snail species, Aeolidia papillosa (A. elegantissima is a favoured prey), Hermissenda crassicornis (eats hydroids), Cadlina luteomarginata (eats sponges), and Pomaulax (Astraea) gibberosa (eats algae), and then pressing them to the tentacles of A. elegantissima for 3sec.  A clean (non-mucus covered) cover slip, also touched to the tentacles, is used as a control.  Counts of number of discharged nematocysts clinging to the cover-slips show that Aeolidia mucus elicits significantly fewer nematocysts to discharge, as compared with the other species.  The study is notable in that it seems to answer the question as to whether the mucus inhibits discharge of the nematocysts or whether the nudibranch is just immune to the stings.

In other feeding encounters with the same 4 species as noted in Research Study 1 above, Aeolidia papillosa appears at first to be stung on contact with the tentacles of Anthopleura elegantissima then, after momentarily retreating, it returns to attack and eat the sea anemone.  The other 2 nudibranch species Hermissenda crassicornis and Cadlina luteomarginata are stung by the anemone, and eventually captured and eaten.  The shelled Pomaulax (Astraea) gibber-osa withdraws into its shell on contact with the anemone's nematocysts.  On the basis of Aeolidia’s behaviour, the authors speculate as to whether an “acclimation” period is necessary, such as is thought to occur with some species of anemonefishes.  Thus, does Aeolidia modify the chemical nature of its mucus after being initially stung in order to continue its attack later without being stung? Mauch & Elliott 1997 Veliger 40: 148.

NOTE the technique is not precise and the authors discuss the statistical problems inherent in their methodology, yet the results are interesting and should stimulate further investigations

Aeolidia papillosa investigaates the column of a sea anemone 1.3X

More recent studies on this idea of chemical camouflage, using Aeolidia papillosa and different sea anemones, show that the predator is able to modify its mucus to be more effective against different prey.  The experiment involves Aeolidia being allowed to feed on different sea anemones¹ for several days, then testing the effectiveness of its mucus in stimulating nematocyst discharge on the prey's tentacles.  When the predator eats Metridium senile for several days its mucus elicits 50% less nematocyst
discharge than in controls² (see graph on Left).

Similarly, when Aeolidia feeds on Urticina for several days, nematocyst discharge into this anemone's mucus is reduced by almost 70% over that into the CONTROL probes (see graph on Right). Note in both experiments that tests of Aeolidia's mucus on anemones that the predator is NOT eating lead to no significant difference in discharge between test and control probes.

Finally, if Aeolidia is allowed to feed on Urticina for several days and then switched to Metridium for a further 2wk, tests of its mucus against both prey show that nematocyst discharge by Metridium becomes relatively less, while that by Urticina increases (see graph on lower Left). Within 10-15d, discharge in Metridium is almost completely inhibited, while that in Urticina is about 5-fold greater than at Day 0 when the switch is made.  Whether the snail chemically modifies the mucus itself, or whether it incorporates chemicals³ from its prey is not known, nor is the identity of the chemical(s) known. Greenwood et al. 2004 Biol Bull 206: 113.

NOTE¹ the study is done in Maine. Anemone species used in the study include Metridium senile and a species of Urticina (felina) not found on the west coast. A third species, Aulactinia stella, also not found on the west coast, is not included here

NOTE² the test probes consist of short lengths of monofilament line either coated in gelatin (a protein = CONTROL) or in Aeolidia mucus (the probe is rubbed over the snail’s back to coat it with mucus).  The number of nematocysts dicharging into the probes are compared to assess the inhibitory effect, if any, of the snail’s mucus. The protocol is not perfect, as differences likely exist in the quantity of gelatin and mucus on the probes. Also, to remove some of the variability, perhaps a "before and after" test procedure could have been used. N numbers are also small in some of the tests (e.g., 2-3), possibly risking acceptance of the null hypothesis when it may have been rejected were sample sizes to have been larger

NOTE³  if the snail if fed 2 different species of sea anemones, its mucus will inhibit discharge from both species