Research Study 1

Fig. 1.  Note that the species name should read "senile"

Fig. 2.  Aeolidia papillosa investigates the column of a sea anemone

How do Aeolidia papillosa (Figs. 1 - 2) 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 four 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 four species as noted 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. In contrast, the other two nudibranch species Hermissenda crassicornis and Cadlina luteomarginata are stung by the anemone, and eventually captured and eaten. The shelled Pomaulax gibberosa 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?

NOTE the technique is imprecise and the authors discuss the statistical problems inherent in their methodology, yet the results are interesting and may stimulate further investigation

Research Study 2

Later investigation on the idea of chemical camouflage, using Aeolidia papillosa and different sea anemones, shows 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 shown in CONTROLS² (see Fig. 1).  Similarly, when Aeolidia feeds on Urticina for several days, nematocyst discharge into the mucous probe is reduced by almost 70% over that into the CONTROL probes (Fig. 2). 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 (Fig. 3). 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 certain chemicals³ from its prey is not known, nor is the identity of the chemical(s) known. 

NOTE¹ the study is done in Maine. Anemone species used in the study include Metridium senile and a species of Urticina  not found on the west coast. A third species used, 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 discharging into the probes is 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, and also in the replicatablity of the probe device (is this even tested?). Also, to remove some of the variability, perhaps a "before and after" test procedure could have been used. Sample  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.  The results are so interesting that it would repay the effort to repeat the experiments using west-coast species, perhaps taking special care to do them with greater statistical rigour

NOTE³ if the snail is fed simultaneously on two different species of sea anemones, its mucus will inhibit discharge from both species

Fig. 1.  Effect of a diet of Metridium on subsequent nematocyst discharge into probes coated in mucus from Aeolidia papillosa, compared with that into probes coated in mucus from Urticina, not eaten by Aeolidia prior to the tests

Fig.  Fig. 1.  Effect of a diet of Urticina on subsequent nematocyst discharge into probes coated in mucus from Aeolidia papillosa, compared with that into probes coated in mucus from Metridium, not eaten by Aeolidia prior to the tests

Fig. 3.  Effect of diet switching on nematocyst discharge into probes coated in mucus from Aeolidia papillosa