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| Defenses | |||
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| Nematocysts | |||
Defenses of nudibranchs and their relatives include nematocysts, considered in this section, and |
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Research study 1 |
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Each cnidosac is lined with a single-layered epithelium composed mainly of When Flabellina and other nematocyst-bearing aeolids are disturbed or irritated, as by a potential predator, they curl their bodies and bristle their cerata. Special muscles surrounding the cnidosacs squeeze the nematocysts from their sacs and enclosing cnidophage cells, and force them out a cnidopore at the tip of the ceras. Once they contact seawater they discharge. In theory, some of the nematocysts penetrate the predator and it withdraws. Presumably, those nematocysts that penetrate are ones that are “backed up” against the cnidosac or the discharged cell mass; otherwise, a free-floating capsule would likely discharge its thread ineffectually. NOTE bristling makes it difficult for a potential predator to avoid touching the cnidopore ends of the cerata. Studies on cnidosac discharge in an east-coast nudibranch that specialises in eating the polyp stages of jellyfishes show that release of nematocysts from a cnidosac is in bunches, thus allowing for second, and perhaps more, discharges. The scientists assess the potential defensive efficacy of these discharges by touching the cerata to the soft parts of their own lips (WARNING: exercise caution when using your own tongue and lips to assess the stinging potential of nematocysts, especially those from hydroids. Even west-coast anemone species that can be touched by the finger with impunity can produce long-lasting and potentially serious allergic responses if touched to more sensitive skin areas). NOTE action and reaction being equal |
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Research study 2 |
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Research study 3 |
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NOTE in a typical aeolid nudibranch there are about 3000 nematocysts within each cnidosac. So, with a total of 100 cerata, there would be some 300,000 functional nematocysts for the nudibranch to use in its own defense. |
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Research study 4 |
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NOTE apparently stopcock grease works equally well |
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Research study 5 |
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How does the sequestration of undischarged nematocysts work? How can a nematocyst pass by the jaws and radula without discharging? One often-read explanation is that mucus secreted by the nudibranch inhibits discharge of the nematocysts. As nematocysts take a few days to develop, another idea is that the functionally mature ones discharge when the snail eats them, while the functionally immature ones are moved into the digestive-gland diverticula and then to the cnidosacs. Development of the nematocysts is then completed within the cnidosacs. Feces of aeolids are usually filled with discharged nematocysts. The means by which a host would differentiate between discharged and undischarged capsules in the stomach or digestive gland is not known. NOTE the gut lining of nematocyst-eating nudibranchs typically is well vacuolated, that is, containing balloon-like inclusions. Another possibility, then, is that these vacuolations provide insulation against the stings |
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Research study 6 |
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NOTE1 an anemone-eating aeolid with some features in common with the west-coast Aeolidiella chromosoma. A typical cnidosac in Spurilla contains over 3000 nematocysts. The study is included here with the hope that someone will do a similar study on a west-coast aeolid NOTE2 a point not discussed by the authors is that the proportions listed here don’t seem high enough to account for a steady movement of nematocysts through the cnidosac via a casting out or absorption as they senesce. For example, if a nematocyst matures in 2-4d and has a life span in the cnidosac of, say, 10d, then wouldn’t proportions from base to tip be more likely to be in the ranges of 40, 10, and 1%? NOTE3 these ideas contrast an earlier theory that non-functioning and/or senescent nematocysts are disposed of by the host voluntarily casting off its cerata. Ceratal autotomy does indeed happen, but whether this is its function is not known. It would seem a wasteful strategy, especially in view of the cycling option noted here |
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Research study 7 |
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One of the earliest studies on nematocyst cycling and selection of specific nematocysts to sequester involves Flabellina (Coryphella) verrucosa eating hydroids in New Hampshire. This species is cosmopolitan in distribution and also inhabits the west coast, so this study is included here just to stimulate research interest. In their study, the authors collect Flabellina, examine what kinds of nematocysts they have in their cnidosacs, and later divide their collection into 3 feeding groups. The examination shows that the dominant nematocyst at the time of collection is a kind known as a microbasic eurytele. When given 3 different hydroids to eat in separate experiments, the researchers find:
Thus, in all 3 treatment groups the originally dominant nematocyst type, the microbasic eurytele, is replaced by a different type. The authors note that several hydroid species are present in the collection area that could have been eaten by the nudibranchs, so they must have been specialising on one or more species that possess this type of nematocyst. The authors conclude by commenting that nematocysts may be selected on bases of toxicity or how readily they lend themselves to the mechanics of transport through the gut. NOTE the authors also note that Aeolidia papillosa eating the plumose anemone Metridium senile selectively sequesters 2 types of nematocysts from 6 types available NOTE the authors of this Research Study and the one to follow have different ideas of the nematocyst complement in this hydroid species |
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Research study 8 |
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The authors note a prevalence of what appears to be predator-induced damage to cerata in one of their study populations of Hermissenda, so identification of the modus operendi of various “cerata-attacking” species in the laboratory could lead to better identification of Hermissenda crassicornis with damaged, missing, and |
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Research study 9 |
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Considerable variability exists in nematocyst complement in F. verrucosa from different geographical areas and, when switched to different diets, Flabellina’s nematocyst complement changes to match that of the new diet within 2wk. For example, within 2wk on a diet of scyphistomae, over 96% of the nematocyst complement is made up the 2 nematocyst types common in that prey (holotrichous isorhizas and heterotrichous microbasic rhopaloid heteronemes). On a diet of tunicates, which contains no nematocysts, the cnidosacs of Flabellina still retain some hydroid-derived nematocysts after 2wk. Although there is no supporting evidence, the author suggests that the quick turnover of nematocysts with change in diet in Flabellina may give it the potential to “arm” itself quickly against specific predators. |
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In contrast, upstream presence of green crabs Carcinus maenas does not influence uptake. An ability to modify the nematocyst complement in its cnidosac would theoretically enable a nudibranch to adapt its defensive weaponry to combat predators specific to an area in which it lives. Are the sea stars and fishes tested really predators of Flabellina? This we may never know, but the research question addressed is unique and the results certainly point to a need for further work on other species, especially west-coast ones. The interesting thing about the research approach is that in the absence of knowledge of natural predators of nudibranchs, why not let the nudibranch itself tell us which are its predators? NOTE1 according to the author the nematocyst complements in the two genera of hydroids are mutually exclusive, and thus present a range of nematocysts (at least 9 different types) from which Flabellina can choose NOTE2 Flabellina verrucosa has a circumboreal distribution, and is found in the north Pacific and north Atlantic regions. This explains the inclusion of several east-coast studies in this section of the ODYSSEY NOTE3 the author provides uptake data for all 9 nematocyst types. Of these, only microbasic mastigophores differ significantly between control and experimental treatments, and then only for 2 of the 3 potential predators tested. Two other nematocyst types show slight but significant reductions in the treatment group; all other pairings are not significantly different |
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Many species of aeolid nudibranchs, including Flabellina trilineata shown in the photograph, sequester undischarged nematocysts, apparently for use in their own defense. The nematocysts are housed in special sacs or cnidosacs located in the tips of the cerata. 
phagocytosing cells called cnidophages. Nematocysts that are eaten are sorted in the stomach and shunted into the diverticlua of the digestive gland that extend into the cerata. From there they pass through a narrow canal that opens into each cnidosac. On entry into the lumen of the cnidosac the nematocysts are phagocytosed by the cnidophages. Within the cnidophages the nematocysts may lie parallel with one another, or be arranged in circles or bouquets depending upon the species (drawing below Right). 
A neat technique for sampling nematocysts released from aeolid cnidosacs is described for Aeolidia papillosa. A glass microscope slide is coated in 
If the above description is accurate, then one should find immature nematocysts within newly developing cnidosacs, and this is indeed the case. Studies in Florida on 
In tests at Bamfield Marine Sciences Centre, British Columbia of apossible defensive roles of nematocysts in aeolid nudibranchs, the cerata of Hermissenda crassicornis are placed in contact with sea stars Crossaster papposus and Pycnopodia helianthoides. Although discharged nematocysts could be seen in the tube feet of Crossaster, the authors conclude that the nematocyst do not play a major defensive role against these and other potential predators (also tested are hermit crabs and sculpins). Rather, they suggest that the cnidosacs may function primarily as a storage device for safely sequestering nematocysts that could damage the digestive system, with ceratal autotomy being used to rid the body of these unwanted nematocysts. This reiteration of an old idea for the function of the cnidosacs underscores the need for further research. 
predatory attacks in the field. 
Gulf of Maine show that while it is a generalist predator of hydroids, it also eats jellyfish scyphistomae. About 70% of Flabellina’s nematocysts come from the hydroids Tubularia crocea and Eudendrium spp. Most of these are of a type known as heterotrichous microbasic euryteles, which are predominant in these species (see drawing on Left). 
The aeolid nudibranch Flabellina verrucosa primarily eats hydroids and jellyfish scyphistomae, from which it extracts nematocysts for incorporation into its cnidosac. If fed on hydroids Tubularia spp. and Obelia geniculata the