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Defenses of nudibranchs and their relatives include nematocysts, considered in this section, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

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Research study 1

photograp of aeolid nudibranch Flabellina trilineatadrawing of representative gut system of an aeolid nudibranchMany 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

Each cnidosac is lined with a single-layered epithelium composed mainly of phagocytosing cells called drawing of a cross-sectional view of a cnidosac of an aeolid nudibranchcnidophages.  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). 

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. Kälker & Schmekel 1976 Zoomorph 86: 41.

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).  Cargo & Burnett 1982 Veliger 24: 32.

NOTE action and reaction being equal

Research study 2

photograph of opisthobranch Glaucus atlanticus
Unfortunately, the above "defensive" scenario has never been demonstrated in controlled experiments.  There are anecdotal reports of fishes shaking their heads after biting at the cerata of an intended prey aeolid nudibranch, but most or all such reports lack scientific rigour.  There is no question that the cnidosac-borne nematocysts can, in fact, sting (see the first "NOTE" in Research Study 1 above).  Swimmers in Australia are reported being stung by contact with swimming nudibranchs GlaucusGlaucus preys on Portuguese-man-of-war Physalia spp. and related siphonophores, and is known to sequester highly toxic nematocysts in its cnidosacs. The Glaucus observation is certainly convincing evidence for a defensive role for the nematocysts but, of course, how it functions in nature is not known. Thompson & Bennett 1969 Science 166: 1532.


Research study 3

drawing of a ceras of an aeolid nudibranch showing cnidosac and sphincter muscle
Within the cells of the cnidosac the nematocysts are contained within vacuoles.  Many nematocysts are contained in each cell.  On stimulation of a ceras, special muscles at the base of the cnidosac contract.  This ruptures the integrity of the cnidosac, and squeezes cells and nematocysts out at the top, either through a permanent hole, the cnidopore, or through a thin membrane covering the pore.  On contact with the outside environment the nematocysts discharge, but only if the cell containing them is itself ruptured.  Greenwood & Mariscal 1984 Tissue & Cell 16: 719.

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.  Greenwood & Mariscal 1984 Mar Biol 80: 35.

Research study 4

photograph of aeolid nudibranch Aeolidia papillosaA neat technique for sampling nematocysts released from aeolid cnidosacs is described for Aeolidia papillosa.  A glass microscope slide is coated in saliva, dried to tackiness, then gently pressed on the cerata, which have been caused to bristle by poking the animal.  The cnidosacs release their contents and the nematocysts discharge onto the saliva, allowing close examination of them at a later time, perhaps after staining or other treatment.  The author suggests that the mechanics of how the nematocysts discharge when this method is used supports the hypothesis that the primary function of the sequestered nematocysts is for defense.  Gaulin 1982 Veliger 25: 171.

NOTE  apparently stopcock grease works equally well

Research study 5

How does the sequestration of undischarged nematocysts work?  How can a nematocyst pass by the jaws and radula without discharging?  One often reads explanations that mucus secreted by the nudibranch inhibits discharge of the nematocysts.  However, 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. Greenwood & Mariscal 1984 Mar Biol 80: 35.

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

Research study 6

drawing of ceras of aeolid nudibranch showing % immature nematocysts in each one-thirdIf the above description is true, then one should find immature nematocysts within newly developing cnidosacs, and this is indeed the case.  Studies in Florida on Spurilla1 neopolitana in which cerata are experimentally removed from recently fed individuals reveal an initially higher proportion2 of immature nematocysts in the lower portions of the cnidosacs than in the upper portions (see diagram on Left).  This suggests a movement and a maturation within the cnidosacs from the digestive gland at the lower end to the cnidopore at the upper end.  Nematocysts typically mature in 2-4d and have a finite life span.  Presumably, the old non-functioning nematocysts3 are cast out or absorbed by the host. If Spurilla is starved for 3d, the number of immature nematocysts in their cnidosacs drops by one-fifth, from 150 to 30, suggestive of an interrupted “supply line”.  Greenwood & Mariscal 1984 Mar Biol 80: 35; Drawing from Kälker & Schmekel 1976 Zoomorph 86: 41 for Flabellina trilineata.

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

Research study 7
  photograph of an aeolid nudibranch Flabellina verrucosa

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:

1. when eating the hydroid Hydractinia echinata with 4 types of nematocysts, the dominant one in the cnidosacs after several days is a type known as a microbasic mastigophore.
2. when eating the hydroid Tubularia crocea with 4 types of nematocysts, the dominant one in the cnidosacs after several days is a type known as a stenotele (a toxic “killing” kind of nematocyst). 
3. when eating the hydroid Obelia geniculata that has a single type of nematocyst known as a basitrichous isorhiza, after several days this becomes the dominant one in the cnidosacs.

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. Day & Harris 1978 Veliger 21: 104.

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 study and Research Study 10 to follow have different ideas of the nematocyst complement in this hydroid species

Research study 8

photograph showing size disparity between an aeolid nudibranch Hermissenda crassicornis and a sea star Pycnopodia helianthoidesphotograph of sea star Crossaster papposusIn 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. 

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 allow identification of photograph of aeolid nudibranch Hermissenda crassicornis with missing ceratapredatory attacks in the field.  Miller & Miller & Byrne 2000 Invert Biol 119: 167.


Hermissenda crassicornis with damaged, missing, and
perhaps regenerating cerata, cause not known 3X

Research study 9

list of nematocyst found in the cnidosacs of Flabellina verrucosa with hydroid origins indicated
More recent studies on nematocyst complement in Flabellina verrucosa in the drawing of heterotrichous microbasic euryteles, the predominant nematocyst in the hydroid prey of a nudibranch Flabellina verrucosaGulf 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). 

Although only about 0.5% of Flabellina’s nematocysts come from scyphistomae (see green dots in table), these polyps are selected over any others in choice tests, perhaps because of the toxicity of their nematocysts. 

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.  Frick 2005 Mar Biol 147: 1313.

NOTE tunicates used are Botrylloides violacaeus and Didemnum sp.

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photograph of aeolid nudibranch Flabellina verrucosaSo, what have we learned from these studies?  What features of a nematocyst are important in its selection by a nudibranch, specifically, by Flabellina verrucosa? Consider these answers, then CLICK HERE for explanations.


Transport suitability. 


Predator type. 

Research study 10


histrogram showing uptake of nematocysts by the aeolid nudibranch Flabellin verrucosa in the presence or absence of different potential predatorsThe 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 nematocysts1 sequestered reflect what is available in the prey, as has been seen in other studies.  Even though the predators of Flabellina are not known, a question asked by a researcher in Maine2 is whether the presence of a potential predator might influence the type of nematocyst it sequesters.  The answer is ‘yes’, as the upstream presence of sea stars Crossaster papposus and cunner fishes Tautogolabrus adspersus (an Atlantic species) over a 2-wk period in laboratory experiments causes significantly increased uptake of highly toxic microbasic mastigophore3 nematocysts over control animals (see graph on Left). 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 this research idea is that in the absence of knowledge of natural predators of nudibranchs, why not let the nudibranch itself tell us which are its predators?  Frick 2003 Biol Bull 205: 367.

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 in part explains the inclusion of several east-coast studies in this section of the ODYSSEY; however, the main reason is to encourage west-coast scientists to research this interesting topic

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

Research study 11

Learned-avoidance behaviour in predators of nematocyst-bearing nudibranchs has not been well studied, mainly owing to lack of knowledge of who these predators might be. One exception to this last is the large notaspidean (side-gilled) opisthobranch Pleurobranchaea californica, which is known to attack and eat a number of different nudibranchs. Research at Hopkins Marine Station, California shows that contact of the big predator with the much smaller nematocyst-bearing aeolid Flabellina iodinea can elicit rapid aversive responses in the former, a behaviour quickly learned and remembered (experienced Pleurobranchaea show strong avoidance behaviour for several days after exposure). The aposematic learning response is not visual, for the predator lacks the capability for this; rather, it is a response to chemical odour of the prey. In support of this the authors note that some attacks stop prior to contact, suggesting response of the predator to water-borne stimuli, and this is confirmed in other experiments involving extracts of Flabellina. These observations add to a growing body of knowledge of associative learning, memory, and aposematic odour-aversions in opisthobranch molluscs. Noboa & Gillette 2013 J Exp Biol 216: 3231. Photograph series courtesy the authors. Photograph of F. iodinea courtesy Gary McDonald, Long Marine Laboratory, Santa Cruz, California and CalPhotos.

NOTE termed aposematic odour-learning. Much of the previous research on aposemetism, that is, warning to potential predators that attacking a certain prey might lead to unpleasant consequences, has been colour-based, as seen in the "advertisement" colours of toxic bees, wasps, ants, and butterflies, but odour-based (chemical) warnings are known from a few species to work in a similar way

photographs of the differing responses Pleurobranchaea responding to nematocyst stings from aeolid nudibranchs Flabellina iodinea and Hermassenda crassicornis californica
photograph of aeolid nudibranch Flabellina iodinea
Note the conspicuous size difference between predator and prey. The Left photo-pair shows aversive behaviour to F. iodinea. The Right photo-pair shows the same predator 20min later attacking and eating another nematocyst-defended aeolid, Hermissenda crassicornis. Not all naive predators show the same aversive responses to Flabellina; about half of the 40-or so Pleurobranchaea tested in the study simply attack and eat up the prey   Flabellina iodinea showing brightly coloured (to our eyes) cerata with tips containing nematocyst-filled sacs. Next to nothing is known about the how the sacs function in defense in these or other aeolid nudibranchs 3X
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