Part 3 – Defensive Attributes
In the Aeolidina, (the aeolids) the cerata have certainly reached the apogee of development, therefore most of the discussion in this Part will pertain to that group. Defences in the aeolids are concentrated in the cerata and normally at the ceratal tips. This is to be expected as the cerata, especially the tips, are the first to be encountered by, and most easily presented to, a predator. The presentation of the cerata and the behaviour adopted by the nudibranch ensures that the attention of any predator is diverted away from the head and vital viscera towards the defence laden and “expendable” cerata. There are several mechanisms through which cerata are utilised for defensive purposes.
Cerata function defensively:
– by being sacrificed through the process of autotomy;
– through sudden erection towards a threat as a startle or deimatic display but by also directing the cerata tips towards that threat;
– by possessing or actively exuding defensive toxic or antifeedant chemicals;
– by mimicking through structure and/or colour their host;
– possibly by Mullerian mimicry creating an aposematic group within their habitat;
– and, most remarkably, sequestering, storing and discharging when threatened, stinging cells obtained from their cnidarian prey.
Cerata can be autotomised, that is, deliberately cast off, if the nudibranch is aggravated, and subsequently regenerated. Different species have different thresholds, some casting them off much more readily than others. However it has been recorded that once a high threshold has been reached further cerata are more readily cast off. Also, different species are known to respond not only to the degree of the stimulus but also to the type of stimuli. For example, some species react more readily to pinching of cerata while others to pulling. This is thought to be associated with the type of predator most commonly encountered by that species such as fish – mouthing/pulling, or crabs – pinching. The cerata are cast off at a fracture line called the autotomy plane, located at the base of each ceras and which is quickly sealed off by specialised sphincter musculature to minimise damage. (Read the NudiNote: Autotomy – The Self Sacrifice Defence that includes details on how this is achieved.) If a ceras is seized by a predator the nudibranch can cast it off leaving the predator holding just the ceras, while it makes its escape. Alternatively if the nudibranch is attacked or otherwise threatened it can cast off cerata that will then wriggle vigorously for sometime, up to several hours in some species, creating a diversion and acting as a decoy that is more attractive than the retreating nudibranch. The detached cerata of some species, e.g. the Phyllodesmium, also produce a sticky epithelial secretion of an acidic nature. This causes the now detached cerata to attach quite tenaciously, either to the surrounding substrate rather than drift off in the current or surge, or even attach directly on to the predator where the acidic nature of the secretion acts as a further deterrent.
The cerata are without doubt an essential, though not primary, component for the survival of the aeolid nudibranch so it is interesting that they can be sacrificed so readily in some species. Factors mitigating their loss include: the loss is not immediately life threatening, the wound is quickly sealed to prevent undue fluid loss or ingress of pathogens, there are numerous cerata so the loss of several is not catastrophic and they are regenerated fairly quickly (One studied species: fully functional in ~24 days, fully matured so as to be indistinguishable from others in ~40 days). It is not unusual to see aeolids with regenerating cerata at various stages of regrowth (personal observation). As a renewable defence item there is no doubt that there is an energy cost to be considered in the replacement of the autotomised cerata however it is probably worth comparing that cost to the energy outlaid producing and maintaining a shell and operculum, especially considering the additional benefits cerata provide. In most species the autotomy of cerata is a last resort strategy.
– Deimatic display:
Some aeolids, when threatened, will erect and bristle their cerata towards the perceived threat. This sudden erection of the cerata is a startle display known as a form of deimatic behaviour. This display is often enhanced by the bold colouration and its pattern possessed by some. Additionally it serves to concentrate the ceratal tips where the stinging cells are held (see below), especially as they are mustered toward the aggressor. The bristling/mustering action that follows the initial startling is a form of aggressive defence.
– Nematocyst defence:
The aeolid nudibranchs have evolved an astounding defensive attribute whereby they sequester the nematocysts (stinging cells), a type of cnidocyst, of their cnidarian prey, translocate some of them undischarged through their gut to the tips of their cerata wherein they are stored, within a special sac – the cnidosac, from where they may be released, as required, for their own defensive use. At this point the nematocysts are referred to as kleptocnides (kleptocnidae). This is a unique capability of the Aeolidina, not found anywhere else in the Mollusca, apart from the Hancockia genus, of the Dendronotina group of nudibranchs, wherein it is believed to have evolved separately. Confusion also exists around whether the Dendronotina species Embletonia gracilis possesses cnidosacs and/or stinging cells due to conflicting reports from taxonomists. (Some flatworms have also been reported to store sequestered cnidocysts in cells on their dorsal surface.)
Depending upon the source consulted cnidarians possess a number of different types of cnidocysts (~30) and to generalise, the nematocysts or penetrating stinging variety are just one, but also the most diverse, of three major types and the one that concerns us here. The others fall into two broad categories – sticky types called ptychocysts and coiling types called spirocysts. These last two are of no use to the nudibranch being employed by cnidarians to form attachment to the substrate or maintain retention of a captured item of prey. A nematocyst is a fine thread-like tubule with barbs. It is coiled up within a capsule and is actually stored inside out at that point. When fired it evaginates penetrating its target, delivering a dose of toxic chemicals (usually stinging to us) and remains attached to the target by the barbs located at the base and sometimes even along the length of the tubule.
The detail of nematocyst uptake, transport, storage as kleptocnides and deployment in defence will be the subject of another NudiNote but a quick outline will be provided here. By some method of differentiation the aeolid nudibranchs are able to select the type of nematocysts and their level of maturity suitable for their future intended purpose from the ingested food. Additionally, some research has indicated that certain aeolids are capable of altering, from time to time, the type of nematocyst they sequester through their diet, in response to the type of threat (chemical based) they detect in the region. Ingested as part of their diet, but undischarged, nematocysts are transported to the cnidosac located at the tip of each ceras at the terminus of the digestive diverticulum. Here they are engulfed by cnidophage cells within which they are nurtured and mature and are now termed kleptocnides. The kleptocnides discharge on contact with sea water when squeezed out of the cnidosac via the cnidopore when the cnidosac is stimulated to contract. The “cloud” of discharging kleptocnides engaging an aggressor provides an effective deterrent to some predators.
In three Aeolidina genera in particular, Favorinus, Phyllodesmium and “Phestilla” the cnidosacs are present but not functional, that is, they lack functional kleptocnide content. This is thought to be directly related to their diet. Favorinus prey on sea slug eggs so there is no opportunity to sequester nematocysts and Phyllodesmium prey upon octocorals, that although possessing nematocysts, are not thought to be sufficiently potent to employ in defence by those sea slugs. (Phyllodesmium jakobsenae is the exception but interestingly the kleptocnides have only been recorded in the smaller posterior-situated cerata.) This low potency of prey nematocysts might be a similar situation with “Phestilla” that prey on hard corals such as Porites, Pavona and dendronphyllids such as Tubastraea. In another two genera, the monospecific Fiona preying on gooseneck barnacles and the small genusTergipes rasping at the exposed polyp tissue of hydroids, the cnidosac itself is absent. The small Bulbaeolidia alba is unusual too in being reported to have a cnidosac but lacking not only kleptocnides but also lacking an a cnidopore, or exit from the cnidosac to the exterior. Diet therefore can be seen to play a major role in determining the defensive methods employed.
Not all the Aeolidina species have had their cnidosacs examined in-depth to ascertain their structure, however their variability, even within families, is well known.
– Glandular defence:
A number of different types of glands have been identified in the epidermal and sub-epidermal layers of aeolid cerata. Secretion, depending upon the gland, may be continual or by a larger amount promptly and specifically upon stimulation. Some are doubtless of a defensive nature and it is conjectured that they may work in unison. Others may help to reduce abrasion and also provide a continual “every day” cleansing function keeping the surface of the cerata free of harmful bacteria, chemicals and undesirable settling epibionts. Across the aeolid families there is much variation in quantity of glands and therefore their importance in defence. As previously mentioned most of the Phyllodesmium possess a cnidosac but lack the kleptocnides therein. It is thought that in lieu of sequestration of nematocysts their defence is focused upon the accumulation and concentration of secondary metabolites from their octocoral prey (the only aeolid family with this food source) that are known themselves to harbour antifeedant toxins. A change in diet has brought about a change in defensive method. The uptake of these chemicals is believed to provide to the sea slug not only an antifouling but also antifeedant defensive properties. (In the research, the actual extraction of the secondary metabolites was made via “crude extract” and not from any identifiable glands, meaning those chemicals could be in the body tissues themselves.) Investigations on Phyllodesmium guamensis have revealed that the highest concentrations are to be found in the cerata with moderate to nonexistent concentrations in the mantle and internal organs. Concentration refers to a degree of magnitude above the level found in the octocoral prey.
– Mimicry and crypsis:
While some aeolids seemingly advertise their presence and use bright colours and patterning on their cerata to startle potential predators (deimatic display) many others adopt a different approach by having cerata that exhibit colours that match their prey or, at the least, have transparent cerata that allow the colour of the recently consumed prey, in the digestive diverticulum, to be revealed, thereby matching their background. Even the colour of the symbiotic zooxanthellae obtained from prey and farmed in the cerata (see Part 2) helps match the background – in Phyllodesmium especially.
Some Phyllodesmium in particular have developed incredible mimicry of their host octocorals by developing cerata that have an uncanny appearance to the tentacles of their host. Not only do they match in colouration but the shape and size too, are most similar. Phyllodesmium rudmani has taken this structural mimicry to the highest level such that only for the fact that the tips of the cerata do not rhythmically open and close in the same manner as their feeding Xenia octocoral prey they would be absolutely indistinguishable from it. As it is, even when specifically searching for this species in amongst the host’s tentacles, if the host is not feeding then the only give-away is the different appearance of the rhinophores.
Although it has not generally been thought that the bright colours possessed of some upon their cerata are intended to act as a warning when in a passive state, at least one study (on Cratena peregrina) has drawn the conclusion that aposematism is at work, through Mullerian mimicry, among groups of aeolids exhibiting similarly coloured cerata.
– Cerata defence in non-aeolids
Some cladobranch nudibranchs other than aeolids – Hancockia and Embletonia have already been mentioned above in relation to cnidosacs. But there are others that autotomize their cerata as a defensive strategy including: Melibe, Tethys, Doto and Janolus (but not in Caldukia the close relative of Janolus) for example.
In the Sacoglossa the uptake of chloroplasts from their algal diet causes the cerata of some to take on the green colour of the food source making them quite cryptic in situ. Others however are boldly coloured and patterned and this may serve as a warning to potential predators that their cerata contain glands that release toxic/distasteful chemicals derived or synthesised through their diet. Species of Cyerce, for example, possess defensive glands sometimes along the ceratal edges and visible on the ceratal faces of others.
A number of Sacoglossa species autotomize their cerata when distressed including: some Mourgona, Sohgenia, Cyerce and Polybranchia of the Caliphyllidae family, some Costasiella, Ercolania and Placida of the Limapontiidae family and Hermaea and Aplysiopsis of the Hermaeidae family. Those that do autotomize their cerata often do so with copious amounts of mucus that is laden with defensive toxins.
So it can be seen that the importance of cerata in the defence of certain sea slugs cannot be overstated and while a particular species may not employ all of the above mentioned functions there is no doubt that several are usually used in unison to effect an avoidance response in many predators. However no defensive method can be considered absolute i.e. preventing predation completely. They serve but to mitigate the frequency or severity of predation on individuals or the species as a whole. The specialist predator will always have developed means to overcome or circumvent a specific type of defence.
(Note: where an sp. number is used in this NudiNote it refers to a species shown on this site.)
David A. Mullins – March 2022
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