“The Sting’s the Thing”
Sea Slug Predators of Anemones and Hydroids
Have you ever inadvertently brushed past a stinging hydroid during a dive and some small part of your skin, being exposed, receives a searing sting? Or perhaps you have been enjoying the surf and a bluebottle’s tentacles have wrapped around your arm or chest and put a sudden and painful end to the day’s activities? It certainly beggars belief then, to learn, that there are animals that actually feed upon these, and related tormenting denizens of our marine world, ingesting those polyp’s stinging cells and all seemingly without repercussion. The tormentors are cnidarians and they belong to a large group that range from jellyfish to the reef-building corals.
The sea slugs that prey upon soft and hard corals have been discussed in a previous NudiNote (see The Coral Eaters). There are other animal groups, also belonging to the cnidarians (Phylum Cnidaria), that sea slugs prey upon, including anemones, hydroids and siphonophores, that are discussed here.
Cnidarians and Nematocysts
A characteristic defining feature of all the cnidarians is the possession of cnidocytes. These are specialised single use cells that are chiefly concerned with: catching prey, or as a defence against predators, or even defending territory. They operate by forcefully ejecting a cnidocyst or nematocyst. Across the phylum there are approximately 30 types of nematocysts grouped into three main categories – penetrating, sticky and coiling. Those nematocysts that penetrate are hollow, usually barbed, and inject a toxin to immobilise their prey. These are the stinging cells. These nematocysts, according to the species that possess them, have a range of penetration power and also toxicity, the toxicity of course being dependent on the target’s susceptibility.
Sea Slug Cnidarian Predators
Almost all of the sea slugs that prey on cnidarians are cladobranch nudibranchs. Not all Cladobranchia however feed on cnidarians but the greater majority do. None of the Dorididea (dorid) nudibranchs feed on cnidarians. There is evidence that some species of the Pleurobranchoidea e.g. Pleurobranchaea spp, feed upon cnidarians. These sea slugs are generalist foragers not specialist feeders though, with the cnidarians being just one (but significant) of the wide range of animal types found within their stomach upon investigation.
Within the Cladobranchia:
The Aeolidina nudibranchs, all feed upon cnidarians of one type or another (except the oophagous Favorinus species) and we have previously dealt with those members preying on corals;
Most of the Dendronotina too, are cnidarian predators and we have also discussed those members that prey on corals;
Above: Bornella anguilla eating hydroids. At one point the jaws of the nudibranch can be seen to be working upon the polyps.
Above: This video clip shows the jaw action of Bornella anguilla, gripping the hydroid branches and dragging them into the mouth for stripping by the radula.
Of the remaining group, the Arminina, we have discussed those members that feed on corals and sea pens and none of the remaining prey on other cnidarians.
As to be expected, those that prey on anemones and hydroids need to have developed ways and means to avoid the potential harm, that crawling over and ingesting animals that possess such powerful toxic stinging cells, would engender.
– Those nudibranchs that prey on hydroids have a narrow body and foot that enables them to grip and move along the polyp-free stem or on the opposite side to the polyps on a branch, a combination of anatomy and behaviour that serves to minimise contact with those surfaces that have the reactive stinging cells.
– Conversely those, like the Cerberilla, that burrow into the sand hunting for the likes of cerianthids (tube anemones) have a broad foot for that purpose and long oral tentacles for tactile sensing, but smallish rhinophores for water sampling.
– The buccal cavity and oesophagus of many nudibranchs that prey upon cnidarians have a hard cuticle covering for protection of the underlying epithelium.
– Mucus secretions not only assist the movement of food particles through the digestive system but can also act as a barrier against epithelial contact by essentially just smothering the nematocysts. It has been shown that the nudibranch’s mucus can also be augmented with compounds that are able to inhibit or reduce nematocyst discharge through a process of acclimation to a specific prey’s nematocysts. So if the targeted prey changes so too does the inhibitory chemistry of the mucus.
– Although many nudibranchs are in possession of them some aeolids in particular have a high concentration of intracellular ovoid discs, called spindles, within their outer skin and stomach epithelium. Upon receiving a discharge of nematocysts the spindles, composed of “granular” chitin, are released entangling the nematocyst’s tubules thus impeding their progress and reducing their effect. This is analogous to the reactive armour on modern military tanks that explodes outwards upon detecting an incoming projectile. Once again, nature is way ahead of us.
Nearly all of the Aeolidina and many of the Dendronotina use their jaws to slice or crop off segments of their cnidarian prey, the radula being used to drag those pieces further into the oesophagus. Species of Doto, belonging to the Dendronotina, however, feed by a different method, slashing open the stalks of the hydroids and sucking out the internal fluids.
To generalise, those that hunt across the substrate for anemones and small hydroids tend to have longer oral tentacles the better to cover and sample a broader swathe of territory while those that live on large hydroids, that is, residing and spending their whole adult lifespan on their prey, may have oral tentacles that are quite short. Compare, for example, the oral tentacle lengths of Cerberilla ambonensis with Lomanotus sp., both pictured here.
A highly adapted aeolid genus is Glaucus. They have adapted to the pelagic lifestyle by swallowing and maintaining an air pocket in their stomach thus making them buoyant to float, upside down as it happens, on the surface and feed on the floating siphonophores such as Physalia – Bluebottles, Velella – By-the-wind sailors and Porpita – Blue buttons.
A recently described phenomenon that should be mentioned here is kleptopredation. Research on the feeding process of the aeolid nudibranch Cratena peregrina indicates that it is stimulated to feed upon polyps of its hydroid prey that have recently captured and are handling plankton. This plankton digestion is estimated to form at least half of the nudibranch’s diet. Additionally, this method of predation may afford a second benefit to the aeolid nudibranch in that the prey has just expended its nematocysts on a recent capture. The nudibranch is thus avoiding, or at least reducing, their effect upon them. This process has yet to be further investigated to ascertain if it applies to a wider species population.
Sequestration of Nematocysts
In dining on cnidarians most of the aeolids have not only evolved to manage the hypersensitive nematocysts to avoid being stung or injured but to also co-opt these stinging cells for their own defensive use. While this is a complex process, the detail of which is grist for a future NudiNote, a little information here will help place this strange capability into perspective. The nudibranch is able to sort the ingested nematocysts, selecting the type and maturity that are suitable for their purpose. Research has shown that immature nematocysts that are, as yet, incapable of discharging are selected for sequestration. This sequestration then involves transport through the digestive system and out to the tips of their cerata and storing in a cnidosac within special cells, called cnidophages, wherein they mature and are held ready for defensive use. At this point they are technically referred to as kleptocnides (which could be translated as “stolen stinging cells”). A “typical” aeolid cnidosac contains about 3,000 kleptocnides, so the defensive potential is significant. Muscles in the cnidosac contract and force the kleptocnides out of their cnidophages and the cnidosac where, resulting contact with seawater causes them to discharge. Not all are discharged at once allowing for more than one release. Interestingly, some research has indicated that certain aeolids are capable of altering the type of nematocyst they sequester through their diet, in response to the type of threat they detect in the region.
The possession of and therefore the availability of these stinging cells in the cerata for defensive purposes is complemented by behaviour to increase their effectiveness. Many nudibranchs with charged cnidosacs will suddenly erect and direct their cerata towards a perceived threat. This sudden startling action itself is termed deimatic or warning behaviour and depending upon species can be further enhanced by the possession of bold colouration and pattern on the cerata. The directing of the apex of the cnidosac-charged cerata towards the threat maximises the discharged concentration of the kleptocnides if extruded through the cnidopore.
Of the aeolids, the oophagus Favorinus, the octocoral predators Phyllodsemium (Phyllodesmium jakobsenae apart) and the hard-coral predators Phestilla, whilst possessing cnidosacs, lack the kleptocnide content. There are other exceptions among the various genera that lack kleptocnides including Bulbaeolidia alba for example. A number of non-Aeolidina cladobranchs also possess cnidosac-like structures but only members of the Dendronotina family Hancockiidae have been recorded with kleptocnides present within.
David A. Mullins – August 2021
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– This NudiNote has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #388 (June 2021): 44-46 by David A. Mullins.