BECKYHITCHIN
The cutest kleptomaniacs
Becky Hitchin takes a deep dive into the science governing the defence mechanisms of some of our beloved nudibranchs
Nudibranchs are so cute , they deserve to have anime hearts floating above them as they shift in and out of our super-macro camera focus . But cute they are not to anything wanting to feed on them . Nudibranchs are far more devious than they look , with a range of defensive mechanisms to deter predators , and super sneaky ways of gaining the best food around .
There ’ s a group of nudibranchs called aeolids . Think of Flabellina , Fjordia , typical nudibranch shape , big wafty cerata coming out of their backs containing a dark stripe of digestive tract . These tend to graze on animals that contain stinging cells ( cnidarians ), and therein lie a few interesting and clever stories .
Firstly , they have an ability to steal stinging cells from their prey and use them in their own defence . Instead of digesting the stinging cells , they store them – in fully functional stinging glory – in their own cerata . So how does this work ? Aeolids in the UK can be seen eating hydroids such as the oaten pipe hydroid , Tubularia indivisa , as well as soft corals .
Their body shape , often with a long and slender foot , may help them move between polyps without getting stung themselves . They use their jaws to slice or crop off segments of their cnidarian prey , and use their version of a tongue , called the radula , to drag those pieces – including the stinging cells - further into the oesophagus . They have a range of protective devices to prevent themselves being stung as the stinging cells move through the body .
Many aeolids have a hard cuticle covering their mouth area and oesophagus . It ’ s made of chitin – the same rigid substance that forms exoskeletons of arthropods such as crustaceans and insects , and the beaks of cephalopods . The skin and stomach cells of aeolids also have a special type of protective barrier – ovoid discs called ‘ spindles ’ that sit in the cells , again made of chitin .
If nematocysts discharge against a nudibranch ’ s skin or digestive tract , these spindles stop the damage going any further than the outermost layer of skin , absorbing the impact of the discharging nematocysts and keeping the toxins at a safe distance from the underlying muscle layers of the nudibranch .
There is also a role for mucus . Experiments show that aeolid mucus seems capable of reducing the amount of nematocysts fired from an item of prey , though it remains unclear exactly how that comes to pass . It does seem , however , that if a nudibranch moves to a different prey species , the mucus will also change to inhibit the release of stinging cells of the new prey . Does the nudibranch alter its own mucus for this ? Or acquire some compounds from the prey during the feeding process ? Much still needs to be learned .
The nematocysts go from the mouth into the digestive system of the aeolid . After 2-3 hours , the nematocysts are present in the digestive glands of the cerata where they are ready to be turned against predators . They are engulfed into cells within the cnidosac – a terminal structure on the end of each cerata – where they can remain waiting for several weeks . A ‘ typical ’ aeolid cnidosac could contain about 3,000 nematocysts ! When needed , the nematocysts are ejected from the tip of the cerata , giving any predator a mouthful , or skinful , of unpleasant toxins . It even seems that nudibranchs have at least some control of how many nematocysts they expel , and how quickly .
How can they better that superpower , you may ask yourself ? With something called kleptopredation , that ’ s how . It ’ s unique to aeolids , and only discovered recently . Not only can aeolids arm themselves with stolen stinging cells , they can feed themselves with stolen food . When they eat their chosen hydroid , it appears that they choose to ingest polyps that are stuffed full of plankton – the hydroid ’ s own food . Research has shown that up to 50 % of a particular aeolid ’ s energy content comes not from the cnidarian itself , but the cnidarian ’ s own prey .
So next time you see a small , red nudibranch , with cerata wafting in the current , remember that not only are you watching an innovative , voracious predator , but also a cunning thief as well . �
Facelina auriculata in the kelp at Handa Island , west Scotland , by Becky Hitchin
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