Steal Their Defence And Make It Your Weapon – The Sea Slug Strategy

Nudibranchs are peculiar, punky underwater beasties. Their name literally means naked gill, as their respiratory organs are all on the outside. One branch of the nudibranch family tree - the aeolid nudibranchs - is particularly cool. Unlike other animals they nom on anemones - something most marine critters steer clear of.

If you've ever been rock pooling and given an anemone a gentle poke you will have felt an odd tingly/sticky sensation in your fingers. This is because anemone tentacles are equipped with stinging cells that fire on contact. The stinging cells - better known as nematocysts - are a formidable device.

Inside is a tightly sprung stinging contraption made up of a sticky thread and toxin-filled barbs. On the outside is a sensory probe, the trigger for this biological weapon. When something brushes past the sensory hair, it triggers a rapid change in the nematocyst wall. Water rushes in, increasing the pressure and propelling the barbed thread out so fast that it can pierce the hard outer parts of a crustacean! (Here's some cool footage of nematocyst fire).

Nematocysts are usually a pretty effective way of keeping predators at bay, but nudibranchs have many adaptations that let them take on the challenge. For starters, their mouth and throat are lined with a tough substance called chitin - the toxin-loaded barbs can't break through this so the nudibranchs are able to chomp and swallow anemone tentacles without any trouble.

Further on in the nomming process, nudibranchs use tiny discs of chitin, called spindles, to protect their tummy. These little discs are the biological sandbags in sea slug defence, helping to keep toxins away from more sensitive parts of the body. In their stomach and their intestine the spindles become tangled with the nematocyst threads and act as a physical barrier to their toxins.

Spindles are also found in their skin, where they help resist small stings. Anything bigger and the damage is rapidly repaired.

Another addition to the nudibranch armoury is mucus. Yum. Unlike trails left by other molluscs on your garden path, nudibranch mucus can cut down nematocyst fire - but only in certain anemones. This is because they are wonderfully well-adapted to their target prey, their mucus only has an effect on the anemones they normally eat - pop some mucous on a foreign anemone and it will do you no favours in battle.

So what happens to all the nommed nematocysts that haven't fired?

The nematocysts that have made it to the intestine without going off don't all come out the other end. Instead, many go on to become part of the nudibranch's own defences. The sea slug digestive system is a higgledy-piggledy wiggle of a thing, with prongs going right up into each of its fleshy appendages (cerata). And at the end of each is booty sack especially for the stolen nematocysts. But this is no ordinary booty sack. Inside, special cells lie waiting to trap the stolen goods - cnidophages (a name literally meaning "nematocyst eating"). These cells hold the nematocysts prisoner until the nudibranch needs to ward of a potential predator.

Should the slug sense an attack, the stolen nematocysts (aka kleptonidae, the booty, prisoners, tiny toxic grenades - take your pick) are squeezed out of the cerata, discharged, and either disarm or deter the attacker. Wonderful.