Last week I introduced the nudibranchs, a group of beautiful and surprising sea slugs. In this post, I’ll be talking about how nudibranchs steal weapons from their prey and use them to their own advantage.
Nudibranchs like to eat Cnidaria. This group of simple but fascinating animals includes corals, sea anemones, jellyfish and others. Cnidaria means “nettle”, and what makes cnidarians unique is their sting. Their tentacles are covered in special cells called cnidocytes. Each cnidocyte contains a device that you might call a microscopic biological cannon. These come in several types with different payloads. Nudibranchs are after the type called the nematocyst. It’s something like a venomous hypodermic harpoon.
Nematocysts are famously deadly. Jellyfish don’t attack humans, but some who’ve swum too close to a box jellyfish (pictured right) have met their end, and stings from other cnidarians can cause extreme pain or death. Nematocysts are even more dangerous to sea creatures like fish and crustaceans, which are cnidarians’ prey. They’re so effective that they have made cnidarians successful predators across the world despite moving slowly (if at all) and having no bones, no teeth, usually no eyes and never much of a brain. When the elegant Glaucus atlanticus (pictured below left) steals the nematocysts of the notorious Portuguese man o’ war, it becomes even more dangerous than their owner by concentrating them at the ends of its bunched cerata.
Nematocysts are tough, elastic capsules filled with a long, coiled tube. They have an opening covered by a flap next to a sensitive hair. The tube ends in a barbed spike and the capsule is filled with venom. When the hair is stimulated, an electrical current rushes over the capsule, causing water to surge inside. Immense pressure builds and in nanoseconds the coiled tube explodes through the opening and into the flesh of a victim. Research has shown that the barb accelerates with more than 5,000,000 g. It releases toxins that incapacitate nerves and muscles, rendering the target either a harmless nuisance or a helpless meal. You can see microscopic footage of the process here.
Over millions of years nudibranch sea slugs have taken their prey’s best defence and turned it into their own secret weapon. How they do this is still a little bit mysterious. Have a look at Phyllodesmium poindimiei on the right (and on the move here). P. poindimiei eats Carijoa coral. It digests almost all of it, but somehow plenty of nematocysts are preserved. They are transported out of the main gut into extensions that run to the tips of the slug’s cerata (its horns). That’s right – nudibranchs’ cerata are full of intestine. In P. poindimiei, you can see this particularly clearly through its translucent flesh. At the ends of the cerata are organs called cnidosacs, which collect the nematocysts, preserve them, and can squeeze them out at attackers.
This process has made marine biologists very curious for many years, as nudibranch nematocysts were first discovered over 100 years ago. How do the slugs avoid getting stung? Why don’t they digest the nematocysts? How they keep bits of other animals’ cells alive inside their bodies? And how do they make use of them? Today, quite a lot is known about this. For example, nudibranchs’ skin, gut lining and digestive tract are full of tiny hard discs that seem to act like “biological ‘sandbags’”, protecting them against stings. And nudibranch mucus – their slime – seems to stop nematocysts from firing. Once nematocysts get to the cnidosacs, they are engulfed by special cells that can hold onto them for weeks. When it needs them, a slug squeezes out a batch and they only fire once they’ve left its body.
Evolution seems to have an inspired imagination, and at first glance it’s hard to work out how it would have gotten this particular idea. However, marine biologists have a hunch. Cnidosacs actually digest nematocysts, very slowly, and some nudibranchs (like Dendronotus albus, pictured) have cerata without cnidosacs that just collect and digest nematocysts. It looks like cerata originally evolved to digest and dispose of toxic stings away from the body of the slug. That happens to be where they would attract the attention of predators. Offering a predator an easy picking from one’s body packed with a dose of deadly poison is a pretty good survival strategy. In fact, nudibranchs can perform autotomy, which is when an animal loses a part of its body deliberately. If you disturb them, some of their cerata drop off. It probably doesn’t take a fish too long to learn that those morsels are not as appetising as they look. In this case, those nudibranchs which have evolved cnidosacs and can squirt nematocysts at predators have just added a finishing touch.
There’s something else I’ve been keeping from you: stings aren’t the only things sea slugs can steal. The next thing is even more impressive. But it will have to wait until the new year! In January, my Seabeasts of the Week will be animals that live like plants. Nudibranchs will make one more special appearance. Be there!
Finally, here are a few resources for anyone particularly interested in nudibranchs:
Check out the New Week Nudibranch thread on The Featured Creature, where you can find regular image-rich posts about specific nudibranchs.
An interesting review on nematocysts in nudibranchs.
And you’ve got to look at these swimming Spanish shawls.