The Art Of Wiggling Away: An Anemone's Route to Freedom

A couple of month's back I described how sea slugs steal stinging cells from anemones when they snack on them - a crafty tactic that lets the slug turn their prey's defence into their own weapons when they're having lunch. Fortunately for anemones, encountering a hungry nudibranch doesn't always result in their demise.

Anemones are sit and wait predators. They do just that. Stay put and spend long periods waiting for the next tasty morsel to come to them. When something does come by they aren't very selective. After all, if you can't chase your own food down, you have to handle what you're given. While they're waiting for the next unsuspecting victim to fall within their grasp, anemones use a particularly efficient suction cup called a pedal disk to keep themselves steady. This disk sticks them to the rocks, reef, or whatever they're sitting on and is why we often think of them as sedentary.

Anemones aren't incapable of motion though, and this is where their escape plans come into play.

Nudibranchs can pick out their prey from chemical cues in the water, but anemones have no such warning of their predator's approach. In fact, they are not even aware of an attack until the sea slug takes its first bite of tentacle! So, in the presence of their nudibranch nemesis, most primitive anemones pop off their substrate and vigorously beat their tentacles to swim out of reach. It's an inefficient approach: flapping their squishy tentacles in wild abandon and barely staying buoyant for more than 15 seconds, but it works. Those 15 seconds buy then enough time to live a long and happy life under the sea - at least until the next sea slug comes along. For some of the tiniest anemone species, these 15 seconds may be the start of a much longer period of roaming the ocean in turbulent waters - a process that could help their population spread more widely.

Another approach is to pop their pedal disk off their perch and stretch their body (if it's long enough) up, down and side to side in an awkward belly dance to safety. This spurt of wiggly action takes the anemone as much as 15 metres away from the monstrous nudibranch. Not bad for the ocean equivalent of a couch potato.

Nudibranchs are just one marine predator. Can you think of any others that might feast on anemones? And how do they get past their stinging tentacles?