Sea anemone stinging strategy

Some sea anemones are predators: they lash out selectively with venomous harpoons to grab morsels to eat. Other species, though, are less discriminating: they’re more like a stinging nettle, and they envenomate anything that comes within stinging distance. One strategy better suits an exposed animal that wants to ward off everything, while the other approach rewards a species where predation is less of a problem and conserving energy is what matters most. And, speaking with Chris Smith, Lily He, from Harvard University, has worked out how - and why - both approaches are achieved...

Lily - The stinging cells have a really large organelle that actually takes up most of the cell's composition. And that stinging organelle is called a nematocyst. It has a stinging barb that is inverted and coiled inside that cyst. And then when there are changes in osmotic or hydrostatic pressure within that cyst, the stinging tube can actually shoot out at a really fast rate. It actually can penetrate things like shrimp or predators.

Chris - They're essentially then cell-sized hypodermics?

Lily - Yeah, that's a really good analogy for it! Yes.

Chris - And how might the animal's strategy of using them differ then?

Lily - So that was actually part of our study here where we looked at a couple different anemones. And these two different anemones are evolutionarily highly related. But they can have very different behaviour. And so like for example, one type of anemone requires both like a touch and like a chemical signal simultaneously - both signals - to be able to sting. And then the other anemone actually only requires like a touch signal to be able to sting. We looked at these two different animals and did some mathematical modeling and were able to find that for one of the anemones - the one that requires both that touch and chemical stimulus or signals to be able to sting - is primarily stinging for predation in the sense that they're mostly stinging things that they would eat; whereas the other species of anemone would be primarily stinging for self-defense. So it would be stinging things like butterflyfish or any animal that's trying to eat it.

Chris - And have you got a feel or did you manage to find out how that distinction is achieved? So the predatory species only discharges when both the chemical stimulus and the touch stimulus are present, whereas the other one is more indiscriminate, it's just defending itself. Did you manage to find out what the physiological basis of that discrimination is?

Lily - Our current hypothesis is that there is a certain type of protein that's found in both types of anemones but there are differences. And so what we found was that in one species this protein generally serves as sort of like a gate for sort of mediating these signals. And so in the case of that predatory anemone, that protein is serving as sort of a gate or a filter. And so it only is activated in the case of those simultaneous chemical and touch signals. And then in the other case that anemone that's primarily stinging for defensive purposes or self-defense that anemone uses a different variation of that protein that isn't as sensitive to chemical signals. And so it is essentially able to be activated upon just a touch signal.

Chris - Would it be possible to swap 'em round - the particular protein you're referring to - between the two and then demonstrate you get the reverse behaviour?

Lily - Yeah, we would've loved to do that. That was a dream experiment to be able to knock down or deactivate production of the protein in the predatory anemone and then have that predatory anemone somehow be able to produce or express the protein that's found in the anemone that's stinging primarily for defense. But, unfortunately, it wasn't necessarily practical to do those experiments. Some of the current genetic manipulations that are required for that are actually not really like fully developed at this point.

Chris - So work for another day then to, to follow that one up. Mm-Hmm. <Affirmative>, does it fit or can you pull together the evolutionary path by which these individual creatures have arrived at the strategy they've got? Does it fit with where we find them, their hunting strategy, which presumably it does, and how they came by this particular adaptation?

Lily - That anemone that's primarily predatory it actually usually buries itself in the mud or sand and really only has its tentacles, which is where all the stinging cells are exposed in the environment. And so it has a very different lifestyle from the other anemone, which stings primarily for defense. And that other anemone is usually exposed to the environment doesn't necessarily hide or bury itself because it actually has these algal endosymbionts, so that means that these algae live inside the anemones tissues and provide actually most of the nutrients for the animal. And so the animal needs to provide some sort of protection for the algae. And in turn the algae provide a lot of nutrients for the animal. And we sort of hypothesise that the algae is providing enough nutrients so that this animal can actually have essentially less discriminate stinging. And that's largely driven by the idea that the animals that sting for mostly for defense do not have sort of as high of an energy barrier to producing new stinging cells.