Close to dead calm on the Yorkshire hem of the North Sea today. The waves are barely 10cm high and the water is so clear that, standing knee-deep between each half-hearted surge, I can see sand grains shifting on the bottom.



There’s a commotion further out – cries of warning, then curiosity. Something is collected cautiously in a bucket and brought to me for identification. Even close up against the blue plastic, it’s difficult to see clearly – an ovoid emptiness where the sand swirling in the water does not go. Then suddenly, its fringes light up pink, gold and green.

It’s not a jellyfish, this glimmering blob, but a ctenophore or comb jelly, one of a group thought to be more than 500m years old and about as different from other mobile multicellular animal life as it’s possible to be. The fairy fire that flickers along its transparent body is an illusion worked by ranks of tiny cilia packed into rows, called combs.

Beroe cucumis will not sting. Photograph: Amy-Jane Beer

The microstructure of the combs gives them the optical properties of a photonic crystal – an opalescent ability to separate and reflect particular wavelengths of light. As the cilia move, the reflected wavelength (and hence colour) changes. By day the trick works with borrowed sunlight, and in darkness it plays with the bioluminescence generated by the animal’s own body.

Within minutes they’re everywhere, lolling with the gentle slosh of the tide and flickering between presence and absence. Now we see them, now we don’t. They range in size from grape to kiwifruit, and appear both globby and crystalline, like balls of glass being blown.

I lift one (they don’t sting) and glass becomes flesh. For all its delicacy, it has heft. In my hand it is flabby and helpless, but in open water it will command surprising muscular control. In fact, this species, Beroë cucumis, and its relatives are formidable predators of other ctenophores. The mouth at one end can expand well beyond the circumference of the body, and the animal becomes all gape, capable of engulfing prey its own size. The size disparity in this swarm makes me wonder if all but the largest are living dangerously.

Follow Country diary on Twitter: @gdncountrydiary



