For the past few weeks, social media has been awash with photos of jellyfish, the heatwave drawing swarms of the UK’s five most common species – moon, compass, barrel, lion’s mane and blue – inshore in search of plankton blooms. With images of pulsating, tentacle-trailing behemoths fresh in my mind, it was disconcerting to plunge into the sea and feel the sensation of something brushing against my bare arms and face.
I paused mid-stroke, waiting for the prickling, itching or burning pain that would indicate a sting, but none came so I carried on swimming. Then something bumped against my left shin. Taking a breath, I ducked beneath the surface and peered around. The water was cloudy with silt but I could just make out small, shadowy orbs drifting across my field of vision like eye floaters.
As I surfaced, my friend called out that there was something squishy in one of her aqua shoes. Treading water, she managed to pull it off and I poked around inside, stringy tentacles clinging to my fingers as I flushed out the invader. It was a sea gooseberry (Pleurobrachia pileus), a species of comb jelly. These gelatinous creatures are often assumed to be jellyfish. In fact, they comprise a unique phylum known as Ctenophora, whereas jellyfish are members of the phylum Cnidaria. While jellyfish paralyse their prey by injecting venom from barbed stinging cells, comb jellies are equipped with adhesive cells, called colloblasts, that release a sticky, mucus-like substance to entrap plankton. Their two feathery tentacles can be up to 20 times the length of their 2.5cm spherical bodies, and it was these trailing fishing lines that had been harmlessly snagging our skin.
We swam in to find that the receding tide had left hundreds of glistening sea gooseberries strewn across the sand. Up close, we could see that their translucent bodies were ribbed like pumpkins, with eight equally spaced longitudinal comb rows. These paddle-like structures consist of successive plates of large, fused cilia (hair-like projections) known as ctenes. These ctenes beat to propel the jellies through the water. In the bright sunshine, they also refracted the light, sending rainbow-like waves of colour rippling along the creatures’ bodies.
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