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As a child, I was obsessed with my DK Eyewitness Ocean book, particularly the “dangerous creatures” pages. My favourite was the Portuguese man-of-war, illustrated by a silhouette of a man being dwarfed by a jellyfish-like creature with 30-metre tentacles.

They seemed a world away, exotic in both location and possibility. So when they began beaching in greater numbers on Cornish shores around 2010, it initially felt exciting. But they weren’t the beasts of my fantasies; they were smaller, lifeless, carried here on the Gulf Stream and discarded by the waves like oceanic jetsam.

Strandings of Portuguese men-of-war on British coasts are now a nearly annual occurrence, mainly in autumn and winter, and probably due to rising sea temperatures and changing currents. I keep my distance from this individual; the man-of-war sting, although rarely fatal, can be intensely painful even after they’ve died. Probably blown in on recent strong south-westerlies, it’s around 15cm, with an iridescent balloon-float, almost pasty-like in shape with a pink, crimped seam. The bright blue underside, once fringed with tentacles, has been trimmed.

The Portuguese man-of-war is a complex creature. They are not singular animals, nor are they jellyfish, but rather colonies of multicellular organisms called zooids, each performing a function: the pneumatophore bladder floats on the surface, enabling movement; dactylozooids make up the trailing tentacles, which ensnare and sting fish, paralysing them; and gastrozooids liquefy and digest – the perfect symbiotic storm.

And they haven’t come alone. Carried by the same currents, violet sea snails have begun arriving too, floating on rafts of self-spun bubbles. These small molluscs feed on other surface dwellers, such as the Portuguese man-of-war. Some years ago, I found an empty shell here, an ice-cream swirl of white and purple, both predator and prey succumbing to the same fate on our sands.

The rising tide will soon carry the Portuguese man-of-war back out to sea to decompose. I understand now that its enchantment isn’t in its danger, but in its alien beauty and complex biology. But drifting effortlessly across the ocean comes at a cost, and every stranding is a reminder of how climate change is impacting our coastlines.

• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024, is available now at guardianbookshop.com