Nick Baker’s Hidden Britain
The popular naturalist, author and TV presenter reveals a secret world of overlooked wildlife
Tiny Dancer
GREEN HYDRA
The immortal freshwater organism that tangles prey with its toxic, harpoon-flinging tentacles
IN GREEK MYTHOLOGY THE HYDRA WAS a poisonous, many-headed serpent with frustratingly effective powers of regeneration – cut off one of its heads and two would grow in its place. A monster of myth, right? Well, actually, no. Hydras do exist. You can find them in your local pond and they’re as terrifying as the mythological one, just a bit smaller.
The green hydra (Hydra viridissima) is the only one of our four native species that’s green. It’s also less than 10mm long, so the only things it’ll be terrorising are water fleas and small fish fry.
Think of them as skinny sea anemones. They’re part of the Cnidaria group of animals, which also includes jellyfish and corals. They’re quite simple creatures, little more than a flexible tube with a mouth at one end, surrounded by a crown of tentacles. They have no heart, brain, eyes or gills. But don’t let the simplicity of this particular cnidarian fool you: hydras are fascinating creatures.
Set up an aquarium and add some weed, rocks or branches that were previously submerged in a pond. Let things settle down and then take a close look at those items. If you’re lucky, you’ll see small, thread-like creatures extending off them and feeding. Those are hydras. If you’ve got a lot of them, they’ll appear as a greenish fuzz. But if you disturb them, they’ll retract to become a minuscule blob of jelly.
Hydras attach themselves by their bases and waft their delicate, flexible and stretchy tentacles around in what reminds me of a slow-motion ribbon dance. Look closely at a stretched-out tentacle and you’ll see this predator’s secret – what looks like a string of beads. Each ‘bead’ is an amazing organ known as a cnidocyte, from cnid, the Greek for nettle.
Within each cnidocyte is a sac-like structure called a nematocyst – essentially a tightly packed, harpoon-like barb. When a hapless creature bumps into a trigger hair, called a cnidocil, the harpoon fires. A tiny explosion accelerates the barb to 40,000 G-force in about 700 nanoseconds. Its prey doesn’t stand a chance – as each barb penetrates it delivers a potent neurotoxin. The more the hydra’s victim struggles, the more tangled it becomes, and the more nematocysts are deployed.
Hydras can take prey items much bigger than themselves. Watching a hydra manoeuvre a large water flea towards its mouth before engulfing it alive is akin to a cat being pulled into a hosepipe – horrific, yet you can’t help but watch.
Three other kinds of nematocysts are used by hydras. One that ensnares prey by producing a long, sticky filament; another that attaches to hard surfaces and enables the cnidarian to move like a caterpillar; and another that is a spiky weapon like the head of a mace, thought to be used in defence.
Another weird quality of hydras is their powers of procreation and regeneration. Populations can rapidly increase by budding – they simply grow others from their bodies. Perhaps the most bizarre of their talents is the fact they can live forever – by budding, but also if you chop up a hydra, each fragment will form into a separate animal.
A graceful and delicate, immortal, toxic, harpoon-flinging blob of fascination better than anything in Greek myth, and in a pond near you.
The colour of green hydra is caused by a symbiotic alga called Chlorella vulgaris, which lives within its body.
LOOK CLOSER
Cut and come again
Small insects, annelids and crustaceans are fodder for hydras, but these cnidarians do have a rather surprising foe. Anchistropus emarginatus is an ectoparasite that is immune to the stinging cells of hydras and uses spines and strong limbs to tear chunks out of their tentacles. Don’t shed any tears for the hydra though – it’ll grow more!