{"id":15926,"date":"2022-05-04T00:00:00","date_gmt":"2022-05-03T22:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcwildlife\/?post_type=purple_issue&p=15926"},"modified":"2022-05-11T13:56:57","modified_gmt":"2022-05-11T11:56:57","slug":"hidden-britain-green-hydra","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcwildlife\/2022\/05\/04\/hidden-britain-green-hydra\/","title":{"rendered":"Hidden Britain: Green Hydra"},"content":{"rendered":"\n
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\"\"<\/figure><\/div>\n\n\n\n

Nick Baker\u2019s <\/strong>Hidden Britain<\/span><\/h4>\n\n\n\n

The popular naturalist, author and TV presenter reveals a secret world of overlooked wildlife<\/span><\/strong><\/p>\n<\/div><\/section>\n\n

Tiny Dancer<\/h2>\n\n
GREEN HYDRA<\/span><\/strong><\/h5>\n\n

The immortal freshwater organism that tangles prey with its toxic, harpoon-flinging tentacles<\/p>\n\n

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A green hydra ensnares a water flea (top right) using its cnidocyte-covered tentacles, which contain barbs (inset)<\/figcaption><\/figure><\/div>\n\n

IN GREEK MYTHOLOGY THE HYDRA WAS a poisonous, many-headed serpent with frustratingly effective powers of regeneration \u2013 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\u2019re as terrifying as the mythological one, just a bit smaller.<\/p>\n\n

The green hydra (Hydra viridissima) <\/em>is the only one of our four native species that\u2019s green. It\u2019s also less than 10mm long, so the only things it\u2019ll be terrorising are water fleas and small fish fry.<\/p>\n\n

Think of them as skinny sea anemones. They\u2019re part of the Cnidaria group of animals, which also includes jellyfish and corals. They\u2019re 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\u2019t let the simplicity of this particular cnidarian fool you: hydras are fascinating creatures.<\/span><\/p>\n\n

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\u2019re lucky, you\u2019ll see small, thread-like creatures extending off them and feeding. Those are hydras. If you\u2019ve got a lot of them, they\u2019ll appear as a greenish fuzz. But if you disturb them, they\u2019ll retract to become a minuscule blob of jelly.<\/p>\n\n

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\u2019ll see this predator\u2019s secret \u2013 what looks like a string of beads. Each \u2018bead\u2019 is an amazing organ known as a cnidocyte, from cnid, <\/em>the Greek for nettle.<\/p>\n\n

Within each cnidocyte is a sac-like structure called a nematocyst \u2013 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<\/span> G-force in about 700 nanoseconds. Its prey doesn\u2019t stand a chance \u2013 as each barb penetrates it delivers a potent neurotoxin. The more the hydra\u2019s victim struggles, the more tangled it becomes, and the more nematocysts are deployed.<\/span><\/p>\n\n

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 \u2013 horrific, yet you can\u2019t help but watch.<\/p>\n\n

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.<\/p>\n\n

Another weird quality of hydras is their powers of procreation and regeneration. Populations can rapidly increase by budding \u2013 they simply grow others from their bodies.<\/span> Perhaps the most bizarre of their talents is the fact they can live forever \u2013 by budding, but also if you chop up a hydra, each fragment will form into a separate animal.<\/p>\n\n

A graceful and delicate, immortal, toxic, harpoon-flinging blob of fascination better than anything in Greek myth, and in a pond near you.<\/p>\n\n

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The colour of green hydra is caused by a symbiotic alga called Chlorella vulgaris, <\/em>which lives within its body<\/strong>.<\/p>\n\n

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LOOK CLOSER<\/span><\/strong><\/h5>\n\n\n\n

Cut and come again<\/strong><\/strong><\/h4>\n\n\n\n
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The flesh-eating Anchistropus emarginatus<\/em><\/figcaption><\/figure><\/div>\n\n\n\n

Small insects, annelids and crustaceans are fodder for hydras, but these cnidarians do have a rather surprising foe. Anchistropus emarginatus <\/em>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\u2019t shed any tears for the hydra though \u2013 it’ll grow more!<\/p>\n<\/div><\/section>\n\n

<\/p>\n\n

<\/p>\n\n

ILLUSTRATIONS BY PETER DAVID SCOTT\/THE ART AGENCY<\/p>\n","protected":false},"excerpt":{"rendered":"

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