Tentacled Terror

According to ancient Greek mythology Hydra was the nine-headed, swamp-dwelling beast eventually slain by Heracles. The Hydra in these photographs came from the shallow water at the edge of a swamp too but – disappointingly – only had one head. Pond-swelling Hydra often have several heads because they bud off new individuals from the central column of their body, although I’ve never seen one with more than three heads. These animals are hard to spot because even big ones are only a few millimetres long and they cling to waterweeds, where they’re perfectly camouflaged by their green colouration. That’s due to symbiotic algae that live in their gut wall, and because the algae generate oxygen Hydra can often thrive in stagnant, oxygen-starved conditions – around the swampy edges of shallow ponds for example. The best way to find Hydra is to put a handful of waterweed in a glass container and stand it on a well-lit window ledge – the Hydra will often move to the glass surface and anchor themselves there, waving their tentacles. If you’re lucky you might also witness one of their ways of moving – by somersaulting, tentacle-end over foot-end. Each of those tentacles is equipped with stinging poisonous hairs (nematocysts) that are fired out when an unfortunate pond-creature blunders against their trigger hairs and is paralysed by the toxin – you can find a diagram showing how they work here http://upload.wikimedia.org/wikipedia/commons/b/b0/Nematocyst_discharge.png

The specimen in these photographs shows the amazing way in which Hydra can shape-shift, contracting into a blob crowned with contracted tentacles (top photo) or elongating its column and tentacles when it’s in hunting mode. Some of these pictures were taken with a microscope equipped with polarising optics, which generate the coloured background. You can watch videos of a brown species of Hydra here http://www.arkive.org/brown-hydra/hydra-oligactis/video-09.html