Hydra are Cnidarians, related to jellyfish but more akin to really tiny, freshwater sea anemones that have lost a lot of weight. They are just a few millimetres in length and look like a little stem with 1 to 12 tentacles waving around from the top. The name comes from the mythical Greek Hydra, which was a serpent creature with lots of heads and was eventually heroically killed by Heracles during his adventures of killing and capturing all sorts of baddies in order to be cleansed of the sin of killing all his children when he went mad one time.
Our Hydra are nowhere near as terrible as the Greek one and people usually only pick them up to look at them under a microscope. Some of those people are scientists, so they may well hurt a fly, but not a great deal more than that.
One similarity is that neither Hydra moves too much, but ours isn't the guardian of the entrance to the Underworld. Hydra stick to a surface by secreting adhesive fluids from a foot called the basal disc and largely stay there. I say "largely" because they actually can move around if they want to. They can slide along on that sticky foot or simply unstick it and move with the currents. Even better, they can get around like a slimy, mucus covered slinky. This involves bending double and gripping the substrate with their mouth and tentacles before releasing their foot and somersaulting so that their foot is again on the ground and they can get right way up again. It's slow going for something so tiny, but it's pretty impressive for something related to creatures that can barely control their travel at all; mouth and tentacles usually mean just one thing for jellyfish and sea anemones. Which brings us to feeding.
They may be utterly minuscule, but Hydra are predators and their tentacles are still armed with cnidocytes to inflict a sting. They are completely harmless to humans but are deadly to the mini crustaceans that make up their diet. Tiny copepods or Daphnia swim around and bump into a tentacle. The cnidocyte reacts by firing a minute harpoon and injecting venom. The tentacle will then coil around the crustacean, ensuring it comes into contact with more and more cnidocytes to make sure it really is completely dead. The mouth is at the centre of the tentacles and is horribly expandable to get food in. Actually, the whole body of the Hydra is expandable so that it can enclose prey twice as big as itself. Being so small, Hydra breathe and excrete through their skin. Bits of food that are indigestible are simply spat out through the same mouth.
If there is a lot food, Hydra can reproduce asexually. They go through budding, where an even tinier version of themselves grows from the side of their body and is later released into the water. In tougher times, individual Hydra develop both testes and ovaries. The testes release gametes into the water to fertilise eggs which will be kept until the adult dies and poignantly sinks to the bottom, perhaps to the heart rending sound of a tiny violin. The eggs are pretty tough and won't hatch until conditions improve.
If disturbed, Hydra can retract their tentacles so that they look like buds on the stem, They can also retract their entire body so that the whole thing looks like a little drop of jelly, and if things get really bad, they can also regenerate tissue. If you cut a Hydra in half, the two halves will become two whole Hydras again but they will each remain about half as big as the original Hydra. They don't so much grow new cells as change and move the cells they have left into whatever they need to be one Hydra again. That's SO weird! I wonder how small they can get? And they've actually beaten the Greek Hydra here! She could only grow two heads once one was cut off.
Everything we've seen is done without a brain, Hydra just have a nerve net that connects all the parts that need to know stuff about the world around them. Maybe brains are overrated? I think I'll still hang on to mine. For now, at least.
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