The Hydra of mythology was a massive monster.

If you cut off one of its heads, two more would grow back in its place, and its teeth had the power to raise skeletons. It lived in the lake of Lerna in the Argolis.

We found our Hydra at a shopping mall. See, hydras are micro-animals found in freshwater around the world, often attached to duckweed and wood floating on unpolluted ponds and streams. And while James, our enterprising master of microscopes, does often collect his various microbial samples from the great outdoors, in Poland where James lives, the ponds and rivers are too cold most of the year for hydras.

But he’d heard that hydras are a problem for aquarium owners, thus a trip to the mall. This is the first hydra he found. The microcosmos is a many splendored thing, and sometimes what it uncovers is a toxin-tentacled, potentially immortal hunter hitching a ride on an aquatic snail inside a shopping mall aquarium.

The good news is, it doesn’t have any teeth so I think we’re safe from the skeletons. Hydra belong to the phylum Cnidaria, which makes them a relative of jellyfish and sea anemone. Their bodies are small, about 10-20 millimeters in length, and also quite simple, composed of two cellular layers around a central cavity, which acts sort of like the hydra’s stomach.

At the end of this cavity, the hydra’s body gives way to tentacles, which surround the hydra’s mouth. Like our snail-bound friend from earlier, hydras usually secure themselves to hard surfaces, including stones and bits of plants. There, the hydra waits for food, its tentacles stretching up to three times the length of its body in search for prey like crustaceans, insects, and even small fish.

Much of the hydra’s hunting ability lies in those tentacles, which are armed with little projectile cellular capsules called nematocysts. Here, you can see some nematocysts that were discharged by a hydra, which happened due to mechanical stimulation of the nematocysts as the sample was being prepared. The cylindrical body of the nematocyst is attached to a long thread, which can serve different functions.

One type of nematocyst is equipped with a thread that is designed to wind around the hydra’s prey, while the thread of another kind of nematocyst has an opening at the tip, which allows it to puncture the surface of the hydra’s prey to inject a neurotoxin. When prey brushes up against the hydra’s tentacles, the nematocysts are discharged quickly, and by quickly, I mean very fast. They experience around 5,410,000 g of force in as little a time as 700 ns.

That incredible speed helps the thread of the nematocyst act like a harpoon and catch its prey. When the hydra isn’t feeding, its mouth is sealed with the rest of its body in a continuous sheet of cells. So to eat, the hydra essentially has to tear a hole open in its body to let the prey in, which it does by sending neuronal signals that stretch the mouth apart until finally, it rips open.

Even more remarkable, hydras can open their mouths wider than their body, which lets them eat prey that is bigger than them. Inside the hydra, the prey is broken down into particles that are taken up by surrounding digestive cells. Now hydra, let’s just get this out of the way, don’t have an anus, so the remaining undigested bits of food go out the same way they came in: through the mouth.

You may have noticed that the hydras we’ve shown are either brown or green. You may have guessed, based on occurrences of the color green in previous episodes, that it suggests something about chloroplasts. Indeed, green hydra—which are their own distinct group called Chlorohydra—are occupied by unicellular algae, which take up residence inside of vacuoles within the hydra’s cells, performing photosynthesis and providing sugars in exchange for other nutrients and protection from the hydra.

Hydras have fascinated biologists for centuries thanks to their spectacular regeneration abilities. They can withstand all manner of violence: cut a hydra in two, and you’ll end up with two hydras. Cut it into twenty pieces, and you’ll have twenty hydras.

Turn the hydra inside-out, and it will recover. Scientists have even blended hydras down to their cellular components, spun them in a centrifuge so they pack together, and watched as the cells sort themselves back into hydra. For the organism, this all boils down to a unique ability to continuously renew their body through stem cells, which is why hydra are sometimes called the eternal embryo.

This is not to say that they cannot die—if food runs low or water quality becomes poor or something eats them, then yes, the hydra too will succumb. But scientists studying hydra over several years have found no sign of aging in the organisms, which means that barring all of those challenges we mentioned earlier, hydras may be immortal. But as mortal beings ourselves, the possibility of hydra’s immortality will likely remain just that to us—an unconfirmed likelihood.

The simplicity of the hydra’s body belies the complexity of its life, from algal symbionts, to its periodically torn-open mouth, to its endless renewal and regeneration. All this, from an organism we found in a shopping mall aquarium. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

If you want to see more from our Master of Microscopes, James, check out. Jam and Germs on Instagram. Thank you so much to all of our patrons on Patreon for helping make this show possible and better every episode.

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