Hydra

Not immortal in the sense of being unkillable, but close to biologically ageless. A hydra can be eaten, starved, crushed, or poisoned like any animal, and most wild hydras die young from those causes. What makes it unusual is that it shows no measurable ageing, its chance of dying and its ability to reproduce do not decline as the years pass, whereas in almost every other animal both get steadily worse. The reason is that a hydra's body is made largely of stem cells that divide constantly and replace every part of the animal every few weeks, so it never accumulates old, worn tissue. Laboratory studies that tracked hydras for years found no rise in death rate, and statistical models based on that data suggest a well-kept hydra could in principle live for hundreds or thousands of years.

A hydra hangs in the water with its tentacles spread like a net and waits. When a small animal such as a water flea brushes a tentacle, it triggers thousands of stinging capsules called nematocysts, which fire barbed threads into the prey in a fraction of a millisecond and inject a paralysing toxin. Other stinging cells fire sticky or coiling threads that grip the prey. The tentacles then bend inward and stuff the catch through the single mouth at the top of the body, which stretches enormously, into the hollow gut, where it is digested. Anything indigestible is later spat back out of the same mouth, since a hydra has no separate anus.

Hydra usually appear in a tank because they were introduced with live or frozen food, plants, or water from another aquarium, and they thrive when there is a lot of surplus food, so the first step is to feed less and siphon out uneaten food and detritus. They will sting and eat baby shrimp and small fish fry but ignore adult fish. For a serious infestation, options include manual removal, a big water change with substrate cleaning, adding fish that eat hydra such as some livebearers and certain gouramis and loaches, or, in a fishless or plant-only tank, treatments based on the deworming drug fenbendazole, which kills hydra and planaria but also harms snails and shrimp, so it is used with care.

They are both cnidarian polyps and look alike, a column topped by a ring of tentacles around a mouth, but a hydra is much smaller and lives only in fresh water, while sea anemones are marine and range from a few millimetres to over a metre. A hydra is typically a few millimetres to a centimetre tall, has only six to twelve tentacles, can glide or somersault to move, and reproduces mainly by budding off clones. Sea anemones are generally larger and heavier, have many more tentacles, are usually firmly anchored, and belong to a different cnidarian class, the Anthozoa, which never has a jellyfish stage. Hydra are in the class Hydrozoa, most of whose members do produce tiny jellyfish, though hydra themselves have lost that stage.