Head to head
Hydra vs Sea Anemone
A hydra and a sea anemone have the same basic design, a tube anchored at one end with a mouth ringed by tentacles at the other, and both catch prey by firing stinging cells. They belong to different branches of the cnidarian tree, and they differ enormously in size, habitat, and lifespan.
| Hydra | Sea Anemone | |
|---|---|---|
| Habitat | Fresh water only | Seas and oceans only |
| Size | A few mm to ~1 cm | A few mm to over 1 m across |
| Cnidarian class | Hydrozoa | Anthozoa (with the corals) |
| Jellyfish stage | Lost; polyp only | Never had one |
| Tentacles | Usually 6 to 12 | Dozens to hundreds |
| Main reproduction | Budding off clones | Spawning eggs and sperm; also splitting |
| Movement | Glides, somersaults, can detach and drift | Mostly fixed; slow creep, rare swimming |
| Lifespan | Negligible ageing; potentially indefinite | Often decades; some clones far older |
Hydra vs Sea Anemone: common questions
Is a hydra just a tiny freshwater sea anemone?
It is a reasonable first impression, since the body plan is nearly the same, but they sit on different branches of the cnidarian family tree. A hydra is a hydrozoan, in the same class as the Portuguese man o war and many small colonial jelly-like animals, a group whose defining feature is a life cycle that usually includes a small medusa or jellyfish stage. The hydra is unusual in having lost that stage and living only as a polyp. A sea anemone is an anthozoan, the class that also contains the hard and soft corals and the sea pens, none of which ever produce a medusa. So the hydra and the anemone arrived at the same tube-and-tentacles shape from different starting points.
Can either one hurt a person?
Sea anemones can, mildly. Most are harmless to touch and feel merely sticky, because their stinging cells cannot get through thick skin, but some species pack a real sting that causes burning, welts, and a rash, and a few tropical anemones can give a nastier reaction, so the rule on a reef is to look and not handle. Hydra are no threat to people at all: they are millimetres long and their stinging threads cannot penetrate human skin, so you would never feel one. Hydra are only a problem inside an aquarium, where they can kill baby shrimp and newly hatched fish.
Which lives longer, a hydra or a sea anemone?
Both are famous for longevity, in different ways. A hydra shows what biologists call negligible senescence: in the lab its risk of dying and its fertility do not measurably worsen over years, because it keeps rebuilding its whole body from stem cells, and statistical models suggest an individual could persist for a very long time in ideal conditions. Individual sea anemones also live a long time, commonly decades, and captive specimens have been kept for 60 to 80 years, while anemones that reproduce by splitting can form clones that are, in a sense, centuries old. Neither is truly immortal, since both can be eaten, starved, or killed by bad conditions, but a well-kept hydra arguably has the edge on paper.