Hydra
A freshwater animal smaller than a grain of rice that shows no measurable signs of aging
No photo available for Hydra
Superpower
The hydra does not appear to age. Its cells are in a constant state of renewal — stem cells throughout the body continuously divide and differentiate, replacing every cell in approximately 20 days. In laboratory conditions with adequate food, hydra show no increase in mortality rate over time and no decline in reproductive ability. They are biologically immortal in the technical sense: negligible senescence. They can also regenerate from any fragment — cut a hydra in half and you get two hydras.
Overview
Hydra are tiny freshwater polyps — typically 1–20mm long — belonging to the same phylum as jellyfish and sea anemones. They live attached to underwater plants and rocks in ponds and slow rivers worldwide and are sufficiently common that most people have unknowingly shared a pond with millions of them. Their biological immortality was documented in a 2015 study that maintained 2,256 hydra over 8 years and found no evidence of aging. They are effectively impossible to accidentally kill.
Found in
Freshwater ponds, lakes, slow rivers, and streams worldwide. Found on the undersides of aquatic vegetation and rocks in clean, cool water. Present on every continent except Antarctica.
Things worth knowing
- 1
If you cut a hydra into many small pieces, each piece regenerates into a complete hydra. The animal has no defined head or tail — any fragment can become either. A piece of body wall with no head, no foot, and no nerve ring will grow both within days.
- 2
Hydra are ferocious predators for their size — they use nematocysts (the same stinging cells as jellyfish) on their tentacles to capture and consume water fleas, mosquito larvae, and small fish fry.
- 3
Hydra move by somersaulting — attaching their tentacles to a surface, releasing their base, swinging over, reattaching, and repeating. They also float upside down at the surface using a gas bubble secreted from their foot.
- 4
The hydra nervous system is one of the simplest known — a diffuse nerve net with no brain, no centralized ganglia, and no defined sense organs. Despite this, hydra respond to light, gravity, and chemical signals with coordinated whole-body behavior.
- 5
Hydra reproduce both sexually and by budding — a miniature hydra grows from the parent's body wall, develops tentacles, and eventually detaches. Under good conditions a single hydra can bud multiple offspring simultaneously.