When food is plentiful, manyHydrareproduce asexually by producing buds in the body wall, which grow to be miniature adults and break away when they are mature.

When a hydra is well fed, a new bud can form every two days. When conditions are harsh, often before winter or in poor feeding conditions,sexual reproductionoccurs in someHydra. Swellings in the body wall develop into either ovaries or testes. The testes release free-swimming gametes into the water, and these can fertilize the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating, and, as the adult dies (due to starvation and/or cold), these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into nymphHydra. SomeHydraspecies, likeH. circumcinctaandH. viridissima, arehermaphrodites[6]and may produce both testes and ovaries at the same time.

Many members of theHydrozoago through a body change from apolypto an adult form called amedusa.