Some of us age more gracefully than others, but perhaps no animal group does it better than the tiny freshwater polyps known as hydras. In 1998 one biologist ventured that the tentacled creatures, by continually renewing their own cells, may stave off aging altogether to achieve a kind of biological immortality.

More recently, the species Hydra magnipapillata was one of a few dozen organisms included in a study of aging diversity. Whereas female fertility in humans spikes early, then tapers off, and mortality rises sharply as we age, plenty of organisms follow a different path. The water flea, for example, experiences fluctuations in fertility throughout its life span and a more gradual rise in mortality. But the hydra takes the prize for life-cycle oddity. The polyp's mortality appears to remain low for an indefinite period, the researchers reported in Nature. (Scientific American is part of Nature Publishing Group.) In a controlled laboratory setting, they estimated that 5 percent of a hydra population would still be alive after 1,400 years.