Negligible Senescence

Some animals barely seem to age at all — biologists call this negligible senescence. The rougheye rockfish can live past 200 years and keeps reproducing the whole time. The Greenland shark drifts through Arctic water for an estimated 400 years, making it the longest-lived vertebrate known.

In these animals, getting older does not raise the odds of dying. Their fertility holds, their bodies do not visibly break down, and time simply passes.