Jellyfish lack bones, shells, or hard teeth, which makes their fossil record patchy. Even so, discoidal and medusa-like impressions in Ediacaran and Cambrian rocks, combined with the diversity of living cnidarians, point to a very ancient origin. Modern Discomedusae include some of the largest jellyfish, with individuals that can weigh hundreds of kilograms.

On Haeckel's plate of Discomedusae, one jellyfish trails a storm of tentacles under the historical name Desmonema annasethe. Haeckel named it after his wife, Anna Sethe, who died shortly before he encountered the animal; the name is no longer treated as a valid species in modern taxonomy. The image is both scientific and personal—a grief pressed into taxonomy. Every time the plate is reproduced, that private loss quietly rides along with the swirling animals.

Jellyfish are often framed as simple, almost alien animals, yet their lives unfold through currents, light, prey, and changing water conditions. They pulse, drift, and sometimes gather in seasonal blooms. Watching one can feel like watching a living oscillation, a reminder that some life strategies involve working with motion rather than resisting it.

Some coastal regions have experienced conspicuous jellyfish blooms, but long records do not establish a simple global rise. Warming, eutrophication, fishing pressure and habitat change may alter bloom conditions in particular systems; the strength and direction of those effects vary, and causal evidence remains limited. These ancient animals can still become local indicators of rapidly changing food webs in the same oceans that shaped them across deep time. Their persistence is not a story of remaining unchanged or ‘low-tech’; it is the continuing success of responsive body plans and life cycles that evolution has kept reshaping.