Half-billion-year hunters that are mostly water and sting
Jellyfish are the free-swimming medusa stage of many Medusozoa within Cnidaria—gelatinous bells of mesoglea trailing stinging tentacles. They pulse for propulsion, capture prey with nematocysts, and cycle through planula larvae and often long-lived polyps before a brief sexual medusa. Fossils imply they are at least 500 million years old, perhaps 700.
True scyphozoans are marine; some lookalike hydrozoans invade fresh water. Most coastal medusae grow fast, breed within months, then die, while the seabed polyp may persist. Medusozoans share tetramerous symmetry—parts in fours. Classes include Scyphozoa ("true" jellies), Cubozoa (box jellies), and Hydrozoa; anthozoan corals and anemones lack a medusa. Stalked staurozoans break the free-swimming rule. A group is a smack or smuck.
The English name dates to 1796 and once swept in comb jellies; aquaria pushed "sea jellies" to dodge the backbone implication of "fish." Authorities disagree whether ctenophores or salps count. Humans press and salt rhizostome species as Asian delicacies; Australian researchers have called them sustainable, protein-rich, low-energy food. Green fluorescent protein from bioluminescent species became a transformative gene reporter in microscopy.
Stings injure thousands of swimmers yearly, from mild pain to rare fatalities. Blooms foul nets and clog seawater intakes at power and desalination plants. Life's rhythm—egg, planula, polyp with possible asexual budding, then medusa—lets one polyp seed many jellies when conditions favour blooms, which is why a creature mostly water can still reshape coastal ecology overnight.
Mesoglea is ninety-five percent water with collagen and wandering amebocytes; oxygen diffuses through skin without lungs or heart. Rhopalia carry statolith gyroscopes and pigment ocelli that track light—some mangrove species look upward while commuting between swamp and lagoon. Burgessomedusa from the mid-Cambrian Burgess Shale is the oldest clear free-swimming medusa fossil, anchoring jellyfish among the earliest multi-organ animal groups.
Source: Jellyfish
