scyphozoans

Scyphozoa, commonly known as true jellyfish, is a class of marine cnidarians distinguished by a predominantly free-swimming medusa body form, with a relatively reduced polyp stage compared to hydrozoans. Scyphozoans occur in oceans worldwide, from coastal shallows to deep seas, and function as important predators of plankton and small invertebrates. They can form seasonal blooms that influence local ecosystems.

Anatomy and structure: The body is bell-shaped (the umbrella) with a thick mesoglea. Tentacles surround the margin, and the interior gastrovascular cavity is divided by radial canals. Cnidocytes bearing nematocysts are present on tentacles and oral lobes for prey capture and defense. Sensory organs called rhopalia, often containing statocysts and ocelli, are located at the margins of the bell.

Life cycle: Scyphozoans exhibit alternation of generations. Fertilized eggs hatch into free-swimming planula larvae, which settle as polyps (scyphistoma). The polyp reproduces asexually by strobilation, producing several disk-like ephyrae that mature into adult medusae. The medusa is typically the dominant, sexually reproducing stage in most species.

Ecology and behavior: They are largely carnivorous, feeding on zooplankton, crustaceans, and small fishes. Scyphozoans swim by rhythmic bell contractions and can form extensive blooms under favorable conditions. Some species possess venomous nematocysts capable of delivering painful stings to humans, though effects vary by species.

Taxonomy and notable genera: The class includes orders such as Semaeostomeae, Rhizostomeae, and Coronatae. Notable genera include Aurelia (moon jelly), Cyanea (lion’s mane), and Chrysaora (sea nettles). Scyphozoans contribute to marine food webs and are studied for their ecological and evolutionary significance.