Medusozoa

Medusozoa is a clade within the phylum Cnidaria that includes all cnidarians capable of producing a medusa, the umbrella-shaped, free-swimming stage common to jellyfishes and their relatives. It encompasses four classes: Scyphozoa (true jellyfishes), Cubozoa (box jellyfishes), Hydrozoa (hydrozoans, including many colonial forms), and Staurozoa (stalked jellyfishes). In contrast to Anthozoa, medusozoans typically have a life cycle that includes a medusa stage, though the degree of medusa prominence varies among groups; Staurozoa, in particular, are sessile as adults and lack a free-swimming medusa stage.

Anatomically, medusozoans are cnidarians with cnidocytes that bear nematocysts. Their body plans may be bell-shaped (scyphozoans, cubozoans) or polyp-like (some hydrozoans). Most lineages begin life as planula larvae after fertilization; in many hydrozoans, a sexually produced medusa is produced by a polyp colony, whereas in scyphozoans and cubozoans the medusa is the dominant reproductive stage.

Diversity and ecology: Medusozoans are widespread in marine environments worldwide, from shallow coastal waters to deep seas; freshwater species exist in some hydrozoans. The box jellyfishes (Cubozoa) are among the most venomous marine animals; other medusozoans produce stings of varying intensity. Ecologically, they are important predators of zooplankton and are prey for various species; some form complex colonial organisms, such as Physalia physalis in Hydrozoa.

Taxonomy and evolution: Medusozoa has undergone revisions with molecular data; their evolution reflects transitions between sessile and pelagic life stages; their fossil record extends back to the Cambrian.