Hydromedusae
Introduction

Hydromedusae, the planktonic, adult stage of the Hydroidomedusae, belong to the phylum Cnidaria. Cnidarians are diploblastic, acoelomate metazoa, they have a tissue level of construction, lacking real organs. Their digestive system or coelenteron is sack-like or branched, with a single orifice serving as both mouth and anus. They are usually radially (more rarely bilaterally) symmetrical with the primary body axis oral-aboral. Cnidaria are fundamentally tentacle-bearing animals, covered with stinging organs or cnidae (cnidocysts, spirocysts, and ptychocysts), after which the phylum is named (Desmoneme cnidocyst, Microbasic eurytele cnidocysts, Macrobasic eurytele, Cnidocyst types). Two basic structural types can be recognised among the Cnidaria: the polyp, which is benthic, sessile, and generally modular (that is, consisting of a series of morphologically similar structural units), and the medusa, planktonic, free-swimming and solitary. Cnidarian species may be represented by medusae or polyps, or have both stages in their development. They typically have a ciliated, motile gastrula - the planula larva.

Symmetry in the Hydroidomedusae is tetramerous, polymerous, or rarely bilateral. They typically undergo a polymorphic succession, beginning with the polyp, an asexual benthic, generally sessile stage, which is followed by the medusa (= hydromedusa), a sexual planktonic free-swimming stage (Development, bottom). In many forms the medusae are reduced to gonophores, which no longer leave the hydroid colonies, and in a few others the medusae give rise directly to other medusae (Development, top). In Hydroidomedusae the asexual budding of a new medusa or gonophore usually involves formation of a medusary nodule or entocodon (except in the Narcomedusae) (Medusa bud). The hydroids can be solitary but usually form colonies by budding. The colonies, with interconnected coelenteron, often produce individual polyps specialised for different functions (defensive dactylozooids, reproductive gonozooids, nutritional gatrozooids, etc.). The hydromedusae are solitary, free-swimming, and are provided with a velum (except Obelia) occluding the umbrella aperture (craspedote medusae) (Medusa, Medusa 2, Closed statocyst, Bell margin, Bell margin 4).

With a few exceptions, sexes are separate. Sex cells generally mature in the ectoderm (Medusa, Medusa 2). The gastrula larva, or planula, resulting from fertilised eggs, has only two embryonic cell layers, the ectoderm and the endoderm. Hydroidomedusae have a simple mouth which opens directly into the gastrovascular cavity without an actynopharynx, and their gastric cavity lacks septa similar to those of the Scyphozoa and Anthozoa (Medusa, Medusa 2, Side view P. rubiginosa). Their mesoglea is acellular, the cnidocysts being usually restricted to the ectoderm.

The hydromedusae are essentially carnivorous, but some may feed on bacteria, protozoans, phytoplankton, algae, and even dissolved organic matter. Some species harbour symbiotic intracellular algae from which they may derive some nutrients. They are among the most important planktonic predators.

The ca. 700 described species of hydromedusae live in all seas, at all latitudes and all depths. Only a few have colonised brackish or fresh water.