Podocoryna M. Sars, 1846

In Norwegian waters, Podocoryna hydromedusae are bell-shaped or almost spherical jellyfish that range in size from 1 to 4 mm. They are the only Norwegian jellyfish with a combination of simple tentacles (i.e. not in groups) and lips extending into oral arms that end in clusters of stinging cells. Podocoryna hydromedusae are called medusa buds while they are still developing, because they look like small bulbs budding from the gonozooids. After liberation from the colony the hydromedusae are able to swim immediately.

Podocoryna hydroids have the typical characteristics of the family, resembling each other and other hydractiniids so closely that identification is often not possible. In some exceptional colonies, the distribution of the spines may give some indication of the species. Fully mature colonies are usually smaller than those in Hydractinia and Schuchertinia and they may have a more delicate appearance as well as lacking strong and conspicuous spines, but identification without the hydromedusae is still unreliable.

The hydroids can be confused with each other and with young colonies of other hydractiniid genera. The hydromedusae are easier to identify, but in Norwegian waters young Podocoryna jellyfish may be difficult to tell apart from young specimens of Rathkea octopunctata and Lizzia blondina. The tentacles of larger hydromedusae of the latter two species will be in groups, in contrast to the always singly occurring tentacles of Podocoryna, and Lizzia possess oral tentacles originating above the mouth, rather than oral arms. The jellyfish of R. octopunctata and L. blondina also reproduce asexually by having buds of small medusae on the manubrium, a characteristic that is never present in Podocoryna medusae.

Podocoryna colonies grow on a wide array of substrates including a variety of shells (either inhabited by snails or hermit crabs), polychaete tubes, barnacles, and other animals. In Norway the colonies are found mostly in shallow (<200 m) waters, while the hydromedusae are often caught near the surface especially in spring, summer and autumn.

Traditionally, the presence of hydromedusae was the defining characteristic of Podocoryna, but recent studies have shown that at least one species in the group (P. pruvoti) lacks the jellyfish stage and instead reproduces through reduced medusoids. In addition, some species currently placed in other hydractiniid genera also produce medusae (e.g. Hydractinia pacifica). Because of this, the genus Podocoryna is currently delimited based on DNA data.

Podocoryna was for many years considered a synonym of Hydractinia, and as a result a lot of information for Podocoryna species can be found under the latter name. In addition, the genus name was historically misspelled as Podocoryne, and a bulk of records for Norway and North Atlantic waters exist under this incorrect name.