Class Hydrozoa

Class Hydrozoa

Hydrozoa includes nearly 3,200 species; most are marine, although some freshwater species are known (see the figure below). Animals in this class are polymorphs, and most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable.

The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal.

Many hydrozoans form colonies that are composed of a branched colony of specialized polyps that share a gastrovascular cavity, such as in the colonial hydroid Obelia. Colonies may also be free-floating and contain medusoid and polypoid individuals in the colony as in Physalia (the Portuguese Man O’ War) or Velella (By-the-wind sailor). Even other species are solitary polyps (Hydra) or solitary medusae (Gonionemus). The true characteristic shared by all of these diverse species is that their gonads for sexual reproduction are derived from epidermal tissue, whereas in all other cnidarians they are derived from gastrodermal tissue.

(a) Obelia, (b) Physalia physalis, known as the Portuguese Man O‘ War, (c) Velella bae, and (d) Hydra have different body shapes but all belong to the family Hydrozoa. (credit b: modification of work by NOAA; scale-bar data from Matt Russell)