Hydrozoa

Hydrozoa is a class within the phylum Cnidaria, which includes sea anemones, corals, and jellyfish. The vast majority are marine species, but freshwater hydrozoans are known, for example, Cordylophora lacustris and Craspedacusta sowerbyi.

Two main body forms exist in hydrozoans: a polypoid structure which is sessile, remaining in the same place, and a medusoid form which is free-swimming. The polyp consists of a stalklike structure that bears a number of tentacles surrounding a mouth. The medusa

KEY TERMS

Medusoid —The generative bud of a sessile hydro-zoa that resembles a Medusa’s head.

Pelagic —Refers to the open ocean.

Polyp —Mature hydrozoa distinguished by a cylindrical body that has an oral opening surrounded by tentacles and an arboreal end that may be fixed in substrate.

Sessile —Attached to a firm substrate.

is a bell-like structure, with the tentacles surrounding the mouth underneath. Some species are solitary, but the majority is colonial. In the latter, the colony arises from a single basal root, which rests on the substrate and from which individual polyps arise. Both colonial and individual species lack a hard outer skeleton.

A special feature among colonial species is the presence of individual polyps that perform separate roles. Some polyps are, for example, specialized for feeding (gastrozooids), while others are responsible for reproduction (gonozooids) or defense. While most colonial sessile species are small and feed by filtering zooplankton from the surrounding water currents, some of the medusoid forms are quite large and capable of feeding on small fish. Floating pelagic species such as Porpita and Velella, or the by-the-wind-sailor, as it is commonly known, resemble small jellyfish and may reach 1.5-2.5 in (4-6 cm) in diameter. These are colonial species made up of large numbers of gastrozooids and gonozooids; the body is modified into a flattened structure with a float on the upper surface to provide buoyancy. Some species, such as Velella have an additional small “sail” on the upper surface to catch the wind and assist further with dispersal.