The Hydrozoa are a class of the phylum Cnidaria, which comprise ~3,700 nominal species. Hydrozoans are characterized by a complex life cycle,which can be viewed as the succession of three different stages: larva (non-feeding planula); polyp (feeding and asexual); medusa (feeding and sexual). As with many marine taxa, the life cycle has both benthic and pelagic stages, but with an important difference: in hydrozoans, it is the adult stage (medusa) that is pelagic, not the usual feeding larva.
The polyp stage is generally colonial, although few species are solitary. In the colonial forms, single individuals are connected through stolons and new polyps are formed by asexual budding. Medusae also bud asexually from stolons or stems.
Throughout the Hydrozoa there are many well-documented cases where the planktonic medusa stage has been repeatedly lost or greatly reduced. In these cases, the reduced medusae (sporosac) become sexually mature while still attached to the benthic polyp colony. In these cases the Hydrozoa effectively lose the pelagic stage of their life cycle. Intermediate stages between fully functional planktonic medeusae and sporosac are also found in Hydrozoa: species can form eumedusoid - reduced medusae unable to feed - that live in the plankton few hours to days, release gametes and die.
Both polyp and medusa stage are characterized by strong seasonality: benthic colonies are often active only during certain months of the year, and may survive unfavorable seasons as quiescent stolons. Moreover, the active colonies show a dramatic seasonality in terms of medusa production. Medusae are produced mostly within a short time window during the year. The length and seasonality of this window differs among species.