(The wreck Pollux - South Trøndelag / 5.20.2002 / 27 meters)

Phylum: Cnidaria

Class: Hydrozoa

 

Norwegian: hydrozoer

The systematics of Hydrozoa is highly complex and has been revised as late as in 2009. Many hydrozoans are colonial. Some form branched colonies attached to a surface, others are free floating (pelagic). Most hydrozoans observed by divers in Norwegian waters, are hydroids. A hydroid has typically three life-cycle stages:

The first stage is a tiny pelagic larva. The larva settles and grow into a, often colonial, hydroid stage. The colony may be branched and the branches bear the individual animals (also called zooids or hydroid polyps). Many species form large, conspicuous colonies by asexual reproduction. Finally, the polyps convert to radially symmetrical medusae. The medusae are released from the colony and become pelagic. This is usually considered their adult stage, when reproduction is accomplished sexually. By utilizing both reproduction strategies the species secure a large number of off-springs as well as a necessary genetic variation.

Almost all hydrozoans are marine animals. Three orders of Hydrozoans dominate the Norwegian marine fauna: