Hydroids (genusObelia) are from the classHydrozoa which consists of mainly marine and some freshwater species and have both the polyp and medusa stages in their life cycle. The phylum the Obelia belongs to is Cnidaria, which are all aquatic and mainly marine organisms that are relatively simple in structure.
Obelias are naturally found underwater in the ocean throughout the world. They are marine colonial hydrozoans that are found growing on algae and hard substrata in the subtidal. You can find them preferentially no deeper than 200 m from the water surface, growing in intertidal rockpools and at extreme low water of spring tides. These colonies also attach themselves to artificial substrata (pilings, harbour installations, buoys, bridge supports), bivalve cultures, and floating debris. Obelia is a cold water organism where water movement plays an important role to supply adequate food, gas exchange, remove waste products, prevent excessive siltation, and provide suitable substratum. Many hydroids are likely to be plentiful where water movement is sufficient but not too strong that it will cause damage. Hydroids tend to occur in low light conditions, possibly due reduced competition from algae and/or settlement preferences of their planulae larvae. .
The Obelia life cycle first starts out with the sessile, asexually producing polyp colony. During this stage of life, Obelia are suspended to substrate surfaces. On this mature colony there are individual hydranths called gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism by feeding; and the reproductive polyp Biology; Thomson Learning Inc; ISBN 0-534-39175-3 (6th edition, hardcover, 2002)
