Hydroids
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Hydroida
Family: Campanulariidae
Genus: Obelia
Species
Obelia dichotoma
Obelia geniculata
Obelia longissima

Table of contents
1 LIFE OF AN OBELIA
2 REFERENCES
3 EXTERNAL LINKS

LIFE OF AN OBELIA

The common name given to the Obelia is hydroids. Hydroids are from the class Hydrozoa 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.

HABITAT

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.

LIFE CYCLE

The Obelia life cycle first starts out with the sessile, asexually producing polyp colony. On this mature colony there are gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism; and the reproductive polyp gonozooids that have medusa buds. The next generation of the life cycle begins when the medusae are released from these gonozooids, producing free swimming male and female medusae velum with gonads, a mouth, and tentacles. The medusae reproduce sexually releasing sperm and eggs that will fertilize to form ciliated swimming larva called a planula. The planulae will be in the free-swimming form for a while and eventually attach itself to some solid surface, thus forming a new generation of polyps by asexual budding.

REFERENCES

EXTERNAL LINKS