Obelia is a genus of hydrozoans, a class of mainly marine and some freshwater animal species that have both polyp and medusa stages in their life cycle.

Hydrozoa belongs to the phylum Cnidaria, which are aquatic (mainly marine) organisms that are relatively simple in structure with a diameter around 1mm.

In mature colonies there are individual hydranths called gastrozooids, which can be found expanded or contracted, to aid in the growth of this organism by feeding.

The medusae reproduce sexually, releasing sperm and eggs that fertilize to form a zygote, which later morphs into a blastula, then a ciliated swimming larva called a planula.

The planulae are free-swimming for a while but eventually attach themselves to some solid surface, where they begin their reproductive phase of life.

Scientists are trying to pinpoint their food preferences, but experiments have shown Obelia feeding on crustaceans under lab conditions, and ciliates.

Defense and the capture of prey are helped by unique stinging cells called cnidocytes that contain nematocysts triggered by the cnidocil.

Estimates of divergence times and distinctive haplotypes provide evidence of glacial refugia around Iceland and southeastern Canada.

The more recent expansion of these haplotypes demonstrates the southward movement of hydroid populations, possibly due to climate change.

[6] Obelia are distinguishable from others in Campanulariidae from their size in length and diameter, as well as their smaller hydrothecal cusps and relatively thinner perisarc thickness.

O. longissima have longer first and second order branches, in addition to a greater variation in hydrothecal cusp length than others in the genus.