[1] 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.

[3] 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.

[7] The vertical portion or main stalk of the hydranth form is called the hydrocalus, and it has two layers – the coenosarc and the perisarc.

The manubrium is hollow, contains the mouth, and is located at the distal end of the body and is surrounded by approximately 24 feeding tentacles.

When the medusa buds are ready to be released from the blastostyle, the lateral wall will burst at the distal end.

Male and female gonangia will release their sperm and eggs into the water, and fertilized zygotes undergo complete cleavage until they become planular larvae.

Research has shown that the mouth opening and tentacle contraction are both governed by spontaneous electrical potentials, rather than knowingly.

[14] O. geniculata are typically voracious predators, and can consume a wide variety of bacteria, crustacean zoopankton, and icthyplankton.

In Korea, colonies of O. geniculata were found to infest areas of Saccharina japonica, a kelp that is widely consumed by humans in China and Japan.

When there is too much O. geniculata encrusted on the kelp, it is no longer deemed suitable for human consumption by degrading the taste and quality.

[17][18] In Spain, O. geniculata was the most abundant epiphyte on two other types of kelp - Undaria pinnatifida and Saccharina latissima.