[3] It is one of the first species to colonise newly submerged surfaces and it can disperse widely because of the long-lasting larval stages and its ability to raft, attached to floating objects.

It can sometimes be found in rock pools but mostly occurs in the shallow sublittoral zone and has been reported at depths as great as 500 metres (1,600 ft) off Patagonia.

It grows on hard surfaces such as rocks, boulders, stones, shells, the stipes of kelp, floating objects and man-made structures[2] and is part of the fouling community.

[3] The polyps of Obelia longissima resemble tiny sea anemones and have a ring of small tentacles which they spread in the current to intercept passing food items.

Experimentally, gemmule production has been triggered by a sudden change in temperature of 5 °C (9 °F), lack of aeration of the water or an over-abundance of food.

Under certain conditions reproductive structures called gonothecae form at junctions of older branches and gonophores develop inside these.

Male or female medusae develop inside the gonophores, become detached and drift planktonically with the currents for one to four weeks.