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Obelia exhibit a microphagous feeding habit by concentrating bacteria in their gastric cavity, which is then digested as it spreads throughout the canal system.

These medusary buds differentiate Obelia from others in the family Campanulariidae as development starts from a bud within the gontheca. Eventually the buds are lost and further development is consistent with that of other hydranths

Obelia use their tentacles to transfer prey to the manubrium, but occasionally the prey are caught directly on the manubrium with oral lips that grab at them.

Obelia are found from the intertidal zone to about 100 m below the surface.

The environment can influence the morphology of Obelia spp.

Knowledge about the phylogeny and taxonomy of Campanulariidae is limited. Hydrozoan species are typically identified based on characteristics such as colony size, branching pattern, the number of pedicel rings, and the length of the hydrotheca.

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. Some morphological traits are hard to distinguish across species, so observing a combination of these traits will help with identification. Other useful observable characteristics are branching pattern of colonies and length of trophosome. There are variations and exceptions to these, which makes identification even more difficult. (7)

O. geniculata is characterized by a thicker perisarc with more variation that other species of ''Obelia. O. longissima'' have longer first and second order branches, in addition to a greater variation in hydrothecal cusp length than others in the genus. O. bidentata differs from the previous species due to their more cylindrical and longer hydrothecal cusps. (7)

Phylogeny
Estimates of divergence times and distinctive haplotypes provide evidence of glacial refugia around Iceland and southeastern Canada. In one study, O. geniculata was first documented in these areas in the 1990s, but in the 2000s were discovered in Massachusetts and Japan. There are three reciprocally monophyletic clades of Obelia, one branch for the North Atlantic, one for Japan, and one for New Zealand. There seems to be an ancestral haplotype that occurs in the North Atlantic populations from Massachusetts, New Brunswick, and Iceland. The population from Woods Hole, MA shows lessgenetic diversity than the New Brunswick population. The more recent expansion of these haplotypes demonstrate the southward expansion of hydroid populations, possibly due to climate change. The North Atlantic haplotypes differ from the populations in Japan and New Zealand. The Pacific populations have more haplotype diversity than the four populations from the North Atlantic in total. This indicates that the North American population is more recently established than the Pacific population. The minimum estimated age of the New Brunswick population is between 47 and 143 thousand years old. Including the Massachusetts population, this number is between 82 and 150 thousand years, but Iceland has the oldest estimated population with the minimum age ranging from 68 to 204 thousand years old.

Updated Bucci Citation: Ferdinando, B., Bucci, C., Colucci, A.M.R., Gravili, C., Stabili, L. (2007) "Obelia (Cnidaria, Hydrozoa, Camanulariidae): a microphagous, filter feeding medusa"