REPRODUCTION AND LIFE CYCLE
Obelia colony propagates by the asexual method of budding producing hydranths (polyps and blastostyles). These represent asexual generation. The blastostyles are specialized reproductive zooids forming medusae by budding. Medusae constitute the sexual or the reproductive individual possessing four gonads. These gonads may be either male or female.
Gonads: Gonads are situated on the sub-umbrellar surface of medusa, one in the middle of the course of each radial canals. They mature after the medusae escape from the gonotheca. Each gonad is an ovoid, knob-like body of undifferentiated interstitial cells between the epidermis (ectoderm) and gastrodermis (endoderm). Epidermis is continuous with that of the sub-umbrella and gastrodermis is continuous with that of radial canals. The space between two layers is filled with a mass of sex cells, which become differentiated into haploid ova or sperms in female and male medusae respectively. On maturation of gametes, the endodermal/gastrodermal wall surrounding the gonad ruptures and the germ cells are set free in sea water.
Fertilization: Fertilization is of cross-fertilization type. It takes place either in seawater or the sperms may be carried by water currents to female medusae, where they fertilize the ova. The parent medusae die soon after liberating the gametes.
Development: The zygote or fertilized egg undergoes equal holoblastic division resulting in a solid ball of cells, the morula. Morula is transformed into blastula by the process of blastulation. The blastula is hollow, having a central cavity called blastocoel, which is surrounded by a single layer of cells, the blastomeres. During gastrulation, the blastocoel gets filled with the cells which are budded off from the wall of blastula. Thus, a solid gastrula (stereogastrula) is formed. Its outer most layer is known as ectoderm and inner mass of cells as endoderm. The gastrula elongates, the ectodermal cells develop cilia and endoderm develops enteron by a process of splitting or delamination. This ciliated gastrula represents the planula larva. It leads a free- swimming existence. After leading an active free-swimming existence, the planula larva settles down, attaches itself by its anterior end to solid substratum and undergoes metamorphosis. Its anterior end forms basal disc for attachment, while the posterior or distal end forms manubrium with a mouth surrounded by a circlet of tentacles. The larva now closely resembles a polyp or hydra, hence called hydrula. The basal part of hydrula is elongated into hydrorhiza from which vertical and lateral branches are budded off and a complex branching Obelia colony is formed. This colony, later, develops medusa buds on the blastostyles to start sexual generation.
The occurrence of free-swimming medusa and ciliated larva in a life-history of a fixed or sessile organism such as obelia, is of great significance; as it helps in the dispersal of organisms and prevents overcrowding of the species.
