Fig. 1.  Outline of reproductive cycle

The life cycle of a hydroid such as Obelia and related colonial species involves two phases: an attached feeding or polyp phase, and a free-swimming reproductive or medusa, phase (Fig. 1).  Reproduction begins in late spring or early summer with appearance of the reproductive polyps.  These bud off small medusae that feed and grow for several months.  Sexes are separate in hydrozoans, as in all cnidarians.  Later in the season the medusae reproduce sexually.   Eggs are released, fertilised by sperm from other medusae, and the resulting planula larva settles to the sea bottom to metamorphose into a juvenile polyp.  This grows, begins to divide asexually, and eventually takes on the familiar bushy form of the colony.  This textbook description applies only to a few types of colonial species; most solitary species vary in degree of separation of feeding and reproductive polyps, and in the free-living extent of the medusa stage.

NOTE  41 species of hydroids are identified from San Juan Island, Washington from their medusae, but little research work has been done on their reproduction.