The life cycle of Sertularia marginata Kirchenpauer, 1864 (Cnidaria: Hydrozoa): a medusoid-producing sertulariid

Abstract

Laboratory observations showed that Sertularia marginata, a nearly cosmopolitan coastal water hydroid, produces medusoids with gametes around the manubrium. The dioecious, short-lived and rudimentary medusoid has a velum with a circular aperture, but lacks radial and circular canals, mouth, tentacles, statocysts and ocelli. Many small microbasic mastigophore nematocysts are evenly distributed on the exumbrella, except in a band near the umbrella margin. The gametes do not fill completely the sub-umbrellar cavity and are liberated with vigorous contraction of the bell, usually soon after the release of the medusoid. Spent medusoids can live for more than two hours. Fertilization occurs after release. Amphisbetia operculata is the only other known sertulariid which releases medusoids. The occurrence of medusoids in the family Sertulariidae could prove to be more common than hitherto suspected, as the life cycles of other species are closely investigated.