Gonangium development and medusoid of Nemalecium lighti (Hargitt, 1924) (Cnidaria: Hydrozoa, Haleciidae)

Abstract

Based on live specimens of Nemalecium lighti collected in the coasts of La Réunion (Indian Ocean) and São Sebastião (Southeastern coast of Brazil) and kept in the laboratory, we observed the release of short-lived medusoids. The gonangia pass through six developmental phases: growing, ripening, migrating, stripping, liberating and spawning. The medusoids are tall, lack tentacles, bulbs, circular and radial canals, and the sexual products are packed around the eccentric manubrium; they are provided with a velum and with a subumbrellar ectoderm rich in transverse striated muscle fibers. There are refringent and isotropic corpuscles within vacuolated and ciliated large cells located around the aperture of the medusoid, which possibly function as statoliths. The corpuscles are similar to those already described for the families Plumulariidae and Aglaopheniidae. The gametes are liberated shortly after the release of the medusoid from the gonotheca. The female medusoid spawned 40-62 ova; spermatozoa exhibited a semicircular nucleus, and planulae were formed c. twelve hours after fertilization. Colonies with medusoids of only one sex or with both male and female medusoids.