Sexual reproduction—A rare occurrence in the life history of the complanate form ofScytosiphon(Scytosiphonaceae, Phaeophyta) from southern Australia

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Abstract
Studies of zooids produced by complanate Scytosiphon thalli throughout the growing season over a period of several years show that the simultaneous production of male and female gametes is rare. Gametogenesis occurs in late winter and spring (Aug.-Oct.). Gametes are negatively phototactic, and the structural and behavioral differences between the sexes are slight. In any mixture of gametes, not all engaged in mating reactions, the proportion of gametes which fuse to form zygotes varies considerably. In culture gametes and zygotes develop into ralfsioid sporophytes which are morphologically indistinguishable, but those derived from zygotes give rise to F2 gametophytes under the same conditions (L:D [light: dark] 10:14 h at 12.degree. and 20.degree. C) in which parthenosporophytes give rise to sporophytic progeny. The effects of L:D, 8:16 and 16:8 h (16.degree. C) on the reproduction of sporophytes are described. The effects of various daylengths on the reproduction of gametophytes in culture are related to seasonal variation in the reproduction of wild thalli. Various difficulties associated with identifying asexual reproduction are discussed. On the basis of differences in gamete size and the morphology of the sporophyte and gametophyte generations, complanate Scytosiphon is distinguished taxonomically from cylindrical S. lomentaria, although the significance of the distinction requires further investigation.