The development of the embryo of Hormosira banksii (Turner) Decaisne (Phaeophyceae, Fucales) in laboratory culture

Abstract

The early development of the embryo of Hormosira banksii, the common intertidal alga at Leigh, was studied using light microscope and scanning and transmission electron microscopic techniques. The development was compared to that of other Fucales. High liberation of eggs and synchronous fertilization was achieved in July-October (Winter and Spring) when the mean sea water temperature is 15°C. Liberation of eggs could be obtained at high temperatures but viability of the eggs was low and growth abnormal. The post-fertilization changes in the egg involved the adhesion and establishment of the embryo. Fertilization took place 5-30 minutes after the release of gametes from conceptacles. Zygotes had adhered to the substratum 3-4 h after fertilization which coincided with the low tide exposure of the subject population. The Golgi apparatus was involved in secretion of fucoidin, resulting in the adhesion of the zygote. Polarity in the spherical zygote was established 12-16 h after fertilization. A second firm attachment was made by exudation of mucilage at the tip of the rhizoid, which then differentiated and elongated while secreting further mucilage. The secondary rhizoids developed within one week and increased in number with time to establish the young plant as an attached alga. The embryo body wall became thicker and tougher with-time to protect the embryo from desiccation and injury. Optimal growth occurred in temperature and salinity corresponding to winter levels (15°C, 30°/oo), and with the optimal 3-5 h of emersion per 12 h, coinciding with conditions in its intertidal habitat. The results of egg liberation and culture experiments demonstrated that plants are well adapted to intertidal conditions, where good growth could be obtained in Winter.