OOGENESIS AND STALK MEDIATED FERTILIZATION IN PHYLLOSPORA-COMOSA, (LABILLARDIERE) AGARDH,C. (SEIROCOCCACEAE, PHAEOPHYTA)

Species of the family Seirococcaceae have been described as producing an oogonial collar and egg stalk. Oogonial collar, stalk and egg struc ture were investigated in Phyllospora comosa (Labillardiere) C. Agardh using light and electron microscopy techniques. Fertilization was obs erved using videomicroscopy. Oogonia develop a layered wall and an 'oo gonial collar' of the inner wall layer around the oogonium apex. A sin gle egg is extruded from each oogonium and held in close proximity to the receptacle surface by an elongate, mucilaginous stalk. The upper c omponent of the stalk is derived from the inner oogonial wall layer an d collar, with a persistent attachment to the outer oogonial wall laye r or stalk base in the region of previous collar development. Upon rel ease supernumerary nuclei are segregated as vestigial cells. Eggs are fertilised within the stalk matrix. Spermatozoid contact with stalks a ppears to be random, and spermatozoid shape appears to facilitate pene tration of the stalk. It is suggested that stalk evolution in the Seir ococcaceae may be related to the subtidal habitat of species in the fa mily. The oogonial collar provides a release mechanism in the absence of periodic drying of the frond as occurs in intertidal habitats. Rete ntion of eggs on stalks may retain eggs in an optimal position for fer tilisation through successive sperm releases.