Fine structure of spermatial surface in the red alga Antithamnion nipponicum (Rhodophyta)

SUMMARY In the ceramiacean red alga Antithamnion nipponicum Yamada et Inagaki, the structure of the spermatial covering and appendages was examined using confocal laser scanning microscopy, scanning and transmission electron microscopy. The liberated spermatium was subspherical, ca 4.5 μm in size with a colorless covering 2.7–3.0‐μm thick. Two flexible, ribbon‐like appendages arose from the periphery of the spermatial covering. The appendages averaged 80 μm in length and were 0.5–0.6 μm width in most parts. Each appendage consisted of a number of thin longitudinal fibrils. Concanavalin A conjugated with fluorescein isothiocyanate, colloidal gold orferritin, bound specifically with the inner layer of spermatial covering and spermatial appendages. When the liberated spermatia were incubated with mature female gametophytes, the spermatial appendages entangled around the tricho‐gyne.