Urochordates are marine animals, and spermatozoa of many urochordates
show chemotactic behavior toward conspecific eggs during fertilization. Sperm
chemotaxis of the ascidian Ciona intestinalis has been particularly well investigated.
The sperm-activating and sperm-attracting factor (SAAF) of the phlebobranchian
ascidians are secreted from the egg cells, and identified as polyhydroxylsterolsulfates.
The molecular structure of SAAFs differs in different species, and the differences may
cause species-specific chemotactic responses. Ascidian spermatozoa appear to sense a
SAAF concentration decrease, resulting in a transient increases in intracellular Ca2+ in
the flagellum and quick changes in the swimming direction of the sperm. In this
chapter, we will introduce the features and molecular mechanisms of sperm chemotaxis
in urochordates, and particularly those in ascidians.
Keywords: Ascidian, Ascidia sydneiensis, Calaxin, Ciona intestinalis,
Fertilization, Flagellar beating, SAAF, Sperm chemotaxis.
