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Naturally occurring polysialic acids (polySias), a unique homopolymer of sialic acid (Sia) residues, have a large structural diversity, primarily arising from the diversities in Sia components as well as in the intersialyl linkages. The α2, 8-linked polySia on the neural cell adhesion molecules (NCAM) is the most extensively studied and recognized as a regulator of cell-cell interactions during development and differentiation in vertebrate brain. However, little is known about the occurrence and functions of polySia other than the α2, 8-linked polySia on NCAM. Recently, it has been shown that polySia occurs in nature more frequently than previously recognized, as analytical methods to detect polySia structures have been greatly improved. Using these methods, two different types of polySia, α2, 8-and α2, 9-linked polySia, have been discovered on the sea urchin sperm, although the α2, 5Oglycolyl-linked polySia was …