Spermatozeugma and sperm ultrastructure of Thysanocardia nigra, Ikeda, 1904 (Sipuncula; Sipunculidea)

Abstract

Summary This study presents details of the ultrastructure of spermatozeugmata and spermatozoa of the peanut worm, Thysanocardia nigra, from the Sea of Japan. Spermatozoa are tightly packaged into unencapsulated conical bundles, spermatozeugmata, with as many as 300–350 cells comprising a single packet. Within an individual spermatozeugma, cells are aligned side by side such that head of the bundle is comprised of the heads of spermatozoa and the internal cavity of the bundle harbors numerous flagella that together form a tail for the spermatozeugma. The integrity of the packet appears to be maintained by specialized junctions between adjacent cells. The spermatozeugmata break up when discharged into the sea water. Each spermatozoon is characterized by a rounded head, consisting of the acrosome, nucleus, and midpiece. The midpiece contains mitochondria, centrioles, and some residual cytoplasm that contains dark granules. The method of “sperm packaging” and morphology of spermatozeugmata are unique to the Sipuncula. The ultrastructural modifications of spermatozeugmata are discussed as possible adaptations for the reproductive mode and unique excretory system of sipunculans.