Abstract
The sperm and spermatogenesis in the aphallic hydrothermal vent gastropod Rhynchopelta concentrica are described by transmission electron microscopy. Only euspermatozoa were found in the specimens studied. Spermatozoa are filiform introsperm, confirming that this species has internal fertilization. The sperm have an elongate nucleus (which has an intranuclear canal for most of its length) capped by a needle-like acrosome that is invaginated posteriorly. A small basal plate lies between the base of the acrosome and the nucleus. The mid-piece consists of numerous small mitochondria with poorly developed cristae housed in a cytoplasmic sleeve, the inner region of which forms an electron-dense tube extending along the axoneme. As is typical of vetigastropod and neritimorph sperm, there is no glycogen piece. Spermatogenesis is similar to that described for many gastropods, although aspects of nuclear and mid-piece/flagellum formation resemble those described for neritimorphs.
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