Dimorphic spermatozoa of the hydrothermal vent prosobranch Alviniconcha hessleri : systematic importance and comparison with other caenogastropods

Abstract

Ultrastructural features of euspermatozoa and paraspermatozoa of Alviniconcha hessleri Okutani & Ohta are described and compared with available data for other gastropods. Euspermatozoa are filiform, 130-135 .mu.m long, with a conical acrosomal vesicle (featuring an apical bleb), a curved, rod-shaped nucleus, an elongate midpiece (with 10-11 helically coiled mitochondrial elements), an annular complex, a glycogen piece and an end piece. An unusual sheath of dense material of uncertain origin is associated with the nucleus and the midpiece. Apart from this sheath, most other features of Alviniconcha euspermatozoa occur in euspermatozoa of the Neotaenioglossa (in particular groups such as the Loxonematoidea (Abyssochrysidae), Vermetoidea, Calyptraeoidea, Littorinoidea, Truncatelloidea) and certain Neogastropoda. Paraspermatozoa of Alviniconcha consist of large, moderately electron-dense vesicles, smaller, highly electron-dense bodies and occasional mitochondria all enclosed by the plasma membrane. These cells lack axonemes, recognizable nuclear material and an acrosome. Although similar 'nurse cell' type paraspermatozoa associate with euspermatozoa to form spermatozeugmata in the Littorinoidea and Abyssochrysidae, this phenomenon was not observed in Alviniconcha. Sperm ultrastructure indicates that Alviniconcha belongs within the Neotaenioglossa and is more closely allied to the Littorinoidea-Loxonematoidea than to the Calyptraeoidea or Hipponicoidea. Specifically, sperm data do not support a close relationship between Alviniconcha and the Architaenioglossa, the Cerithioidea or the Trichotropidae, but are consistent with a recent proposal to include the genus within the loxonematoidean family Provannidae