TRANSFER OF THE GASTROPOD FAMILY PLESIOTROCHIDAE TO THE CAMPANILOIDEA BASED ON SPERM ULTRASTRUCTURAL EVIDENCE

Abstract

Using sperm ultrastructure the systematic placement and affinities of the caenogastropod family Plesiotrochidae are re-examined. The simultaneous hermaphrodite, Plesiotrochus crinitus Thiele, 1930, produces both euspermatozoa (uniflagellate, fertile sperm) and paraspermatozoa (bi- or triflagellate, infertile sperm). Features of each type of sperm clearly indicate that the Plesiotrochidae are closely related to the Campanilidae (Campaniloidea) and are not, as previously believed, referable to the superfamily Cerithioidea. Significant sperm synapomorphies of Plesiotrochus (Plesiotrochidae) and Campanile (Campanilidae) include the morphology of the eusperm midpiece (seven to nine straight mitochondria surrounded by a segmented, accessory sheath of membrane-bound vesicles) and morphology of the anucleate parasperm head (axial core of mitochondria surrounded by a bilaterally symmetrical arrangement of axonemes and dense vesicles). The characteristic substructure of the cerithioidean eusperm midpiece (four straight mitochondria each containing parallel, cristal plates) is not observed in Plesiotrochus or Campanile. Euspermatozoa of Plesiotrochus differ from Campanile principally in details of the acrosomal complex (Plesiotrochus with apical bleb, probable absence of an accessory membrane; Campanile without apical bleb, accessory membrane well developed), the transverse profile of all midpiece mitochondria (thin in Plesiotrochus; thick in Campanile), and morphology of the annulus (double ring in Plesiotrochus; single ring in Campanile). In addition, all observed paraspermatozoa of Plesiotrochus are anucleate, whereas in Campanile anucleate and nucleate paraspermatozoa are present. On the basis of sperm synapomorphies of Plesiotrochus and Campanile, the Plesiotrochidae are transferred from the Cerithioidea to the Campaniloidea.