Spermatozoa of the Australian ‘greenlip’ abalone Haliotis laevigata Donovan: ultrastructure and comparison with other gastropods, especially other Haliotidae (Vetigastropoda, Molluscs)

Summary

Spermatozoa of a commercial Australian abalone, Haliotis laevigata Donovan, 1808 (the ‘greenlip’ abalone) are examined for the first time using transmission electron microscopy. As is characteristic of other externally fertilizing molluscs, the spermatozoa of H. laevigata are of the structurally simple type (classic ect-aquasperm). In anterior- posterior sequence, each spermatozoon consists of a conical acrosomal vesicle (length 1.35–1.38 μm, deeply invaginated posteriorly and associated with a subacrosomal axial rod and flocculent deposits), a barrel-shaped nucleus (length 2.1–2.2 μm, with prominent lacunae), a short midpiece (consisting of a pair of orthogonally arranged, triplet substructure centrioles surrounded by a five spherical mitochondria), nine satellite fibres (connecting the distal centriole to the plasma membrane) and a 40–44 μm long flagellum (featuring a 9 + 2 microtubular pattern axoneme). Total length of spermatozoa ranges from 44–48 μm. The contents of the acrosomal vesicle can be divided into anterior and posterior portions based on differing electron densities and the occurrence anteriorly of a thin, ‘truncated cone’ structure at the periphery of the vesicle. Preliminary examination of spermiogenic stages indicates that like other vetigastropods the acrosomal vesicle of H. laevigata develops through coalescence of several moderately large, electron-dense, pro-acrosomal vesicles. Comparison with other species of Haliotis which have been studied reveals that spermalozoa of H. laevigata (type of subgenus Schismotis) most closely resemble those of the Asian species H. japonica aquatilis and H. diversicolor supertexta (members of subgenus Sulculus). Such spermatozoa differ from those of other investigated haliotids (H. discus, H. mfescens of subgenus Nordotis and H. midae of subgenus Euhaliotis) all of which have appreciably longer acrosomal and nuclear regions. The use of sperm ultrastructure for probing species relationships within the Haliotidae appears to be a most promising area for future research, particularly when directed at type species of nominal subgeneric units.

Key words:

• Spermatozoa
• spermiogenesis
• abalone
• reproduction
• Haliotis
• Gastropoda
• ultrastructure