Phylogenetic trends and variation in the ultrastructure of the spermatozoa of sympatric species of South African patellid limpets (Archaeogastropoda, Mollusca)

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Abstract
Phylogenetic parsimony analysis of sperm ultrastructure allows recognition in South African patellid limpets of four monophyletic groups: the argenvillei group (3 species); the aphanes group (2 species); the concolor group (6 species) and a more heterogeneous cochlear group (3 Patella species and dubiously associated with these, species of the genera Helcion, Nocella and Celiano). Of 5 pairs of spermatological sister-species, each species is extensively sympatric with its sister-species. Sperm of such sympatric species may be morphologically almost identical or may show structural differences which would be expected to block hybridization. Where, as in the argenvillei and aphanes groups, the sperm within each group are structurally similar, it is likely that immunological and biochemical rather than structural features of the gametes act as species barriers preventing hybridization. Where contrasting morphological specializations of the sperm of sister-species are demonstrated, these are considered adaptations for fertilization in concert with but probably secondary to immunological/biochemical factors. They are presumed to augment rather than initiate reproductive isolation. Examples of such structural differences are the development of an elongate anterior acrosomal process in H. pruinosus relative to its sister-species H. pectunculus and, independently, in P. granularis relative to its plesiomorphic precursor P. longicosta. Trends in sperm evolution, from a morphology in the argenvillei and aphanes groups which is close to that of ‘primitive’ sperm, include elongation of the nucleus and development of a pointed conical acrosome in the cochlear group; development of a rounded posterior acrosome lobe in the concolor group; and numerous other described changes. Patellid species which are at least partly sympatric do not maintain reproductive isolation by staggering of the spawning time in terms of the time of the year, though the degree of interspecific, and intraspecific, synchrony during the day requires further investigation.