Relevance of sperm ultrastructure to the classification of giant clams (Mollusca, Cardioidea, Cardiidae, Tridacninae)
Jennifer L. Keys, John M. Healy, 2000. "Relevance of sperm ultrastructure to the classification of giant clams (Mollusca, Cardioidea, Cardiidae, Tridacninae)", The Evolutionary Biology of the Bivalvia, E. M. Harper, J. D. Taylor, J. A. Crame
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Examination of sperm ultrastructure in six out of eight of the living species of giant clams (‘Tridacnidae’) supports reduction in status of the Tridacnidae to a subfamily of the Cardiidae (Tridacninae), as suggested by recent cladistic analyses based on shell, anatomical and molecular characters. Tridacninae spermatozoa are all of the aquasperm type, featuring, in anterior–posterior sequence: a conical acrosomal vesicle, an oblong to rod-shaped nucleus, a short midpiece region (proximal and distal centrioles surrounded by four round mitochondria); and a flagellum (9 + 2 pattern axoneme). Substantial differences occur between species with respect to the shape and length of the nucleus, and in the spatial relationship between the acrosomal complex and the nuclear apex. Although the two extant genera can be distinguished on sperm features – Tridacna, nuclear peg associated with acrosome, centriolar connective absent; Hippopus, nuclear peg absent, connective present – no defining feature of the Tridacninae can be detected. Within Tridacna, the species T. (Chametrachea) maxima and T. (C.) crocea are distinguished from other species of the genus by a much finer nuclear peg and a considerably smaller acrosome. In contrast, and against expectation, T. (C.) squamosa shows acrosomal and nuclear dimensions very close to those obtained for T. (Tridacna) gigas. Recently proposed phylogenies and classificatory changes for ‘tridacnids’ are discussed in the light of available sperm data.
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The Evolutionary Biology of the Bivalvia
Bivalves are key components of Recent marine and freshwater ecosystems and have been so for most of the Phanerozoic. Their rich and long fossil record, combined with their abundance and diversity in modern seas, has made bivalves the ideal subject of palaeobiological and evolutionary studies. Despite this, however, topics such as the early evolution of the class, relationships between various taxa and the life habits of some key extinct forms have remained remarkably unclear.
In the last few years there has been enormous expansion in the range of techniques available to both palaeontologists and zoologists and key discoveries of new faunas which shed new light on the evolutionary biology of this important class.
This volume integrates palaeontological and zoological approaches and sheds new light on the course of bivalve evolution. This series of 32 original papers tackles key issues including: up to date molecular phylogenies of major groups; new hard and soft tissue morphological cladistic analyses; reassessments of the early Palaeozoic radiation; important new observations on form and functional morphology; analyses of biogeography and biodiversity; novel (palaeo)ecological studies