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Relevance of sperm ultrastructure to the classification of giant clams (Mollusca, Cardioidea, Cardiidae, Tridacninae)

By
Jennifer L. Keys
Jennifer L. Keys
Department of Zoology and Entomology, The University of Queensland, Brisbane 4072 Qld, Australia (e-mail: jkeys@zen.uq.edu.au)
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John M. Healy
John M. Healy
Department of Zoology and Entomology, The University of Queensland, Brisbane 4072 Qld, Australia (e-mail: jkeys@zen.uq.edu.au)
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Published:
January 01, 2000

Abstract

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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Contents

Geological Society, London, Special Publications

The Evolutionary Biology of the Bivalvia

E. M. Harper
E. M. Harper
Cambridge University, UK
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J. D. Taylor
J. D. Taylor
The Natural History Museum, UK
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J. A. Crame
J. A. Crame
British Antarctic Survey, UK
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Geological Society of London
Volume
177
ISBN electronic:
9781862394254
Publication date:
January 01, 2000

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