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Sperm ultrastructure of nine species of protobranch bivalves, representing three of four extant orders (Solemyida, Nuculida, Nuculanida), is discussed. Greatest diversity occurs in Solemyida (acrosomal vesicle low-conical, tall-conical, or very elongate, with radial plates; nucleus rod-shaped, teardrop-shaped, or very elongate; four, five, or six mitochondria) and the least in Nuculida (acrosomal vesicle low- to tall-conical; lacking radial plates; nucleus rod-shaped, five or six mitochondria) followed by Nuculanida (short, conical acrosomal vesicle with radial plates; spheroidal nucleus; four or five mitochondria). The wide variety of shapes in Solemyidae suggests taxonomic potential, especially in resolution and/or recognition of supraspecific taxa, but no diagnostic family characters were identified. Taxonomic potential exists for Nuculida (acrosomal shape) and Nuculanida (mitochondrial number). Protobranch sperm is highly diverse, and no defining character of the whole group was found. Support was found for the Nuculida and Nuculanida as natural groups but not for their close relationship. Nuculanida and Solemyida exhibit radial plates in the acrosomal vesicle but otherwise share no derived characters. The striking similarity of most sperm features of Nuculanida with certain pteriomorphians, especially Pectinoidea, a relationship also suggested by some mitochondrial DNA sequence data, poses interesting questions concerning their relationships and/or shared functional constraints.
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