| Summary: | Sperm ultrastructure is described for the ocean quahog Arctica islandica (Linnaeus, 1767) (Arcticidae), a long-lived, and commercially and phylogenetically important marine bivalve from the North Atlantic, and for Neotrapezium sublaevigatum (Lamarck, 1819), an Indo-Pacific member of the only other family of Arcticoidea (Trapezidae). Spermatozoa of A. islandica consist of (in anterior to posterior sequence): an elongate-conical, deeply invaginated, acrosomal vesicle (length 2.0 ± 0.2 μm; invagination occupied by a granular subacrosomal material); a straight, anteriorly-tapered, rod-shaped nucleus (length 6.6 ± 0.4 μm); a short (approximately 0.8 μm) midpiece consisting of two orthogonally arranged centrioles, surrounded by four (approximately 75% of spermatozoa observed) or, less commonly, five (approximately 25% of spermatozoa observed) spherical mitochondria; nine satellite fibres connecting the distal centriole to mitochondria and the plasma membrane; and a flagellum (length 60 ± 5.0 μm, with 9+2 axoneme), originating from the distal centriole. Contents of the acrosomal vesicle of A. islandica are differentiated into a very electron-dense basal ring (with reticulate structure) and two less electron-dense zones. Spermatozoa of N. laevigatum (Lamarck, 1819) differ substantially from those of A. islandica and are characterized by: a rounded-conical, deeply invaginated, acrosomal vesicle (length 0.43 ± 0.2 μm), with a curved basal ring and two less conspicuous components; a barrel-shaped nucleus (length 1.6 ± 0.5 μm) with a broad apical depression accommodating the base of the acrosomal vesicle; a midpiece composed of five (approximately 80% of spermatozoa observed) or four (approximately 20% of spermatozoa observed) mitochondria. Centriolar and flagellar details are essentially as for A. islandica, and putative glycogen deposits are associated with the distal centriole and mitochondria in both species. Sperm data corroborate recent transcriptomic analyses separating Arcticidae and Trapezidae in different imparidentian clades. Based on sperm morphology, A. islandica would appear more closely related to the Glauconomidae of the Cyrenoidea than to the Trapezidae, Veneroidea or any other previously examined group of euheterodonts, suggesting that it could be the only living member of the Arcticoidea. The relationships of the Trapezidae remain uncertain, with apparent sperm similarities to members of several groups of euheterodonts (e.g. Tellinoidea, Pholadoidea, Galeommatoidea), while several potentially closely related key taxa (e.g. Glossidae) remain unstudied for sperm characters.
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