Abstract
The spermatozoa of Thunnus thynnus and Euthynnus alletteratus consist of an acrosome‐less head (comprising the ovoid nucleus and the short midpiece) and a long flagellar tail that contains the conventional 9 + 2 axoneme and lacks lateral fins. The centrioles are arranged at approximately right angles and lie outside of a shallow nuclear groove. The flagellum inserts laterally on the nucleus, therefore the spermatozoon is asymmetrical. The midpiece contains a few mitochondria which are separated from the axoneme by the cytoplasmic canal; they are spherical in T. thynnus and elongate, somewhat irregular in E. alletteratus. Although the main ultrastructural characteristics of the spermatozoa appear to indicate a great homogeneity in the sperm morphology within the family Scombridae, small species‐specific divergences may be of use in systematics.
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