Sperm morphology of the black marlin (Makaira indica) differs from scombroid sperm

To assess the relationships of billfishes with other scombroids, sperm morphology of the black marlin, Makaira indica (Cuvier, 1832), was compared with that of published data for scombroids such as bluefin tuna, Thunnus thynnus (Linnaeus, 1758), little tunny, Euthynnus alletteratus (Rafinesque, 1810) and sailfish, istiophorus platypterus (Shaw in Shaw and Nodder, 1792). Testicular tissue samples were collected from four spawning black marlin from the Coral Sea, Australia, and processed for standard transmission electron microscopy. The ultrastructure of black marlin (and sailfish) sperm revealed morphological features characteristic of the simple type I teleost sperm. In contrast, the more advanced type II sperm ultrastructure is exhibited by the bluefin tuna, little tunny, and other scombroids. This lends support to the hypothesis that the billfishes are distinct enough from tunas and other scombroids (Scombroidei) to warrant placement in a separate suborder: Xiphioidei.