Morphometric and structural analysis of Florida manatee spermatozoa

Sperm characteristics, such as sperm morphology and sperm morphometry are important in assessing sperm quality. This is especially important for the management and conservation of endangered and exotic species, like the Florida manatee, where information of this nature is extremely limited. In this study, we fill this knowledge gap to better understand the reproductive physiology of Florida manatees by conducting the first extensive analysis of sperm morphometry and ultrastructure. Sperm were retrieved from the vas deferens of nine recently deceased Florida manatees. Computer-aided sperm morphology analysis (CASMA) was used for morphometric analysis and laser-scanning confocal microscopy and electron microscopy were used for structural and ultrastructural characterization. Our findings reveal new morphometric and structural data for the Florida manatee spermatozoon. Twelve morphometric features of Florida manatee sperm were quantified with some approximately 1.5–2 times larger than those previously reported. Ultrastructurally, the Florida manatee spermatozoon followed a mammalian structural pattern with an ovate-shaped head, midpiece containing 84–90 mitochondria, and a flagellum. However, unique ultrastructural features were identified. Distinct, rectangular-like enlargement of four outer dense fibers surrounding the axoneme was evident, which may provide additional tensile strength to counteract the forces on sperm transiting the female reproductive tract. Likewise, strong localization of F-actin fibers within the midpiece may function to maintain sperm integrity within the female reproductive tract. These findings highlight the potential effects of sexual selective pressures on sperm size and structure in the Florida manatee and provide avenues for research on the occurrence of sperm competition in this species.