17 Temporal ultrastructure changes in staghorn coral (Acropora cervicornis) sperm: implications for fertility

Coral reefs around the world are under siege from stressors due to climate change, causing warming and more acidic waters. These stressors can reduce reproduction; however, reproduction is key to successful adaptation to survive climate change. Towards that end, the basic morpho-structure of coral sperm during fertilisation was examined. On Florida reefs, the annual spawning event for the threatened staghorn coral, Acropora cervicornis, occurs over 2 to 4 nights during August. Gamete bundles are released that break down within ∼30 min to liberate both sperm and eggs. Sperm quality, including total sperm concentration, motility, and longevity, vary depending on night of spawn, coral genotype, and environmental events such as storms. Nevertheless, sperm must become motile within a limited time window to fertilise an egg before degradation begins. To investigate the effect of time on sperm ultrastructure, gamete bundles were collected from the surface of the water with transfer pipettes and transferred to 50-mL conical tubes. Following bundle breakdown, sperm were maintained at ambient temperature and monitored for acquisition of motility. Once motility was observed, 200-µL aliquots of sperm suspensions collected at time 0 and 4 h were placed into 1.8 mL of 4% (v:v) freshly prepared glutaraldehyde in filtered seawater and stored at 4°C until processing. Fixed sperm were centrifuged and processed for transmission electron microscopy. Photomicrographs revealed a bullet-shaped nucleus located towards the proximal end of the cell, and electron-dense layers of annulate lamellae. Mitochondria were observed within the sperm head at the distal end, and an intercentriole ligament was observed between the distal and proximal centrioles. A cytoplasmic collar surrounded the proximal end of the flagellum which consisted of a standard 9 + 2 microtubule arrangement. Morphologic changes observed at 4 h included breakdown of the intercentriole ligament, formation of vesicles from the annulate lamellae, and loss of definition on nucleus integrity, possibly associated with commencement of DNA decondensation. Characterisation of the ultrastructure of staghorn coral sperm (1) provides baseline information on sperm integrity for this species, allowing future comparisons when examining potential negative environmental effects; and (2) confirms post-release ultrastructural changes. Findings have useful applications for assisted reproduction for conservation of staghorn coral and indicate that coral sperm have similar complexities of post-release changes as illustrated in sperm from other taxa.