Morphological assessment of free-spawned sperm in scleractinian coral: a relationship between cell morphology and motility

Although the quality of the spermatozoa is a key factor for reproductive success, there is few information describing the sperm quality for most marine invertebrates, especially corals. Given the high mortality rates and several threats currently faced by coral reefs, ways of assessing coral spermatozoa viability are needed. Thus, in this study we proposed a morphological classification for coral spermatozoa using Mussismilia harttii as model. In addition, we evaluated the relationship of morphology with the percentage and duration of motility. Spermatozoa were first classified into two main morphological categories: normal and abnormal. Then, we classified morphological abnormalities in the head and flagellum together with proposing subcategories that better described it. In general, 68.77 ± 6.50% of the spermatozoa had their whole structure intact, while 31.23 ± 6.51% had morphological abnormalities. Most abnormalities were in the flagellum (five types in total), while only two abnormalities were located in the head. Coiled flagellum and shortened flagellum were the most representative abnormalities, accounting for 24% of the total detected. We observed that reduction of sperm motility is directly related to increased morphological abnormalities and they are influenced by the cell ageing. Our results reinforce how the characterization of sperm morphology can be a practical and useful tool that can help to better understand the effects of multiple stressors on coral reproduction. Graphical abstract