Factors enhancing fish sperm quality and emerging tools for sperm analysis

• Factors affecting sperm quality: epigenetics, nutrition and cryopreservation
• Revision of methods in fish sperm analysis
• Perspectives for future application in aquaculture related industries

With this review we try to give a comprehensive overview of the current methods used in research to assess sperm quality. In addition, we identify some of the most important factors for enhancing sperm production and quality, including, broodstock nutrition, epigenetics and sperm management (cryopreservation). Sperm quality can be assessed by analyzing different parameters from simple methods to very sophisticated approaches involving molecular tools. Parameters related with sperm composition or function (e.g. spermatozoa plasma membrane lipids, seminal plasma composition, motility activation) have successfully characterized a sperm sample but could not respond to the causes behind sperm defects. Reactive oxygen species (ROS) are one of the causes of the impairment of sperm traits. High contents of ROS are capable of producing cell apoptosis, DNA strand breakages, mitochondria function impairment, and changes in membrane composition due to sugars, lipids, and amino acid oxidation, affecting at later times sperm fertilization ability. Recently, the importance of spermatozoa RNAs in the fertilization and early embryo development has been clearly demonstrated in different species, including fish. Spermatozoa delivers more than the paternal genome into the oocyte, carrying also remnant mRNA from spermatogenesis. These RNAs have been found in sperm from human, rodent, bovine, and recently in several fish species, demonstrating the important predictive value of spermatozoa transcripts present only in those samples with high motility or from males with higher reproductive performance. The content of those transcripts can be changed during gametogenesis process influencing their content in spermatozoa.We will focus this review on sperm quality markers, in new trends on sperm analysis, and in the use of these tools for the identification of factors enhancing gamete quality. Basic research in this field is helping to develop appropriate quality evaluation methodologies and early biomarkers of reproductive success, with potential future industrial applications.