Marine invertebrate sperm: assessment of sperm quality using computer-aided sperm analysis

Abstract

The Southern African marine ecosystems are dominated by a variety of marine broadcast spawners. In this study, five species of marine invertebrates, namely Parechinus angulosus (Echinoidea), Choromytilus meridionalis (Bivalvia), Crassostrea gigas (Bivalvia), Donax serra (Bivalvia) and Haliotis midae (Gastropoda) were investigated. The sperm morphology and base-line data were gathered concerning the sperm concentration, motility and sperm kinematic parameters using computer-aided-sperm analysis (CASA). Sperm morphology and sperm motility play an important role in determining sperm quality as they relate to fertilization success. More specifically, head length and total sperm length have been used to find associations with swimming speed for best rates of fertilization. The implementation of CASA allows detailed quantification of the nature of the sperm swimming track, percentage motility groupings and detailed kinematics for rapid-, medium-and slow swimming sperm subpopulations. The analysis of testicular sperm, taken directly from the gonads, and sperm activated by the introduction to sea water through the swim-up technique was determined. Using the CASA motility module, the behaviour of sperm was studied using Choromytilus meridionalis that was thermally induced to spawn and the activity of sperm in the presence of egg-water was determined using Parechinus angulosus as a model. A helical swimming pattern was a distinctive feature found in all species studied. However, species-specificity was found in the diameters and kinematic differences of the helical pattern, with Parechinus angulosus sperm creating the largest diameter tracks and Crassostrea gigas sperm the smallest diameter tracks, including a characteristic serrated helix. Parechinus angulosus and Haliotis midae maintained the progressive helical pattern, post-activation of 60 min, while mostly straight-line forward progressive sperm was evident in Choromytilus meridionalis and Crassostrea gigas sperm. The CASA quantitative sperm track features had a negative association with sperm morphology (head width, tail and total length), but has a strong positive association with sperm speed. It was evident from the Parechinus angulosus model that a decrease (thinner) in sperm head-width and shorter tail resulted in faster swimming sperm, creating a swimming track with a larger circumference and diameter which covered a bigger surface area. In conclusion, this finding shows that head shape played a significant role in sperm hydrodynamics and how it could relate within the fertilization environment. The different patterns of sperm motility could be related to a search strategy in quickly locating eggs and behaviour to reproductive strategies such as broadcast spawning and spermcasting. It is with great interest that this study elucidates the immense contribution CASA has made to being able to correlate so many of these sperm features and that these associations make biological sense.