Quantitative analysis of sperm plane circular movement in the blue mussels Mytilus edulis, M. trossulus and their hybrids.

Fertilization success of free spawning organisms such as Mytilus species depends on gamete interactions. Therefore, gamete traits such as sperm movement are important for determining fertilization success in free spawning organisms. Since little is known about sperm movement pattern in Mytilus species, the purpose of this study was to investigate sperm movement pattern of blue mussel M. edulis, M. trossulus and their hybrids using computer-assisted sperm movement video analysis. Sperm of all genotypes were found to conduct circular movement in a two-dimensional plane. Furthermore, new sperm movement parameters, real time radius (R), angle change rate (θ) and the center of circular track (O(t)) were developed to verify and quantitatively describe the plane circular movement pattern using software (Image-J) that may be widely applied to sperm movement study in other organisms. Angle change rate was positively correlated to fertilization success. However, no correlation between fertilization and real time radius was detected. Although no interspecific differences were found in the radius, the F¹ (first generation) hybrid sperm had a lower angle change rate than M. edulis and M. trossulus. Published studies have shown that sperm circular movement is more prevalent in aquatic broadcast spawning species than in species with mating behavior or internal fertilization. Therefore, a two-dimensional circular movement pattern in sperm may represent a trait that increases fertilization success for broadcast spawning species by either increasing gamete interaction rate at a small scale and/or avoiding swimming further away from the eggs before sperm detects the chemoattractant gradient.