Gamete Traits Research Articles

Population density effects on gamete traits and fertilisation dynamics under varying sperm environments in mussels.

Gamete traits can vary widely among species, populations and individuals, influencing fertilisation dynamics and overall reproductive fitness. Sexual selection can play an important role in determining the evolution of gamete traits with local environmental conditions determining the strength and direction of sexual selection. Here, we test for signatures of post-mating selection on gamete traits in relation to population density, and possible interactive effects of population density and sperm concentration on sperm motility and fertilisation rates among natural populations of mussels. Our study shows that males from high-density populations produce smaller sperm compared with males from low-density populations, but we detected no effect of population density on egg size. Our results also reveal that females from low-density populations tended to exhibit lower fertilisation rates across a range of sperm concentrations, although this became less important as sperm concentration increased. Variances in fertilisation success were higher for females than males and the effect of gamete compatibility between males and females increases as sperm concentrations increase. These results suggest that local population density can influence gamete traits and fertilisation dynamics but also highlight the importance of phenotypic plasticity in governing sperm-egg interactions in a highly dynamic selective environment.

Fertilization Mode Covaries with Body Size.

AbstractThe evolution of internal fertilization has occurred repeatedly and independently across the tree of life. As it has evolved, internal fertilization has reshaped sexual selection and the covariances among sexual traits, such as testes size, and gamete traits. But it is unclear whether fertilization mode also shows evolutionary associations with traits other than primary sex traits. Theory predicts that fertilization mode and body size should covary, but formal tests with phylogenetic control are lacking. We used a phylogenetically controlled approach to test the covariance between fertilization mode and adult body size (while accounting for latitude, offspring size, and offspring developmental mode) among 1,232 species of marine invertebrates from three phyla. Within all phyla, external fertilizers are consistently larger than internal fertilizers: the consequences of fertilization mode extend to traits that are only indirectly related to reproduction. We suspect that other traits may also coevolve with fertilization mode in ways that remain unexplored.

Transgenerational plasticity as a mechanism of response to marine heatwaves in the purple sea urchin, Strongylocentrotus purpuratus

Kelp forests of the California Current System have experienced prolonged marine heatwave (MHW) events that overlap in time with the phenology of life history events (e.g., gametogenesis and spawning) of many benthic marine invertebrates. To study the effect of thermal stress from MHWs during gametogenesis in the purple sea urchin (Strongylocentrotus purpuratus) and further, whether MHWs might induce transgenerational plasticity (TGP) in thermal tolerance of progeny, adult urchins were acclimated to two conditions in the laboratory – a MHW temperature of 18°C and a non-MHW temperature of 13°C. Following a four-month long acclimation period (October–January), adults were spawned and offspring from each parental condition were reared at MHW (18°C) and non-MHW temperatures (13°C), creating a total of four embryo treatment groups. To assess transgenerational effects for each of the four groups, we measured thermal tolerance of hatched blastula embryos in acute thermal tolerance trials. Embryos from MHW-acclimated females were more thermally tolerant with higher LT50 values as compared to progeny from non-MHW-acclimated females. Additionally, there was an effect of female acclimation state on offspring body size at two stages of embryonic development - early gastrulae and prism, an early stage echinopluteus larvae. To assess maternal provisioning as means to also alter embryo performance, we assessed gamete traits from the differentially acclimated females, by measuring size and biochemical composition of eggs. MHW-acclimated females had eggs with higher protein concentrations, while egg size and lipid content showed no differences. Our results indicate that TGP plays a role in altering the performance of progeny as a function of the thermal history of the female, especially when thermal stress coincides with gametogenesis. In addition, the data on egg provisioning show that maternal experience can influence embryo traits via egg protein content. Although this is a laboratory-based study, the results suggest that TGP may play a role in the resistance and tolerance of S. purpuratus early stages in the natural kelp forest setting.

Examination of the effects of inbreeding on reproductive capacity in the Pacific oyster, Crassostrea gigas

The Pacific oyster (Crassostrea gigas) has been widely cultured as a commercially important bivalve. Intercrossing of inbred strains is regarded as an efficient method to increase yield (heterosis), so the development of inbred lines is common in the breeding strategies of C. gigas. However, inbreeding usually reduces physiological efficiency or reproductive capacity. The effects of inbreeding on gonadal development and reproductive capacity remain to be explored in oysters. In the present study, the monthly gonadal development of an inbred and a control population was observed. Compared with the control population, the delayed gonadal development in the inbred population may require more time and cost for breeders in the artificial breeding process. We investigated how inbreeding influenced female and male gametic traits, including reproductive effort, egg diameter, fertilization rate, hatching rate, sperm motility, sperm longevity, and sperm curvilinear velocity, as well as sperm competitiveness based on a male's share of paternity. We found no inbreeding depression for egg diameter, fertilization rate, or sperm motility. However, maternal inbreeding decreased reproductive effort and hatching rate, and paternal inbreeding decreased sperm longevity, curvilinear velocity, and competitiveness. Our results show evidence of inbreeding depression in the gonadal development and reproductive capacity of C. gigas, which could help us in a better understanding and management of inbreeding in oyster aquaculture.

Reproducing in hot water: Experimental heatwaves deteriorate multiple reproductive traits in a freshwater ectotherm

Abstract Heatwaves are occurring at an increasing frequency and intensity under ongoing climate change. As many reproductive traits—including mating behaviour and gamete traits—are sensitive even to small temperature changes, the impact of heatwaves on reproduction and sexual selection processes is likely to be vast. Also, evaluating whether the sexes respond differently to these extreme events is crucial to understand the impact on fecundity and the consequences at the population level. Nonetheless, our knowledge of the effects of heatwaves on these key aspects of animal life is still limited. Here, we expose recently mated male and female guppies Poecilia reticulata to an experimental heatwave (32°C, 6°C above the control, for 5 days) to determine its effects on several traits, including sexual behaviour, condition, ornamentation and fertility. Using this design, in contrast to alternative experimental setups, we had the possibility to attribute the effects of the heatwave to males' and females' reproductive traits independently. Overall, our results indicate that heatwaves can drastically affect key reproductive traits and unravel sex‐specific responses. In males, there was no effect of the heatwave on survival, but both pre‐ and postcopulatory reproductive traits were affected. After the heatwave, we detected a decrease in orange colouration (the most important ornament on which female choice is based) and the overall level of sexual activity, and a shift in the preferred mating tactic towards forced copulation attempts. The latter suggests implications in sexual conflict dynamics, as forced copulations override female mate choice. Also, after the heatwave, males had more sperm but of lower quality, and, in addition, an increased variance in sperm number. Overall, heatwaves may thus result in a compromised ability to secure matings and fertilizations. In females, the heatwave significantly affected survival, with increased mortality in the short term, and impaired fecundity, with many females from the heatwave treatment not reproducing at all. The negative effects of heatwaves on key reproductive traits unravelled by our study could have major implications for population dynamics and persistence. It highlights the need for further studies on how these extreme events affect reproduction, to improve our understanding of the impacts of climate change. Read the free Plain Language Summary for this article on the Journal blog.

Density-dependent patterns of multivariate selection on sperm motility and morphology in a broadcast spawning mussel.

Sperm cells exhibit extraordinary phenotypic variation, both among taxa and within individual species, yet our understanding of the adaptive value of sperm trait variation across multiple contexts is incomplete. For species without the opportunity to choose mating partners, such as sessile broadcast spawning invertebrates, fertilization depends on gamete interactions, which in turn can be strongly influenced by local environmental conditions that alter the concentration of sperm and eggs. However, the way in which such environmental factors impact phenotypic selection on functional gamete traits remains unclear in most systems. Here, we analyze patterns of linear and nonlinear multivariate selection under experimentally altered local sperm densities (densities within the capture zone of eggs) on a range of functionally important sperm traits in the broadcast spawning marine mussel, Mytilus galloprovincialis. Specifically, we assay components of sperm motility and morphology across two fertilization environments that simulate either sperm limitation (when there are too few sperm to fertilize all available eggs), or sperm saturation (when there are many more sperm than required for fertilization, and the risk of polyspermy and embryonic failure is heightened). Our findings reveal that the strength, form, and targets of selection on sperm depend on the prevailing fertilization environment. In particular, our analyses revealed multiple significant axes of nonlinear selection on sperm motility traits under sperm limitation, but only significant negative directional selection on flagellum length under sperm saturation. These findings highlight the importance of local sperm densities in driving the adaptation of sperm phenotypes, particularly those related to sperm motility, in broadcast spawning invertebrates.

Evolutionary consequences of environmental effects on gamete performance.

Variation in pre- and post-release gamete environments can influence evolutionary processes by altering fertilization outcomes and offspring traits. It is now widely accepted that offspring inherit epigenetic information from both their mothers and fathers. Genetic and epigenetic alterations to eggs and sperm-acquired post-release may also persist post-fertilization with consequences for offspring developmental success and later-life fitness. In externally fertilizing species, gametes are directly exposed to anthropogenically induced environmental impacts including pollution, ocean acidification and climate change. When fertilization occurs within the female reproductive tract, although gametes are at least partially protected from external environmental variation, the selective environment is likely to vary among females. In both scenarios, gamete traits and selection on gametes can be influenced by environmental conditions such as temperature and pollution as well as intrinsic factors such as male and female reproductive fluids, which may be altered by changes in male and female health and physiology. Here, we highlight some of the pathways through which changes in gamete environments can affect fertilization dynamics, gamete interactions and ultimately offspring fitness. We hope that by drawing attention to this important yet often overlooked source of variation, we will inspire future research into the evolutionary implications of anthropogenic interference of gamete environments including the use of assisted reproductive technologies. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Extreme temperatures compromise male and female fertility in a large desert bird

Temperature has a crucial influence on the places where species can survive and reproduce. Past research has primarily focused on survival, making it unclear if temperature fluctuations constrain reproductive success, and if so whether populations harbour the potential to respond to climatic shifts. Here, using two decades of data from a large experimental breeding programme of the iconic ostrich (Struthio camelus) in South Africa, we show that the number of eggs females laid and the number of sperm males produced were highly sensitive to natural temperature extremes (ranging from −5 °C to 45 °C). This resulted in reductions in reproductive success of up to 44% with 5 °C deviations from their thermal optimum. In contrast, gamete quality was largely unaffected by temperature. Extreme temperatures also did not expose trade-offs between gametic traits. Instead, some females appeared to invest more in reproducing at high temperatures, which may facilitate responses to climate change. These results show that the robustness of fertility to temperature fluctuations, and not just temperature increases, is a critical aspect of species persistence in regions predicted to undergo the greatest change in climate volatility.

Evaluation of gamete quality indicators for Arctic charr Salvelinus alpinus

The reproductive performance of farmed Arctic charr exhibits substantial individual variation. The causes behind this variation have not yet been properly explained, despite the species relatively long history in aquaculture. The aim of the present study was to provide better understanding of the issue by evaluating parent and gamete traits in relation to fertility and offspring viability under routine hatchery conditions. Are losses mainly due to failed fertilisation or mortality? Are maternal or paternal factor more predominant? The study covered sex hormone status of the broodstock, egg traits (egg size and wateriness of egg batches), milt traits (volume, density, and sperm motility characteristics), and symmetry of early cell division.We found that the majority of offspring loss occurred as embryo mortality, although a significant additional loss could be assigned fertilisation failure. Fertilisation and mortality rates were not inter-correlated. Neither of the evaluated egg traits, egg size and wateriness of newly stripped egg batches, proved to be useful indicators of egg quality. With regard to sperm traits, milt density as well as several computer assisted sperm analysis (CASA) parameters, i.e. swimming speed (VAP, VSL, and VCL) and beat cross frequency (BCF), were correlated with reproductive performance and could potentially be used as quality indicators for Arctic charr. Together, male 17,20β-P levels and BCF explained 69.5% of the between male variation in proportion of successfully eyed eggs.The results support previous indications that post-ovulatory aging of oocytes is contributing to the impairment of reproductive performance in farmed Arctic charr. In addition, a significant paternal factor to this impairment has been exposed. However, the cause and mechanism involved are still far from clear.

Cryptic species diversity and reproductive isolation among sympatric lineages ofStrongylocentrotussea urchins in the northwest Atlantic

Distinguishing between intra- and inter-specific variation in genetic studies is critical to understanding evolution because the mechanisms driving change among populations are expected to be different than those that shape reproductive isolation among lineages. Genetic studies of north Atlantic green sea urchins Strongylocentrotus droebachiensis (Müller, 1776) have detected significant population substructure and asymmetric gene flow from Europe to Atlantic Canada and interspecific hybridization between S. droebachiensis and Strongylocentrotus pallidus (Sars, 1871). However, combined with patterns of divergence at mtDNA sequences, morphological divergence at gamete traits suggests that the European and North American lineages of S. droebachiensis may be cryptic species. Here, we use a combination of cytochrome c oxidase subunit I ( COI) sequences and single nucleotide polymorphisms (SNPs) to test for cryptic species within Strongylocentrotus sea urchins and hybrids between S. droebachiensis and S. pallidus populations. We detect striking patterns of habitat and reproductive isolation between two S. droebachiensis lineages, with offshore deep-water collections consisting of S. pallidus in addition to a cryptic lineage sharing genetic similarity with previously published sequences from eastern Atlantic S. droebachiensis. We detected only limited hybridization among all three lineages of sea urchins, suggesting that shared genetic differences previously reported may be a result of historical introgression or incomplete lineage sorting.

Effectiveness of a nonlethal method to quantify gamete production in freshwater mussels

AbstractStudies on the reproductive biology of freshwater mussels have contributed to conservation of this group, but methods to study early reproductive stages are either lethal (e.g., histological technique) or useful for only qualitative assessments (e.g., nonlethal syringe technique). Using 2 common mussels (Quadrula apiculata and Quadrula verrucosa) and 2 rare mussels (Quadrula petrina and Quadrula houstonensis) distributed across 3 sites in the Navasota River and San Saba River, Texas, we validated the effectiveness of the syringe technique to quantify gamete production by examining: 1) if estimates of gamete traits (sperm concentration, egg size, and egg concentration) obtained with the syringe technique were correlated to estimates of gamete traits (sperm density, egg size, and egg density) obtained with a histological technique; and 2) if survival, growth, and body condition of individual mussels sampled with the syringe technique were negatively affected in a 2-y mark–recapture field experiment....

Sperm chemotaxis promotes individual fertilization success in sea urchins.

Reproductive success fundamentally shapes an organism's ecology and evolution, and gamete traits mediate fertilization, which is a critical juncture in reproduction. Individual male fertilization success is dependent on the ability of sperm from one male to outcompete the sperm of other males when searching for a conspecific egg. Sperm chemotaxis, the ability of sperm to navigate towards eggs using chemical signals, has been studied for over a century, but such studies have long assumed that this phenomenon improves individual male fitness without explicit evidence to support this claim. Here, we assessed fertilization changes in the presence of a chemoattractant-digesting peptidase and used a microfluidic device coupled with a fertilization assay to determine the effect of sperm chemotaxis on individual male fertilization success in the sea urchin Lytechinus pictus We show that removing chemoattractant from the gametic environment decreases fertilization success. We further found that individual male differences in chemotaxis to a well-defined gradient of attractant correlate with individual male differences in fertilization success. These results demonstrate that sperm chemotaxis is an important contributor to individual reproductive success.

Inbreeding depression of sperm traits in the zebra finch Taeniopygia guttata.

Inbreeding depression, or the reduction in fitness due to mating between close relatives, is a key issue in biology today. Inbreeding negatively affects many fitness-related traits, including survival and reproductive success. Despite this, very few studies have quantified the effects of inbreeding on vertebrate gamete traits under controlled breeding conditions using a full-sib mating approach. Here, we provide comprehensive evidence for the negative effect of inbreeding on sperm traits in a bird, the zebra finch Taeniopygia guttata. We compared sperm characteristics of both inbred (pedigree F=0.25) and outbred (pedigree F=0) individuals from two captive populations, one domesticated and one recently wild-derived, raised under standardized conditions. As normal spermatozoa morphology did not differ consistently between inbred and outbred individuals, our study confirms the hypothesis that sperm morphology is not particularly susceptible to inbreeding depression. Inbreeding did, however, lead to significantly lower sperm motility and a substantially higher percentage of abnormal spermatozoa in ejaculate. These results were consistent across both study populations, confirming the generality and reliability of our findings.

C-14: Comparative cryobiology of germplasms in non-traditional species

One of the greatest challenges to retaining viability of frozen gametes and reproductive tissues is understanding and overcoming species-specificities. This is important because there is significant diversity in the cryobiological properties and requirements among cell types and tissues themselves. Systematic studies can lead to successful post-thaw recovery, especially after determining (1) membrane permeability to water and cryoprotectant, (2) cryoprotectant toxicity, (3) tolerance to osmotic changes and (4) resistance to cooling and freezing temperatures. While species-dependency ultimately dictates the ability of specific cells and tissues to survive freeze–thawing, there are commonalities between taxa that allow a protocol developed for one species to provide useful information for another. This is the primary reason for performing comparative cryopreservation studies among diverse species. Our laboratory has compared cellular cryotolerance, especially in spermatozoa, in a diversity of animals – from coral to elephants – for more than 30 years. Characterizing the biophysical traits of gametes and tissues is the most efficient way to develop successful storage and recovery protocols. Such data are only available for a few laboratory, livestock and fish species, with virtually all others (wild mammals, birds, reptiles and amphibians) having gone unstudied in this arena. Nonetheless, when a rare animal unexpectedly dies, there is no time to understand the fundamentals of biophysics. In these emergencies, it is necessary to rely on experience and the best data from taxonomically-related species. Fortunately, there are some general similarities among most species, which, for example, allow adequate post-thaw viability. Regardless, there is a priority for more information on biophysical traits and freezing tolerance of distinctive biomaterials, especially for oocytes and gonadal tissues, and even for common, domesticated animals.

Assisted reproductive technology results: why are live-birth percentages so low?

The present bioessay aims to analyze the impact of parental age, cause of infertility, embryo chromosomal anomalies, assisted reproduction technology (ART) treatments, and environmental and occupational exposures to xenobiotics on ART results, particularly on live-birth percentages per transfer. Special attention is paid to analyzing the effects of these factors on the mitochondrial, genetic, and epigenetic traits of gametes and embryos to ascertain the molecular/cellular mechanisms responsible for the relatively low percentages of live births reported year after year in ART cycles. The bias of age distribution of women attending fertility clinics toward the late thirties and beyond and the high incidence of mosaicism found in pre-implantation embryos emerge as the two biggest players in this scenario. Parental reproductive aging and some causes of infertility are associated with mitochondrial, genetic, and epigenetic alterations to gametes. ART treatments such as ovarian stimulation, gamete/embryo cryopreservation, oocyte in vitro maturation, intracytoplasmic sperm injection, in vitro culture system, and embryo biopsy may also induce epigenetic changes in gametes and/or pre-implantation embryos. Finally, exposure to numerous environmental chemicals is linked to sperm genetic and epigenetic defects. Whereas the selective transfer of euploid blastocysts may improve implantation and pregnancy percentages, especially in reproductively older women, it does not guarantee the total absence of mitochondrial and/or epigenetic defects in embryos. The presence of induced and/or inherited DNA epigenetic disturbances in ART offspring is unlikely to be prevented, even by replacing the whole cytoplasm of oocytes using nuclear-genome-transfer technology.

Inbreeding depression in male gametic performance.

One key objective in evolutionary ecology is to understand the magnitude of inbreeding depression expressed across sex-specific components of fitness. One major component of male fitness is fertilization success, which depends on male gametic performance (sperm and pollen performance in animals and plants, respectively). Inbreeding depression in male gametic performance could create sex-specific inbreeding depression in fitness, increase the benefit of inbreeding avoidance and reduce the efficacy of artificial insemination and pollination. However, there has been no assessment of the degree to which inbreeding generally depresses male gametic performance and hence post-copulatory or post-pollination fertilization success. Because inbreeding depression is understood to be a property of diploid entities, it is not clear what degree of inbreeding depression in haploid gametic performance should be expected. Here, we first summarize how inbreeding depression in male gametic performance could potentially arise through gene expression in associated diploid cells and/or reduced genetic diversity among haploid gametes. We then review published studies that estimate the magnitude of inbreeding depression in traits measuring components of sperm or pollen quantity, quality and competitiveness. Across 51 published studies covering 183 study traits, the grand mean inbreeding load was approximately one haploid lethal equivalent, suggesting that inbreeding depresses male gametic performance across diverse systems and traits. However, there was an almost complete lack of explicit estimates from wild populations. Future studies should quantify inbreeding depression in systematic sets of gametic traits under naturally competitive and noncompetitive conditions and quantify the degree to which gamete phenotypes and performance reflect haploid vs. diploid gene expression.

Assessing risks of invasion through gamete performance: farm Atlantic salmon sperm and eggs show equivalence in function, fertility, compatibility and competitiveness to wild Atlantic salmon.

Adaptations at the gamete level (a) evolve quickly, (b) appear sensitive to inbreeding and outbreeding and (c) have important influences on potential to reproduce. We apply this understanding to problems posed by escaped farm salmon and measure their potential to reproduce in the wild. Farm Atlantic salmon (Salmo salar) are a threat to biodiversity, because they escape in large numbers and can introgress, dilute or disrupt locally adapted wild gene pools. Experiments at the whole fish level have found farm reproductive potential to be significant, but inferior compared to wild adults, especially for males. Here, we assess reproductive performance at the gamete level through detailed in vitro comparisons of the form, function, fertility, compatibility and competitiveness of farm versus wild Atlantic salmon sperm and eggs, in conditions mimicking the natural gametic microenvironment, using fish raised under similar environmental conditions. Despite selective domestication and reduced genetic diversity, we find functional equivalence in all farm fish gamete traits compared with their wild ancestral strain. Our results identify a clear threat of farm salmon reproduction with wild fish and therefore encourage further consideration of using triploid farm strains with optimized traits for aquaculture and fish welfare, as triploid fish remain reproductively sterile following escape.

Viability Assessment for Artificial Gametes: The Need for Biomarkers of Functional Competency1

Assisted reproductive technology (ART) has resulted in more than 5 million births worldwide. However, mainstream ART techniques are not always successful for an estimated 30% of infertile patients in whom gametes are nonviable. Most patients would clearly prefer genetic parenthood, currently possible only via the use of donated gametes or, in future, via the clinical use of artificial gametes (AGs) incorporating parental DNA. Despite much recent progress in the derivation of AGs, significant obstacles remain. Although it is possible to create artificial cells exhibiting some of the molecular and physiological traits of human gametes, they do not yet exhibit the same level of functionality as their in vivo counterparts. Most current effort pays scant attention to confirmation of molecular integrity and clinical applicability of AGs. Here we discuss the various clinical parameters used to assess gamete and embryo viability and discuss markers of gamete function that may be used within future studies attempting to derive AGs. The use of AGs may prove controversial to some members of the general public, and, as such, there is significant need for an appropriate ethical and legal framework governing the clinical use of such cells. However, provided these issues can be successfully overcome, it is highly likely that AGs will represent powerful biological tools for reproductive science, a valuable training resource for embryologists and for potential use in the clinical treatment of human infertility.

Positive Darwinian selection in gamete recognition proteins of Strongylocentrotus sea urchins

Gamete recognition proteins commonly experience positive Darwinian selection and evolve more rapidly than nonreproductive proteins, but the selective forces responsible for their adaptive diversification remain unclear. We examined the patterns of positive selection in the cognate interacting pair of proteins formed by sperm bindin and its egg receptor (EBR1) and in two regions of the sea urchin sperm receptor for egg jelly suREJ3 gene (exons 22 and 26) among four species of Strongylocentrotus sea urchins (S.purpuratus, S.droebachiensis, S.pallidus and S.franciscanus). The signatures of selection differed at each reproductive protein. A strong signal of positive selection was detected at bindin in all lineages even though the species compared had highly variable gamete traits and experience different intensities and forms of sexual selection and sexual conflict in nature. Weaker selection was observed at EBR1 but the small region studied precluded a clear understanding of the extent of sexual conflict between bindin and the EBR1 protein. At the suREJ3 locus, diversifying selection was observed in exon 22 but not exon 26, suggesting that these regions experience different selective pressures and evolutionary constraints. Positive selection was also detected within S.pallidus at suREJ-22 because of the presence of 12 amino acid replacement mutations segregating at frequencies >0.10. Our results suggest that sexual conflict may be the predominant evolutionary mechanism driving the rapid diversification of reproductive proteins between, and polymorphism within, strongylocentrotid sea urchins.

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.