Sperm utilization pattern in the honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Queen honeybees (Apis mellifera) mate with a large number of drones on their nuptial flights. Not all drones contribute equally to the queens offspring and the queens utilization pattern of spermatozoa from different drones has an important impact on the genetic composition of the colony. Here we study the consequences of sperm use for the fitness of the queens mates with microsatellite DNA-fingerprinting. Eight queens were instrumentally inseminated with semen of six or seven drones. Each drone contributed either 0.5 µl or 1.0 µl semen, respectively, and we analyzed both the impact of the insemination sequence and the amount of semen on the sperm utilization. Our data show no significant effect of the insemination sequence but a strong impact of the semen volume of a drone on the frequency of his worker offspring in the colony. This effect was not linear and the patriline frequencies of the drones contributing larger semen volumes are disproportionately enhanced. If these observations are also valid for natural matings, drone honeybees should maximize the number of sperm but not apply specific mating tactics to be first or last male in a mating sequence.Communicated by R. PageAn erratum to this article can be found at     

Sperm swimming speed and energetics vary with sperm competition risk in bluegill (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>)

Under sperm competition, a males fertilization success depends largely on the ejaculate characteristics of competing males. Theoretical models predict that, in external fertilizers, increased risk of sperm competition should result in selection for increased sperm swimming speed. To test this prediction, we studied the behavior of sperm from parental and sneaker male bluegill (Lepomis macrochirus), a fish species characterized by high levels of cuckoldry due to alternative reproductive tactics of males (parentals and cuckolders). Because cuckolders (sneakers and satellites) always spawn in the presence of a parental male, but the reverse is not true, cuckolders experience the greater risk of sperm competition. We show here that the spermatozoa of sneakers have faster initial swimming speeds but shorter periods of motility than the sperm of parental males. Moreover, we show that sperm swimming speeds shortly after activation (when most fertilization occurs) are correlated with starting ATP levels in spermatozoa, suggesting that sperm competition has selected for higher energetic capacity in the sperm of sneakers. Thus, the higher energetic capacity and initial swimming speed of sneaker sperm may explain why, despite having fewer sperm per ejaculate than parentals, sneakers fertilize more eggs than parental males when they compete to fertilize a clutch of eggs.Communicated by L.W. Simmons     

Female control of reproductive skew in cooperatively breeding brown jays (<Emphasis Type="Italic">Cyanocorax morio</Emphasis>)

Brown jays (Cyanocorax morio) are long-lived, social corvids that live in large, stable, territorial groups (mean = 10 individuals). In this study, I determined the distribution of reproductive success within groups using multi-locus DNA fingerprinting. Breeding females produced virtually all (99%) of the young within their own nests. Reproduction within groups was highly skewed towards a single primary female, although long term data indicate that secondary females (female breeders that were usually younger and subordinate to the primary female) were sometimes successful. The high reproductive skew observed for females was associated with primary female aggression. Successful reproduction by secondary females may have been due to parental facilitation or the inability of primary females to completely suppress secondary females. Multiple paternity occurred in 31–43% of broods and extra-group paternity occurred in a minimum of 22% of broods. Patterns of paternity also varied between years, since females often switched or included new genetic mates. Although male consorts of nesting females fathered relatively few offspring (20%), they still had a higher chance of fathering offspring than any other single group male. Reproduction was less skewed for males than females as a result of female mating patterns. Female reproductive patterns are consistent with some of the predictions and assumptions from optimal skew models, while male reproductive patterns are not. The factors affecting skew in species with complex social systems such as incomplete control by breeders over subordinate reproduction, female control of paternity, and resource inheritance have not been well incorporated into reproductive skew models.Communicated by: J. Dickinson     

Patterns of sexual cohabitation and female ejaculate storage in the American lobster (<Emphasis Type="Italic">Homarus americanus</Emphasis>)

Little is known of the time and ejaculate allocation strategies during mating of American lobster, Homarus americanus. This study investigated sexual cohabitation and female ejaculate accumulation patterns in a laboratory mating experiment, as well as female seminal receptacle load in exploited populations in the waters of the Magdalen and Anticosti Islands, in eastern Canada. In the laboratory experiment, the length of sexual cohabitation was proportionate to female size for large but not for small males. Also, large males cohabited with pre- and postmolt females longer than small males. These different time investment strategies can be explained by different mutual benefits. In the field and laboratory, larger females accumulated more ejaculate than smaller ones. This suggests that male lobsters tailor ejaculate to female size, a reliable index of her reproductive potential. Moreover, similarly-sized females accumulated more ejaculate when mated with large compared to small males. Comparison of receptacle loads between wild-mated and laboratory-mated females suggests that the former were mating mainly with smaller males, although some evidence of positive size-assortative mating existed, especially at the less exploited Anticosti site. The results are discussed in the context of evolutionary theory and of proposed management measures to increase egg production in exploited populations.Communicated by T. Czeschlik     

Male-male interference competition decreases spawning rate in the European bitterling (<Emphasis Type="Italic">Rhodeus sericeus</Emphasis>)

We investigated the consequences of male-male interference competition associated with alternative male mating tactics in a freshwater fish, the European bitterling (Rhodeus sericeus). Male bitterling defend territories around living mussels and attract females to lay their eggs in the gill cavities of mussels. We experimentally manipulated spawning-site abundance and male density at two spatial scales. We showed that the total number of eggs spawned by females was constrained by the number of mussels available for oviposition. The effect was mediated by behavioral interactions among competing males because of variation in the Operational Sex Ratio (OSR) in close proximity to a mussel and not by a direct limitation in mussel capacity to accommodate the eggs. Both total and local male densities affected spawning behavior, and interacted in their effect on female spawning rate. Territorial male aggression caused courtship interruptions that prolonged the time until successful spawning and increased with male density. However, territoriality broke down at the highest male density, with a consequent stabilizing effect on spawning rate.Communicated by J. Krause     

Varying levels of female promiscuity in four <Emphasis Type="Italic">Apodemus</Emphasis> mice species

Sexual selection in most vertebrates is based on the evolution of fitness optimization strategies such as multiple-male mating (MMM). Several ecological correlates of MMM have been identified in bird and fish populations; however, only few studies have documented the effects of environmental change on promiscuity in mammals. In this study, the 127 pregnant females from four central European and ecologically diverse species of field mice (genus Apodemus) were studied to assess the role of ecological factors that may have shaped the evolution of particular mating systems. MMM was found in all analyzed species: in Apodemus uralensis and Apodemus flavicollis, up to two males could be identified as the fathers of a particular litter, while three males sired 9.1% of analyzed litters of Apodemus sylvaticus and 20.6% of Apodemus agrarius. Furthermore, there were obvious differences between species in relative testes size and the proportion of multiple sired litters during those seasons when the opportunity for multiple mating was high. The species with the smallest testes and the least promiscuous was A. uralensis (only 43.5% of multiple sired litters), while the species with the biggest testes and the most promiscuous was A. agrarius (69.2%). MMM was significantly associated with higher litter size in A. flavicollis, and the probability of MMM strongly increased with season in A. agrarius and with abundance in A. uralensis. These results indicate that ecological factors are associated with MMM rates in Apodemus field mice and more research is needed to fully understand the evolution of mating strategies at different levels of biological resolution.     

Sperm traits in the quacking frog (<Emphasis Type="Italic">Crinia georgiana</Emphasis>), a species with plastic alternative mating tactics

In species where males use alternative reproductive tactics and male phenotypes are confronted with different risks of sperm competition, theory predicts that between-male-type differences in sperm expenditure may evolve. In the frog Crinia georgiana big males can monopolize females, whereas small males often engage in polyandrous matings. Consequently, big males may experience a lower risk of sperm competition than do small males. We tested if the predictions from theoretical models can be applied to the mating system of C. georgiana. Our results showed that small males do not have larger testes relative to their body size compared to their larger counterparts and that the efficiency with which sperm number, size, motility, and longevity are produced by the testes does not differ between small and large males in the predicted way. These results are not in alignment with predictions from a loaded raffle model of sperm competition on sperm expenditure in males with alternative phenotypes. The plasticity in mating tactics used by C. georgiana males and a high intraseasonal variation in male densities may have prevented the evolution of enhanced sperm performance in smaller males. A fair raffle in the sperm competition game played by C. georgiana males could also explain the observed patterns in sperm traits. Future investigations determining the parameters responsible for the deviation from theoretical predictions in this system will test the degree to which current theoretical models can indeed be applied to species with plastic reproductive tactics.     

Auditory defence in the peacock butterfly (<Emphasis Type="Italic">Inachis io</Emphasis>) against mice (<Emphasis Type="Italic">Apodemus flavicollis</Emphasis> and <Emphasis Type="Italic">A. sylvaticus</Emphasis>)

Morphological and behavioural traits can serve as anti-predator defence either by reducing detection or recognition risks, or by thwarting initiated attacks. The latter defence is secondary and often involves a ‘startle display’ comprising a sudden release of signals targeting more than one sensory modality. A suggested candidate for employing a multimodal defence is the peacock butterfly, Inachis io, which, by wing-flicking suddenly, produces sonic and ultrasonic sounds and displays four large eyespots when attacked. The eyespots make small birds retreat, but whether the sounds produced thwart predator attacks is largely unknown. Peacocks hibernate as adults in dark wintering sites and employ their secondary defence upon encounter with small rodent predators during this period. In this study, we staged predator–prey encounters in complete darkness in the laboratory between wild mice, Apodemus flavicollis and Apodemus sylvaticus, and peacocks which had their sound production intact or disabled. Results show that mice were more likely to flee from sound-producing butterflies than from butterflies which had their sound production disabled. Our study presents experimental evidence that the peacock butterfly truly employs a multimodal defence with different traits targeting different predator groups; the eyespots target birds and the sound production targets small rodent predators.     

Histological investigation of the reproductive cycles of the limpets <Emphasis Type="Italic">Patella </Emphasis><Emphasis Type="Italic">vulgata</Emphasis> and <Emphasis Type="Italic">Patella ulyssiponensis</Emphasis>

This study devised a staging system for, and monitored, the gonad development of the limpet species Patella vulgata and Patella ulyssiponensis on the South West coast of Ireland using histological techniques. Maturation began in the males of both species in January and in the females it began in March. There was no statistical difference in gonad development between sexes and between species. Spawning in the male P. vulgata occurred from September to December 2003 and in September and October 2004. In female P. vulgata spawning occurred from October to December 2003, no spawning of females was observed in 2004. In male P. ulyssiponensis spawning occurred in November and December 2003 and from September 2004 to December 2004. Spawning was observed from November 2003 to January 2004 and in September 2004 in female P. ulyssiponensis. Sex ratios also varied between the species and between months sampled. Nevertheless more males were observed in both species.     

Courting virtual rivals increase ejaculate size in sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>)

Sperm competition occurs when sperm of two or more males compete to fertilize a given set of eggs. Game-theory models of sperm competition predict that males, which face an increased risk of sperm competition, will invest more sperm in a mating. In the pair-spawning three-spined stickleback ( Gasterosteus aculeatus), males attempt to steal fertilizations (i.e. to sneak) when they are in the courting phase, but not when they are parental. The behaviour of neighbouring males may therefore indicate the risk of sperm competition. We visually confronted males before spawning with computer animations of the same virtual stickleback showing two different behaviours: either courting (high risk of sperm competition) or brood-caring (low risk of sperm competition). We show that males invest significantly more sperm (absolutely and relatively with respect to available sperm in the testes) after the courting stimulus. The relative investment ratio between the courting and the brood-caring treatment was on average 1.75. Three-spined sticklebacks thus seem to assess the risk of sperm competition by the behaviour of neighbours and to adjust their ejaculate accordingly. This result suggests that the evolutionary force of sperm competition has led to precise mechanisms of future risk assessment.     

Genetic analysis of tuna populations,<Emphasis Type="Italic"> Thunnus thynnus thynnus</Emphasis> and<Emphasis Type="Italic"> T</Emphasis>.<Emphasis Type="Italic"> alalunga</Emphasis>

The genetic population structures of Atlantic northern bluefin tuna ( Thunnus thynnus thynnus) and albacore ( T. alalunga) were examined using allozyme analysis. A total of 822 Atlantic northern bluefin tuna from 18 different samples (16 Mediterranean, 1 East Atlantic, 1 West Atlantic) and 188 albacore from 5 samples (4 Mediterranean, 1 East Atlantic) were surveyed for genetic variation in 37 loci. Polymorphism and heterozygosity reveal a moderate level of genetic variability, with only two highly polymorphic loci in both Atlantic northern bluefin tuna ( FH* and SOD- 1*) and albacore ( GPI- 3* and XDH*). The level of population differentiation found for Atlantic northern bluefin tuna and albacore fits the pattern that has generally been observed in tunas, with genetic differences on a broad rather than a more local scale. For Atlantic northern bluefin tuna, no spatial or temporal genetic heterogeneity was observed within the Mediterranean Sea or between the East Atlantic and Mediterranean, indicating the existence of a single genetic grouping on the eastern side of the Atlantic Ocean. Very limited genetic differentiation was found between West Atlantic and East Atlantic/Mediterranean northern bluefin tuna, mainly due to an inversion of SOD- 1* allele frequencies. Regarding albacore, no genetic heterogeneity was observed within the Mediterranean Sea or between Mediterranean and Azores samples, suggesting the existence of a single gene pool in this area.     

Comparative life-history study on sympatric hyperiid amphipods (<Emphasis Type="Italic">Themisto pacifica</Emphasis> and <Emphasis Type="Italic">T</Emphasis>. <Emphasis Type="Italic">japonica</Emphasis>) in the Oyashio region,western North Pacific

Life-history features of the sympatric amphipods Themisto pacifica and T. japonica in the western North Pacific were analyzed based on seasonal field samples collected from July 1996 through July 1998, and data from laboratory rearing experiments. T. pacfica occurred throughout the year, with populations peaking from spring to summer. In contrast, T. japonica were rare from autumn to early winter, but became abundant in late winter to spring. Mature T. pacifica females and juveniles occurred together throughout the year, indicating year-round reproduction. Mature T. japonica females were observed only in spring, and juveniles occurred irregularly in small numbers, suggesting limited, early-spring reproduction in this study area. Size composition analysis of T. pacifica identified a total of eight cohorts over the 2 years of the study. Due to the smaller sample size and rarity of mature females (>9.6 mm) and males (>7.1 mm), cohort analyses of T. japonica were not comparable. Laboratory rearing of specimens at 2°C, 5°C, 8°C and 12°C revealed that a linear equation best expressed body length growth by T. pacifica, while a logistic equation best expressed body length growth by T. japoncia. Combining these laboratory-derived growth patterns with maturity sizes of wild specimens, the minimum and maximum generation times of females at a temperature range of 2–12°C were computed as 32 days (12°C) and 224 days (2°C), respectively, for T. pacifica, and 66 days (12°C) and 358 days (2°C), respectively, for T. japonica. The numbers of eggs or juveniles in females marsupia increased with female body length and ranged from 23 to 64 for T. pacifica and from 152 to 601 for T. japonica. Taking into account the number of mature female instars, lifetime fecundities were estimated as 342 eggs for T. pacifica and 1195 eggs for T. japonica. Possible mechanisms for the coexistence of these two amphipods in the Oyashio region are also discussed.Communicated by O. Kinne, Oldendorf/Luhe     

Intraspecific competition in<Emphasis Type="Italic"> Fucus serratus</Emphasis> and<Emphasis Type="Italic"> F</Emphasis>.<Emphasis Type="Italic"> evanescens</Emphasis> (Phaeophyceae: Fucales) germlings: effects of settlement density,nutrient concentration,and temperature

Survival and growth of early post-settlement stages are critical for the development of seaweed populations. Fucoid germlings commonly settle in dense monospecific aggregates, where intraspecific competition and environmental variables (e.g. nutrient concentration and temperature) may affect survival and growth. Using factorial experiments, we determined the effects of settlement density (~10, ~50 and ~250 germlings cm^–2), nutrient enrichment (from ~10 to ~40 µM N and from ~0.5 to ~2.5 µM P), and temperature (7°C and 17°C) on Fucus serratus and F. evanescens germlings in laboratory cultures over 3 months. Settlement density, nutrient concentration and temperature interactively affected growth of germlings, and the magnitude of this interaction varied between the two species. This represents the first record of such factorial interactions in Fucus spp. germlings. Intraspecific competition, estimated as the relative reduction in germling growth and survival from low to high densities, increased with decreasing nutrient concentration and increasing temperature in both species. While temperature and nutrient concentration had little effect on germling size distributions, size inequality and skewness generally increased with germling density, indicating that a few large individuals gained dominance and suppressed many smaller ones at high density. Self-thinning increased with settlement density and depended on nutrient concentration and species at high density. At high density, self-thinning increased with decreasing nutrient levels in F. evanescens, but not in F. serratus. At low density, nutrient enrichment increased germling growth in F. evanescens, but not in F. serratus, whereas growth in both species was stimulated by nutrient enrichment at higher densities. These results suggest that germling growth and self-thinning are more sensitive to variation in nutrient concentration in F. evanescens than in F. serratus. The potential implications of our findings for the understanding of eutrophication-related abundance changes in both species in southern Norway are discussed.Communicated by L. Hagerman, Helsingør     

Heart rate reflects osmostic stress levels in two introduced colonial ascidians <Emphasis Type="Italic">Botryllus schlosseri</Emphasis> and <Emphasis Type="Italic">Botrylloides</Emphasis><Emphasis Type="Italic">violaceus</Emphasis>

The influence of abiotic factors on the establishment and success of invasive species is often difficult to determine for most marine ecosystems. However, examining this relationship is critical for predicting the spread of invasive species and predicting which habitats will be most vulnerable to invasion. Here we examine the mortality and physiological sensitivity to salinity of adult colonies of the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. Adult colonies of each species were exposed to abrupt changes in salinity (5, 10, 15, 20, 25, 30 psu) in the laboratory. Salinity ranges used in the laboratory corresponded with those of the field distributions of B. violaceus and B. schlosseri in the Great Bay Estuary, NH. Heart rate was used as a proxy for health to assess the condition of individual colonies. Heart rates were monitored daily for approximately 2 weeks. Results revealed that both species experienced 100% mortality after 1 day at 5 psu and that their heart rates declined with decreasing salinity. Heart rates of B. schlosseri remained consistent between 15 and 30 psu and slowed at 10 psu. Heart rates of B. violaceus remained constant between 20 and 30 psu, but slowed at 15 psu. These laboratory results corresponded to the distribution of these species in estuaries, indicating salinity is a key factor in the distribution and dominance of B. schlosseri and B. violaceus among coastal and estuarine sites. Furthermore, physiological differences to salinity were found between colonies of B. schlosseri in the Venetian Lagoon and colonies in Portsmouth Harbor, suggesting adaptation to environmental variables.     

Predation on egg capsules of <Emphasis Type="Italic">Zidona</Emphasis><Emphasis Type="Italic">dufresnei</Emphasis> (Volutidae): ecological implications

Among the diverse patterns of energy allocation to the offspring of gastropods, the presence of egg capsules to protect embryos is common. Females of the edible snail Zidona dufresnei attach egg capsules to hard substrates in shallow Argentine Patagonian waters (40°45′S, 64°56′W) during spring-summer. Embryonic development takes about 30 days at 22°C. In this study, three likely capsule predator species and the marks left by each on egg capsule walls were identified in laboratory experiments in February 2010. Abundances of predators and egg capsules with evidence of predation were assessed in the field in the summers of 2010 and 2011. Under laboratory conditions (N = 10 replicates per treatment and control), the predation rate by the chiton Chaetopleura isabellei was the highest (up to 90%), followed by the gastropod Tegula patagonica and the crab Neohelice granulata (~20% each). Nearly 60% of 41 capsules found in the field showed signs of predation. According to the marks identified in the laboratory, C. isabellei was responsible for 79% of this predation, and T. patagonica for the rest. Predation appears to be important during the encapsulated early life and could be an agent for selecting for resistant capsule walls and a relatively shorter development time.     

Geographical genetic structure and phylogeography of the <Emphasis Type="Italic">Sargassum horneri</Emphasis>/<Emphasis Type="Italic">filicinum</Emphasis> complex in Japan,based on the mitochondrial <Emphasis Type="Italic">cox3</Emphasis> haplotype

The genetic structure and phylogeography of the brown seaweed Sargassum horneri/filicinum complex in Japan were studied based on the mitochondrial cox3 haplotype. The cox3 haplotypes found were divided into three clades in a statistical parsimony network, among which there were large numbers of steps. Contrary to the reported large amount of drifting S. horneri along the Japanese coast, the three clades were dividedly distributed on the Japanese coast: the northern Pacific, the central Pacific, and western Japan. The western Japan S. horneri had haplotypes that were phylogenetically closer to those of S. filicinum than to the northern and central Pacific S. horneri populations. The S. filicinum populations were included within the western Japan clade and grouped together with the S. horneri samples from western Japan. Taken together with the unstable morphological diagnosis, this result suggests that S. filicinum should be reduced into a synonymy of S. horneri. The TMRCA analysis suggested that the divergence time of each clade may go back to the last interglacial period and a skyline plot suggested that the last glacial maximum had only a small effect on the population size of S. horneri. The geographic subdivision of the three groups, in spite of a large amount of drifting mats, suggests a limited contribution of drifting mats to gene flow on a large geographic scale. On a small geographic scale, a small number of haplotypes were shared between S. horneri-type and S. filicinum-type populations. This result suggests that populations of these two types are partially, though not completely, isolated from each other, possibly by selfing in S. filicinum-type populations or by a difference in peak reproduction.     

Feeding selectivities of the marine cladocerans<Emphasis Type="Italic"> Penilia avirostris</Emphasis>,<Emphasis Type="Italic"> Podon intermedius</Emphasis> and<Emphasis Type="Italic"> Evadne nordmanni</Emphasis>

We conducted grazing experiments with the three marine cladoceran genera Penilia, Podon and Evadne, with Penilia avirostris feeding on plankton communities from Blanes Bay (NW Mediterranean, Spain), covering a wide range of food concentrations (0.02–8.8 mm³ l^–1, plankton assemblages grown in mesocosms at different nutrient levels), and with Podon intermedius and Evadne nordmanni feeding on the plankton community found in summer in Hopavågen Fjord (NE Atlantic, Norway, 0.4 mm³ l^–1). P. avirostris and P. intermedius showed bell-shaped grazing spectra. Both species reached highest grazing coefficients at similar food sizes, i.e. when the food organisms ranged between 15 and 70 µm and between 7.5 and 70 µm at their longest linear extensions, respectively. E. nordmanni preferred organisms of around 125 µm, but also showed high grazing coefficients for particles of around 10 µm, while grazing coefficients for intermediate food sizes were low. Lower size limits were >2.5 µm, for all cladocerans. P. avirostris showed upper food size limits of 100 µm length (longest linear extension) and of 37.5 µm particle width. Upper size limits for P. intermedius were 135 µm long and 60 µm wide; those for E. nordmanni were 210 µm long and 60 µm wide. Effective food concentration (EFC) followed a domed curve with increasing nutrient enrichment for P. avirostris; maximum values were at intermediate enrichment levels. The EFC was significantly higher for P. intermedius than for E. nordmanni. With increasing food concentrations, the clearance rates of P. avirostris showed a curvilinear response, with a narrow modal range; ingestion rates indicated a rectilinear functional response. Mean clearance rates of P. avirostris, P. intermedius and E. nordmanni were 25.5, 18.0 and 19.3 ml ind.^–1 day^–1, respectively. Ingestion rates at similar food concentrations (0.4 mm³ l^–1) were 0.6, 0.8 and 0.9 g C ind.^–1 day^–1.Communicated by O. Kinne, Oldendorf/Luhe     

Patterns of gamete incompatibility between the blue mussels<Emphasis Type="Italic"> Mytilus edulis</Emphasis> and<Emphasis Type="Italic"> M</Emphasis>.<Emphasis Type="Italic"> trossulus</Emphasis>

Previous research on gametic incompatibility in marine invertebrates suggests that for highly dispersive marine invertebrate species, barriers to fertilization among closely related taxa are often incomplete and sometimes asymmetric. The nature of these barriers can dramatically affect the patterns of gene flow and genetic differentiation between species, and thus speciation. Blue mussels, in the genus Mytilus, are genetically distinct in allopatry yet hybrids are present wherever any two species within the group co-occur. The present study sampled M. edulis (L.) and M. trossulus (Gould) in May and June 2001 from the East Bay section of Cobscook Bay, Maine, USA (latitude 44°56′30″N; longitude 67°07′50″W), where the two species are sympatric. Gamete incompatibility was investigated in a series of laboratory fertilizations carried out in July 2001. The proportion of fertilized eggs typically exceeded 80% at sperm concentrations of 10³–10⁴ ml^?1 among intraspecific matings (n=18), but was <30% even at sperm concentrations in excess of 10⁵–10⁶ ml^?1 for interspecific matings (n=13). Further analysis indicated that approximately 100- to 700-fold higher sperm concentrations were required to achieve 20% fertilization in interspecific matings relative to intraspecific matings, indicating strong barriers to interspecific fertilization. The proportion of fertilized eggs did not follow this general pattern in all matings, however. The eggs from two (out of five) M. edulis females were almost as receptive to M. trossulus sperm as they were to M. edulis sperm. In contrast, the eggs from all M. trossulus females (n=3) were unreceptive to M. edulis sperm, suggesting that fertilization barriers between these species may be asymmetric. Given the experimental design employed in this study, the results are also consistent with a strong maternal or egg effect on the level of interspecific gamete compatibility in M. edulis.