Sex‐Specific Spawning Behavior and Its Consequences in an External Fertilizer

Quantitative study of the behavior of two broadcast spawners, the sea urchins Strongylocentrotus intermedius and Mesocentrotus nudus, during mass spawning events in situ

Background

The spatial distribution of spawners and temporal parameters of spawning in motile invertebrates with external fertilization might influence reproductive success. However, to date, data on the prespawning and spawning behaviors of broadcast spawners in the field have been scarce and mostly qualitative. The present study was intended to clarify the behavioral adaptations of two sea urchin species, Strongylocentrotus intermedius and Mesocentrotus nudus, using quantitative analysis of their behavior during mass spawning events under natural conditions.

Methods

We analyzed in situ video recordings of sea urchin behavior obtained during six spawning seasons (2014–2019). The total number of specimens of each sea urchin species and the numbers of spawning males and females were counted. Quantitative parameters of sea urchin spawning (numbers of gamete batches, release duration of one gamete batch, time intervals between gamete batches and total duration of spawning) and movement (step length of spawners and nonspawners before and during spawning and changes in distances between males/nonspawners and females) were determined.

Results

For each species, 12 mass spawning events were recorded in which 10 or more individuals participated. The temporal dynamics of the numbers of males and females participating in mass spawning were well synchronized in both species; however, males began to spawn earlier and ended their spawning later than females. In both species, the most significant intersex difference was the longer spawning duration in males due to the longer pause between gamete batches. The total duration of gamete release did not differ significantly between sexes. The average duration of sperm release during mass spawning events was longer than solitary male spawning. Males and females showed significant increases in the locomotion rate 35 min before the start of spawning and continued to actively move during spawning. An increase in movement rate before spawning in males and females was induced by environmental factor(s). Nonspawners of both species showed increased locomotion activity but in the presence of spawning neighbors and less prominently than spawners. On a vertical surface, both echinoids moved strictly upward. On flat surfaces, males, females and nonspawners of both echinoids became closer during spawning.

Discussion

We showed that two sea urchin species with planktotrophic larvae display similar behavioral adaptations aimed at enhancing reproductive success. The high sensitivity of sea urchins, primarily males, to some environmental factors, most likely phytoplankton, may be considered a large-scale adaptation promoting the development of mass spawning events. The longer spawning duration in males and increased movement activity before and during spawning in both sexes may be considered small-scale adaptations promoting approach of males and females and enhancing the chances of egg fertilization.

Morphological stasis masks ecologically divergent coral species on tropical reefs

Coral reefs are the epitome of species diversity, yet the number of described scleractinian coral species, the framework-builders of coral reefs, remains moderate by comparison. DNA sequencing studies are rapidly challenging this notion by exposing a wealth of undescribed diversity, but the evolutionary and ecological significance of this diversity remains largely unclear. Here, we present an annotated genome for one of the most ubiquitous corals in the Indo-Pacific (Pachyseris speciosa) and uncover, through a comprehensive genomic and phenotypic assessment, that it comprises morphologically indistinguishable but ecologically divergent lineages. Demographic modeling based on whole-genome resequencing indicated that morphological crypsis (across micro- and macromorphological traits) was due to ancient morphological stasis rather than recent divergence. Although the lineages occur sympatrically across shallow and mesophotic habitats, extensive genotyping using a rapid molecular assay revealed differentiation of their ecological distributions. Leveraging "common garden" conditions facilitated by the overlapping distributions, we assessed physiological and quantitative skeletal traits and demonstrated concurrent phenotypic differentiation. Lastly, spawning observations of genotyped colonies highlighted the potential role of temporal reproductive isolation in the limited admixture, with consistent genomic signatures in genes related to morphogenesis and reproduction. Overall, our findings demonstrate the presence of ecologically and phenotypically divergent coral species without substantial morphological differentiation and provide new leads into the potential mechanisms facilitating such divergence. More broadly, they indicate that our current taxonomic framework for reef-building corals may be scratching the surface of the ecologically relevant diversity on coral reefs, consequently limiting our ability to protect or restore this diversity effectively.

Synchronized emergence under diatom sperm competition

Appropriate timing of mating is crucial for the success of individuals. However, we know little about factors that explain variation in mating time in unicellular organisms. Unicellular eukaryotes often have facultative sexuality, that is, the less frequent sex is occasionally induced after long clonal reproduction. Thus, males originated from clonemates could be non-negligible mating rivals. Using a centric diatom whose clonal cells differentiate into either male or female, we analysed whether males (spermatogonium) compete or cooperate with each other. By analysing differentiation timing with hypotheses based on evolutionary game theory, we estimated that a substantial part of the variation in the mating timing of the diatom can be explained by results of optimization through interactions among selfish individuals rather than cooperation among clonemates. However, the competition is fiercer than expected owing to excessive synchronization, which was realized by adjustment of meiotic duration: cells completed mitotic division in the earlier mating phase took longer to enter into meiosis, whereas late-dividing cells entered into meiosis more quickly. Adjacent cells tended to synchronize, and model analyses suggest that cell-cell interaction can create a gap between the optimal and actual decisions. Our results provide insights into the evolution of cellular decision making and its restriction.

Life-history traits of Farlowella hahni (Siluriformes, Loricariidae) in streams of the Ivinhema River Basin, Upper Paraná Basin

Abstract Brazilian laws allow the catch of some species as ornamental fish; however, in the absence of biological information, such catch can threaten species conservation. Therefore, this study aimed to offer population and reproductive information about Farlowella hahni in streams of the Ivinhema River Basin, Upper Paraná Basin. To accomplish this, a total of 165 F. hahni individuals were analyzed, including 109 females and 48 males, resulting in an important sex ratio difference from 1:1 (χ 2=19.97; p<0.001), indicating a predominance of females throughout the year. Females reach higher standard length than males. Males presented isometric Length-Weight Relationships, while females have positive allometric growth. Gonadosomatic index presented higher values from October through December. The mean fecundity was 100.3±36.4 of vitellogenic oocytes and was positively correlated with standard length (r=0.51; p=0.01). The data presented herein are the first for this species and suggest a need to carefully manage the F. hahni population, considering its low abundance and fecundity.

Releasing small ejaculates slowly increases per-gamete fertilization success in an external fertilizer: Galeolaria caespitosa (Polychaeta: Serpulidae)

The idea that male reproductive strategies evolve primarily in response to sperm competition is almost axiomatic in evolutionary biology. However, externally fertilizing species, especially broadcast spawners, represent a large and taxonomically diverse group that have long challenged predictions from sperm competition theory-broadcast spawning males often release sperm slowly, with weak resource-dependent allocation to ejaculates despite massive investment in gonads. One possible explanation for these counter-intuitive patterns is that male broadcast spawners experience strong natural selection from the external environment during sperm dispersal. Using a manipulative experiment, we examine how male reproductive success in the absence of sperm competition varies with ejaculate size and rate of sperm release, in the broadcast spawning marine invertebrate Galeolaria caespitosa (Polychaeta: Serpulidae). We find that the benefits of Fast or Slow sperm release depend strongly on ejaculate size, but also that the per-gamete fertilization rate decreases precipitously with ejaculate size. Overall, these results suggest that, if males can facultatively adjust ejaculate size, they should slowly release small amounts of sperm. Recent theory for broadcast spawners predicts that sperm competition can also select for Slow release rates. Taken together, our results and theory suggest that selection often favours Slow ejaculate release rates whether males experience sperm competition or not.

Chemical Ecology of Chemosensation in Asteroidea: Insights Towards Management Strategies of Pest Species

Within the Phylum Echinodermata, the class Asteroidea, commonly known as starfish and sea stars, encompasses a large number of benthos inhabiting genera and species with various feeding modalities including herbivores, carnivores, omnivores and detritivores. The Asteroidea rely on chemosensation throughout their life histories including hunting prey, avoiding or deterring predators, in the formation of spawning aggregations, synchronizing gamete release and targeting appropriate locations for larval settlement. The identities of many of the chemical stimuli that mediate these physiological and behavioural processes remain unresolved even though evidence indicates they play pivotal roles in the functionality of benthic communities. Aspects of chemosensation, as well as putative chemically-mediated behaviours and the molecular mechanisms of chemoreception, within the Asteroidea are reviewed here, with particular reference to the coral reef pest the Crown-of-Thorns starfish Acanthaster planci species complex, in the context of mitigation of population outbreaks.

Chemicals released by male sea cucumber mediate aggregation and spawning behaviours

The importance of chemical communication in reproduction has been demonstrated in many marine broadcast spawners. However, little is known about the use of chemical communication by echinoderms, the nature of the compounds involved and their mechanism(s) of action. Here, the hypothesis that the sea cucumber Holothuria arguinensis uses chemical communication for aggregation and spawning was tested. Water conditioned by males, but not females, attracted both males and females; gonad homogenates and coelomic fluid had no effect on attraction. Male spawning water, but not female spawning water, stimulated males and females to release their gametes; the spermatozoa alone did not induce spawning. H. arguinensis male spawning water also induced spawning in the phylogenetically related H. mammata. This indicates that males release pheromones together with their gametes that induce spawning in conspecifics and possibly sympatric species. Finally, the male pheromone seems to be a mixture with at least one labile compound (biological activity is lost after four hours at ambient temperature) possibly including phosphatidylcholines. The identification of pheromones in sea cucumbers offers a new ecological perspective and may have practical applications for their aquaculture.

Do Sperm Really Compete and Do Eggs Ever Have a Choice? Adult Distribution and Gamete Mixing Influence Sexual Selection, Sexual Conflict, and the Evolution of Gamete Recognition Proteins in the Sea

The evolution of gametic compatibility and the effectiveness of compatibility, within and across species, depend on whether sperm from different males directly compete for an egg and whether eggs ever have a choice. Direct sperm competition and egg choice depend on whether sperm from different males arrive at an egg in the brief interval between first sperm contact and fertilization. Although this process may be relevant for all sexually reproducing organisms, it is most easily examined in aquatic external fertilizers. When sperm are released into the sea, packets of seawater at the spatial scale relevant to single eggs might contain sperm from only one male, eliminating the potential for direct sperm competition and egg choice. Field experiments and a simple heuristic model examining the degree of sperm mixing for the sea urchin Strongylocentrotus franciscanus indicate that degree of competitive fertilization depends on density and distribution of competing males and that the nature of this competition influences whether males with high- or low-affinity gamete recognition protein genotypes have higher reproductive success. These results provide a potential explanation for the generation and maintenance of variation in gamete recognition proteins and why effectiveness of conspecific sperm precedence can be density dependent.

Environmental and biological cues for spawning in the crown-of-thorns starfish

Sporadic outbreaks of the coral-eating crown-of-thorns starfish are likely to be due, at least in part, to spatial and temporal variation in reproductive and settlement success. For gonochoric and broadcast spawning species such as crown-of-thorns starfish, spawning synchrony is fundamental for achieving high rates of fertilization. Highly synchronized gamete release within and among distinct populations is typically the result of the entrainment of neurohormonal endogenous rhythms by cues from the environment. In this study, we conducted multiple spawning assays to test the effects of temperature change, reduced salinity and nutrient enrichment of seawater, phytoplankton, gametes (sperm and eggs), and the combined effect of sperm and phytoplankton on the likelihood of spawning in male and female crown-of-thorns starfish. We also investigated sex-specific responses to each of these potential spawning cues. We found that (1) abrupt temperature change (an increase of 4°C) induced spawning in males, but less so in females; (2) males often spawned in response to the presence of phytoplankton, but none of the females spawned in response to these cues; (3) the presence of sperm in the water column induced males and females to spawn, although additive and synergistic effects of sperm and phytoplankton were not significant; and (4) males are more sensitive to the spawning cues tested and most likely spawn prior to females. We propose that environmental cues act as spawning ‘inducers’ by causing the release of hormones (gonad stimulating substance) in sensitive males, while biological cues (pheromones) from released sperm, in turn, act as spawning ‘synchronizers’ by triggering a hormonal cascade resulting in gamete shedding by conspecifics. Given the immediate temporal linkage between the timing of spawning and fertilization events, variability in the extent and synchronicity of gamete release will significantly influence reproductive success and may account for fluctuations in the abundance of crown-of-thorns starfish.

Sensitivity of sea urchin fertilization to pH varies across a natural pH mosaic

In the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male-female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61-8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm-egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.

The Evolution of Reproductive Phenology in Broadcast Spawners and the Maintenance of Sexually Antagonistic Polymorphism

Reproductive phenology is a crucial life-history trait that evolves in response to external environmental conditions and frequency- and density-dependent interactions within species. Broadcast spawners-which represent a large fraction of aquatic biodiversity-evolve phenologies that balance strong density-dependent fertilization success against abiotic environmental conditions that are required for successful reproduction. The overall balance between these processes may be particularly complex in dioecious species, where selection on reproductive timing potentially differs between the sexes. Here, we develop a population genetic model of reproductive phenology in a dioecious broadcast spawning species and show that environmental variability and density-dependent fertilization dynamics naturally give rise to profound sex differences in selection on gamete release strategies. The frequency-dependent nature of sperm competition generates sexually antagonistic selection on reproductive timing and facilitates the maintenance of genetic variation in phenological traits. Selection in females favors monomorphic spawning phenologies that maximize net fertilization success and offspring survival across environmental conditions, whereas selection in males often favors polymorphic phenologies that are primarily shaped by sperm competition. Our model helps explain several well-documented empirical observations in aquatic species, including high intraspecific variance of reproductive phenologies, sex-specific spawning phenologies, and spawning during environmentally suboptimal times.

Sexual dimorphism in an under-ice spawning fish: the burbot (Lota lota)

Sexual dimorphic characteristics arise in response to differing selective pressures on the sexes and can be used to attract mates or signal reproductive readiness. How sexual dimorphism is expressed where visual cues may be of limited use is an underexplored aspect of reproductive ecology. The burbot (Lota lota (L., 1758)) is a common boreal fish that is not overtly sexually dimorphic. It spawns mid-winter in a light-limited under-ice environment. We examined a variety of morphological and reproductive characteristics in burbot from a northern lake over one full year to assess both seasonal and sex-based variation. Spawning occurred under ice in early February. Seasonal variation was more pronounced in females for many of the traits examined. Growth, fin lengths, swim bladder mass, and liver lipid concentration did not differ between the sexes. Male burbot had significantly higher body condition, larger gas glands, and smaller livers. Males also had significantly larger gonads than females, unusual for boreal fishes. The high gonadal investment of male burbot suggests that sperm competition may be intense in this species. This study demonstrates that sexual dimorphism can be subtle and is present in a seemingly monomorphic species—the burbot.

The effect of alternative mating tactics on the fertilization success of a hermaphroditic seabass

In the simultaneously hermaphroditic marine fish, Serranus subligarius, male role individuals are known to pair spawn, group spawn and streak spawn. While the effects of these common mating tactics on mating success in the male role have been well studied, their consequences for the reproductive success of the individuals taking the female role have received little attention. To investigate those consequences, I observed mating behaviors and quantified fertilization success in natural and experimental settings during the summers of 2005-2008 at three sites with different local population densities. I observed focal individuals in 15-min increments and recorded the total number of spawns, number of streak spawns, size of participating spawners, and fertilization rate. The occurrence of small-sized individuals in the local population is associated with higher frequencies of streaking behavior; these small fish are most often first-year individuals reaching sexual maturity late in the spawning season (August/September). Spawns that included one or more streak spawners had a significantly lower average fertilization rate (89 %) than pair spawns without a streak spawner (97 %). This pattern was confirmed with a field manipulation experiment in which spawning events that included streakers again showed lower fertilization rates (93 %) than spawning events that did not include streakers (98 %). These lower fertilization rates occurred despite the fact that spawns that included multiple males produced, on average, 20 % more sperm than produced in spawns with only a single male. These results indicate that females incur a significant fitness cost when streakers invade a spawning event, a cost not attributable to sperm limitation or any direct effects on the female.

Contemporary evolution of sea urchin gamete-recognition proteins: experimental evidence of density-dependent gamete performance predicts shifts in allele frequencies over time

Species whose reproductive strategies evolved at one density regime might be poorly adapted to other regimes. Field and laboratory experiments on the sea urchin Strongylocentrotus franciscanus examined the influences of the two most common sperm-bindin alleles, which differ at two amino acid sites, on fertilization success. In the field experiment, the arginine/glycine (RG) genotype performed best at low densities and the glycine/arginine (GR) genotype at high densities. In the laboratory experiment, the RG genotype had a higher affinity with available eggs, whereas the GR genotype was less likely to induce polyspermy. These sea urchins can reach 200 years of age. The RG allele dominates in larger/old sea urchins, whereas smaller/younger sea urchins have near-equal RG and GR allele frequencies. A latitudinal cline in RG and GR genotypes is consistent with longer survival of sea urchins in the north and with predominance of RG genotypes in older individuals. The largest/oldest sea urchins were likely conceived at low densities, before sea-urchin predators, such as sea otters, were overharvested and sea-urchin densities exploded off the west coast of North America. Contemporary evolution of gamete-recognition proteins might allow species to adapt to shifts in abundances and reduces the risk of reproductive failure in altered populations.

The role of structured stirring and mixing on gamete dispersal and aggregation in broadcast spawning

Broadcast-spawning benthic invertebrates synchronously release sperm and eggs from separate locations into the surrounding flow, whereupon the process depends on structured stirring by the flow field (at large scales), and sperm motility and taxis (at small scales) to bring the gametes together. The details of the relevant physical and biological aspects of the problem that result in successful and efficient fertilization are not well understood. This review paper includes relevant work from both the physical and biological communities to synthesize a more complete understanding of the processes that govern fertilization success; the focus is on the role of structured stirring on the dispersal and aggregation of gametes. The review also includes a summary of current trends and approaches for numerical and experimental simulations of broadcast spawning.

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.

Influence of sperm and phytoplankton on spawning in the echinoid Lytechinus variegatus

The cues triggering large-scale broadcast-spawning events in marine invertebrates are not fully understood. Using the sea urchin Lytechinus variegatus, we tested the effectiveness of a variety of potential spawning cues in eliciting a spawning response. In the laboratory, during two consecutive spawning seasons, about 400 isolated sea urchins were exposed to phytoplankton, sperm, or eggs, singly or in combination. The likelihood of spawning, time to spawning, and spawning behavior were recorded for both sexes. Sperm was most successful at inducing spawning. No response to eggs was noted. Phytoplankton alone did not trigger spawning, but when a phytoplankton cue was followed by the addition of sperm, spawning behavior was induced, the time between addition of sperm and spawning was reduced, and the variance among individuals in the time of spawning initiation was reduced. Males spawned sooner in response to cues than females and rarely spawned spontaneously in phytoplankton or control treatments. A semilunar pattern in the sensitivity to spawning cues was noted. During time periods when sea urchins were less ripe, the ratio of spawning males to spawning females increased. Our results indicate that seasonal and lunar cycles, together with the presence of phytoplankton, increase the sensitivity of these sea urchins to spawning cues and the precision of their responses to conspecific sperm.

Incomplete reproductive isolation in the blue mussel (Mytilus edulis and M. trossulus) hybrid zone in the Northwest Atlantic: role of gamete interactions and larval viability

Reproductive isolation in free-spawning organisms may involve only small changes in the gamete surface molecules that control fertilization, linking gamete incompatibility and speciation. Most studies have focused on species in which natural hybrids are absent and reproductive isolation is complete, but how gamete incompatibility evolves remains unclear. Reproductive isolation is incomplete between two sympatric mussel species (Mytilus edulis, M. trossulus) that hybridize in nature. In this study prezygotic and postzygotic components of reproductive incompatibility were examined in laboratory crosses. Conspecific crosses showed significantly greater rates of fertilization than heterospecific crosses, although some females of both species showed heterospecific gamete compatibility. The proportion of fertilized eggs developing into normal larvae was not significantly different between conspecific and heterospecific crosses, but survival of normal larvae was greater for conspecific crosses. Mixed-species sperm experiments suggested that conspecific sperm preference may further limit hybridization. The different components of reproductive incompatibility and total incompatibility varied among females of both species. Although our study has shown that partial reproductive isolation between M. edulis and M. trossulus involves both prezygotic gamete interactions and postzygotic larval survival, further research is required to determine the potential role of gamete incompatibility in the evolution of complete reproductive isolation between these species.

Reproductive biology of Echinometra lucunter (Echinodermata: Echinoidea) in a northeast Brazilian sandstone reef

The edible sea urchin Echinometra lucunter (Linnaeus, 1758) is a very common species on the sublittoral-midlittoral in Brazilian rocky shores. The aim of this work was to describe the gametogenesis and reproductive strategy of the E. lucunter population at Muro Alto beach in the Northeast coast of Brazil from August 2004 to August 2005. A total of 240 specimens were collected on the sandstone reef flat from a tidepool during spring low tides. The overall sex ratio was1.12:1,withoutsignificanttemporalvariationexceptinOctober2004. Firsts sexual maturity ocurred in individuals from a diameter of 20.8 mm. There was not a significant difference in gonad index between females and males during the sampling period. The female's gonad index variation was associated with a well-defined spawning, corroborated by the histological analysis of the gonads, which demonstrates sex differences of the gamete production. By contrast, the males showed no clear pattern. It is suggested that continuous reproduction with seasonal peaks in the E. lucunter population occurs at Muro Alto beach.

Gonadossomatic relation and reproductive strategy of Brycon opalinus (Cuvier, 1819) in the Serra do Mar State Park--Núcleo Santa Virgínia, Atlantic Forest, Brazil

The objective of this study was to analyze the gonadossomatic relation of males and females of Brycon opalinus (the Pirapitinga do Sul) in the Serra do Mar State Park--Núcleo Santa Virgínia. The study area included three rivers: Paraibuna, Ipiranga, and Grande. From the gonadossomatic relation, it was possible to determine the reproductive season and strategy of this species. The gonadossomatic relation was high throughout the year for both males and females. Sperm competition and dilution of the ejaculate in the spawning locations favored substantial development of the testicles. The sex ratio of Brycon opalinus was always in favor of females. The existence of sexual dimorphism in this species was confirmed from the larger size of the females.

Sperm release strategies in marine broadcast spawners: the costs of releasing sperm quickly

When under competition for fertilisations, males are thought to increase their reproductive success by releasing as many sperm as possible into the reproductive arena and in many species, this prediction holds. For marine invertebrates, which utilise the ancestral strategy of broadcast spawning eggs and sperm, however, it appears that males tend to release their sperm more slowly than females release their eggs. Marine invertebrate eggs typically have a relatively slow permanent block to polyspermy (whereby eggs become impermeable to further sperm attachment), and for several minutes after fertilisation, sperm can continue to attach to a fertilised egg. We hypothesised that releasing sperm slowly minimises the 'wastage' of sperm on already fertilised eggs. We simulated different sperm release rates in a flume using the broadcast spawning polychaete, Galeolaria caespitosa. Sperm release rates strongly affected overall fertilisation success: higher release rates resulted in lower fertilisation rates. Laboratory studies confirmed that the 'permanent' block to polyspermy in G. caespitosa took less than a minute to form but this lag was sufficient to result in some sperm wastage. Thus upstream, fertilised eggs that have not formed a permanent block to polyspermy can remove sperm from the pool that would otherwise fertilise downstream sibling eggs. We suggest that while electrical blocks to polyspermy evolved in response to excess sperm, permanent blocks to polyspermy could have evolved in response to sperm limitation (insufficient sperm).

THE QUICK AND THE DEAD? SPERM COMPETITION AND SEXUAL CONFLICT IN SEA

Our view of sperm competition is largely shaped by game-theoretic models based on external fertilizers. External fertilization is of particular interest as it is the ancestral mode of reproduction and as such, relevant to the evolution and maintenance of anisogamy (i.e., large eggs and tiny, numerous sperm). Current game-theoretic models have been invaluable in generating predictions of male responses to sperm competition in a range of internal fertilizers but these models are less relevant to marine broadcast spawners, the most common and archetypal external fertilizers. Broadcast spawners typically have incomplete fertilization due to sperm limitation and/or polyspermy (too many sperm), but the effects of incomplete (<100% fertilization rates) fertilization on game-theoretic predictions are unclear particular with regards to polyspermy. We show that incorporating the effects of sperm concentration on fertilization success changes the predictions of a classic game-theoretic model, dramatically reversing the relationship between sperm competition and the evolutionarily stable sperm release strategy. Furthermore, our results suggest that male and female broadcast spawners are likely to be in conflict at both ends of the sperm environment continuum rather than only in conditions of excess sperm as previously thought. Across the majority of the parameter space we explored, males release either too little to too much sperm for females to achieve complete fertilization. This conflict could result in a coevolutionary race that may have led to the evolution of internal fertilization in marine organisms.