The relationship between mating system and simultaneous hermaphroditism in the coral reef fish, Hypoplectrus nigricans (Serranidae)

The behaviour and reproductive ecology of the simultaneously hermaphroditic coral reef fish, Hypoplectrus nigricans (Serranidae), the black hamlet, was investigated in an effort to elucidate the relationship between simultaneous hermaphroditism and mating behaviour. The major features of the mating system are as follows. (1) Hamlets spawn only in pairs, one partner releasing eggs and the other fertilizing them. There is no selfing. (2) Eggs are planktonic. There is no parental care. (3) The major courtship display serves to advertise that an individual has ripe eggs. (4) A clutch is not released in a single spawn but is parcelled over several, usually with the same partner. (5) Partners generally alternate sex roles with each spawn: that is, they take turns giving up parcels to be fertilized. This active reciprocation of release of eggs, called egg trading, means that reproductive success as a male depends upon the ability to reproduce as a female. Since most of the reproductive effort of an individual is spent on female functions, egg trading provides a fecundity advantage to hermaphroditism, making it evolutionarily stable relative to dioecy (separate sexes). The advantage is analogous to that which parthenogenetic organisms have over sexual ones. The fecundity advantage under egg trading can account for the maintenance, but not the origin of simultaneous hermaphroditism, since hermaphroditism must be established before egg trading can evolve.     

Sexual selection and reproductive success in hermaphroditic seabasses

Mating behavior in simultaneously hermaphroditic seabasses hasbeen often cited as an example of cooperation among unrelatedconspecifics. The predominant mating behavior in this groupinvolves egg trading, where individuals reciprocally fertilizeparcels of eggs from a partner. Egg trading has been suggestedas a good example of a tit-for-tat cooperative mating strategy.Although simultaneous hermaphroditic fishes are often held upas strong examples of cooperation in mating behavior, a closerexamination reveals significant sexual selection and sexualconflict between male and female roles among individuals. Inthe 7 species where data exist, there is a significant increasein male reproductive success with individual size, and in allbut 1 species success through male function increases fasterthan reproductive success through female function. Despite thismale-size advantage in simultaneous hermaphrodites, most speciesmaintain their hermaphroditism for their entire life, and theincreased male allocation while engaging in biased forms ofreciprocation appear to increase the evolutionary stabilityof hermaphroditism in these species. Thus, egg-trading behavioris probably more complicated than was initially recognized,with individuals releasing different numbers of eggs in spawns,spawning at different rates as males and females, and partitioningmale effort between pair and alternative mating tactics. Thedepartures from equal reciprocity can probably be best understoodby including aspects of traditional mating-system theory, withindividuals increasing male mating success through a varietyof behavioral tactics.     

SEX ALLOCATION AND MALE FITNESS GAIN IN A COLONIAL,HERMAPHRODITIC MARINE INVERTEBRATE

While simultaneous hermaphroditism occurs in most animal phyla, theories for its adaptive significance remain untested. Sex allocation theory predicts that combined sexes are favored in sedentary and sessile organisms because localized gamete dispersal and local mate competition (LMC) among gametes promote decelerating fitness “gain curves” that relate male investment to reproductive success. Under this LMC model, males fertilize all locally available eggs at low sperm output, additional output leads to proportionally fewer fertilizations, and combined sexes with female-biased sex allocation are favored. Decelerating male gain curves have been found in hermaphroditic flowering plants, but the present paper reports the first analysis in an animal. The colonial hermaphroditic bryozoan Celleporella hyalina forms unisexual male and female zooids that can be counted to estimate absolute and relative gender allocations. I placed “sperm donor” colonies—each with different numbers of male zooids, and each homozygous for diagnostic allozyme alleles—among target maternal colonies on field mating arrays, and estimated donor fertilization success by scoring allozyme markers in target-colony progeny. Fertilization success increased with numbers of donor male zooids, but linear and not decelerating curves fit the data best. Mean sex allocation was not female biased, consistent with nondecelerating male gain. Sperm donors, moreover, did not monopolize matings as expected under high LMC, but rather shared paternity with rival colonies. Hence localized water-borne gamete dispersal alone may not yield decelerating male gain and favor the maintenance of hermaphroditism; relaxed sperm competition in low density populations might also be required. In free-spawning marine organisms, males cannot control access to fertilizations, intense sperm competition may be commonplace, and high male sex allocation may be selected to enhance siring success under competition.     

The effect of cryptic female choice on sex allocation in simultaneous hermaphrodites

Sex allocation theory for simultaneous hermaphrodites has focused primarily on the effects of sperm competition, but the role of mate choice has so far been neglected. We present a model to study the coevolution of cryptic female choice and sex allocation in simultaneous hermaphrodites. We show that the mechanism of cryptic female choice has a strong effect on the evolutionary outcome: if individuals remove a fixed proportion of less-preferred sperm, the optimal sex allocation is more female biased (i.e. more biased towards egg production) than without cryptic female choice; conversely, if a fixed amount of sperm is removed, sex allocation is less female-biased than without cryptic female choice, and can easily become male biased (i.e. biased towards sperm production). Under male-biased sex allocation, hermaphroditism can become unstable and the population can split into pure males and hermaphrodites with a female-biased allocation. We discuss the idea that the evolution of sex allocation may depend on the outcome of sexual conflict over the fate of received sperm: the sperm donor may attempt to manipulate or by-pass cryptic female choice and the sperm recipient is expected to resist such manipulation. We conclude that cryptic female choice can have a strong influence on sex allocation in simultaneous hermaphrodites and strongly encourage empirical work on this question.     

Reproductive behavior,egg trading,and correlates of male mating success in the simultaneous hermaphrodite,Serranus tabacarius

Synopsis Serranus tabacarius (Serranidae), the tobaccofish, is a simultaneous hermaphrodite which belongs to a group of seabasses that exhibit a wide variety of social and mating systems. The reproductive behavior of tobaccofish is similar to other hermaphroditic seabasses, with individuals assuming sex-specific spawning behaviors that allow for the assignment of male and female roles in a mating sequence. Virtually all matings involved pairs of individuals, although streaking, an alternative male mating tactic, was observed once. Pairs engage in egg trading, where individuals divide their daily clutch into a series of sequentially released parcels and take turns releasing eggs for their partner to fertilize. Individuals mate over a late afternoon spawning period with a number of partners sequentially. Larger individuals have both more total matings and more spawning partners. Egg trading is not symmetrical, the number of male and female matings for an individual in a spawning sequence is often unequal. Overall, the ratio of male to female matings increases with individual size. Large individuals are socially dominant, chase conspecifics during the reproductive period, and are more likely to end a spawning bout with a partner immediately after mating in the male role. In addition, larger individuals are less likely to reciprocate female matings by a partner, either by only mating once (as a male) in a spawning bout or by mating consecutively as a male within a series of matings. Although larger individuals show this relative specialization in the male role, they maintain their simultaneous hermaphroditism and obtain a substantial percentage of their mating success through female function. Egg trading appears to reduce the opportunity for large individuals to specialize as pure males, and thus interacts with the environmental potential for polygamy in shaping the mating system and sex allocation pattern in this species.     

Hermaphroditism: What's not to like?

Hermaphroditism is rare and phylogenically in decline among animal species. The evolutionary basis for this development is not well understood. This paper focusses on self-incompatible simultaneous hermaphroditism in animals. It proposes that such hermaphroditism is not stable in sufficiently heterogeneous populations, suggesting a possible reason for why hermaphroditism is rare among evolved animal species. The argument turns on the Bateman principle, namely that male reproductive success (RS) is limited by partner availability, while female RS is not. We show that: low-quality individuals do better if female; secondary sexual differentiation may be important for understanding the existence of males; and that hermaphroditic mating is reciprocal. Reciprocity may be key to understanding promiscuity and attendant phenomena such as cryptic female choice, sperm competition and love darts-common features of hermaphroditic mating. We also argue that hermaphrodites are especially vulnerable to male violence, suggesting a reason for the rarity of trioecy. Finally, we propose that external fertilization, and the scope for streaking, may be one reason fish are the only simultaneously hermaphroditic vertebrates.     

Social situation,sperm competition and sex allocation in a simultaneous hermaphrodite parasite,the cestode Schistocephalus solidus

Evolutionary theory predicts an influence of mating group size on sex allocation in simultaneous hermaphrodites. We experimentally manipulated the social situation during reproduction in a simultaneous hermaphrodite parasite, the tapeworm Schistocephalus solidus, by placing worms as singles, pairs or triplets into an in vitro system that replaces the final host. We then determined the reproductive allocation patterns after 24 h (i.e. before the start of egg release) and after 72 h (i.e. around the peak of egg release rate) using stereology. After 24 h, sex allocation strongly depended on worm volume (which is determined in the second intermediate host), but was not significantly affected by the social situation experienced during reproduction. After 72 h, worms in groups had less vesicular sperm (i.e. sperm to be used in future inseminations) than singles. They also stored significantly more received sperm in their seminal receptacles than singles, suggesting that more sperm had been transferred in groups. Moreover, worms in triplets stored significantly more received sperm than worms in pairs, suggesting that they either mated more often and/or transferred more sperm per mating. This suggests a behavioural response to the increased risk of sperm competition in triplets. We further discuss the relative importance of sex allocation decisions at different life‐history stages.     

Sperm competition and the dynamics of X chromosome drive: stability and extinction

Several empirical studies of sperm competition in populations polymorphic for a driving X chromosome have revealed that Sex-ratio males (those carrying a driving X) are at a disadvantage relative to Standard males. Because the frequency of the driving X chromosome determines the population-level sex ratio and thus alters male and female mating rates, the evolutionary consequences of sperm competition for sex chromosome meiotic drive are subtle. As the SR allele increases in frequency, the ratio of females to males also increases, causing an increase in the male mating rate and a decrease in the female mating rate. While the former change may exacerbate the disadvantage of Sex-ratio males during sperm competition, the latter change decreases the incidence of sperm competition within the population. We analyze a model of the effects of sperm competition on a driving X chromosome and show that these opposing trends in male and female mating rates can result in two coexisting locally stable equilibria, one corresponding to a balanced polymorphism of the SR and ST alleles and the second to fixation of the ST allele. Stochastic fluctuations of either the population sex ratio or the SR frequency can then drive the population away from the balanced polymorphism and into the basin of attraction for the second equilibrium, resulting in fixation of the SR allele and extinction of the population.     

Sperm trading and sex roles in the hermaphroditic opisthobranch sea slug Navanax inermis : eager females or opportunistic males?

The sea slug Navanax inermis (Gastropoda, Opisthobranchia) is a well‐known example of a simultaneous hermaphrodite in which mating partners trade sperm. According to previous work by others, sperm trading follows from a general preference for the female role when the expected variance in reproductive success in the male role is higher and the risk of failure therefore larger. However, this view contradicts theoretical and empirical studies of other systems, which predict a general preference for the male role; sperm trading is assumed to follow from the fact that individuals benefit from sperm receipt as a nutritional compensation for sperm investment. In this study, we investigate the behaviour of N. inermis in more detail. In addition to observations of regular pairs, we also paired individuals with a partner that had been isolated for 33 days in order to induce changes in sex‐role preference in the non‐isolated partner. We also collected all clutches produced throughout the study to check for signs of infertility as a consequence of allosperm depletion. Fertility of field‐collected and isolated individuals suggested that sperm depletion occurs under natural conditions and may be caused by a lack of partners. Although this argues in favour of female preference, low mating rates both reduce variance in male reproductive success and remove the intent to use sperm for nutritional purposes, thus eliminating the conditions under which both hypotheses are intended to operate. The observational data indicate that animals are eager to mate as males, particularly at the beginning of a mating session. Intromissions lasted longer when a simultaneous intromission was received from the partner. Increases in intromission were recorded in non‐isolated individuals with partners which were previously isolated and therefore more attractive as females. This response would not have been expected were the female role the preferred one. A summary of the arguments concerning preference for either gender suggests some preference for the male role, but indicates that sexual preferences may actually change or become neutral within an individual, even in the course of a mating session. Overall, our results clearly confirm previously published observations of sperm trading in N. inermis . © 2003 The Linnean Society of London. Biological Journal of the Linnean Society, 2003, 78 , 105?116.     

No Plastic Responses to Experimental Manipulation of Sperm Competition per se in a Free‐Living Flatworm

In the absence of sperm competition evolutionary theory predicts low mating rates and low ejaculate expenditure per mating, and sex allocation theory for simultaneous hermaphrodites predicts a strongly female‐biased sex allocation. In the presence of sperm competition a shift towards a more male‐biased sex allocation and a higher ejaculate expenditure are predicted. The free‐living flatworm Macrostomum lignano has been shown to respond plastically in mating rate, testis size, and sperm transfer to manipulation of the social group size, a proxy of the strength of sperm competition. However, manipulation of social group size may manipulate not only sperm competition, but also other factors, such as food supply and metabolite concentration. In this study we therefore manipulated sperm competition per se by repeatedly exposing individuals to partners that have either mated with rivals or not, while keeping the social group size constant. Our results suggest that M. lignano does not have the ability to detect sperm competition per se, as worms experimentally exposed to the presence or absence of sperm competition did not differ in sex allocation, sperm transfer or mating behavior. A response to our manipulation would have required individual recognition, the ability to detect self‐referencing tags, or tags or traces left by rivals on or in the mating partners. We first discuss the possibility that highly efficient sperm displacement may have decreased the difference between the treatment groups and then propose three alternative cues that may allow M. lignano to respond plastically to the social group size manipulation used in earlier studies: assessment of the mating rate, chemical cues, or tactile cues.     

Random mating with respect to mating status in the simultaneously hermaphroditic land snail Arianta arbustorum

In promiscuous species with sperm storage, males are expected to show a preference for mating with virgin and young females to reduce the risk of sperm competition. In the simultaneous hermaphrodite land snail Arianta arbustorum, sperm production precedes egg production by 2–4 weeks, resulting in a short period of protandric hermaphroditism before shell growth is completed. In natural populations, copulating pairs involving individuals which have not yet completed shell growth (virgins) have been observed. We ran a series of mate-choice experiments to examine whether virgin and nonvirgin (experienced) individuals of A. arbustorum discriminate between virgin and nonvirgin mating partners. We also assessed the number of sperm delivered to partners with different mating status. Neither virgin nor nonvirgin snails showed any preference for mating with a virgin partner. In all test situations mating was random and the number of sperm delivered was not adjusted to the mating status of the partner. Mating success was mainly determined by the activity of the individual. The random mating pattern does not imply random fertilization of eggs because the presence of a sperm-digesting organ and the morphology of the sperm storage organ allow a selective storage and use of sperm in A. arbustorum.     

Dimorphic ejaculates and sperm release strategies associated with alternative mating behaviors in the squid

Sperm competition is a powerful postcopulatory selective force influencing male adaptations associated with increasing fertilization success, and it is usually related to the evolution of different strategies of ejaculate expenditure between individuals. Ejaculates may also be influenced by additional selective pressures associated with sperm competition, such as timing between insemination and fertilization, female reproductive tract morphology, and fertilization environment. Also, males that adopt alternative mating tactics may face distinct sperm competition pressures, which may lead to the evolution of intraspecific diversity in ejaculates. In loliginid squids, males with alternative reproductive tactics (sneakers and consorts) differ not only in mating behavior, but also transfer spermatophores into two distinct sites within the female. Here, we compared structure and functioning of spermatophores between sneakers and consorts in the squid Doryteuthis plei applying microscopy techniques and in vitro experiments. Sneakers and consorts exhibit differences in spermatophore structure that lead to distinct spermatophoric reactions and spermatangium morphologies. Moreover, in sneakers, sperm release lasts longer and their sperm show an aggregative behavior not detected in consorts. Slow sperm release may be a strategy to guarantee longer sperm provision, given the wide interval between sneaker mating and egg release. For consorts, in turn, intense and quick sperm discharge may be advantageous, as timing between mating and egg‐laying is relatively short. Within the complex squid mating system, factors such as (i) different fertilization sites and (ii) interval between mating and egg release may also influence sperm competition, and ultimately shape the evolution of divergent ejaculates between dimorphic males.     

Reproductive costs of mating with a sibling male: sperm depletion and multiple mating in Cephalonomia hyalinipennis

Polygynous parasitoid males may be limited by the amount of sperm they can transmit to females, which in turn may become sperm limited. In this study, I tested the effect of male mating history on copula duration, female fecundity, and offspring sex ratio, and the likelihood that females will have multiple mates, in the gregarious parasitoid Cephalonomia hyalinipennis Ashmead (Hymenoptera: Bethylidae: Epyrinae), a likely candidate for sperm depletion due to its local mate competition system. Males were eager to mate with the seven females presented in rapid succession. Copula duration did not differ with male mating history, but latency before a first mating was significantly longer than before consecutive matings. Male mating history had no bearing on female fecundity (number of offspring), but significantly influenced offspring sex ratio. The last female to mate with a given male produced significantly more male offspring than the first one, and eventually became sperm depleted. In contrast, the offspring sex ratio of first‐mated females was female biased, denoting a high degree of sex allocation control. Once‐mated females, whether sperm‐depleted or not, accepted a second mating after a period of oviposition. Sperm‐depleted females resumed production of fertilized eggs after a second mating. Young, recently mated females also accepted a second mating, but extended in‐copula courtship was observed. Carrying out multiple matings in this species thus seems to reduce the cost of being constrained to produce only haploid males after accepting copulation with a sperm‐depleted male. I discuss the reproductive fitness costs that females experience when mating solely with their sibling males and the reproductive fitness gain of males that persist in mating, even when almost sperm‐depleted. Behavioural observations support the hypothesis that females monitor their sperm stock. It is concluded that C. hyalinipennis is a species with a partial local mating system.     

Gregariousness and protandry promote reproductive insurance in the invasive gastropod Crepidula fornicata: evidence from assignment of larval paternity

According to the size-advantage hypothesis, protandric sequential hermaphroditism is expected when the increase in reproductive success with age or size is small for males but large for females. Interestingly, some protandrous molluscs have developed gregarious strategies that might enhance male reproductive success but at the cost of intraspecific competition. The gastropod Crepidula fornicata, a European invading species, is ideal for investigating mating patterns in a sequential hermaphrodite in relation to grouping behaviour because individuals of different size (age) live in perennial stacks, fertilization is internal and embryos are brooded. Paternity analyses were undertaken in stacks sampled in three close and recently invaded sites in Brittany, France. Paternity assignment of 239 larvae, sampled from a set of 18 brooding females and carried out using five microsatellite loci, revealed that 92% of the crosses occurred between individuals located in the same stack. These stacks thus function as independent mating groups in which individuals may reproduce consecutively as male and female over a short time period, a pattern explained by sperm storage capacity. Gregariousness and sex reversal are promoting reproductive insurance in this species. In addition, females are usually fertilized by several males (78% of the broods were multiply sired) occupying any position within the stack, a result reinforcing the hypothesis of sperm competition. Our study pointed out that mating behaviours and patterns of gender allocation varied in concert across sites suggesting that multiple paternities might enhance sex reversal depending on sperm competition intensity.     

Mating Behaviour of the Sperm Trading Sea Slug Chelidonura hirundinina: Repeated Sex Role Alternation Balances Reciprocity

In animals with separate sexes, male fitness usually increases with the number of matings, whereas female fitness more directly depends on the amount of accessible reproductive resources. In simultaneous hermaphrodites, such differences in fitness pay‐offs between male and female sexual function can result in a preference to copulate in one particular sex role, generating conflicts over mating roles if mates share the same preference. Sperm trading, i.e. the conditional exchange of sperm between mates as found in some hermaphrodites, is often considered a possible solution for the conflict over mating roles. Conditional sperm exchange has recently been demonstrated in Chelidonura hirundinina (Opisthobranchia, Aglajidae), but its functional causes remain obscure. Based on a detailed characterization of mating in this species, we here assess two potential benefits of sperm trading, the balancing of sperm exchange between partners, and the acquisition of information about the partner’s fecundity. We found that the number of sperm droplets exchanged between partners varied more between than within pairs, supporting the first hypothesis. Moreover, larger individuals donated more sperm droplets and are known to produce more eggs. As body size is tightly linked to fecundity, a sperm recipient may use the number of received sperm droplets as an honest signal of the female quality of the sperm donor. Our findings thus help to elucidate how sperm trading may contribute to optimizing the investment of costly sperm in response to partner quality.     

Testing three models on the adaptive significance of protandric simultaneous hermaphroditism in a marine shrimp

Protandric simultaneous hermaphroditism, as reported for shrimps in the genus Lysmata, is a sexual system in which individuals invariably reproduce as males first and later in life as simultaneous hermaphrodites. I tested three models (i.e., sex-dependent energetic costs, sex-dependent mortality rates and sex-dependent time commitments) in an attempt to explain the adaptive value of protandric simultaneous hermaphroditism in the shrimp L. wurdemanni. Specific assumptions and predictions of each model were evaluated using manipulative experiments. In the laboratory, males grew faster than simultaneous hermaphrodites of the same size and age, an indication that the female function incurs higher energetic costs of reproduction than the male function. Also, large SHPs were more successful in monopolizing food than small males. The sex-dependent growth rate and size-dependent resource holding power agree with predictions of the sex-dependent energetic cost model. The time that simultaneous hermaphrodites required for replenishing their sperm reservoirs after mating as males was much shorter (2 days) than the time required to brood one clutch of embryos (11 days). Also, small simultaneous hermaphrodites experienced heavier mortality due to predatory fishes than large ones. The sex-dependent reproductive time commitment and size-dependent mortality agree with predictions of the sex-dependent time commitment model. Conversely, I found no evidence that the sex-dependent mortality model explains protandric simultaneous hermaphroditism in the studied species. In contrast to model predictions, mortality due to predatory fishes suffered by simultaneous hermaphrodites was not greater than that suffered by males of the same body size. In L. wurdemanni, the relationship between sex-specific investment and reproductive success seems to change during ontogeny in a way that is consistent with an adaptive adjustment of sex allocation to improve age-specific reproductive success.     

Sexual selection: lessons from hermaphrodite mating systems

Over the last 130 years, research has established that (a) sexualselection exists and is widespread in the plant and animal kingdoms;(b) it does not necessarily entail sexual dimorphism; even hermaphroditeshave it; (c) it does not require intelligence or a sophisticatedsense of esthetics; even tapeworms and plants choose mates;and (d) it does not require brawn or even mobility for competition;plants may compete for pollinators, and broadcast spawning invertebratesmay also compete for matings. Although discussions of sexualselection often focus on sexual dimorphism, several phenomenathat are commonly associated with sexual selection are widespreadand highly developed in hermaphrodites. These phenomena include(a) bizarre and expensive courtship and copulatory behavior,(b) multiple mating and sperm competition, (c) rapid evolutionof genitalia, (d) special structures associated with courtship,and (e) sexual polymorphism. The skewed breeding sex ratiosassociated with sequential hermaphroditism have long been recognizedas contributory to sexual selection. In many simultaneous hermaphrodites,although the sex ratio at mating may be one to one, the actualreproductive sex ratio may also be skewed, creating a high potentialfor sexual selection. Reproductive biology in hermaphroditictaxa also involves a lot of complexity unknown in dioecioustaxa, such as sex change, facultative sex allocation and conditionalreciprocity that offers opportunities to enrich our understandingof sexual selection and to test the assumptions and predictionsof theory.     

Bateman's Principle and Simultaneous Hermaphrodites: A Paradox

Bateman's principle states that reproductive success is limiteda) in females by the resources available for egg production;and b) in males, only by access to females and/or eggs. Theprinciple has been used to generate predictions for two aspectsof hermaphroditism; a) the advantage of hermaphroditism andb) sexual conflict. Comparing these predictions to the empiricaldata offers tests of Bateman's principle. Charnov's predictionthat hermaphroditism would occur under circumstances where Bateman'sprinciple does not apply is found to be largely correct. However,the prediction as to the association of hermaphroditism andlow fixed costs is inconsistent with the data. Alternative explanationsthat predict that hermaphroditism is a strategy for reducingvariance in reproductive success may better explain the data.Probability theory demonstrates that where two strategies haveequal mean fitness, which must be the case for male and femalefunction, the strategy with the lower variance in reproductivesuccess must have higher fitness (Gillespie's principle). Bateman'sprinciple predicts that this will be the female role in hermaphrodites.However, Charnov, assuming Bateman's principle, predicted thatsexual conflict stemming from a preference for the male rolewould be important in hermaphrodite mating systems, creatinga paradox. Many hermaphrodite mating systems are based on conditionalreciprocity with a preferred sexual role indicating sexual conflict.The data demonstrate that the preferred role varies among taxa,contrary to the predictions of Bateman's principle. It has beensuggested that Bateman's principle can explain cases in whichthe female role is preferred (sperm-trading) as involving energyrather than gamete trading. However, energetic considerationssuggest that energy trading would only be adaptive if Bateman'sprinciple does not apply, paradoxically. The gamete tradingmodel, based on the prediction that the role that offers controlof fertilization will be preferred, is more consistent withthe data. Application of Bateman's principle to hermaphroditesleads to contradictory predictions and does not offer the basisfor a coherent theory of sexual selection, as Bateman proposed.     

Does egg competition occur in marine broadcast‐spawners?

When the availability of sperm limits female reproductive success, competition for sperm, may be an important broker of sexual selection. This is because sperm limitation can increase the variance in female reproductive success, resulting in strong selection on females to compete for limited fertilization opportunities. Sperm limitation is probably common in broadcast-spawning marine invertebrates, making these excellent candidates for investigating scramble competition between broods of eggs and its consequences for female reproductive success. Here, we report our findings from a series of experiments that investigate egg competition in the sessile, broadcast-spawning polychaete Galeolaria caespitosa. We initially tested whether the order in which eggs encounter sperm affects their fertilization success at two ecologically relevant current regimes. We used a split-clutch-split--ejaculate technique to compare the fertilization success of eggs from individual females that had either first access (competition-free treatment) or second access (egg competition treatment) to a batch of sperm. We found that fertilization success depended on the order in which eggs accessed sperm; eggs that were assigned to the competition-free treatment exhibited significantly higher fertilization rates than those assigned to the egg competition treatment at both current speeds. In subsequent experiments we found that prior exposure of sperm to eggs significantly reduced both the quantity and quality of sperm available to fertilize a second clutch of eggs, resulting in reductions in fertilization success at high and low sperm concentrations. These findings suggest that female traits that increase the likelihood of sperm-egg interactions (e.g. egg size) will respond to selection imposed by egg competition.     

Determinants of female fecundity in a simultaneous hermaphrodite: the role of polyandry and food availability

Classical sexual selection theory assumes that the reproductive success of females is primarily limited by the resources available for egg production rather than by the number of mating partners. However, there is now accumulating evidence that multiple mating can entail fitness costs or benefits for females. In this study we investigated the effect of polyandry (i.e., the mating with different mating partners) and food availability on the reproductive output of the female sex function in an outcrossing simultaneous hermaphrodite, the free-living flatworm Macrostomum lignano. We exposed virgin worms to different group sizes, a treatment that has previously been shown to affect the level of polyandry in this species. Moreover, we manipulated the food availability throughout the subsequent egg laying period, during which the worms were kept in isolation. The number of offspring produced was used as an estimate of female fecundity. We found that food availability, but not group size, had a significant effect on female fecundity. Additionally, female fecundity was positively correlated with the number of stored sperm in the female sperm-storage organ at the time of isolation, but it was not correlated with body or ovary size of the worms. Our results suggest that female fecundity in M. lignano is primarily determined by the resources available for egg production, and not by the level of polyandry, confirming classic sexual selection theory for simultaneous hermaphrodites.