Variation in reproductive characteristics of colonies of the freshwater bryozoan Cristatella mucedo

1. We studied relationships between different forms of reproduction and local variation in the reproductive state of colonies of a common freshwater bryozoan Cristatella mucedo Cuvier (Bryozoa: Phylactolaemata). Four sampling locations in central Finland, including both lotic and lentic habitats, were sampled by scuba diving. The traits studied were occurrence of sexual larvae, colony size, number of resting stages (statoblasts) and number of zooids. 2. While 76.9% of the sampled colonies carried statoblasts at the time of sampling, only 4.5% of the colonies were brooding sexual larvae. Most of the colonies were relatively small with a mean colony size of 16.6 mm. 3. In colonies brooding larvae, the number of statoblasts was positively correlated with the number of larvae. We did not detect a colony size threshold for sexual reproduction or formation of statoblasts. Colonies carrying statoblasts had a lower number of zooids per unit dry weight. 4. We found significant variation in reproductive traits of colonies among the sampling locations, and among lotic and lentic habitats. The observed phenotypic differences may reflect broadly distributed, phenotypically plastic clones. Differences in clonal composition of local populations cannot, however, be ruled out and factors that may provide clonal diversity are discussed.     

Will variation among genetic individuals influence species responses to global climate change?

Increased anthropogenic CO₂ emissions in the last two centuries have lead to rising sea surface temperature and falling ocean pH, and it is predicted that current global trends will worsen over the next few decades. There is limited understanding of how genetic variation among individuals will influence the responses of populations and species to these changes. A microcosm system was set up to study the effects of predicted temperature and CO₂ levels on the bryozoan Celleporella hyalina. In this marine species, colonies grow by the addition of male, female and feeding modular individuals (zooids) and can be physically subdivided to produce a clone of genetically identical colonies. We studied colony growth rate (the addition of zooids), reproductive investment (the ratio of sexual to feeding zooids) and sex ratio (male to female zooids) in four genetically distinct clonal lines. There was a significant effect of clone on growth rate, reproductive investment and sex ratio, with clones showing contrasting responses to the various temperature and pH combinations. Overall, decreasing pH and increasing temperature caused reduction of growth, and eventual cessation of growth was often observed at the highest temperature, especially during the latter half of the 15‐day trials. Reproductive investment increased with increasing temperature and decreasing pH, varying more widely with temperature at the lowest pH. The increased production of males, a general stress response of the bryozoan, was seen upon exposure to reduced pH, but was not expressed at the highest temperature tested, presumably due to the frequent cessation of growth. Further to the significant effect of pH on the measured whole‐colony parameters, observation by scanning electron microscopy revealed surface pitting of the calcified exoskeleton in colonies that were exposed to increased acidity. Studying ecologically relevant processes of growth and reproduction, we demonstrate the existence of relevant levels of variation among genetic individuals which may enable future adaptation via non‐mutational natural selection to falling pH and rising temperature.     

Comparative Studies of Ovicell Anatomy and Reproductive Patterns in Cribrilina annulata and Celleporella hyalina (Bryozoa: Cheilostomatida)

Investigations of the common boreal-arctic cheilostomate bryozoans Cribrilina annulata and Celleporella hyalina have shown that the two species possess similar ovicell structures and reproductive patterns. Both species are characterized by frontal dwarf ovicellate zooids, that are female autozooidal polymorphs in C. hyalina and simultaneous hermaphroditic autozooids in C. annulata. The latter species in addition has ovicellate autozooids of the usual type. Each ovicell is formed from a maternal zooid only, and its cavity is lined by the outer hemispherical fold (ooecium) and the distal zooidal wall. The coelomic cavity of the ooecium is separated from the body cavity of the maternal zooid by a transverse wall with simple pores. Each pore is closed by a cell plug, and the ooecia may be considered as kenozooids. Each oocyte is accompanied by a single nurse cell that degenerates after ovulation. The eggs are macrolecithal in C. annulata and microlecithal in C. hyalina, and the former species is a non-placental brooder whereas the latter forms a placenta. Fertilization is precocious. Possible mechanisms of sperm entry as well as oviposition are discussed. The literature concerning ovicell structure and development in cheilostomates is analysed. It is proposed that the brood chamber of cribrimorphs evolved by a fusion of costae and a reduction of the daughter zooid in ancestral forms. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

On the development and reproduction of Botryllus schlossen (Tunicata) colonies from the eastern Mediterranean Sea: plasticity of life history traits

Summary

Analysis of developmental patterns of Botryllus schlosseri colonies from the eastern Mediterranean coast has been performed on 143 genets under different temperature regimens (15,20,27 °C) for up to 22 weeks. While the average maximal size as the length of the blastogenic cycle (BC) varied in respect to water temperature, ontogeny at all temperatures was characterized by 4 developmental stages: 1. the lag phase (the first 1–3 BCs, 1 bud/BC), 2. the exponential growth phase (5.5–7.8 BCs, up to 3 buds/BC), 3. the plateau phase, (7.4–8.2 BCs, 1 bud/BC), 4. the degenerative or the variable phase. Many (40.6%) colonies were not sexually reproductive, the others were male only (30.1%) or hermaphrodites. Colonies at the peak of reproduction develop 1.2 oocytes/zooid, and up to 57.7% produced >4 clutches. Analysing onset of reproduction with maximal colony size revealed 4 patterns, two for “male only” colonies and two for hermaphrodites that varied at different sea water temperatures. In two patterns, the onset of reproduction precedes colony maximal size, and in the others it starts at the peak size or thereafter. Zooids at the colony's periphery developed almost twice as many buds as did zooids at the center, but produced significantly lower numbers of eggs. Fragmentation was recorded in large colonies and was temperature dependent. Results are compared with the data available on populations from other localities indicating for dramatically different developmental modes characteristic to this cosmopolitan species.     

The effects of ambient flow velocity,colony size,and upstream colonies on the feeding success of bryozoa. I. Bugula stolonifera Ryland,an arborescent species

The effects of ambient flow velocity, colony size, and the presence of upstream colonies on the feeding success of the arborescent bryozoan, Bugula stolonifera (Ryland), were studied. Faster ambient flow velocities were found to reduce feeding of zooids of small colonies but not of large colonies. Zooids from different regions of colonies dominated in feeding at different ambient flow velocities: upstream zooids dominated in feeding from slow ambient flow: zooids from central regions dominated in feeding from fast ambient flow. These results are interpreted with respect to the branching morphology of colonies. Finally, evidence that upstream colonies interfere with the feeding success of zooids of colonies downstream was obtained.     

The effects of ambient flow velocity,colony size,and upstream colonies on the feeding success of Bryozoa. II. Conopeum reticulum (Linnaeus), an encrusting species

The effects of ambient flow velocity, colony size, and the presence of an actively-feeding colony upstream on the feeding success of the encrusting bryozoan Conopeum reticulum (Linnaeus) were studied. Zooids from both large and small colonies showed a reduction in feeding as flow velocity increased, however, the reduction in feeding was less for zooids from large colonies except at very fast ambient flow velocities. The greater pumping capacity of large colonies may result in a relatively greater per zooid feeding success from moving water. The presence of an actively-feeding colony upstream was found to enhance the feeding of zooids on downstream colonies. Diversion of flowing water by actively-feeding colonies upstream may account for the observed enhancement of feeding by zooids on colonies downstream.The results from this study on an encrusting species are compared with results from a previous study on feeding from flow by an arborescent bryozoan, and the feeding performances of these two colony types are related to their respective flow microhabitats.     

Growing or reproducing in a temperate sea: optimization of resource allocation in a colonial ascidian

Relatively little is known about the life cycles of ascidians in temperate seas. Here, we investigated the biological cycle of the colonial ascidian Didemnum fulgens, a dominant species in some shallow localities of the NW Mediterranean Sea. Growth rates and frequencies of fission/fusion events were calculated over a period of 13 months, and the reproductive cycle determined after 32 months of observation. For analyses of reproduction, zooids were dissected in the laboratory and classified into five reproductive categories; these data were used to calculate a maturity index. For growth analyses, underwater photographs of marked colonies were used to estimate the surface area of D. fulgens colonies, calculate monthly growth rates, and document fusion and fission events. Clear seasonal patterns in reproduction and growth were observed, with distinct periods of investment into each function. Gonad maturation started in winter and larval release occurred in early summer, just before maximal sea temperatures were reached. After reproducing, colonies shrank and aestivated during the warmer summer months. Growth occurred during the cooler months, with maximal and minimal growth rates observed in winter and summer, respectively. Fusions and fissions occurred year‐round, although fissions were more frequent in fall (coincident with high growth rates) and fusions in spring (coincident with reproduction). These results add to the mounting evidence that ascidian life cycles in temperate seas are characterized by a trade‐off between investment in reproduction and growth, triggered by seasonal temperature shifts and constrained by resource availability during summer.     

Survival of honeybees in cold climates: the critical timing of colony growth and reproduction

Abstract. 1. The adaptive significance of the timing of growth and reproduction by honeybee, Apis mellifera L., colonies in cold climates was studied by describing the seasonal patterns of food storage, brood rearing, and swarming, and then observing the consequences of experimentally perturbing the seasonal cycles of brood rearing and swarming.
2. Colonies consume large amounts of food over winter (20+ kg of honey), but have only a brief period (about 14 weeks) for food collection each year.
3. The honeybee's striking habits of starting brood rearing in midwinter and swarming in late spring evidently help colonies achieve maximum use of the short summer season. Colonies whose onset of-brood rearing was experimentally postponed until early spring showed greatly retarded colony growth and swarming. Other experiments demonstrated that late swarms starve more often during winter than do early swarms.
4. We conclude that the timings of colony growth and reproduction are essential elements in the honeybee's suite of adaptations for winter survival.     

Evolutionary ecology of colonial reef-organisms, with particular reference to corals

Sedentary reef-organisms such as sponges, colonial coelenterates, bryozoans and compound ascidians produce repeated modules (aquiferous systems, polyps, zooids) as they grow. Modular construction alleviates constraints on biomass imposed by mechanical and energetic factors that are functions of the surface area to volume ratio. Colonies thus may grow large whilst preserving optimal modular dimensions. Among corals, optimal polyp size is smaller in the more autotrophic than in the more heterotrophic species. Modular construction allows flexibility of growth form, which can adapt to factors such as water currents, silting, light intensity and proximity of competitors. Modular colonies have great regenerative capacities, even separated fragments may survive and grow into new colonies. All fragments from a parental colony are genetically identical and large branching corals frequently undergo clonal propagation through fragmentation during storms. Soft corals can also fragment endogenously. By spreading the risk of mortality among independent units, the generation and dispersal of fragments lessens the likelihood of clonal extinction. In spite of their ability to propagate asexually, most benthic colonial animals also reproduce asexually. The selective advantages of the genetic diversity among sexually produced offspring seem not to be linked with dispersal, but probably lie in the biological interactions with competitors, predators and pathogens in the parental habitat. Age at first sexual maturity and the proportional investment of resources in sexual reproduction are related to colonial survivorship. Small branching corals on reef flats grow quickly, attain sexual maturity within 1–4 years, planulate extensively, but reach only small sizes before dying. Massive corals are longer lived and have the opposite characteristics of growth and reproduction. Most sessile reef organisms compete for space, food or light. Faster growers can potentially outcompete slower growers, but are often prevented from doing so by several forms of aggression from competitors and by the damage inflicted by storms. Competitive interactions among sedentary organisms on coral reefs are unlikely to be linear or deterministic, and so the co-existence of diverse species is possible.     

Sexual reproduction of the placental brooder Celleporella hyalina (Bryozoa,Cheilostomata) in the White Sea

The evolution of parental care is a central field in many ecological and evolutionary studies, but integral approaches encompassing various life-history traits are not common. Else, the structure, development and functioning of the placental analogues in invertebrates are poorly understood. Here, we describe the life-history, sexual colony dynamics, oogenesis, fertilization and brooding in the boreal-Arctic cheilostome bryozoan Celleporella hyalina. This placental brooder incubates its progeny in calcified protective chambers (ovicells) formed by polymorphic sexual zooids. We conducted a detailed ultrastructural study of the ovary and oogenesis, and provide evidence of both auto- and heterosynthetic mechanisms of vitellogenesis. We detected sperm inside the early oocyte and within funicular strands, and discuss possible variants of fertilization. We also detail the development and functioning of the placental analogue (embryophore) in the various stages of embryonic incubation as well as embryonic histotrophic nourishment. In contrast to all known cheilostome placentas, the main part of embryophore of C. hyalina is not a single cell layer. Rather, it is a massive “nutritive tissue” whose basal part is associated with funicular strands presumably providing transport function. C. hyalina shows a mixture of reproductive traits with macrolecithal oogenesis and well-developed placenta. These features give it an intermediate position in the continuum of variation of matrotrophic provisioning between lecithotrophic and placentotrophic cheilostome brooders. The structural and developmental differences revealed in the placental analogue of C. hyalina, together with its position on the bryozoan molecular tree, point to the independent origin of placentation in the family Hippothoidae.     

Growth and reproduction of an estuarine population of the colonial hydroidCordylophora caspia (Pallas) in the northern Baltic Sea

Growth and reproduction of the colonial hydroidCordylophora caspia were monitored during the breeding season in natural conditions. In 1987, a life history study was carried out on the upright stems of the main stolon. Mean size of uprights varied cyclically. The first peak coincided with the peak number of sexual hydranths, after which the mean upright length decreased, possibly indicating somatic costs of sexual reproduction. Extrinsic factors like flooding may also have contributed to cyclical changes in upright size. In 1988 and 1989, colonies were reared on experimental plates in the estuary. In 1988, colonies grew until mid July, after which they regressed to a dormant condition and then started growing again in mid August. Predation and space competition are discussed as possible causes for this dormancy in the middle of the growing season. In 1989, colonies grew continually, with the exception of a decline in colony biomass and number of feeding hydranths at the end of July, just following the peak of sexual reproduction. Sexual reproduction started in the early stages of colonial development for all years. During early summer,C. caspia allocated resources simultaneously in colonial growth and sexual reproduction. However, sexual reproduction had a clear peak in mid summer, and thereafter sexual reproduction ceased while colonial growth continued.     

Does the diapause experience of bumblebee queens Bombus terrestris affect colony characteristics?

1. Bumblebee colonies show much variation in the number of workers, drones, and queens produced. Because this variation prevails even when colonies are kept under identical conditions, it does not seem to be caused by extrinsic factors but rather by differences between founding queens. 2. The most likely factor that could cause differences between queens is diapause. Although colonies are raised under standardised conditions, the queens often experience diapause of different length. If there are costs associated with diapause that influence post‐diapause reproduction, the diapause history of the queens could affect colony characteristics. 3. Here, several colony characteristics are compared: number of first and second brood workers; total number of workers, drones, and queens; energy spent on sexuals; sex ratio; rate of worker production; time to emergence of first reproductive; and colony lifetime. Colonies were used where the queens experienced a diapause treatment of 0 (nondiapause queens), 2, and 4 months. 4. Although no proof was found for the existence of costs associated with diapause, the colony characteristics of nondiapause queens were significantly different from those of diapause queens. Colonies of nondiapause queens produced the lowest number of workers but the highest number of young queens. 5. It is argued that these nondiapause colonies are more time‐constrained than diapause colonies because nondiapause colonies produce two generations within the same season and should therefore be more efficient in producing sexual offspring. 6. Moreover, nondiapause colonies should rear a more female‐biased sex ratio because they can be certain of the presence of males produced by other (diapause) colonies.     

Quantitative observations on asexual reproduction of Aeolosoma viride (Annelida,Aphanoneura)

Agametic reproductive activity (via paratomy) of Aeolosoma viride was analyzed throughout the life cycle in individually reared specimens. Aeolosoma viride is organized in linear chains of 3–4 zooids; the main zooid is anterior, and the secondary zooids are positioned posterior to the main zooid in inverse order with respect to their degree of growth, the most advanced being at the posterior end, and those less advanced nearer the main zooid. On average, worms lived 66±10 d and produced 57±6 offspring. A budding area located in the sub‐terminal part of the main zooid produced chaetigers that formed the origin of the secondary zooids. A growth zone was located in the posterior end of each secondary zooids. Fission occurred between the penultimate and the last zooid of the chain. Just before fission, the growth zone of each secondary zooid became a budding area. Agametic reproduction was via multiple paratomy with linear succession of the secondary zooid and terminal fission. The structure of the chain was therefore modulated by the interaction of the processes of budding, growth, cephalic differentiation, and fission, which occurred continuously and on different timescales. Values of parameters describing paratomic activity (interval between origin of the zooids, time to produce a chaetiger, growth time of the zooids, and interval between the fission of the filial chains) are low early in an individual's life, but increase during senescence. Due to its relatively rapid lifecycle and high reproductive activity, A. viride is a convenient experimental organism for the study of agametic reproduction.     

Pseudolunularia gen.n. (Cheilostomata), a IunuIitiform bryozoan from the Indo-West Pacific

A new genus of anascan lunulitiform bryozoan, Pseudolunularia . is introduced for two new species, P. enigma and P. unguis . It is inferred that live colonies both possess basal rhizoids and are supported and stabilized by elongate avicularian mandibles. The autozooids, brooding zooids and avicularia of both species bear a striking resemblance to a parallel pair of species belonging to the genus Lunularia, L. repandus and L. cupulus .     

Zooid size and growth rate of the bryozoan Cryptosula pallasiana Moll in relation to temperature, in culture and in its natural environment

The size of cheilostome bryozoan zooids has been widely discussed for its potential in inferring palaeotemperatures, based on correlations between zooid size and temperature. Studies in both the natural environment and under experimental laboratory conditions have shown that an increase in temperature significantly decreases zooid size in a range of bryozoan taxa. In order to test the effect of temperature on zooid size, the cheilostome bryozoan Cryptosula pallasiana was for the first time successfully cultured under laboratory conditions. C. pallasiana was grown at 14 °C and 18 °C using Rhodomonas sp. as a food organism. Zooid size, tentacle number and growth rate were measured over a period of 26 days. For comparison, zooids from colonies of C. pallasiana collected from the natural environment were measured in winter and summer months. Results showed that colonies grown in laboratory culture had significantly longer and wider zooids at 14 °C than at 18 °C. The specific growth rate of C. pallasiana doubled from 14 °C to 18 °C. Comparison of tentacle number in culture showed a significantly higher value at lower temperatures. This may be related to differing food availability, longer polypide life spans, or a shift of energy use at colder temperatures. In nature the zooids were significantly longer in colonies sampled in July than in January, a clear difference from laboratory results. The utility of cheilostome Bryozoa as indicators of environmental change and their potential for studies of paleotemperature are highlighted.     

Reproductive strategy and the life cycle in graptoloid colonies

Urbanek, A. 1990 10 15: Reproductive strategy and the life cycle in graptoloid colonies. Lethaia , Vol. 23, pp. 333–340. Oslo. ISSN 0024–1164.
Graptoloid colonies were clones composed in all probability of hermaphroditic zooids. Their breeding system approaching that of amphicarpic plants, namely distant out crossing combined with selfing, was exceptionally flexible. Moreover, the balanced coexistence of these extreme means of reproduction may be visualized as an evolutionarily stable strategy (ESS). The evolutionary consequences of such a breeding system might have accounted for the overall high rates of graptoloid evolution as well as for the rapid transformations in large populations. Sexual reproduction of graptoloid colonies was complemented by a multiplication through occasional fragmentation of colonies and subsequent regeneration from the fragments. Fragmentation of colonies played an important role in the survival of graptoloid colonies during catastrophic events such as hurricanes and later in recruitment. Fragmentation followed by regeneration, and only later by sexual reproduction of regenerated fragments, constituted the so-called great cycle (GC), whereas the regular course of events initiated by sexual reproduction in an undisturbed (complete) colony resulted in normal colony formation (astogeny) and is termed the small cycle (SC). Thus the adaptive significance of the sexual process in the life cycle is in the restoration of the perfect' pattern of the complete colony, which offered the best hydrodynamic properties and highest fitness. ▭ Graptolithina, Graptoloidea, breeding sysfem, reproductive strategy, fragmentation, life cycle .     

Enzyme polymorphism inPreissia quadrata (Hepaticae, Marchantiaceae)

Preissia quadrata has a high habitat specificity and a reproductive system including frequent sexual reproduction and absence of specialized asexual propagules. Fertilization is aquatic. Colonies collected in Europe, Asia, eastern and western Canada show the species to be polymorphic at the eleven isozyme loci studied. Most often, thalli of the same colony appeared genetically identical. Partitioning of genetic variation was among rather than within colonies, suggesting a clonal structure of the colonies and a species structure consisting of a series of small populations reproductively isolated from each other. Little genetic exchange between colonies growing nearby was present, but the species does not appear to consist of regionally circumscribed genetically cohesive entities. Some European and Canadian colonies had identical electrophoretic patterns. The problem of understanding the phenomena holding liverwort species together, both morphologically and genetically, remains an open question. The genetic structure of the species might be due to its past history. It might have had a continuous distribution in the northern part of the Northern Hemisphere. If so, little subsequent genetic diversification, perhaps linked to its haploid-dominant life cycle, must be supposed.     

Reproductive synchrony in a temperate damselfish, <Emphasis Type="Italic">Chromis limbata</Emphasis>

Synchronous reproduction has been reported in many tropical damselfishes, and it appears that temperature is the main driver of synchrony at these latitudes. However, reproductive synchrony and its relation to temperature is less well understood in temperate damselfish. The endemic Azores chromis (Chromis limbata) was chosen as a study species to address this topic. First, an ethogram of the reproductive behaviour and embryo development of the Azores chromis is provided. Then, are investigated (1) the reproductive dynamics at the colonial and inter-colonial levels, (2) how these are affected by temperature, and (3) the relation between reproductive dynamics and the social organization of temperate damselfish. In the Azores chromis, reproduction was cyclic with a high periodicity of 7.2 ± 0.63 days on average. Embryo development was synchronous within colonies towards the beginning and the end of incubation, but not at intermediate stages of development. Colonies that were closer were more synchronized than those located further apart, despite contrasting temperature and depth profiles. Embryo size differences were also greater between colonies that were further apart, and embryos tended to grow larger in warmer water. A literature review revealed that reproductive synchrony was common in colonial breeding damselfish species. Although these results suggest that within the breeding season, the timing and length of reproductive cycles cannot be explained by temperature alone, the observed changes in embryo size raise concern about the effects of changing oceanic conditions on this species.     

Trade-offs between sexual and asexual reproduction in a monoecious species Sagittaria pygmaea (Alismataceae): the effect of different nutrient levels

Available resources could influence the trade-offs among different reproductive components in plants. Here, we created three nutrient levels to test the nutrient effects on trade-offs among sexual reproduction, clonal propagation and vegetative growth in a monoecious clonal herb Sagittaria pygmaea. The results of this study showed that the plant exhibited different trade-off patterns among different nutrient levels. When the nutrient level was low, there were weak trade-offs between sexual reproduction and vegetative growth and between clonal propagation and vegetative growth; when the nutrient level was moderate, we found a strong trade-off between sexual reproduction and clonal propagation; but when the nutrient level was high, we found no trade-offs among these three different reproductive components. These results indicated that the plant could adjust its trade-off patterns to fit the nutrient variation and suggested that trade-offs are unlikely to constrain the evolution of reproductive strategy in this species.     

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.