Evolution of marine invertebrate reproductive patterns

A simple model of the evolution of reproductive patterns in marine benthic invertebrates is presented. The aim is to discuss the dichotomous distribution of forms into those which produce a large number of small eggs with planktotrophic development, and those which produce a small number of large eggs with direct or lecithotrophic development. The fecundity of adult individuals is assumed to be inversely proportional to egg size, and the mortality of planktonic larvae is assumed to be density independent and size dependent. The growth of planktonic larvae is assumed to be sigmoid with metamorphosis occurring at a given size. The model concludes that there are at most two evolutionarily stable egg sizes. Depending on larval growth rate and death rate, the metamorphosis size, a smaller egg size, or both may be evolutionarily stable. The predictions of the model are compared to patterns observed in nature. Illustrative data are supplied mainly from prosobranch molluscs.     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively),and the balance between the two reproductive modes may vary widely between and within species.Extensive clonal growth may affect the evolution of life history traits in many ways.First,in some clonal species,sexual reproduction and sex ratio vary largely among populations.Variation in sexual reproduction may strongly affect plant's adaptation to local environments and the evolution of the geographic range.Second,clonal growth can increase floral display,and thus pollinator attraction,while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal.Geitonogamous pollination can bring a cost to plant fitness through both female and male functions.Some co-evolutionary interactions,therefore,may exist between the spatial structure and the mating behavior of clonal plants.Finally,a trade-off may exist between sexual reproduction and clonal growth.Resource allocation to the two reproductive modes may depend on environmental conditions,competitive dominance,life span,and genetic factors.If different reproductive modes represent adaptive strategies for plants in different environments,we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition,while clonal growth should be dominant in stable habitats.Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes.Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives.     

Asexual and sexual reproductive strategies in clonal plants

Most plants can reproduce both sexually and asexually (or vegetatively), and the balance between the two reproductive modes may vary widely between and within species. Extensive clonal growth may affect the evolution of life history traits in many ways. First, in some clonal species, sexual reproduction and sex ratio vary largely among populations. Variation in sexual reproduction may strongly affect plant’s adaptation to local environments and the evolution of the geographic range. Second, clonal growth can increase floral display, and thus pollinator attraction, while it may impose serious constraints and evolutionary challenges on plants through geitonogamy that may strongly influence pollen dispersal. Geitonogamous pollination can bring a cost to plant fitness through both female and male functions. Some co-evolutionary interactions, therefore, may exist between the spatial structure and the mating behavior of clonal plants. Finally, a trade-off may exist between sexual reproduction and clonal growth. Resource allocation to the two reproductive modes may depend on environmental conditions, competitive dominance, life span, and genetic factors. If different reproductive modes represent adaptive strategies for plants in different environments, we expect that most of the resources should be allocated to sexual reproduction in habitats with fluctuating environmental conditions and strong competition, while clonal growth should be dominant in stable habitats. Yet we know little about the consequence of natural selection on the two reproductive modes and factors which control the balance of the two reproductive modes. Future studies should investigate the reproductive strategies of clonal plants simultaneously from both sexual and asexual perspectives. Translated from Acta Phytoecologica Sinica, 2006, 20(1): 174–183 [译自: 植物生态学报]     

The distribution of male and female reproductive success in a broadcast spawning marine invertebrate

Many studies have addressed sexual selection in animals, butfew data are available on animals that release eggs and sperminto the environment for external fertilization. Although thisreproductive mode represents the ancestral condition and isstill a very common reproductive strategy, it is underrepresentedin empirical studies and theoretical treatments. Here I presentdata on the pattern of reproductive success in male and femalesea urchins. The results suggest that the strength of sexualselection and the differences between the sexes in the intensityof sexual selection depend on mate density. In general, despitethe high degree of multiple paternity, the variance in reproductivesuccess appears to be lower in males and higher in females thanit is in polygamous species with internal fertilization. Theseresults may provide insight into the patterns of effective populationsize in marine invertebrates and also more generally the evolutionarytransition from sexual monomorphism to polymorphism in adulttraits.     

Cytotoxicity from sulfide exposure in a sulfide-tolerant marine invertebrate

It is unknown whether sulfide-tolerant marine invertebrates suffer cytotoxicity from sulfide exposure in vivo at environmentally-relevant concentrations. We tested this with the mudflat polychaete Glycera dibranchiata. Exposure of live animals to sulfide up to 2.4 mmol l^− 1 for 24 h did not affect animal survival, and animal condition (gross appearance and activity) was unaffected at sulfide concentrations up to 0.25 mmol l^− 1. However, animal condition was decreased at higher sulfide concentrations (P < 0.0001, n = 33). Coelomic fluid obtained from the animals showed decreased erythrocyte count (P = 0.0035, n = 14), indicating cell loss, and increased propidium iodide and Hoechst 33324 staining (P < 0.0001 and P = 0.0003, respectively, n = 14), indicating loss of plasma membrane integrity, even at sulfide concentrations that produced no change in animal condition. When G. dibranchiata were allowed a 72 h recovery period following 24 h sulfide exposure, there was no overall improvement in condition (P ≥ 0.12, n = 7-8), and worms that had been exposed to 1 mmol l^−  1 sulfide still had an erythrocyte count that was less than half that of control animals (P = 0.028, n = 7). The inability to completely recover the cell count was at least partially due to impaired production of new erythrocytes, since analysis of BrdU incorporation indicated that erythrocyte proliferation was reduced from 2% per day in control animals to 0.12% per day in animals exposed to 1 mmol l^− 1 sulfide (P = 0.010, n = 21). Together, these findings indicate that at least some sulfide-tolerant marine invertebrates experience significant cellular injury and impaired tissue proliferation when exposed to environmentally relevant sulfide concentrations, even when the appearance and behavior of the animal appear unaffected.     

Thiosulfate elimination and permeability in a sulfide-adapted marine invertebrate.

Oxidation of hydrogen sulfide to thiosulfate is one of the best-characterized mechanisms by which animals adapted to sulfide minimize its toxicity, but the mechanism of thiosulfate elimination in these animals has remained unclear. In this study, we examined the accumulation and elimination of thiosulfate in the sulfide-adapted marine worm Urechis caupo. The coelomic fluid of U. caupo exposed to 50-100 micromol L-1 sulfide in hypoxic seawater (Po2 ca. 10 kPa) accumulated (mean+/-SD) 132+/-41 micromol L-1 thiosulfate after 2 h, reaching 227+/-113 micromol L-1 after an additional 4 h in aerated, sulfide-free seawater. In whole-animal thiosulfate clearance studies, the rate of thiosulfate elimination from the coelomic fluid followed a single exponential time course with a half-life of 6 h. The thiosulfate permeability coefficient of isolated preparations mounted in diffusion chambers was 7.6x10-5+/-7. 7x10-5 cm s-1 for the hindgut and 5.5x10-7+/-2.7x10-7 cm s-1 for the body wall. These rates were independent of the direction of net efflux (mucosal-to-serosal or serosal-to-mucosal). Using a simple mathematical model of U. caupo that incorporates the thiosulfate permeability coefficients, the thiosulfate half-life was calculated to be 23 h without hindgut ventilation but less than 1 h with normal hindgut ventilation. Based on this information, we propose that passive thiosulfate diffusion across the hindgut is adequate to explain the observed rates of thiosulfate elimination.     

Seasonal non-indigenous species succession in a marine macrofouling invertebrate community

We report results from a 2-year study on the succession pattern of macrofouling assemblages in the Taranto Sea, an important alien species hotspot in the Mediterranean Sea. Four sets of PVC panels were used as macrofouling collectors; each unit was installed at a different time (April 2013, July 2013, October 2013 and January 2014) and then surveyed quarterly for 1 year. The macrofouling community consisted of 93 sessile invertebrate species, of which 16 were NIS and five were cryptogenic. In both years non-indigenous species (NIS) recruitment occurred mainly in the quarter July/October in concert with the settlement of pioneer autochthonous species. This recruitment is independent of immersion time, occurring on both bare substrates and on previously colonized panels. This increase in NIS coverage is influenced by the development stage of the community, suggesting that NIS grow better without potential competitors. Two sets of NIS were distinguished. The first included abundant ascidians, serpulids, and bryozoans that are structuring components of early communities when favorable conditions exist (i.e. a lack of competitive autochthonous species). After settlement, these species are unable to develop in later-stage communities. The second set of NIS was composed of sabellid worms that settle in early and late communities but, unlike the other NIS, are able to persist and become dominant in late macrofouling communities independent of seasonal changes.     

Phenotypic plasticity and morphological integration in a marine modular invertebrate

Background  

Colonial invertebrates such as corals exhibit nested levels of modularity, imposing a challenge to the depiction of their morphological evolution. Comparisons among diverse Caribbean gorgonian corals suggest decoupling of evolution at the polyp vs. branch/internode levels. Thus, evolutionary change in polyp form or size (the colonial module sensu stricto) does not imply a change in colony form (constructed of modular branches and other emergent features). This study examined the patterns of morphological integration at the intraspecific level. Pseudopterogorgia bipinnata (Verrill) (Octocorallia: Gorgoniidae) is a Caribbean shallow water gorgonian that can colonize most reef habitats (shallow/exposed vs. deep/protected; 1–45 m) and shows great morphological variation.     

Size-specific predation on marine invertebrate larvae

Predation on planktonic larval stages is frequently a major source of mortality for the offspring of benthic marine invertebrates. Mortality rate likely varies with larval size and developmental stage, but few experiments have measured how these factors affect predation rates. I used experimental reductions in egg size to test how variation in larval size affects the likelihood of predation during planktonic development. Blastomeres of the sand dollar Dendraster excentricus were separated at the two-cell stage to produce half-sized zygotes. Larvae resulting from this manipulation were tested for their susceptibility to predation relative to whole-sized siblings at four ages. Individuals from each size class were simultaneously presented as prey items to five predators (crab zoeae, crab megalopae, chaetognaths, solitary tunicates, and postlarval fish) in the laboratory. Four predators consumed significantly more half-sized larvae than whole-sized larvae, but one predator type (postlarval fish) consumed more whole-sized larvae. Predators that consumed more half-sized larvae also preferentially consumed younger larvae. In contrast, postlarval fish showed no significant prey preference based on larval age. These results suggest that assumptions of constant mortality rates during development should be modified to account for the effects of larval size and age.     

Natural female mating rate maximizes hatchling size in a marine invertebrate

1. Males and females often differ in their optimal mating rates, resulting potentially in conflicts over remating. In species with separate sexes, females typically have a lower optimal mating rate than males, and can regulate contacts with males accordingly. The realized mating rate may therefore be closer to the female's optimum. In simultaneous hermaphrodites, however, it has been suggested that the intraindividual optimization between 'male' and 'female' interests generates more 'male'-driven mating rates. 2. In order to assess the consequences of variation in mating rate on 'female' reproductive output, we exposed the simultaneously hermaphroditic sea slug Chelidonura sandrana to four mating rate regimes and recorded the effects on a variety of fitness components. 3. In focal 'females', we found (i) a slight but significant linear decrease in fecundity with mating rate, whereas (ii) maternal investment in egg capsule volume peaked at an intermediate mating rate. 4. Combining the observed fecundity cost with the apparent benefits of larger offspring size suggests that total female fitness is maximized at an intermediate mating rate. With the latter being close to the natural mating rate of C. sandrana in the field, our findings challenge the assumption of 'male'-driven mating systems in simultaneous hermaphrodites. 5. Our study provides experimental evidence for various mathematical models in which female fitness is maximized at intermediate mating rates.     

Induction of antiviral immunity by double-stranded RNA in a marine invertebrate

Vertebrates mount a strong innate immune response against viruses, largely by activating the interferon system. Double-stranded RNA (dsRNA), a common intermediate formed during the life cycle of many viruses, is a potent trigger of this response. In contrast, no general inducible antiviral defense mechanism has been reported in any invertebrate. Here we show that dsRNA induces antiviral protection in the marine crustacean Litopenaeus vannamei. When treated with dsRNA, shrimp showed increased resistance to infection by two unrelated viruses, white spot syndrome virus and Taura syndrome virus. Induction of this antiviral state is independent of the sequence of the dsRNA used and therefore distinct from the sequence-specific dsRNA-mediated genetic interference phenomenon. This demonstrates for the first time that an invertebrate immune system, like its vertebrate counterparts, can recognize dsRNA as a virus-associated molecular pattern, resulting in the activation of an innate antiviral response.     

Variation in multiple paternity in natural populations of a free-spawning marine invertebrate

For free-spawning marine invertebrates, fertilization processes control the genetic diversity of offspring. Each egg can potentially be fertilized by a sperm from a different male, and hence genetic diversity within a brood varies with levels of multiple paternity. Yet, few studies have characterized the frequency of multiple paternity in natural spawns. We analysed patterns of multiple paternity in two populations of the colonial ascidian Botryllus schlosseri using microsatellites. Because previous studies have shown that at moderate to high population densities, competition among male-phase B. schlosseri colonies results in the nearest male dominating the paternity of a brood, we specifically tested the effect of population density on patterns of paternity. Paternity was estimated using three multilocus indices: minimum number of fathers, counts of sperm haplotypes, and effective paternity (K(E)). Multiple paternity was evident in more than 92% of the broods analysed, but highly variable, with a few broods displaying unequal contributions of different males. We found no effect of population density on multiple paternity, suggesting that other factors may control paternity levels. Indirect benefits from increasing the genetic diversity of broods are a possible explanation for the high level of multiple paternity in this species.     

Sulfide permeability in the marine invertebrate Urechis caupo

Summary Hydrogen sulfide can reach toxic concentrations in the burrow-water of the echiuran worm Urechis caupo during low tide. Its two large epithelial surfaces, the thick muscular body wall and the thin-walled hindgut are in constant contact with the environment. Hindgut inflation of up to 2 ml water·g wet weight^-1 causes tissue stretch. To determine if these body surfaces present a barrier to sulfide influx, the total permeability coefficient P_T was measured at different degrees of stretch in diffusion chambers at pH 6.0, 7.0, and 8.0, and specific permeability coefficients P_H2S and P_HS^- were calculated. Both the body wall and the hindgut were more permeable to H₂S than HS^-. The body wall showed no significant increase in sulfide permeability with natural degrees of stretch, and the mean P_H2S and P_HS^- were 0.17 and 0.063 cm·h^-1, respectively. The sulfide permeability of the hindgut was increased by stretch, with the relative permeability of H₂S increasing faster than that of HS^-. Unstretched hindgut mean P_H2S and P_HS^- were 0.095 and 0.11 cm·h^-1, respectively, and stretched hindgut mean P_H2S and P_HS^- were 1.8 and 0.16 cm·h^-1, respectively. A model of sulfide influx in the natural environment indicates that even if the hindgut is kept uninflated, the coelomic fluid of U. caupo would have toxic sulfide concentrations well before the end of a 2-h tidal exposure in the absence of a sulfide elimination mechanism.Abbreviations P permeability coefficient - P_T total permeability coefficient - PD transepithelial potential difference     

Enzymatic hydrogen sulfide production in marine invertebrate tissues

At least some mammalian tissues produce H2S in vitro from L-cysteine at rates sufficient to have physiological effects. To determine whether tissues of macrofaunal invertebrates have the same capacity, we measured H2S production in tissue homogenates of the Manila clam Tapes philippinarum and the lugworm Arenicola marina. Tissue homogenates from both animals produced significant quantities of H2S gas upon addition of L-cysteine and the enzyme cofactor pyridoxal-5PRIME;-phosphate (10 mmol l(-1) and 2 mmol l(-1), respectively), while only tissues from T. philippinarum produced measurable H2S in the absence of added substrate or cofactor. In T. philippinarum tissues, H2S production was completely inhibited by the cystathionine beta-synthase (CBS) inhibitor aminooxyacetic acid (AOAA), suggesting that the majority of H2S production was via CBS pathways, while in A. marina body wall, AOAA inhibited only half of the total H2S production, indicating that the CBS pathway was not the only major source of H2S production. H2S production in tissues of T. philippinarum but not A. marina was doubled by the addition of a second thiol substrate (2.5 mmol l(-1) 2-mercaptoethanol), suggesting the presence of an 'activated serine sulfhydrase pathway', which had previously been demonstrated only in some microfauna.     

The marine invertebrate fauna of a British coastal salt marsh

There are few accounts in the literature of the marine fauna of European salt marshes. The marine invertebrate populations of a coastal salt marsh in eastern England were studied for two years. A total of 32 marine species were recorded. The community was composed of a subset of species from adjacent intertidal sand and mud flats, along with a brackish water component. This gave the marsh a characteristic fauna, which was dominated by infaunal and surface-living deposit feeders. The community showed a seasonal minimum in winter, and the infauna showed an additional summer decrease. This is attributed to increased interstitial salinities. Principal component analysis revealed the importance of the tidal gradient and the structure of the habitat in determining the community. The abundance of both epibenthic and infaunal predators were negatively correlated with the abundance of some prey. Also a small number of other pairwise negative interactions were revealed, but overall there was no strong evidence of interspecific competition exerting much influence on the community. The role of habitat heterogeneity and variation in the lateral movement of adult benthos and larval recruitment as promoters of diversity are discussed.     

Ant reproductive strategies

In contrast to what is generally believed, the reproductive strategies of ants are remarkably diverse and include such different phenomena, as wingless female and male sexuals, reproduction by mated workers, thelytokous parthenogenesis, and complete workerlessness. We review the various reproductive life histories and investigate them in the light of recent models on the evolution of dispersal strategies and multiple-queening. It appears that most deviations from “normal” colony propagation can be explained by a decreased success of dispersal and solitary founding by solitary queens in certain types of habitats. Consequently, alternative reproductive strategies are found especially in those species, in which environmental conditions or a highly specialized way of life are thought to make solitary founding costly. Among the key factors, which determine the success of reproductive strategies, appear to be spatial and temporal distribution of habitats and the availability of nest sites.     

Flexible mating: cross-pollination affects sex-expression in a marine clonal plant

Many functionally hermaphroditic plants have evolved mechanisms to reduce interference between the sex functions and to optimize reproductive output. In addition to physical mechanisms such as the spatial (herkogamy) and temporal (dichogamy) separation of male and female functions, plasticity in sex expression by means of mate-recognition (flexible mating) could be important in plants with variable access to cross-pollen. This applies particularly to clonal plants because of their modular growth form. We experimentally tested for the effects of pollen source and vegetative neighbourhood on instantaneous sex ratio and seed production in the self-compatible clonal marine angiosperm Zostera marina L. To this end, we exposed the (monoecious) flowering shoots to self and cross-pollen and to neighbourhoods of their own and a mix of foreign vegetative shoots. Flowering shoots that had been exposed to cross-pollen showed (1) a significantly lower female/male ratio at peak flowering, evidence for mate-recognition, and (2) a significantly higher seed set by the end of the season. Both effects were independent of the genetic composition of their vegetative neighbourhood. The results suggest that Z. marina maintains a cryptic self-incompatibility system not previously described for angiosperms with sub-aqueous pollination. In Z. marina, and possibly other self-compatible clonal plant species, mate-recognition could be a means of increasing the out-crossing probability for flowering shoots with central positions within their clone.     

Sources of phenotypic variance in egg and larval traits in a marine invertebrate

Contrary to many separate sex systems, the evolutionary ecology of polyandry in simultaneous hermaphrodites, and in particular in those with internal fertilization, has received little attention. Recent studies on the promiscuous gastropod Chelidonura sandrana showed that offspring size, an important determinant of offspring performance in many marine invertebrates, varies with the number of different mating partners. However, the source of this differential allocation by mothers remained unclear. Using a quantitative genetic model, we here tested for parental effects on offspring size and the importance of ‘good gene’ effects on early life history traits. Our analysis revealed no significant sire but strong dam effects for all investigated traits. Moreover, embryo viability tended to increase with egg capsule volume, thus linking offspring size with offspring performance. Our findings suggest that in C. sandrana (1) differential allocation is a maternal effect in response to the number of different partners, and that (2) additive genetic variance is of negligible importance in early life history traits.