Seasonal genetic variation associated with population dynamics of a poecilogonous polychaete worm

Poecilogonous species show variation in developmental mode, with larvae that differ both morphologically and ecologically. The spionid polychaete Pygospio elegans shows variation in developmental mode not only between populations, but also seasonally within populations. We investigated the consequences of this developmental polymorphism on the spatial and seasonal genetic structure of P. elegans at four sites in the Danish Isefjord‐Roskilde‐Fjord estuary at six time points, from March 2014 until February 2015. We found genetic differentiation between our sampling sites as well as seasonal differentiation at two of the sites. The seasonal genetic shift correlated with the appearance of new size cohorts in the populations. Additionally, we found that the genetic composition of reproductive individuals did not always reflect the genetic composition of the entire sample, indicating that variance in reproductive success among individuals is a likely explanation for the patterns of chaotic genetic patchiness observed during this and previous studies. The heterogeneous, unpredictable character of the estuary might maintain poecilogony in P. elegans as a bet‐hedging strategy in the Isefjord‐Roskilde‐Fjord complex in comparison with other sites where P. elegans are expected to be fixed to a certain mode of development.     

Reproductive cycle of the sea cucumber Holothuria (Platyperona) sanctori (Holothuroidea: Echinodermata) in the southwestern Mediterranean Sea: interpopulation variability

This study is a first contribution on the reproductive biology of Holothuria (Platyperona) sanctori from the Algerian coastline in the southwestern Mediterranean. Sampling was conducted at two sites in central Algeria, one (Ain Taggourait, w. Tipasa) where there is little anthropogenic influence and another (Tamentefoust, w. Alger) where there are two major sources of pollution from Oued el Harrach and Oued el Hamiz effluents which contain particulate organic matter. Significant differences in sea cucumber reproduction were observed between the two sites. This could be due to the difference in organic matter noted in the sediment of the two study sites. However, temporal similarities of the gonad index (GI) and the sexual maturity stages suggest that the reproductive cycle of H. (P.) sanctori is annual with the main spawning event taking place from June to October. Spawning is of high intensity in the Bay of Bou Ismail (Ain Taggourait) and spread out over time in the Bay of Algiers (Tamentefoust), reflecting an environment sufficiently rich in food to insure reproduction. We also show that gonad maturation of males and females was synchronized. Temperature is the most likely factor influencing the reproductive cycle and spawning. At the study sites, H. (P.) sanctori had a winter sexual resting phase, followed by maturation during spring, before spawning in summer. However, there was a slight lag to the start of spawning at the two study sites.     

DEVELOPMENTAL MODE AND SPECIES GEOGRAPHIC RANGE IN REGULAR SEA URCHINS (ECHINODERMATA: ECHINOIDEA)

Among marine benthic organisms, the ability to disperse, primarily during the larval stage, is widely thought to influence the extent of species geographic range. Because related species often differ in their modes of larval development (pelagic, feeding larvae; pelagic, nonfeeding larvae; or brooded development), and these can have dramatically different planktonic intervals, the mode of development may influence geographic range. A global survey of 215 regular echinoids shows that species with pelagic, feeding larvae have significantly larger ranges than those with pelagic, nonfeeding larvae, but there is no difference in ranges between species with pelagic, nonfeeding larvae and those with brooded development. These patterns are maintained within the Cidaroida and the Temnopleuroida, which account for the great majority of species with pelagic, nonfeeding development and brooded development. This limited effect of developmental mode on geographic range is found among species occurring predominantly in waters shallower than 100 m. For species occurring deeper than 100 m, there is no significant difference in geographic range related to type of development. The relationship between developmental mode and species range was examined more closely for circa 30 species for which the developmental period was known from laboratory observations. Adjusting the developmental times to a common temperature, 20°C, using realistic values for Q₁₀ from 2.0 to 3.6, showed a highly significant, negative correlation between egg volume and developmental time, indicating the potential for developmental mode to influence the planktonic interval. However, there was no relationship between time in the plankton, estimated from unadjusted developmental times, and extent of species geographic range. These results suggest that developmental mode may influence extent of species geographic ranges indirectly through the consequences of dispersal for gene flow or recovery from disturbance.     

Salinity adaptations in the gastropodsLittorina mandshurica andLittorina squalida at different stages of ontogenesis

Salinity adaptations of the gastropodsLittorina mandshurica andL. squalida were studied at different stages of their life cycles.L. mandshurica exhibited a higher degree of adaptation to variations in salinity thanL. squalida at all developmental stages. Egg masses did not protect embryos from desalination; the larvae inside the egg masses were capable of adapting to diluted seawater. As larvae developed in egg masses, their adaptive capabilities increased. At the time of hatching, the salinity tolerance of larvae differed little, if at all, from that of adults. It is assumed that pelagic egg capsules provide an advantage in the distribution of a species.     

盐度对沙蜇有性繁殖阶段早期发育的影响

近几十年来,沙蜇的频繁暴发给东亚海域的海洋生态系统带来了广泛影响。在秋季,沙蜇成熟的雌雄水母体在沿岸水域聚集产卵,有性繁殖产生的受精卵发育成新的底栖螅状体,为螅状体种群数量进行补充。河口浅滩海域为沙蜇的繁育地,沿岸盐度较低,在秋季降雨期盐度多变,较低、多变的盐度可能对沙蜇有性繁殖阶段的早期发育产生重要作用,从而影响螅状体种群数量的补充。实验设置了4种不同盐度(15、20、25、30)试验组,在不同盐度下对沙蜇受精卵进行培养,探讨盐度对沙蜇早期发育过程中受精卵、浮浪幼虫发育以及早期螅状体生长及存活的影响。试验结果:沙蜇受精卵胚胎发育的适宜盐度为20,发育基本与盐度25、30同步,盐度15受精卵细胞发育迟缓,发育率显著降低;浮浪幼虫发育适宜盐度为20和25,两组浮浪幼虫附着变态率高于盐度15、30,盐度15时浮浪幼虫活力明显降低、发育迟缓,浮浪幼虫在盐度15时水中存活时间较长可达8 d,但附着时间集中在培养后的3、4天,与其他组相同;早期螅状体幼体适宜盐度为20、25、30,早期螅状体存活率、相对增长率及特定生长率均显著高于盐度15,三组间差异不显著。结果表明,盐度显著影响沙蜇有性繁殖阶段...     

Reproductive traits explain contrasting ecological features in sponges: the sympatric poecilosclerids Hemimycale columella and Crella elegans as examples

Our study focused on the Mediterranean species Hemimycale columella and Crella elegans, which have overlapping ecological distributions but contrasting population densities and resilience. We formulated the hypothesis that differential reproductive traits were the main cause underlying these ecological differences in the study area. The issues addressed were whether recruits compete for the substrate, either because their respective reproductive cycles overlap or/and larval performance differs between species, and whether a contrasting investment in reproduction contributed to explain their diverse densities. Both species were simultaneous hermaphrodites and incubated their larvae. The reproductive period was notably shorter in C. elegans than in H. columella, while investment in reproductive tissue was higher in individuals of H. columella than in C. elegans. In contrast, C. elegans larvae harbored larger amounts of lipids and yolk inclusions than H. columella larvae. Moreover, the former contained amazing collagen masses densely packed among their inner cells. H. columella is likely to produce a higher number of poorer equipped larvae while C. elegans seems to produce a lower number of more resistant, better fitted with energy reserves, and thus potentially more successful larvae. These reproductive patterns agree with a higher and stable density of C. elegans in the study site compared with a more variable abundance of H. columella.     

Seasonal and interpopulational variability in fecundity,egg size,and elemental composition (CHN) of eggs and larvae in a grapsoid crab, <Emphasis Type="Italic">Chasmagnathus granulatus</Emphasis>

Reproductive traits at the beginning and the end of the annual reproductive season were compared between two populations of the intertidal crab Chasmagnathus granulatus living in ecologically contrasting habitats: (1) Mar Chiquita (MC) (37°45′S, 57°19′W), a highly productive estuarine coastal lagoon with strong salinity fluctuations. (2) San Antonio Bay (SA) (40°46′S, 64°50′), a physically stable but less productive coastal marine environment. Number, size, and elemental composition (CHN) of eggs and larvae differed significantly between populations. Regardless of the season, more but smaller eggs and larvae were produced in MC, while eggs and larvae from SA revealed higher dry mass and C/N ratios indicating higher lipid content. A latitudinal temperature gradient cannot explain these patterns, suggesting that other environmental factors including salinity, quality or quantity of benthic food sources and productivity may be responsible. In both populations, fecundity and biomass per egg were higher at the beginning as compared to the end of the reproductive season. As a consequence, the reproductive effort was consistently maximal at the beginning of the season. At MC, also variability was found between two successive years. Intraspecific (both interpopulational and seasonal) variations in reproductive and developmental traits may be important for the formation of physiologically different metapopulations along the wide geographic range of C. granulatus.     

Interactions in soft bottom benthic communities: Quantitative aspects of behaviour in the surface deposit feedersPygospio elegans (Polychaeta) andMacoma balthica (Bivalvia)

The surface deposit feeding speciesPygospio elegans andMacoma balthica are dominant members of many sandy bottom communities of northern boreal regions. The feeding mode of both species and the tube-building ofP. elegans are assumed to affect community structure by interactions with other species. The weight of tubes ofP. elegans varied between 2 and 13 g DW/100 cm² at the two stations investigated and during the year, which is equivalent to 230–1500 cm of tubes per 100 cm² of sediment surface. Sediment stability may be affected directly or indirectly by the amount of tubes present.M. balthica shows a linear relation between the maximum size of particles which can be inhaled and animal length. In Kiel Bay, particles>0.5 mm are out of the range of this species. In summer, the potential feeding area (PFA) of aP. elegans population at one station in Kiel Bay was 1.8 times the available surface area. The PFA of three different populations ofM. balthica in Kiel Bay exceeded the available surface area by factors of 2.6, 2.7, and 3.2. These findings indicate strong intra- and interspecific competition for food. Additionally, the feeding of both species may strongly affect the recruitment of benthic species via pelagic larvae. Experiments are proposed to evaluate the significance of the investigated behavioural aspects for community structure. AWI Publication No. 393     

An alternative mode of larval development inScoloplos armiger (O. F. Müller, 1776) (Polychaeta,Orbiniidae)

During a three-year plankton monitoring at the island of Helgoland (German Bight, North Sea), pelagic larvae ofScoloplos armiger (O. F. Müller, 1776) were observed to occur regularly from February to April. Up to the present day, only benthic development within egg cocoons has been described (Anderson, 1959). Planktonic larvae comprise early, ovoid trochophores up to advanced stages with 11 setigers. The main morphological difference between the two larval types is the more numerous differentiation of lateral cilia by the pelagic larvae. Annual alterations in appearance and abundance suggest that reproduction is correlated with water temperature.     

The distribution,abundance and seasonality of pelagic marine invertebrate larvae in the maritime Antarctic

A survey of pelagic larvae was undertaken between November 1992 and February 1995 at Signy Island, Antarctica (60° 43 minutes S, 45° 36 minutes W). A diver-towed net and hand-held plankton pump were used at five sites of varying depths (6 to 30 m) and benthic substrata, in a combination of monthly and fortnightly samples. Overall larval ecological diversity was much higher than expected, with 131 morphologically distinct larval forms collected, apparently representing most of the benthic phyla present. The species richness observed is comparable with levels recorded at temperate latitudes, and higher than Arctic data and the implications for Thorson''s rule (the inferred cline of reduced pelagic larval diversity towards high latitudes) is discussed. Larval abundances were low (mean 2.6 individuals per cubic metre) which were two to six orders of magnitude lower than peaks in comparable data from temperate and tropical zones. We suggest that the low abundances recorded are a reflection of both slow developmental rates and a high dilution of larvae, reducing synchrony and spreading larvae over larger distances. Three seasonal periods, during which different larval types occur, have been identified. Summer, late summer and winter spawning strategies were discernable, and in some groups larvae occurred throughout the year.     

The relative importance of sexual and clonal reproduction for population growth in the perennial herb Fragaria vesca

The relative importance of sexual and clonal reproduction for population growth in clonal plants is highly variable. Clonal reproduction is often more important than sexual reproduction but there is considerable interspecific variation and the importance of the two reproductive modes can change with environmental conditions. We carried out a demographic study on the woodland strawberry (Fragaria vesca), a widespread clonal herb, at 12 sites in Switzerland during 2 years. Study sites were selected in two different habitats, i.e., forest and forest edge. We used periodic matrix models to estimate annual population growth rates and carried out prospective analyses to identify life cycle components that influence population growth rates most. Retrospective analyses were applied to study how the two different habitats affected population dynamics. Furthermore, we tested whether trade-offs between sexual and clonal reproduction occurred. There were large differences in annual population growth rates between sites and large within-site differences between years. Results of the prospective analyses clearly indicate that clonal reproduction is the dominant reproductive pathway whereas sexual reproduction is rather insignificant for population growth. Compared to forest habitats, forest edge habitats had higher population growth rates in the first year but smaller growth rates in the second year. We attribute these differing habitat effects to different water availabilities during consecutive years. No trade-offs between sexual and clonal reproduction were found. In conclusion, population growth of F. vesca relies heavily on clonal reproduction. Furthermore, reproduction and survival rates of F. vesca depend highly on spatio-temporal variation of environmental conditions.     

Benthic–pelagic coupling in the population dynamics of the harmful cyanobacterium Microcystis

1. In eutrophic lakes, large amounts of the cyanobacterium Microcystis may overwinter in the sediment and re‐inoculate the water column in spring. 2. We monitored changes in pelagic and benthic populations of Microcystis in Lake Volkerak, The Netherlands. In addition, sedimentation rates and the rate of recruitment from the sediment were measured using traps. These data were used to model the coupling between the benthic and pelagic populations and to calculate the contribution of overwintering benthic and pelagic populations to the magnitude of the pelagic summer bloom. 3. Changes in the benthic Microcystis population showed a time lag of 3–14 weeks compared with the pelagic population. This time lag increased with lake depth. The largest amount of benthic Microcystis was found in the deepest parts of the lake. These observations suggest horizontal transport of sedimented Microcystis from shallow to deep parts of the lake. 4. Recruitment from and sedimentation to the sediment occurred throughout the year, with highest recruitment and sedimentation rates during summer. Model simulations indicate that the absence of benthic recruitment would reduce the summer bloom by 50%. 5. In spring, the total pelagic population was three to six times smaller than the total benthic population. Yet, model simulations predict that the absence of this small overwintering pelagic population would reduce the summer bloom by more than 64%. 6. Reduction of the overwintering pelagic populations, for instance by flushing, may be a useful management strategy to suppress or at least delay summer blooms of Microcystis.     

Reproductive and recruitment dynamics of clionaid sponges on oyster reefs in North Carolina

Boring sponges belonging to the family Clionaidae have become a destructive nuisance to eastern oyster (Crassostrea virginica) aquaculture and restoration efforts in the southeastern USA. Clionaid sponges colonize the inner layers of oyster shells and remove carbonate material, compromising the quality and marketability of the oyster; however, relatively little is known about reproduction and recruitment of these sponges. Using histological techniques, reproductive activity of clionaid sponges was monitored at two sites (Cedar Island and Masonboro Sound) in coastal North Carolina. Sponge recruitment to limestone tiles (5×5 cm), oyster shells, and clam shells was monitored in 2013 and 2014; recruitment to the limestone tiles was statistically higher than recruitment to clam or oyster shells. Overall, seasonal patterns in reproduction and recruitment of clionaid sponges were generally similar at the two sites. Three species of clionaid sponge were found during field sampling (Cliona celata, C. lobata, and C. truitti), and reproductive activity (eggs and spermatocysts) of these species was observed from April to November, with peak reproduction occurring from June to September for C. lobata and from August to September for C. celata. Recruitment peaked in late summer/early fall. Additionally, the relationship between environmental conditions (temperature, salinity, dissolved oxygen, pH, and chlorophyll a) and clionaid recruitment was explored using a regression model. At Cedar Island, the best‐fit model included salinity and dissolved oxygen, while the best‐fit model at Masonboro Sound included temperature, pH, and salinity. The data from this study show that the primary reproduction and recruitment pulses occur in the fall for local clionaids, and thus mitigation strategies should be applied in the late fall or winter to minimize infestations.     

Temperature influences the reproduction of fiddler crabs at the southern edge of their distribution

Understanding the spatial patterns of reproductive biology might provide predictions about fitness and population stability in different locations within the geographical range of a particular species. Leptuca uruguayensis is a fiddler crab that breeds year round in tropical estuaries but only in summer months in temperate salt marshes. In this study, we examined several reproductive attributes of the southernmost population of L. uruguayensis, including the proportion of ovigerous females, the proportion of surface‐active crabs, the developmental status of the gonads and hepatopancreas, and the fullness of seminal receptacles, and related them to environmental factors such as temperature, photoperiod, and sediment organic matter content. We found that temperature was the environmental factor that was most correlated with the reproductive process of the southernmost fiddler crab, since this environmental factor was related to ovarian development, to the fullness of seminal receptacles, and to the hepatosomatic index. At the southern edge of its distribution, the low temperatures of winter restricted reproduction in L. uruguayensis. These winter temperatures might represent the lower limit of the thermal window of this fiddler crab, limiting its extension toward higher latitudes.     

Reproductive potential of a marine ecosystem engineer at the edge of a newly expanded range

Global climate change is leading to redistribution of marine species and altering ecosystem dynamics. Given recent poleward range extension of the barrens‐forming sea urchin Centrostephanus rodgersii (Diadematidae) from mainland Australia to Tasmania, there is a need to understand the population dynamics of this ecologically important species in the Tasmanian environment. This paper informs possible population dynamics of C. rodgersii in Tasmania by examining its reproductive ecology in this new environment. Reproductive periodicity (gonad index and propensity to spawn) was assessed bimonthly over 18 months at four sites in eastern Tasmania spanning ～2° in latitude. At all sites, C. rodgersii displayed a strong seasonal cycle in gonad production with major spawning occurring in winter (～August) at minimum annual water temperature. Gametes from Tasmanian C. rodgersii were viable as determined by fertilization and early development trials. However, development to the two‐arm stage at ～3 weeks was strongly dependent on water temperature across the 8–20 °C temperature range, with poor development occurring below 12 °C. The range of temperatures tolerated by Tasmanian C. rodgersii larvae was similar to that of larvae from its native New South Wales range, indicating that this species has not undergone an adaptive shift to the cooler Tasmanian environment. There was also no evidence for an adaptive shift in reproductive phenology. Importantly, coastal water temperatures in eastern Tasmania during the peak spawning in August fluctuate about the 12 °C larval development threshold. Recent warming of the eastern Tasmanian coast and further warming predicted by global climate change will result in an environment increasingly favourable for the reproduction and development of C. rodgersii.     

Polymorphism in developmental mode and its effect on population genetic structure of a spionid polychaete, Pygospio elegans

Population genetic structure of sedentary marine species is expected to be shaped mainly by the dispersal ability of their larvae. Long-lived planktonic larvae can connect populations through migration and gene flow, whereas species with nondispersive benthic or direct-developing larvae are expected to have genetically differentiated populations. Poecilogonous species producing different larval types are ideal when studying the effect of developmental mode on population genetic structure and connectivity. In the spionid polychaete Pygospio elegans, different larval types have been observed between, and sometimes also within, populations. We used microsatellite markers to study population structure of European P. elegans from the Baltic Sea (BS) and North Sea (NS). We found that populations with planktonic larvae had higher genetic diversity than did populations with benthic larvae. However, this pattern may not be related to developmental mode, since in P. elegans, developmental mode may be associated with geography. Benthic larvae were more commonly seen in the brackish BS and planktonic larvae were predominant in the NS, although both larval types also are found from both areas. Significant isolation-by-distance (IBD) was found overall and within regions. Most of the pair-wise F(ST) comparisons among populations were significant, although some geographically close populations with planktonic larvae were found to be genetically similar. However, these results, together with the pattern of IBD, autocorrelation within populations, as well as high estimated local recruitment, suggest that dispersal is limited in populations with planktonic larvae as well as in those with benthic larvae. The decrease in salinity between the NS and BS causes a barrier to gene flow in many marine species. In P. elegans, low, but significant, differentiation was detected between the NS and BS (3.34% in AMOVA), but no clear transition zone was observed, indicating that larvae are not hampered by the change in salinity.     

Mixed asexual and sexual reproduction in the Indo‐Pacific reef coral Pocillopora damicornis

Pocillopora damicornis is one of the best studied reef‐building corals, yet it's somewhat unique reproductive strategy remains poorly understood. Genetic studies indicate that P. damicornis larvae are produced almost exclusively parthenogenetically, and yet population genetic surveys suggest frequent sexual reproduction. Using microsatellite data from over 580 larvae from 13 colonies, we demonstrate that P. damicornis displays a mixed reproductive strategy where sexual and asexual larvae are produced simultaneously within the same colony. The majority of larvae were parthenogenetic (94%), but most colonies (10 of the 13) produced a subset of their larvae sexually. Logistic regression indicates that the proportion of sexual larvae varied significantly with colony size, cycle day, and calendar day. In particular, the decrease in sexual larvae with colony size suggests that the mixed reproductive strategy changes across the life of the coral. This unique shift in reproductive strategy leads to increasingly asexual replications of successful genotypes, which (in contrast to exclusive parthenogens) have already contributed to the recombinant gene pool.     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

Effect of desalination on the littoral tecturid gastropods in early development

The effect of decreased salinity on the limpetsCollisella dorsuosa, C. versicolor, C. radiata, andNotoacmea concinna (Mollusca, Gastropoda) is studied at early stages of development. It is shown that, for normal development from fertilization to the veliger stage, the salinity must be not less than 22–24‰. A decrease in salinity depressed the behavior of larvae. At earlier stages, the larvae were less sensitive to changes in salinity than at later stages. At the same time, veligers were better able to adapt to changes in salinity than trochophores. The vertical water distribution of larvae depended on their age. Two days after fertilization, the larvae sank down to start a benthic life. It is proposed that the narrow tolerance range of littoral limpet larvae to changes in salinity is connected with the fact that they spend a relatively small part of their life in plankton.