Age specific fecundity and size of offspring in the prosobranch snail,Viviparus ater

Summary Age specific fecundity and size of offspring were compared in two populations of Viviparus ater, one from Lake Zürich (Switzerland) and one from Lake Maggiore (Italy). Females were caged and their offspring sampled at weekly intervals. Average fecundity per female over one year was 19.6 and 14.7 offspring in Lake Zürich and Lake Maggiore, respectively. Average shell widths of offspring at birth in the two lakes were 8.5 mm and 7.4 mm. Fecundity did not change with age in Lake Zürich but decreased with increasing age of the snails in Lake Maggiore. Fecundity and offspring size were positively correlated with the size of the females in both lakes, but fecundity was higher and offspring were larger in Lake Zürich than in Lake Maggiore irrespective of the size of the females. An analysis of covariance, correcting for differences between the two lakes in the total fresh weight of the offspring produced annually, showed that females from the two lakes distributed their reproductive investment differently. Females in Lake Zürich produced relatively fewer but larger offspring than females in Lake Maggiore. This trade-off was hidden because females in Lake Zürich produced twice as much offspring biomass as females in Lake Maggiore. The possibility that different selection regimes might account for the observed differences is discussed.     

Within-lake dispersal of the prosobranch snails,Viviparus ater and Potamopyrgus jenkinsi

Summary The gastropod species Viviparus ater and Potamopyrgus jenkinsi have both colonized Lake Zürich relatively recently. If it is assumed that they have dispersed in the Lake starting from one location, a model of random diffusion is insufficient to explain their present distribution in Lake Zürich. It is suggested that both species were dispersed by humans. In P. jenkinsi, passive dispersal by birds, fish or on floating algae may also have occurred. To estimate the rate of within-lake dispersal, life tables were constructed and the diffusivity estimated for the two species. Per capita population growth rates were calculated to be 0.00337/d for V. ater and 0.0159/d for P. jenkinsi. Diffusivity was close to 1 m²/d in both species.     

Sex‐specific reproductive investment of summer spawners of Illex argentinus in the southwest Atlantic

Energy investment in reproduction and somatic growth was investigated for summer spawners of the Argentinean shortfin squid Illex argentinus in the southwest Atlantic Ocean. Sampled squids were examined for morphometry and intensity of feeding behavior associated with reproductive maturation. Residuals generated from length‐weight relationships were analyzed to determine patterns of energy allocation between somatic and reproductive growth. Both females and males showed similar rates of increase for eviscerated body mass and digestive gland mass relative to mantle length, but the rate of increase for total reproductive organ weight relative to mantle length in females was three times that of males. For females, condition of somatic tissues deteriorated until the mature stage, but somatic condition improved after the onset of maturity. In males, there was no correlation between somatic condition and phases of reproductive maturity. Reproductive investment decreased as sexual maturation progressed for both females and males, with the lowest investment occurring at the functionally mature stage. Residual analysis indicated that female reproductive development was at the expense of body muscle growth during the immature and maturing stages, but energy invested in reproduction after onset of maturity was probably met by food intake. However, in males both reproductive maturation and somatic growth proceeded concurrently so that energy allocated to reproduction was related to food intake throughout the process of maturation. For both males and females, there was little evidence of trade‐offs between the digestive gland and reproductive growth, as no significant correlation was found between dorsal mantle length‐digestive gland weight residuals. The role of the digestive gland as an energy reserve for gonadal growth should be reconsidered. Additionally, feeding intensity by both males and females decreased after the onset of sexual maturity, but feeding never stopped completely, even during spawning.     

Age and size-dependent variation in the seasonal timing and probability of reproduction among mature female pumpkinseed,Lepomis gibbosus

Synopsis In this study, we used pumpkinseed,Lepomis gibbosus, populations in two east-central Ontario lakes to test for age and size-dependent effects on the probability and timing of reproduction of mature females. Pumpkinseed body size characteristics differed in the two lakes; Little Round Lake harbours a stunted population and Beloporine Lake does not. Age-class distribution of mature females was determined by a maturity assessment on fish collected just prior to spawning and at the mid-spawning period, combined with an early-season capture-mark-recapture survey. Spawning females were collected throughout the breeding season to assess age and size-related temporal trends, and to compare their age and size distribution with that of mature females at large. The proportion of age 2 females in Little Round Lake that actually spawned was considerably lower than the proportion of mature age 2 females at large. Furthermore, age 2 females that spawned in this lake did not do so until late in the breeding season. In contrast, the proportion of young/small females spawning in Beloporine Lake was comparable to the proportion of young/small mature females at large, and both small and large females spawned throughout the breeding season. Small mature females in Little Round Lake may have had no other option but to spawn late in the season because of their poor body condition. In Beloporine Lake, condition factor early in the breeding season in age 2–4 females was higher than that of Little Round Lake females, suggesting that limited energy reserves in the spring may have prevented young Little Round Lake females from early spawning. Our results show that the likelihood and timing of reproduction are both age and size-dependent in some populations. Small individuals that delay seasonal maturation and spawn late in the summer probably contribute little to the population due to the restricted growth and reduced overwinter survival of their progeny.To whom correspondence should be addressed     

Gonad development and size‐at‐maturity of silver therapon Leiopotherapon plumbeus (Kner 1864; Teleostei:Terapontidae) in tropical volcanic lakes in south Luzon,Philippines

Gonad development of the silver therapon Leiopotherapon plumbeus in two volcanic crater  lake  habitats (Sampaloc Lake, Taal Lake) in south Luzon, Philippines was examined during the annual reproductive cycle. The minimum body size‐at‐maturity of fish in these two lake habitats was also compared. Four gonad development stages were characterized as basis for the classification of ovarian (immature, maturing, mature, spawned) and testicular maturation (immature, maturing, mature) phases. The occurrence of all development stages in individual gonads suggest an asynchronous development whereby advanced stages are recruited continuously from a pool of younger stage germ cells to result in elevated female and male GSI throughout the annual cycle due to active gonadogenesis. Together with the increasing occurrence of advanced stage oocytes and spermatozoa from March until October, the elevated GSI of fish may indicate peak gonadal growth during the onset of the dry season (December–January) for eventual spawning from the beginning (May–June) until the end of the wet season (October–November). In both lake habitats, male fish were smaller than females but, regardless of sex, the minimum size‐at‐maturity of fish in Sampaloc Lake was significantly smaller than fish in Taal Lake. Overall, asynchronous development during oogenesis and spermatogenesis allows for year‐round reproduction of silver therapon, with elevated gonad growth in the dry season in preparation for spawning during the wet season. Compared with fish in Taal Lake, a smaller size‐at‐maturity of fish in Sampaloc Lake may be a response of the wild fishery stock to long‐term high fishing mortality and degradation of the lake habitat.     

Fish predation and offspring survival in the prosobranch snail Viviparus ater

In this study we identified some of the predators of the freshwater snail Viviparus ater and estimated offspring survival to the end of the first summer in a natural population. Newborn V. ater were eaten by the fish Barbus barbus, Rutilus rutilus, Scardinius erythrophtalmus and Tinca tinca. Out of 137 guts of Abramis brama caught in Lake Zürich 1 contained shell fragments and an operculum of a newborn V. ater. On a 40×40 m grid near Goldbach, Lake Zürich, we counted 678 adult females of V. ater in June 1988, which together gave birth to approximately 13 300 offspring throughout the summer. In October we found 1348 V. ater of age class 0 on the grid, i.e. approximately 10% of the year's young had survived to the end of their first summer. The grid had been divided into two sections, A and B. In section A, 72 carthenware tiles had been placed as shelters for V. ater. Offspring survival was slightly but significantly higher here (10.9%) than in section B (9.6%). The observation that offspring survival in the natural habitat was 1 order of magnitude lower than in cages suggests that predation is an important cause of mortality of newborn V. ater. Average population density was 2 individuals m^–2 There was a migration from shallow (1–4 m) to deeper water (5–9 m) in September. The average distance between weekly recordings of tagged V. ater was 4.7 m in males and 3.0 m in females.     

Examination of the effect of wastewater on the productivity of Lake Zürich water using indigenous phytoplankton batch culture bioassays

Phytoplankton batch cultures were used to study the effect of biologically and chemically treated wastewater on algal growth in water from Lake Zürich. The question of whether the influence of sewage on the biomass production corresponds solely to the nutrient content of the sewage was also considered. The relationship between net microbial production (y, in 0.1 N KMnO₄ consumption) and total phosphorus concentration (x, in µg P/1) was found to be characterized by the equation y = 2.12 ln x – 4.12. The fact that net microbial production is strongly dependent on total phosphorus concentration emphasizes the significance of the latter for the trophic state of Lake Zürich. We recommend the introduction of additional purification of all sewage entering the lake.     

Differences in demographic parameters among electrophoretic clones of Daphnia obtusa Kurz (Crustacea: Cladocera)

In an 18-month electrophoretic study, a Daphnia obtusa population that recently colonized Lake Orta (a large, severely polluted lake in Northern Italy) showed no genetic variation.Populations from other localities were sampled. From clones established by single females, genotype differences were determined by two life table experiments (using Lake Orta and Lake Maggiore water as a culture medium).As expected, in the Lake Orta water the colonizing clone (LO) had longer survival, produced more newborns and had a larger rate of increase than the other clones. In the Lake Maggiore water the LO clone still fared better than the other clones.     

Reproductive traits of the “ocellated killifish” Floridichthys polyommus Hubbs, 1936 (Pisces: Cyprinodontidae) inhabiting estuary of the Champotón River (Campeche,Mexico)

This study explores the reproductive biology and the influence of environmental factors on the reproductive traits of Floridichthys polyommus inhabiting the estuary of the Champoton River. The analysis included population structure, gonadal developmental stages, reproductive period, size at first sexual maturity, fecundity, sex ratio, and somatic indexes. A spatio‐temporal pattern in the distribution of the seven size classes was detected. The species showed a positive allometric growth. Sex ratio was 1.21:1 (females:males). Size at first maturity was 42.23 mm (females) and 47.8 mm (males). A higher absolute and relative fecundity was detected in Puente Champotón estuary (PCH) with lower salinities than those detected in the Delta (DE). The adaptive significance of this trait could be in response to the heavy osmotic constraint imposed by extreme salinities in the DE. Floridichthys polyommus prefers PCH for spawning, and as a result a higher number of ripe individuals was observed in this site, characterized by the highest nutrient levels, which can boost productivity and food availability for fish. Hepatosomatic index was negatively correlated with gonadosomatic index, suggesting energy transfer from the liver towards gamete production. Floridichthys polyommus exhibits an opportunistic strategy (early maturity, small oocytes, small clutches) suggesting that energy is allocated towards reproduction to compensate for the unpredictable hydrological conditions imposed by the estuary (particularly during the hurricane season). Floridichthys polyommus shows a reproductive strategy exhibited by fish living in unstable systems. A seasonal divergence was observed in the reproductive traits of F. polyommus, while differences between study sites were for fecundity.     

The periodicity of phytoplankton in Lake Constance (Bodensee) in comparison to other deep lakes of central Europe

Phytoplankton periodicity has been fairly regular during the years 1979 to 1982 in Lake Constance. Algal mass growth starts with the vernal onset of stratification; Cryptophyceae and small centric diatoms are the dominant algae of the spring bloom. In June grazing by zooplankton leads to a clear-water phase dominated by Cryptophyceae. Algal summer growth starts under nutrient-saturated conditions with a dominance of Cryptomonas spp. and Pandorina morum. Depletion of soluble reactive phosphorus is followed by a dominance of pennate and filamentous centric diatoms, which are replaced by Ceratium hirundinella when dissolved silicate becomes depleted. Under calm conditions there is a diverse late-summer plankton dominated by Cyanophyceae and Dinobryon spp.; more turbulent conditions and silicon resupply enable a second summer diatom growth phase in August. The autumnal development leads from a Mougeotia — desmid assemblage to a diatom plankton in late autumn and winter.Inter-lake comparison of algal seasonality includes in ascending order of P-richness Königsee, Attersee, Walensee, Lake Lucerne, Lago Maggiore, Ammersee, Lake Zürich, Lake Geneva, Lake Constance. The oligotrophic lakes have one or two annual maxima of biomass; after the vernal maximum there is a slowly developing summer depression and sometimes a second maximum in autumn. The more eutrophic lakes have an additional maximum in summer. The number of floristically determined successional stages increases with increasing eutrophy, from three in Königsee and Attersee to eight in Lake Geneva and Lake Constance.     

PATTERNS OF RESOURCE ALLOCATION IN A DIOECIOUS CAREX (CYPERACEAE)

Allocation to vegetative growth and sexual reproduction was investigated throughout the growing season in the dioecious sedge, Carex picta, under natural conditions and following experimental manipulations. Measurements taken on unmanipulated plants showed that the sexes did not differ in the total amount of biomass they allocated to either growth or reproduction. The relatively equal investment in reproduction by the two sexes is contrary to other studies, the majority of which show greater investment in reproduction by females. Two features of the reproductive biology of C. picta may account for the equal investment: the fruit are relatively inexpensive because they are uniovulate and nonfleshy, and the stamens are relatively expensive because C. picta is wind pollinated. In contrast to the lack of differences in the amount of allocation, there were differences between the sexes in the timing of allocation to growth and reproduction: males allocated more to reproduction and less to growth up to the time of flowering, whereas females showed this pattern during the time of fruit maturation. Defoliation and inflorescence removal experiments showed that a trade-off within plants between growth and reproduction does exist. In addition, the defoliation experiment revealed a difference in the response of the two sexes: defoliated tillers on males showed a reduction in growth, whereas defoliated tillers on females did not. Overall, the data support the idea that differences in the timing of reproductive expenditure are as important as the amount of expenditure in determining many aspects of the life history strategies of the two sexes.     

The influence of intrinsic and extrinsic factors on developmental parameters and their relationships in the marine isopod Idotea linearis (Crustacea)

Developmental and reproductive parameters and their relationships were studied in the marine isopod Idotea linearis. We hypothesized that (1) the temporal patterns of molting and growth undergo complex and sex-specific changes with age as well as with the onset of sexual maturation, and that (2) sexual maturation (and dependent parameters) is controlled by the photoperiod. Both males and females were singly cultured in the laboratory at two alternative photoperiods (constant long and short days, respectively) from hatching until death. Males molted and grew throughout their life, showing a steady increase in stage duration and body size with each molt. Females, in contrast, showed much more complex modifications in molt chronology due to reproductive demands. There was some variability in the stage number, when females reached maturity. Reaching maturity early in the succession of molts was associated with smaller body size at maturity, smaller size of broods, but higher average number of broods per lifetime. Post-puberty molts in females occurred without further growth, and successive broods did not differ in size. The photoperiod strongly affected sexual maturation (and thus in turn molting and growth patterns) in females, while males remained completely unaffected by the photo regime.     

Relationships of reproductive traits and body size with attainment of sexual maturity and age in Blanding's turtles (Emydoidea blandingi)

To place associations among body size, age at maturity, age, and reproductive traits of a long-lived organism in the context of current life history models based on the concept of norms of reaction, we examined data from a mark-recapture study of Blanding's turtles (Emydoidea blandingi) in southeastern Michigan during 24 of the years between 1953 and 1988. Females matured between 14 and 20 years of age. Both the smallest and largest adult females in the population were reproducing for the first time in their lives. This result suggests that a combination of differences in juvenile growth rates and ages at maturity, and not indeterminate growth, are the primary cause of variation in body size among adults. Body size variation among individuals was not related to age at sexual maturity. Females that had slower growth rates as juveniles matured later at similar mean body size compared to those with more rapid growth that matured at an earlier age. As a result, a linear model of age at sexual maturity with growth rates of primiparous females between hatching and maturity was significant and negative (R² = 0.76). Frequency of reproduction of the largest and smallest females was not significantly different. Clutch size did not vary significantly with age among either primiparous or multiparous females. Clutch sizes of primiparous females and multiparous females were not significantly different. However, older females (>55 years minimum age) reproduced more frequently than did younger females (minimum age <36 y).     

Effects of increasing salinity on anArtemia population from Mono Lake,California

Summary Salinity increased from 48 to 93 g/l total dissolved solids (TDS) in Mono Lake, California between 1941 and 1982, and is expected to fluctuate between 169 and 248 g/l at equilibrium by the middle of the next century. In order to predict the consequences of this trend on the Mono Lake ecosystem, we determined effects of salinity on survival, growth, reproduction and hatching ofArtemia monica, Mono Lake's only macrozooplankton species. Seven salainities ranging from 76 to 179 g/l were tested in a long-term experiment to determine both lethal and sublethal responses. The salt tolerance limit for subadultA. monica was between 159 and 179 g/l. Adult size, growth rates, and brood sizes decreased, and female mortality during reproduction increased, as salinity increased. Hatching of diapause eggs was delayed and total percent hatch decreased as salinity increased, and hatching failed at 159 g/l. The life-time reproductive potential of individual females decreased linearly over the seven salinities tested. Based on this study, we predict a decrease in the productivity of theA. monica population in Mono Lake and extinction of the species is probable before the lake reaches equilibrium.     

Size Matters: Individual Variation in Ectotherm Growth and Asymptotic Size

Body size, and, by extension, growth has impacts on physiology, survival, attainment of sexual maturity, fecundity, generation time, and population dynamics, especially in ectotherm animals that often exhibit extensive growth following attainment of sexual maturity. Frequently, growth is analyzed at the population level, providing useful population mean growth parameters but ignoring individual variation that is also of ecological and evolutionary significance. Our long-term study of Lake Erie Watersnakes, Nerodia sipedon insularum, provides data sufficient for a detailed analysis of population and individual growth. We describe population mean growth separately for males and females based on size of known age individuals (847 captures of 769 males, 748 captures of 684 females) and annual growth increments of individuals of unknown age (1,152 males, 730 females). We characterize individual variation in asymptotic size based on repeated measurements of 69 males and 71 females that were each captured in five to nine different years. The most striking result of our analyses is that asymptotic size varies dramatically among individuals, ranging from 631–820 mm snout-vent length in males and from 835–1125 mm in females. Because female fecundity increases with increasing body size, we explore the impact of individual variation in asymptotic size on lifetime reproductive success using a range of realistic estimates of annual survival. When all females commence reproduction at the same age, lifetime reproductive success is greatest for females with greater asymptotic size regardless of annual survival. But when reproduction is delayed in females with greater asymptotic size, lifetime reproductive success is greatest for females with lower asymptotic size when annual survival is low. Possible causes of individual variation in asymptotic size, including individual- and cohort-specific variation in size at birth and early growth, warrant further investigation.     

Sex-related differences in reaction norms in the butterfly Lycaena tityrus (Lepidoptera: Lycaenidae)

We investigate sexual differences in reaction norms in directly developing individuals of the copper butterfly Lycaena tityrus predicted from sexual selection theory. As recent studies on butterflies revealed a high degree of adaptive plasticity in growth and development, which may undermine the basic trade‐offs assumed in life‐history theory, we focus on effects of temperature, trying to drive growth rates to their physiological upper limit and thus disclosing otherwise potentially concealed responses. Development time strongly depended on temperature, leading in accordance with a central assumption in life‐history theory to a larger size at low temperatures, and vice versa. At all temperatures larval development time of males was significantly shorter compared to females, as was predicted by protandry theory. This was partially due to an invariably higher growth rate of males. However, sexes responded in different ways to developmental time constraints caused by increasing temperatures. Despite the shorter larval time of males, both sexes achieved similar body sizes at lower temperatures, because males avoided a reduction in weight due to plastic growth. At high temperatures, in contrast, males were forced to make a trade‐off in which they favoured early emergence over large size, leading to a dramatic weight loss. Weight of females, however, remained similar throughout showing no trade‐off. These different reaction norms reflect divergent selective pressures acting on males and females, which can be explained in relation to the reproductive system. The strong selection for early emergence in males is likely to be due to monandry, discrete non‐overlapping generations (as was already predicted by theory), and territoriality, because prior ownership of a territory seems to be a major advantage for successful reproduction. On the other hand, the preference of females for large body size was expected due to the close relationship between this trait and fecundity. Thus, our results highlight the extraordinary importance of the specific reproductive system, which can influence central life‐history traits in manifold ways.     

Sex ratio and sexual dimorphism in the dioecious Borderea pyrenaica (Dioscoreaceae)

Sex ratio and sexual dimorphism of Borderea pyrenaica, a long-lived dioecious geophyte endemic to the Pyrenees (north-east Iberian Peninsula), were examined in three alpine populations. In this species, age can be estimated and the sex of nonreproductive adult plants identified. Male plants attain sexual maturity earlier, flower more frequently and grow faster than female plants, whereas females allocate a higher biomass to reproduction than males. These results support the hypothesis that female plants incur a higher cost of sexual reproduction and that this higher cost is measurable as reduced vegetative growth and lower flowering frequency. Variation of sex ratio among young, intermediate and old adults within populations suggests, however, that this higher female reproductive investment does not result in sexual differences in mortality. The overall male-biased sex ratio in B. pyrenaica is mainly a consequence of the tendency of males to reproduce at an earlier age and more frequently than females.     

Biology of the marbled lungfish, Protopterus aethiopicus Heckel, in Lake Baringo, Kenya

In this paper we provide life history characteristics for an introduced marbled lungfish (Protopterus aethiopicus) population in Lake Baringo, Kenya, including their length–weight relationship, body condition, fecundity, sex ratio, reproduction and food. These data are compared with those of other populations. Samples were obtained from the commercial fishery between January and October 2001. The lungfish exhibited positive allometric growth, with the length–weight relationship described by the equation: logW = ?6.41 + 3.52 logTL. The mean relative condition factor was close to unity, did not vary much between months and was not significantly different between sexes. The overall sex ratio was skewed in favour of females. The size of fish at first maturity was 70–76 cm total length (TL) for females and 82–88 cm TL for males. Fish in maturity stages III–IV occurred in all monthly samples suggesting year round spawning. The marbled lungfish in Lake Baringo is largely piscivorous and its impact on other fishes certainly warrants further research.     

Daphniopsis studeri (Crustacea: Cladocera) in lakes of the Vestfold Hills,Antarctica

Summary The adult sizes and reproductive capabilities of populations of the cladoceran Daphniopsis studeri Rühe from freshwater and slightly saline lakes in the Vestfold Hills, Princess Elizabeth Land were monitored during December, 1990. The population of Crooked Lake was studied in detail from July 1990–March 1991. Limited samples were also obtained from Heard Island (sub Antarctic). Obligate parthenogenesis involving the production of subitaneous eggs appears to be the case. No males occurred. The overwintering populations of continental Antarctica consist mainly of adult females containing extensive fat reserves. The main reproductive effort occurs in spring. some females produce a smaller second brood during the summer. Reproductive output increases with adult size which is related to the nutrient status of the lake and its productivity. The Heard Island populations have a higher weight to length ratio than the Vestfold populations and have a much greater reproductive output.     

The effects of productivity and seasonality on life history: comparing age at maturity among moose (Alces alces) populations

The productivity hypothesis in respect of an animal species’ geographical range predicts that whereas higher productivity at the equatorial periphery of a species’ range favours superior competitors, lower productivity at the centre of a species’ range favours high reproduction and reduced competitive traits. I test whether life‐history patterns follow this hypothesis, using demographic data from 15 Canadian moose (Alces alces) populations. Two models are contrasted; the first assumes that intraspecific variation in age at maturity is explained proximately by density and juvenile mortality. Age at maturity was found to increase with decreasing juvenile mortality (P = 0.01) and increasing density (P = 0.006). To test the productivity hypothesis, the second model additionally included primary productivity and seasonality as geographical explanatory variables that would ultimately influence age at maturity via juvenile mortality and density. Path analysis indicated that including productivity and seasonality improved the model predictions of variation in age at maturity (R_a² 0.56 and 0.85). In bivariate comparisons, seasonality was negatively associated (P = 0.01) with age at maturity. In the best model, however, primary productivity was the environmental variable that explained 25% of the variance in age at maturity, and forest cover replaced seasonality as an explanatory variable. The positive association between primary productivity and age at maturity is consistent with the productivity hypothesis. Relative to populations that lived at the centre of the species’ range (51°N), moose populations living in relatively high productivity and low seasonality environments (equatorial periphery of species’ range; 48°N) experienced less juvenile mortality, more variable year‐to‐year density, higher relative density and slower life history (slower growth rate, later age at maturity, lower fecundity).