1. The total body length, cephalic length, wet weight and dry weight was measured in juveniles, males and females of Gammarus fossarum and G. roeseli kept in the laboratory. Numbers of flagellar segments on the first and second pairs of antennae of G. fossarum and G. roeseli were quantitatively related to body size and instars. The addition of segments cannot be used to identify particular instars of individuals or to determine their ages in natural populations. 2. At experimental temperatures ranging from 3.8 to 20.2°C, the number of moults, duration of intermoults, maturation times and spedfic growth rates were studied from birth in isolated specimens. Sexual maturity was reached after 9 or 10 moults, at a mean wet weight of c. 5mg for females and c. 7mg for males of G. fossarum, and at c. 10 mg for females and c. 13 mg for males of G. roeseli. At 3.8°C neither spedes reached sexual maturity within 550 days. 3. The mean interval between moults was observed from birth to sexual maturity and was linearly related to moult number and exponentially related to age. The relationship between each intermouU interval, or the maturation time, and the experimental water temperature was described by a power function. Maturation times increased from 96 days at 20.2''C to 355 days at 7.9''C in G. fossarum, and 85 days to 403 days in G. roeseli. 4. Over the range 3.8-20.2°C there was a log-log relationship between temperature and spedfic growth rates. Growth was maximal at 20.2°C in newborn animals as well as in small sexually mature animals; interspedfic differences were highly significant. 5. Increase in body wet weight of G. fossarum and G. roeseli fed ad libitum on a constant mixture of autumn-shed, naturally decaying, tree leaves and aquatic macrophytes was followed to senescence and death. The instantaneous or spedfic growth rate was maximal near birth, at c. 7.98% wet weight day"^ in G. fossarum and 9.03% in G. roeseli. At ^WC, growth conformed to a logistic curve; maximum absolute increments in weight occurred about half-way through a life span of 280-300 days at 20°C, 380-420 days at 16°C and 550-600 days at 12°C. Some individuals lived longer than 850 days at <12°C. The wet weight at birth was 0.112 mg for G. fossarum and 0.123mg for G. roeseli. Asymptotic mean body weights of males and females were, respectively, 61 and 41 mg for G. fossarum and 87 and 58 mg for G. roeseli. However, G. roeseli reached the inflection point of the logistic curve significantly faster than G. fossarum. In the latter spedes, growth and maturation were relatively faster at temperatures below U^C, whereas they were faster in G. roeseli at 16-20°C. Thus G. fossarum is adapted to summer-cool streams and G. roeseli is adapted to summer-warm streams. 211
1. Egg survival (ES, percentage of eggs hatched in vitro), reproductive success (RS, percentage of live young released frotn the brood ptiuch) and brood development time (d, days) in four populations of Gammarus fossarum and two populations of Gammarus roeseli were studied, in the laboratory at water temperatures of 2.(V -26.]°C. Intraspecific differences between populations were not significant, but interspecific differences were found between the two species.
SUMMARY 1. Mathematical functions developed in long‐term laboratory experiments at different constant temperatures were combined with daily water temperatures for 1991–93 in eight Austrian streams and rivers to simulate the complex life histories and reproductive capacities of two freshwater amphipods: Gammarus fossarum and G. roeseli. The functions describe brood development times, hatching success, times taken to reach sexual maturity, growth, and fecundity. The sex ratio was assumed to be 0.5 and an autumn–winter reproductive resting period was based on observations of six river populations. Simulations included summer‐cold mountain streams, summer‐warm lowland rivers, watercourses fed by groundwater or influenced by heated effluents, and varying amplitudes of change within each year. 2. A fortran 77 computer program calculated growth from birth to sexual maturity of first‐generation females born on the first day of each calendar month in 1991, and the numbers of offspring successfully released from the maternal broodpouch in successive broods. At the 1991–93 regimes of temperature, individual G. fossarum released 127–208 offspring and G. roeseli released 120–169 in seven or eight successive broods during life spans of less than 2 years in six rivers. Life spans extended into a third year in the relatively cool River Salzach (mean temperature 7.5 °C). They were not completed in the very cold River Steyr (mean 5.6, range 2.5–7.9 °C), where G. fossarum produced five broods (totalling 120 offspring) and G. roeseli only two broods (totalling 28 offspring) in the 3‐year period. Except in the Steyr, some offspring grew rapidly to maturity and produced several second‐generation broods during the simulation period; in the warmest rivers some third‐generation broods were also produced. Birth dates, early or late in the year, influenced the subsequent production of broods and young, depending on temperature regimes in particular rivers. Total numbers of offspring produced by the second and third generations represent the theoretical reproductive capacities of G. fossarum and G. roeseli. Minimum and maximum estimates mostly ranged from 100 to 17 300, were larger for G. fossarum except in the warmest river (March), where temperatures rose above 20 °C for 56–78 days in summer, and largest (maximum 37 600) in the River Voeckla heated by discharge from a power‐station (mean 11.5 °C). Results from the simulations agree with preliminary assessments of relative abundances for G. fossarum and G. roeseli in several of the study rivers, but in some one or both species appear to be absent. On a wider scale, the present study confirms that G. fossarum is potentially more successful than G. roeseli in cool rivers but indicates that neither species is likely to maintain viable populations in cold rivers strongly influenced by snow and ice‐melt. 3. The potential impacts of future river warming by increases of 1, 2 and 3 °C, due to climate change, vary according to river site, date of fertilisation, the extent of temperature increase, a...
Summary 1. Fecundity of a Dikerogammarus villosus population at Spitz was studied in the Austrian Danube during the 3‐year period 2002–2004. Ovigerous females were absent in October and November, and extremely scarce in December when the reproductive season started again slowly. From January to September pre‐copulatory pairs and egg‐carrying females were present. The reproductive cycle lasted for 9–10 months. 2. Various pigmentation phenotypes of D. villosus have been described in the literature. However, no significant differences were found between the reproductive variables studied here and several colour morphs. Mating was size‐assortative; mean body length of males was about 1.3 times greater than that of their potential mates, and the wet weight was approximately twice as heavy. 3. The relationship between the number of embryos per clutch and the wet weight of females was described by a 3‐parameter power equation. The population mean was 43 eggs with a range of five to 194 eggs. Eighty‐two specimens from 1359 D. villosus females had more than 100 eggs: the smallest of these females was 12 mm long (30 mg) wet weight, and the largest, which was 18 mm long (91 mg), had 194 eggs in embryonic development stage 4. 4. Numbers of embryos in developmental stages 2 (early egg stage) and 7 (newly hatched neonates) differed significantly with body wet weight of ovigerous females (P < 0.05). For an average female in the range 10–12 mm (20–30 mg) the number of juveniles in the brood pouch was 74% of the number of stage 2 eggs. This value can be interpreted as the survival rate of eggs. 5. The overall mean egg volume (EV, ±95% CL) of stage 2 eggs of D. villosus was 0.05 ± 0.001 mm3, and EV increased significantly at each stage of development. At stage 6, egg volume had increased by a factor of 2.6, and averaged 0.13 ± 0.001 mm3. In comparison, G. fossarum and G. roeseli had significantly larger eggs in all developmental stages. 6. Mean egg size of D. villosus (0.063 mm3) was maximal in January. For D. villosus (and G. roeseli) the minimum mean egg size occurred in September. In contrast to G. fossarum and G. roeseli, a second peak in egg size was not observed for D. villosus, and egg size fell more or less successively from January to September. 7. A simple index of fecundity was calculated from the number of stage 2 eggs divided by the female's wet weight. The highest values were observed in April and May, when females from the overwintering generation grew to their maximum body size. Thus the release of a large number of neonates corresponds with the availability of plentiful food and rising water temperatures for juvenile growth in the spring. The lowest value occurred in December. In June the small females of a summer generation appeared, with a naturally low fecundity. 8. The relationship between brood development time and water temperature was studied in the laboratory at a series of constant temperatures. At 16 °C, mean brood development time was 14 days for D. villosus, compared with about 3 weeks for the indigeno...
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