Effects of temperature, age and body size on moulting and growth in the freshwater amphipods Gammarus fossarum and G. roeseli

Abstract

SUMMARY1. 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 in stars. 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 specific 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 species 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 intermoult 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 specific growth rates. Growth was maximal at 20.2°C in newborn animals as well as in small sexually mature animals; interspecific 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 specific growth rate was maximal near birth, at c. 7.98% wet weight day−1 in G. fossarum and 9.03% in G. roeseli. At ≥12°C, 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 0.112mg 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 species, growth and maturation were relatively faster at temperatures below 12°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.