Reproductive potential and lifetime potential fecundity of the freshwater amphipods Gammarus fossarunt and G. roeseli in Austrian streams and rivers

SUMMARY1  Fecundity of Gammarus fossarum populations at six stream sites (S1‐S6) and G. roeseli populations at two sites (S5 and S6) was studied monthly during 1985–1988, distinguishing seven morphological stages of embryonic development. Mean survival of G. fossarum eggs was 65% (60–70%) in nature and 60% (57–63%) in the laboratory; egg survival of G. roeseli was 52% (46–58%) in nature and 41% (35–47%) in the laboratory. Regressions of egg numbers per clutch against body wet weight (WWT) decreased significantly for egg developmental stages 2 to 7 (hatching). 2  In stream populations, ovigerous females of G. fossarum were absent in October (six sites) and November (five sites); ovigerous G. roeseli were usually absent from October until March. 3  Overall mean egg volume (EV) increased significantly from 0,08 ± 0.001 mm 3 and 0.075 ± 0.001 mm 3 for stage 2 eggs, to 0.174 ± 0.012mm 3 and 0.160 ± 0.013 mm 3 for stage 6 eggs of G. fossarum and G. roeseli , respectively. The dry weights of stage 2 eggs, stage 6 eggs and neonates (stage 7) were not significantly different, within and between species; mean dry weight was 36 ± 3 μg. 4  Egg volumes and fecundity indices (FI, weight‐specific number of eggs per clutch) were not significantly different between the same months of four successive years, so the data were pooled to give a single representative ‘year’. At most sites there were marked seasonal fluctuations in these variables. In both species, mean EV was largest for ‘winter’ eggs in December/January and smallest for ‘summer’ eggs in May, EV decreased during some months of the year when FI increased, and vice versa. Mean reproductive effort (RE = EV × FI) declined from high ‘winter’ values to low ‘summer’ values, and this difference was most marked at sites where the major foodstuff available was seasonally limited (autumn‐shed tree leaves). Overall (‘annual’) mean RE was also lowest at these sites (S3 and S4) but was some 13% greater at sites where nutrients and food were present in the greatest quantifies (S5 and S6). Specific maximum potential body sizes and hence maximum potential fecundities (fifty‐nine eggs for G. fossarum and eighty‐seven eggs for G. roeseli ) were attained only at S5 and 56. Mean RE was nearly identical for G. fossarum and G. roeseli from the same sites. 5  At experimental temperatures ( T ) in the range 3.8–20.2°C, females of the same body weight carried significantly different egg numbers per clutch. The relationship between FI and T was described by a second‐order polynomial. The calculated optimum T was 12.1°C for G. fossarum and 16.3°C for G. roeseli . Interspecific differences were highly significant. 6  Theoretical lifetime potential fecundity was approximately 194 eggs for G. fossarum