Intra- and inter-specific variations in egg survival and brood development time for Austrian populations of Gammarus fossarum and G. roeseli (Crustacea: Amphipoda)

SUMMARY. 1. Egg survival (ES, percentage of eggs hatched in vitro ), reproductive success (RS, percentage of live young released from the brood pouch) and brood development lime ( d , days) in four populations of Gammarus fossarum and two populations of Gammarus roeseli were studied, in the laboratory at water temperatures of 2.0–26.1°C. Intraspecific differences between populations were not significant, but interspecific differences were found between the two species.
2. In natural stream populations, the reproductive period of G. fossarum lasted from December to September, that of G. roeseli from March to September.
3. In the experimental temperature range 2–26°C, 73% of the total number (771) of G. fossarum females and 69% of 469 G. roeseli females were ovigerous. Of these, 45% of G. fossarum and 43% of G. roeseli females successfully released live young from their brood pouches.
4. For G. fossarum , the optimum temperatures were 11.4°C for ES, where 76% of the eggs hatched, and 11.8°C for RS, where 77% of the females released live young from their brood pouches. For G. roseli , the optimum temperatures were 13.5°C for ES (51% hatched) and 14.0°C for RS (76% released). Over 50% of eggs hatched at temperatures of 3.6–19.2°C in G. fossarum and at 1 1.9–15.1°C in G. roeseli . Development time increased from 12 days at 21.9°C to 251 days at 2.0°C in G. fossarum , and from 10 days at 24.1°C to 212 days at 4.1°C in G. roeseli .
5. interspecific differences between the effects of water temperature on ES, RS and d are used to explain the different distributional patterns of G. fossarum and G. roeseli in central European running water systems. assuming that other physico-chemical variables are suitable for both species.