Regional variation in amphipod life history: effects of temperature and salinity on breeding

As a test for possible local evolution, Gammarus salinus Spooner (Amphipoda, Crustacea) populations from the Oslofjord, Norway, and from Limfjorden, Denmark, were raised from hatching at the combinations of 8, 12 and 20 °C and 6, 14 and 30%. salinity. At 8 °C all animals died before breeding. Only the Oslofjord amphipods produced a few broods at 12 °C. The Oslofjord specimens reproduced at all salinities at 20 °C, but first and most frequently at 14%. salinity, while those from Limfjorden almost exclusively reproduced at 14%. salinity. Egg volume increased when temperature and/or salinity decreased, egg dry weight increased when salinity increased. Adjusted mean egg number per clutch did not differ significantly between the Oslofjord cultures, but the two lowest means were in cultures with much lower survival. Adjusted means of the Oslofjord and the Limfjorden specimens differed significantly at 14%. Limfjorden females produced smaller broods of larger eggs at a larger body size and at a similar age compared with similarly raised Oslofjord females. Slopes of linear regressions of log egg number per clutch on log body size were never significantly different. Excluding possible hidden maternal effects, it seems that the Oslofjord and Limfjorden G. salinus have evolved differences in breeding traits. Reproducing Gammarus oceaniens Segerstr°le and G. salinus from the Oslofjord were collected and acclimated to continue reproduction at the combinations of 10 and 20 °C and 10 and 30%. salinity. Data corroborated the impression that temperature has a major influence on the determination of egg size, while clutch size is mainly regulated by energy available for breeding. The hypothesis that the balance between many small or a few large eggs per clutch at any time in a population may be genetically controlled was rejected. Physiological, ecological and evolutionary aspects of amphipod life history are discussed.