Effects of diet, body size, age and temperature on growth rates in the amphipod Gammarus pulex

SUMMARY. Increase in body wet weight of Gammarus pulex fed on decaying elm leaves was followed to senescence and death. Growth in juveniles was approximately exponential; from birth to death it conformed to a logistic growth curve, with maximum absolute increments in weight about half-way through a life span of 350–450 days at 15°C. Some individuals lived longer, for up to 640–700 days. The instantaneous or specific growth rate was maximal near birth, at c. 5–6% wet wt day−1, and declined exponentially with increasing size and age. Over the range 4.7–14.8°C there was a log-log relationship between temperature and specific growth rate. Growth was maximal at 20°C in newborn animals and at 15°C in 6–9-mg animals. The specific growth rate of young individuals was fastest on decaying leaves of elm with a well developed flora of fungi and other microorganisms. Leached elm leaves without this flora supported growth at a lower rate. The latter diet was sufficient for survival and growth of newborn individuals; detritus, faeces or other food items were not needed. Isolated specimens grew as fast as those kept in groups. Growth was generally slower on leached leaves of oak and sycamore. In newborn animals fed on the fine roots of aquatic plants (Veronica, Rorippa and Glyceria), growth was as fast as on decaying elm leaves; growth on the green living leaves of the plants was slower, as on detritus from two streams and on a pure culture of an aquatic fungus. Consumption of leached elm leaves was related to leaf thickness. In a full gut the wet weight (1.34–1.37 mg) and volume (3.8–4.1 mm3) (for 20-mg animals) was independent of leaf thickness but dependent on animal size, increasing 4-fold over the range 2–50 mg body wt. Daily consumption (dry wt) was approximately equivalent to 50% body dry wt at 5 mg and 20% at 50 mg body wet wt. Individuals fed on thick leaves ingested 50% more dry weight per day and absorbed more in the gut than when fed on thin leaves, but the relative efficiency of absorption was the same at 36–59% for 10–20-mg animals. Weight-specific absorption in the gut was highest in juveniles and decreased with increasing body weight; relative efficiency of absorption was generally lower in the larger individuals. Assuming an energy value of 5 cal mg−1 dry wt for elm leaves, daily mean energy intake by absorption in thegutof G. pu/ex was2.2 cal mg−1 animaldry wt (9.2 J mg−1) in individuals of 0.4 mgdry wt (2 mg wet wt), decreasing to 0.3 cal mg−1 (1.3 J mg−1) at 10 mg dry wt (50 mg wet wt). Growth in Gammarus is briefly reviewed in the hght of work on other animals and it is emphasized that all aspects of feeding, growth and metabol-ism should be specifically related to size and age of the individuals, using well defined diets.