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 ', 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 mm^) (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^' dry wt for elm leaves, daily mean energy intake by absorption in thegutof G. pu/ex was2.2 calmg"' animaldry wt (9.2 Jmg"') in individuals of 0.4 mgdry wt (2 mg wet wt), decreasing to 0.3 cal mg"' (1.3 J mg~') 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 metabolism should be specifically related to size and age of the individuals, using well defined diets.
Fungal cultures attributed to tbe Saprolegnia parasitica-diclina complex isolated from diseased salmonids, diseased coarse fisb and natural water samples, are compared critically. Tbe two species concept is not sustained and only S. diclina Humpbrey is conserved. Tbis results in a more fiexible framework for comparing isolates witbin tbis species and tbree groups are distinguisbed here, based on growtb cbaracteristics and length/breadth (L/B) ratio of tbe oogonium. Oospore size is also considered in tbis connection. In Windermere (Englisb Lake District) only Saprolegnia diclina Type 1 (L/B ratio 5!2>13%) occurs as a parasite on salmonid fisb. Only S. diclina Type 2 (L/B ratio &2< 12%) occurs as a parasite on perch Perca fluviatilis L. Saprolegnia diclina Type 3 (L/B ratio ^2^510%) is purely sapropbytic. An attempt is made to fit S. parasitica-diclina isolates from natural water samples into tbis scheme so tbat tbe potential infectivity for a particular kind of fisb could be assessed.
A survey ot fish affected with epizoolic ulccralive syndrome taken from outbreaks in countries throughout South and South-East Asia showed that a morphologically typical fungus was consistently present within lesions. Although the majority of the fungal mycelium was dead in most lesions it proved possible to isolate a very delicate and culturally demanding Aphanomyces from such lesions in a few cases. It also proved relatively easy to isolate other members of the Saprolegniaeeae including Aphanomyces from the surface of lesions, but these were considered saprophytes derived from background spore burdens in the water. Sporangium morphology of the putatively pathogenic isolates oi Aphanomyces was different from that of saprophytie Aphanomyces strains and they also had a lower thermal tolerance. When a mycelium from these strains was placed below the dermis of healthy fish, it caused an inflammator)' response and proceeded to migrate down into the tissues of the fish, inducing severe myonecrosis with chronic epithelial reaction. The saprophytie isolates induced a local host response followed by healing of the induced lesion, and destruction or expulsion of the mycelium. It is considered that the speeific slow-growing, thermo-labilc Aphanomyces is the pathogenic fungus which causes so much tissue damage in this disease, although it may not be a primary pathogen in its own right.
Individuals of Gammarus pulex were kept at 15°Cfor periods of 23-70 days on diets comprised of elm or oak leaves, Tricladium and Clavariopsis {Hypho-mycete fungi), Molinia (grass), Zygogonium (alga), Nardia (liverwort). Both the mean interval between moults (mt), and daily increments in body wet weight, were related to diet. The lowest value for mi was 14-6 days on a diet of naturally decaying elm and oak leaves. The largest weight gains also occurred on this diet; the average daily gain in weight (Dw) = 130'8 ixglday, the mean specific gain in weight (Gw) = 1 -69 fxg yjday. On green Molinia and fungi diets Dw ranged from nil to 60-7 tig I day, and mi = 17-4-18-4 days was significantly (P<001) longer than mi on leaf diets. Survival and growth were poor on brown Molinia. Nardia did not support growth or survival. A mixed diet of Tricladium, Zygogonium, Nardia and decaying grasses was not sufficient to promote an increase in body weight, and mi = 21-8 days. This diet represents the commonly available food materials in Mosedale Beck, an acid stream in the upper Duddon catchment. Survival and growth of G. pulex on fungi and leaf diets in media containing low concentrations of potassium ions are also described and discussed briefly in relation to the distribution of G. pulex in the upper catchment of the R. Duddon.
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