The digestive physiology and food requirements of Gammarus pulex (L) (Amphipoda)

Abstract

Several freshwater invertebrates, including Gammarus pulex (L), were shown to possess gut cellulase activity. The nutritive physiology of G. pulex was further investigated in order to elucidate the manner in which plant detritus Is utilised as food. This was approached by studying physical and chemical conditions in the gut and the effects of test diets on ingestion, assimilation and digestive enzyme secretion. The extents to which some of these diets and others supported growth and survival were also examined. The results of an investigation into the digestion of decomposing leaf litter and fine detritus by commercial enzymes are described. Cellulase activity in the midgut glands was endogenous. The pH of the midgut gland fluid was little affe3cted by diet and, along with that of the foregut, was well suited to carbohydrate digestion. The pH at the posterior end of the midgut was better suited to protein digestion. The tract transit time was normally independent of the rate of ingestion. Intact plant cells that were not broken open mechanically were not digested chemically in the gut. The occurrence of enzyme inhibitors in leaf litter was investigated. The assimilation efficiency and rates of ingestion and enzyme secretion varied with different diets, but were not correlated with the nutritive values of the diets. Palatability and physical ease of ingestion controlled rates of ingestion and, probably, enzyme secretion. The assimilation efficiency seemed to depend on the surface area of the gut contents available for enzymic attack. Growth and survival were affected by the water chemistry and the initial size of the animals used, as well as by the nature of the diet. Fine particulate detritus was of little nutritive value and the nutritive value of intact elm litter depended on the amount of microbial growth which it supported. Food nutritive value was not directly related to protein content, although the microbial flora was important in furnishing nitrogen requirements, at least during certain critical phases of growth. An energy budget was calculated and food utilisation in G.pulex and other aquatic invertebrates discussed.