An improved technique for measuring assimilation efficiency by the Cr-C twin tracer method.

Calow and Fletcher have shown that assimilation efficiency can be calculated from the ratios of an assimilated radiotracer (e.g. C) and a non-assimilated tracer (e.g. Cr) in the food and faeces of freshwater pulmonates [Oecologia (Berl.) 9, 155-170 (1972)]. It is suggested that their counting methods have limitations and are not wholly suitable for wide biological application. An alternative technique was tested using third instar larvae of a Kenyan cetoniid, Pachnoda ephippiata (Gerst.) (Coleoptera, Scarabaeidae), fed on an artificial food medium labelled with C-glucose and CrCl. The activity of both tracers in food and faeces was estimated from simultaneous counts of the β emissions recorded by a liquid scintillation spectrometer. The scintillant solution, toluene-Triton X-100-PPO, extracted virtually all of the labelled tracers from 0.2 g samples of the agar-cellulose-water food medium. The calculated assimilation efficiency values for glucose increased from 50.2% to 78.6% over the 6 days following exposure to the labelled food. 90.8% of the total Cr recovered was egested during these 6 days, 76.8% during the first 3. An ecologically more meaningful estimate of assimilation efficiency was calculated by integrating the C and Cr counts over the 15 days that faeces were collected. The applications of this approach to measuring assimilation efficiency are wide. They include the possibility of measuring how much excreta are produced from a given amount of Ingesta and of determining the role of gut-microorganisms in the assimilation of their host's food.