Use of acid-insoluble ash as a marker in absorption efficiency studies with the lemon shark

The suitability of acid-insoluble ash as a marker in digestive efficiency studies with the lemon shark (Negaprion brevirostris) was investigated. Acidinsoluble ash concentration in the experimental diet was increased by adding celite (diatomaceous earth). Acidinsoluble ash was palatable to test animals, mixed homogeneously within the experimental diet, could be easily and precisely quantified in feces and food, and was not produced by lemon sharks. However, acid-insoluble ash content in feces was significantly lower during the first 24 h of fecal production than during subsequent collection periods, and the marker may not have remained uniformly mixed in digesta. Differential passage rates for this marker and food may influence estimates of absorption efficiency. Passage rate of acid-insoluble ash should be investigated in future studies of digestive efficiency when this marker is used.