Aerial respiration facilitates growth in suspension-feeding anuran larvae (Xenopus laevis).

Xenopus laevis tadpoles are midwater suspension-feeders that use buccopharyngeal surfaces for both food capture and aquatic respiration, but also have functioning lungs well before metamorphosis. To examine the effect of aerial respiration on premetamorphic growth and development in this species, tadpoles were raised in normoxic water for from one to three weeks at three different concentrations of food (yeast cells in suspension). At each food concentration the larvae were either allowed or denied access to air. Xenopus larvae reached greater snout-vent lengths, weighed more and developed more rapidly when allowed to breathe air. The difference in growth and development between those tadpoles with and without access to air was proportional to the concentration of particulate matter. In the extreme, tadpoles denied access to air at the highest food concentration all asphyxiated shortly after the start of the experiment. Xenopus tadpoles faced with the functional conflict of using buccopharyngeal surfaces for both feeding and respiration retard ingestion. They do this, however, not by ceasing mucus outflow to their branchial food traps, as has been speculated previously, but rather by capturing food particles in mucus and then expectorating it. Lungs appear to be advantageous to aquatic organisms even in normoxic water in that they allow buccopharyngeal surfaces to be dedicated fully to feeding rather than respiration.