Gills are not the primary site of gas exchange in the suspension-feeding hemichordate Protoglossus graveolens.

The gills are hypothesized to play a key role in early vertebrate evolution by replacing the skin as the primary site of gas exchange. In this scenario, water flow across the gills for suspension feeding was coopted for gas exchange in stem vertebrates, facilitating the evolution of larger, active fishes. This is supported by a stem-vertebrate origin for structures increasing gill capacity for gas exchange. However, these structures might have enhanced an already dominant capacity at ancestral deuterostome gills. To test this, we characterized gill function in the suspension-feeding hemichordate Protoglossus graveolens. We measured oxygen uptake and ammonia excretion in whole worms and worm halves with or without gills at 10°C and 20°C to stimulate gill recruitment. Gills did not enhance oxygen uptake or ammonia excretion, suggesting they are not a primary site of gas exchange. This provides critical functional support for the long-hypothesized vertebrate origin of gills as the primary gas exchange organ.