Gill dimensions in near-term embryos of Amazonian freshwater stingrays (Elasmobranchii: Potamotrygonidae) and their relationship to the lifestyle and habitat of neonatal pups

This comparative study of gill morphometrics in near-term embryos of freshwater stingray potamotrygonids examines gill dimensions in relation to neonatal lifestyle and habitat. In embryos of the potamotrygonids Paratrygon aiereba, Plesiotrygon iwamae, Potamotrygon motoro, Potamotrygon orbignyi, and cururu ray Potamotrygon sp. the number and length of filaments, total gill surface area, mass-specific surface area, water-blood diffusion distance, and anatomical diffusion factor were analysed. In all potamotrygonids, the 3rd branchial arch possessed a larger respiratory surface than the other gill arches. Larger embryos had more gill surface area and large spiracles, which are necessary to maintain the high oxygen uptake needed due to their larger body size. However, the higher mass-specific gill surface area observed in near-term embryos may be advantageous because neonates can use hypoxic environments as refuges against predators, as well as catch small prey that inhabit the same environment. As expected from their benthic mode of life, freshwater stingrays are sluggish animals compared to pelagic fishes. However, based on gill respiratory morphometry (such as gill area, mass-specific gill area, the water-blood diffusion barrier, anatomical diffusion factor, and relative opening of the spiracle), subtypes of lifestyles can be observed corresponding to: active, intermediate, and sluggish species according to Gray’s scale.