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

Wallice Paxiúba Duncan,Maria Isabel Da Silva,Marisa Narciso Fernandes

doi:10.1590/1982-0224-20140132

Wallice Paxiúba Duncan, Maria Isabel Da Silva + Show 1 more

Open Access

https://doi.org/10.1590/1982-0224-20140132

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Abstract

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.

Highlights

Freshwater stingrays (Potamotrygonidae Garman, 1877) occur in major South American rivers that drain into the Atlantic Ocean and Caribbean Sea, with the exception of the Sao Francisco drainages in Brazil
Potamotrygon sp. has a total number of filaments (TNF) similar to that found in P. motoro and P. orbignyi, but they are shorter in Potamotrygon sp. than in the latter species
It was reported that the gill arches of potamotrygonids are not identical structural units in the branchial system (Duncan et al, 2011)

Summary

Introduction

Freshwater stingrays (Potamotrygonidae Garman, 1877) occur in major South American rivers that drain into the Atlantic Ocean and Caribbean Sea, with the exception of the Sao Francisco drainages in Brazil. The distribution of potamotrygonids has been interpreted to be the result of different habitats (including physicochemical composition) that may act as hydrological barriers to the dispersal of potamotrygonid rays (Duncan & Fernandes, 2010). It has been proposed that some innovation in the gill structures may have favoured tolerance of freshwater environments during the evolution and diversification of potamotrygonids throughout the Amazon Basin (Duncan et al, 2010). The structure and dimensions of the potamotrygonid gills are important parameters with which to assess their function (Duncan et al, 2011). Gill measurements, including the length and abundance of gill filaments, the number of respiratory lamellae on the filaments, lamellar bilateral surface area, total gill surface area, mass-specific gill area, and the water-blood diffusion barrier, are species-specific (Hughes et al, 1986). Some respiratory factors, such as the anatomical diffusion factor and diffusing coefficient, are obtained from these measurements and may reflect the gills’ performance under specific environmental conditions (Perry, 1990)

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