Respostas fisiológicas à hipóxia gradual em neonatos e juvenis da arraia cururu Potamotrygon cf. histrix

Abstract

The purpose of this study was identify the response of blood properties and glycogen content in tissues of neonates and juveniles of cururu stingray (Potamotrygon cf. histrix) exposed to progressive decay of dissolved oxygen levels in water (hypoxia). Individuals of cururu stingrays were subjected to three groups: normoxia (NO) where animals were kept in O2 levels up to 80% O2 saturation (about 6.0 mg/L) and two groups exposed to progressive hypoxia, where O2 levels declined (until at least 1 mg/L) by own respiration, hypoxia with access to the surface (HC), where the stingrays had free access to the entire water column, and hypoxia without access to the surface (HS), where a containment screen was installed 5 cm below the surface to prevent animals had access to more oxygenated layer of the water column. Six individuals were used in each treatment, in both experiments with neonates and juveniles. We evaluated the frequency of spiracular movements (FE), which increased in response to hypoxia in both neonates (39 to 74 beats/min) and juveniles (50 to 78 beats/min); hematological parameters, which showed more pronounced responses in neonates, as decreased erythrocyte count and increased in MCH in HS group and increased in MCV on both HC and HS groups. Regarding plasma metabolites, there was an increase in lactate levels in the HC group of juveniles, decreased total cholesterol in neonates and juveniles, and in both treatment groups, as well as increased levels of total protein in HS group of juveniles. In juveniles, increased liver glycogen and muscle glycogen mobilization (more intense in the HC group) were observed in response to progressive hypoxia. However, this pattern was not observed in neonates who showed differences in liver glycogen between groups exposed to hypoxia, but not when compared to normoxic group. Also, similar levels of this sugar in the muscle were observed in the three treatment groups. Neonates and juveniles of Potamotrygon cf. histrix share characteristics observed in other hypoxia-tolerant fish, such as increased gill/espiracular frequency in response to low oxygen levels; their large glycogen reserves (muscle and liver); the ability to adjust some parameters of metabolism, such as hematology, that in the case of cururu stingray occurs in the early stages of life, and increases in circulating lactate levels as a result of activation of anaerobic metabolism. It is concluded that this freshwater stingray species can be inserted in the group of hypoxia-tolerant animals.