Age and growth of the endemic Xingu River stingray Potamotrygon leopoldi validated using fluorescent dyes

Between 2003 and 2005, vertebrae of 151 Xingu River Potamotrygon leopoldi (Potamotrygonidae) (75 males and 76 females) were analysed to derive a growth curve for this species. The disc width (W_D ) was significantly different between sexes, with females measuring 149-700 mm W_D and males 109-500 mm W_D . The average percentage error for vertebrae readings of the whole sample was 2·7%. The marginal increment ratio (R_MI ) showed an increasing trend with the highest value in November, decreasing from December on. The majority of vertebrae displaying R_MI zero, occurred in September, but the annual periodicity of ring deposition throughout the year was not conclusive. Tetracycline (TCN) injected specimens were held in captivity for 13 months and displayed a fluorescent mark in vertebrae confirming a yearly periodicity of band pair formation with the translucent ring deposited in September-October. The Akaike information criterion (AIC) showed that, among the seven models considered, the best fit was obtained for the von Bertalanffy modified with W₀ (where W₀ = W_D at birth) for both sexes. Growth parameters for females were: W₀ = 149 mm; W_∞ = 763·06 mm; k = 0·12 year^{- 1} , whereas for males: W₀ = 109 mm; W_∞ = 536·4 and k = 0·22 year^-1 . Maximal ages were 7·2 years in males and 14·3 years in females. The species shows sexual dimorphism expressed in the growth pattern, size at maturity, longevity and asymptotic sizes. Concern for sustainability is raised due to the construction of the Belo Monte Hydroelectric Power Plant (2015 and 2016) in the State of Pará causing changes to the habitat of this species, which is endemic to the Xingu River and two of its tributaries.