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In cururu stingray (Potamotrygon wallacei Carvalho, Rosa and Araújo 2016) males, plasma progesterone (P 4 ) levels appear to be associated with spermiation events. However, the specific contribution of P 4 in sperm maturation via extratesticular ducts in this stingray species is unknown. With the aim of filling this knowledge gap, this study examined the morphology and the presence of progesterone receptors (PR) in the ducts, and analyzed the relationship of progesterone (P 4 ) with sperm maturation and formation of aggregates. Morphological analysis showed that a columnar pseudostratified epithelium with stereocilia lined all the attached ducts. In active males, the secretory cells of the epididymis and the Leydig glands presented PR; however, these receptors were not found in the distal region of the epididymis (essential for nurturing and capacitation events) of regressing males. In the seminal vesicles of active males, the spermatozoa are parallelly aligned and embedded in a matrix to form the spermatozeugmata. The matrixes are formed by proteins secreted by the ducts and Sertoli cell cytoplasts. These structures presented PR, which suggests that P 4 engages in sperm metabolism during storage. Our findings allude to the potential role of P 4 in regulating the development and function of the attached ducts in different reproductive phases. Furthermore, P 4 seems to be an essential component for regulating sperm progress, protein secretion, aggregate formation, and maintenance of sperm during storage in this freshwater stingray., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Understanding the ecological role of species with overlapping distributions is central to inform ecosystem management. Here we describe the diet, trophic level and habitat use of three sympatric stingrays, Hypanus guttatus, H. marianae and H. berthalutzae, through combined stomach content and stable isotope (δ 13 C and δ 15 N) analyses. Our integrated approach revealed that H. guttatus is a mesopredator that feeds on a diverse diet of benthic and epibenthic marine and estuarine organisms, principally bivalve molluscs, Alpheus shrimp and teleost fishes. Isotopic data supported movement of this species between marine and estuarine environments. H. berthalutzae is also a marine generalist feeder, but feeds primarily on teleost fishes and cephalopods, and consequently occupies a higher trophic level. In contrast, H. marianae is a mesopredator specialized on shrimps and polychaetas occurring only in the marine environment and occupying a low niche breadth. While niche overlap occurred, the three stingrays utilized the same prey resources at different rates and occupied distinct trophic niches, potentially limiting competition for resources and promoting coexistence. These combined data demonstrate that these three mesopredators perform different ecological roles in the ecosystems they occupy, limiting functional redundancy., (© 2022 Fisheries Society of the British Isles.)

In the present work, we examined the morphology, dimensions, cytochemical staining reactions and ultrastructure of blood cells from three freshwater stingray species, Potamotrygon wallacei, Potamotrygon motoro and Paratrygon aiereba, living in the waters of the middle Rio Negro basin (Barcelos, Amazonas, Brazil). We identified erythrocytes, erythroblasts, thrombocytes and four types of leukocytes (basophils, heterophils, lymphocytes and monocytes) in the blood of these stingray species. In all the freshwater stingray species studied, the shapes and dimensions of these cells were similar to those of marine elasmobranchs. Positive PAS staining occurred in heterophils and thrombocytes, and weak staining occurred in lymphocytes and monocytes, while metachromasia only occurred in basophils. Positive Sudan Black B staining was observed in thrombocytes and lymphocytes, and weak staining occurred in heterophils. Basophils and heterophils were the only cells with positive bromophenol blue staining, while no peroxidase staining was observed in any of the four leukocyte types. This is the first study to establish the dimensions and cytochemical staining profiles of blood cells in Amazonian stingray species. Because these elasmobranch species are exported as ornamental fish to countries worldwide, this study can contribute to establishing standards for blood constituents that may be helpful in assessing the health and welfare of these fish in artificial systems., (© 2021. The Author(s).)

This study analyzed sex steroid hormones and morphology of the reproductive tract in free-living adult males of the cururu stingray (Potamotrygon wallacei). These stingrays were captured during the transition between drought and rising waters in the middle Rio Negro basin, which coincides with the reproductive period (spermatogenesis and copulation) of this endemic species. Levels of the plasma steroid hormones testosterone (T), 17 β-estradiol (E 2 ), and progesterone (P 4 ), the gonadosomatic and hepatosomatic indices, as well as histological characteristics of the testis and epididymis were evaluated. Three reproductive maturational stages were observed: regression males, capable to reproduce males, and active males. Plasma T levels were increasing and parallel with the rising water period in the middle Rio Negro basin and were associated with the progress of spermatogenesis. This was not observed for E 2 , which did not vary significantly among the different reproductive stages. The high P 4 concentrations in males in regression and males capable of reproducing appeared to regulate the progression of spermatogenesis. Increases in this hormone in the plasma of active males was associated with spermiogenesis and spermiation events. These findings have not been previously observed in Amazonian freshwater stingrays and demonstrate that the reproductive activity of P. wallacei males is concomitant to the period of rising waters in the middle Rio Negro basin, and is regulated by the action of gonadal steroids, mainly testosterone., (Copyright © 2021 Elsevier Inc. All rights reserved.)

The objective of this study was to compare and correlate the ecology of neonates and young individuals of Potamotrygon wallacei, Potamotrygon motoro and Paratrygon aiereba with regard to their hematological profile and the physicochemical parameters of the water that they inhabit. Principal component analysis (PCA) on the complete blood count revealed total variation of 72.92%, thus demonstrating a differentiation system for oxygen demand. On the other hand, P. motoro was considered to be an intermediate species, given that its complete blood count characteristics interacted with both P. wallacei and with P. aiereba. The interaction among the biochemical variables was shown to total 64.67% of the factors. This allowed differentiation of P. wallacei from P. aiereba, while P. motoro maintained an intermediate position. These characteristics of differentiation within the preferred environment corroborate the PCA of the present study and confirm that these species can be differentiated through considering the complete blood count and biochemical parameters. The PCA on water properties showed 68.57% differentiation, mainly comprising the x axis (49.44%). It can be affirmed that P. motoro has the capacity to inhabit the preferential areas of P. wallacei and P. aiereba, as well as occupying localities in which other stingrays are not found. In conclusion, P. wallacei presents patterns differentiating it from P. aiereba, while P. motoro is a species that presents intermediate characteristics. The latter can be considered to be a more broadly distributed species regarding its ecophysiological characteristics.

Intraerythrocytic parasites are frequently found in fish, including elasmobranchs. The Amazonian rivers present well defined annual hydrological cycles that results in drastic modifications of the environmental conditions with deep implications in the life cycle of the whole associated biota in those fluvial systems. The freshwater stingray Potamotrygon wallacei (stingray cururu) is a new species restricted to the Middle Rio Negro basin and it is subject to strong alterations in their natural habitats (igapós) a result of the constant variations in the water level of Rio Negro. This work demonstrates the occurrence of intraerythrocytic parasite Cyrilia sp. in this stingray species. Additionally, the prevalence and quantification of hemoparasites in different phases of Rio Negro were also established. Field sampling was carried in the Archipelago of Mariuá, Middle Rio Negro, involving different stages of the water cycle. The intraerythrocytic parasites were quantified by direct counting in blood smears using a total counting of 2000 erythrocytes in each blood smear. The presence of parasites intraerythrocytic generates changes in the morphology of blood cell. The largest amount of the hemoparasites was recorded in the drought period. We observed a decreasing tendency in the number of parasites in the blood between the drought periods and inundation. We concluded that the level of Negro River influences the incidence of intraerythrocytic parasites in the cururu stingray and the drought represents the period of larger susceptibility to the infestation.