Hypoxia alters the upper thermal limits and blood physiology in zebrafish, Danio rerio.

Hypoxic aquatic environments occur more frequently as a result of climate change, thereby exerting challenges on the physiological and metabolic functions of aquatic animals. In this study, a model fish, zebrafish (Danio rerio) was used to observe the climate-induced hypoxic effect on the upper thermal limit (critical thermal maximum; CTmax), hemoglobin, and blood glucose levels, and abnormalities of erythrocytes at cellular and nuclear level. The value of CTmax decreased significantly under hypoxia (39.10 ± 0.96 °C) compared to normoxia (43.70 ± 0.91 °C). At CTmax, hemoglobin levels were much lower (9.33 ± 0.60 g/dL) and blood glucose levels were significantly higher (194.20 ± 11.33 mg/L) under hypoxia than they were under normoxia and at the beginning of the experiment. Increased frequencies of abnormalities in the erythrocytes at both cellular (fusion, twin, elongated, spindle and tear drop shaped) and nuclear (micronucleus, karyopyknosis, binuclei, nuclear degeneration and notched nuclei) levels were also found under hypoxia compared to normoxia. These results suggest that hypoxic conditions significantly alter the temperature tolerance and subsequent physiology in zebrafish. Our findings will aid in the development of effective management techniques for aquatic environments with minimum oxygen availability.

Hypoxia reduced upper thermal limits causing cellular and nuclear abnormalities of erythrocytes in Nile tilapia, Oreochromis niloticus.

Global warming is a threat across the world that leads to estimates of the upper thermal limits of ectothermic species. Increased water temperature up-regulates oxygen consumption and metabolic rates, and alters the physiological processes. In this study, we identified the critical thermal maxima (CTmax) and physiological responses under normoxia and hypoxia in Nile tilapia, Oreochromis niloticus. CTmax was 41.25 °C under hypoxia and 44.50 °C under normoxia. Compared to normoxia, lower values of hemoglobin (Hb) and red blood cells (RBCs) were observed at the CTmax under hypoxia. In contrast, higher values of white blood cells (WBCs) and blood glucose (Glu) levels were observed at the CTmax under hypoxia. Consequently, higher frequencies of micronucleus, cellular and nuclear abnormalities of erythrocytes were observed at the CTmax under hypoxia. These results suggest that high temperature tolerance and subsequent physiology are significantly affected by the oxygen supply in Nile tilapia. As climate vulnerability is intensifying day by day, this data will be helpful in successful management practice for the aquatic environment having low oxygen content.

High temperature acclimation alters upper thermal limits and growth performance of Indian major carp, rohu, Labeo rohita (Hamilton, 1822).

Increase in water temperature due to anthropogenic and climatic changes is expected to affect physiological functions of fish. In this study, we determined high temperature tolerance (CTmax) of a common aquacultured Indian major carp, rohu, Labeo rohita fingerlings (15.96 ± 0.72 g BW, 11.56 ± 0.42 cm TL) followed by acclimatization at three temperatures (30, 33, 36 °C). To determine the CTmax, we analyzed the major hemato-biochemical indices - hemoglobin (Hb), red blood cell (RBC), white blood cell (WBC), blood glucose levels, and erythrocytic nuclear abnormalities (ENAs) and erythrocytic cellular abnormalities (ECAs) of peripheral erythrocytes in the fish sampled at the start and end point at each acclimated temperature. Significantly decreased CTmax of the fish was found at 36 °C compared to 30 °C and 33 °C. The fish in the highest (36 °C) temperature were found with significantly lower Hb and RBC content and significantly higher WBC and blood glucose levels than that of the fishes in the lowest (30 °C) temperature both at the start and end points. The highest frequencies of ENAs and ECAs were found in the highest (36 °C) temperature group compared to the lowest (30 °C) temperature group at both the points. We also evaluated growth performance of the rohu fingerlings reared in the three temperatures for 60 days. The growth parameters - final weight gain, percent weight gain and specific growth rate were the highest at 33 °C and the lowest at 36 °C. The present study revealed that the highest temperature (36 °C) tested here may be hazardous to rohu and the temperature should be kept below 36 °C in the aquaculture setting to avoid physiological damage and growth and production loss to the fish.

Sumithion induced structural erythrocyte alteration and damage to the liver and kidney of Nile tilapia.

Indiscriminate use of pesticides in agricultural land poses a potential threat to many non-target organisms, including fish. In the present study, we explored the toxicological effects of sumithion on Nile tilapia (Oreochromis niloticus) after exposure at different concentrations (0.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mg/L) in triplicate for 96 h. The 96-h LC50 value of the pesticide for the test fish was calculated by probit analysis, which was 2.579 mg/L. We also investigated the haematological parameters, erythrocytic alteration and histopathological responses of Nile tilapia. Red blood cell (RBC) and haemoglobin (Hb) level were significantly declined, whereas white blood cell (WBC) and blood glucose level were increased dramatically in the sumithion treated fish. The frequencies of erythrocytic cellular and nuclear alterations were significantly elevated in the fish after exposure to sumithion compared with the control group. The substantial histopathological alterations were observed in sumithion-exposed fish, including pyknotic nucleus, melano-macrophage centers and severe congestion of blood vessels in the liver and patch degeneration, vacuolation and intense form of pyknotic nuclei in the kidney. Observed alterations in this study exhibit that sumithion negatively impacts on Nile tilapia. It is recommended that the use of this pesticide in the agriculture field be monitored and controlled.

Acute effects of chromium on hemato-biochemical parameters and morphology of erythrocytes in striped catfish Pangasianodon hypophthalmus.

Chromium is considered the most detrimental pollutant to the aquatic organisms. The present experiment was conducted to determine the acute toxicity of chromium in view of its effects on hemato-biochemical parameters and the structure of erythrocytes in striped catfish, Pangasianodon hypophthalmus. Fish were exposed to seven different concentrations (0, 10, 20, 30, 40, 50 and 60 mg/L) of chromium, each with three replications for 96 h. After 96 h of exposure, the survived fish were sacrificed to measure hemato-biochemical parameters (hemoglobin, Hb; red blood cell, RBC; white blood cell, WBC; packed cell volume, PCV; mean corpuscular volume, MCV; the mean corpuscular hemoglobin, MCH and blood glucose). In addition, erythrocytic cellular abnormalities (ECA) and erythrocytic nuclear abnormalities (ENA) of peripheral erythrocytes were assayed. No mortality was observed up to 10 mg/L, but 90% and 100% mortality was observed at 50 mg/L and 60 mg/L, respectively after a 96 h exposure period. The 96 h LC50 value through probit analysis was 32.47 mg/L. Hb (%), RBC (×106/mm3) and PCV (%) significantly decreased at 20, 30 and 40 mg/L of chromium, whereas WBC (×103/mm3), MCV (µm3) and MCH (pg) showed the opposite scenario. Blood glucose (mg/dL) levels significantly increased at 10, 20, 30 and 40 mg/L of chromium compared to 0 mg/L. Frequencies of ECA and ENA significantly increased with increasing chromium concentrations. This study indicates that chromium is highly toxic to striped catfish.