Physiological and behavioural responses to acid and osmotic stress and effects of Mucuna extract in Guppies

Abstract

Variation in pH (acidification) and salinity conditions have severe impact at different levels of biological organization in fish. Present study focused to assess the effects of acidification and salinity changes on physiological stress responses at three different levels of function: i) hormonal and oxidative response, ii) osmoregulation and iii) reproduction, in order to identify relevant biomarkers. Second objective of the study was to evaluate the efficacy of plant (Mucuna pruriens) extract for alleviating pH and salinity related stress. Guppies (Poecilia reticulata) were exposed to different pH (6.0, 5.5, 5.0) and salinity (1.5, 3.0, 4.5 ppt) for 7, 14 and 21 days. Following exposure to stress for respective duration, fish were fed diet containing methanol extract of Mucuna seeds (dose 0.80 gm/kg feed) for 7, 14 and 21 days to measure their possible recovery response. Stress hormone (cortisol), hepatic oxidative stress parameters [superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GRd), glutathione peroxidise (GPx), glutathione S-transferase (GST), malondialdehyde (MDA), glutathione (GSH)], gill osmoregulatory response (Na⁺-K⁺ATPase activity), sex steroid profiles and mating behaviours (gonopodial thrust and gestation period) were estimated. Cortisol and MDA levels increased with dose and duration of acid and salinity stress, and cortisol levels were higher in males than in females. Effect on Na⁺-K⁺ATPase activity was more intense by salinity stress rather than pH induced stress. Both acid and salinity stress reduced sex steroid levels, and mating response was highly affected by both stresses in a dose- and duration-dependent manner. Mucuna treatment reduced stress-induced alteration of cortisol, MDA, Na⁺-K⁺ATPase activity and reproductive parameters. Dietary administration of Mucuna seed extract decreased the intensity of environmental stressors at all three functional levels. Mucuna treatment was more effective against salinity stress than acid stress. Thus, cortisol, oxidative stress marker MDA and Na⁺-K⁺ATPase could be effective indicators for acid and salinity stress in wild and domestic fish populations. Dietary administration of Mucuna extract may limit the detrimental effects of acidification and salinity variations that are the inevitable outcomes expected under global climate change conditions.

Keywords: Biomarkers; Cortisol; Plant extract; Salinity; Stress response; pH.