Physiological, biochemical and behavioural responses of freshwater crayfish (Paranephrops zealandicus) to hypoxia and water nutrients. (2015)
AuthorsBroughton, Rebeccashow all
Hypoxia is a decline in water oxygen levels and is often caused by aquatic nutrient enrichment. It is recognised as one of the foremost environmental issues impacting and influencing species distribution. This population level effect is likely mediated by the physiological, biochemical and behavioural impacts associated with reduced oxygen. Most aquatic animals are able to maintain a constant oxygen uptake with small reductions in water oxygen, and are thus able to fuel the various processes driven by aerobic metabolism. However, as hypoxia persists or strengthens, regulation fails and aerobic metabolism is impaired leading to impacts such as the loss of acid-base homeostasis. The nutrient loading that often drives hypoxic episodes (through increased primary production) may impose additional stress that could also impact hypoxic responses and tolerance. The New Zealand freshwater crayfish Paranephrops zealandicus often inhabits streams that are in close proximity to farmland and are therefore prone to agricultural and pastoral run-off events. Consequently, crayfish will be faced with the challenge of energy homeostasis in conditions where aerobic capacity is compromised. The current study has investigated the physiological, biochemical and behavioural responses of P. zealandicus to hypoxia and high nutrients, to determine the potential impacts of agriculturally-impacted waters on this economically and culturally important species. Freshwater crayfish were moderately tolerant of hypoxia, regulating oxygen consumption rates (MO2) down to a critical oxygen tension of 45 mmHg. Contrary to the standard crayfish response, no distinct bradycardia developed under progressive hypoxia. Intraspecific variability in the MO2 responses impacted the results, with individual P. zealandicus showing a variety of oxyregulating and oxyconforming patterns. Exposure to prolonged hypoxia (six hours) did not further affect MO2 or heart rate. Both of these responses, however, were impacted by the severity of hypoxia. Crayfish under severe hypoxia (10 mmHg) showed a 79% decrease in MO2, relative to the 50 % fall found under moderate hypoxia (30 mmHg). High ammonia (30 mg L-1) and high nitrite (20 mg L-1) did not further exacerbate the physiological responses of crayfish under severe hypoxia. Similarly, the addition of these nutrients did not alter markers of biochemical perturbation (acid-base status, anaerobic metabolism endpoints) relative to severe hypoxia alone, which induced a significant acidosis (pH increase, carbon dioxide decrease), indicative of a loss in acid-base homeostasis. Analysis of anaerobic metabolism, as represented by haemolymph lactate accumulation, showed a heavy reliance of P. zealandicus on anaerobic metabolism under severe hypoxia, which may have driven the observed haemolymph acidosis. Given the biochemical and physiological impacts of severe hypoxia (10 mmHg) on crayfish, it might have been anticipated that behavioural responses would be enacted that would permit crayfish to evade these consequences. There was a lack of hypoxia avoidance shown in a behavioural choice chamber experiment. There was, however, a strong emergence response observed under extreme hypoxia (4.2 mmHg). The avoidance and emergence experiments suggested that behavioural responses are only initiated at very low oxygen partial pressures (PO2) in P. zealandicus. That the emersion response occurred at levels significantly lower than the calculated critical oxygen tension (PCRIT; the point at which oxyregulation fails and the animals MO2 falls in concert with falling PO2) indicates that behavioural responses may only be used when absolutely necessary. The ability of P. zealandicus to endure hypoxia through a variety of physiological and biochemical response, and only eliciting behavioural responses as a last resort, has important environmental implications. Tolerance will allow P. zealandicus to reduce any unnecessary risks associated with emergence (e.g. aerial desiccation and risk of predation) for as long as possible. Even if hypoxia worsens its ability to emerge will nevertheless provide it with a means of escape. The results of the current study are relevant for understanding and managing the population declines and recolonisation opportunities in this ecologically and culturally important species.