Abstract
The effect of altered oxygen transport potential on behavioural responses to environmental hypoxia was tested experimentally in snapper, Pagrus auratus, treated with a haemolytic agent (phenylhydrazine) or a sham protocol. Standard metabolic rate was not different between anaemic and normocythaemic snapper (Hct=6.7 and 25.7 g dl(-1), respectively), whereas maximum metabolic rate, and hence aerobic scope (AS), was consistently reduced in anaemic groups at all levels of water P(O(2)) investigated (P<0.01). This reduction of AS conferred a higher critical oxygen limit (P(crit)) to anaemic fish (8.6±0.6 kPa) compared with normocythaemic fish (5.3±0.4 kPa), thus demonstrating reduced hypoxic tolerance in anaemic groups. In behavioural choice experiments, the critical avoidance P(O(2)) in anaemic fish was 6.6±2.5 kPa compared with 2.9±0.5 kPa for controls (P<0.01). Behavioural avoidance was not associated with modulation of swimming speed. Despite differences in physiological and behavioural parameters, both groups avoided low P(O(2)) just below their P(crit), indicating that avoidance was triggered consistently when AS limits were reached and anaerobic metabolism was unavoidable. This was confirmed by high levels of plasma lactate in both treatments at the point of avoidance. This is the first experimental demonstration of avoidance behaviour being modulated by internal physiological state. From an ecological perspective, fish with disturbed oxygen delivery potential arising from anaemia, pollution or stress are likely to avoid environmental hypoxia at a higher P(O(2)) than normal fish.
Highlights
Aquatic animals reside in environments characterised by low oxygen solubility and some face the continual threat of environmental hypoxia (Graham, 1990)
Effects of anaemia on aerobic physiology and avoidance behaviour According to our measures of haematological O2-carrying capacities and rates of O2 consumption (MO2), normocythaemic and anaemic snapper differ in their aerobic physiology, and this in turn appears to exert a strong influence over hypoxia avoidance behaviour
By experimentally manipulating the oxygen-carrying capacity, aerobic scope and critical O2 limit of P. auratus, and exposing them to range of PO2 choices, we have provided compelling evidence that physiological state has a strong influence over the hypoxic avoidance response of fish
Summary
Aquatic animals reside in environments characterised by low oxygen solubility and some face the continual threat of environmental hypoxia (Graham, 1990). Metabolic demand for oxygen has necessitated adequate functional designs that are challenged further by increasingly prevalent episodes of environmental hypoxia (Diaz and Rosenberg, 2008; Gilbert et al, 2010), with negative effects manifested from the cellular level through to the scale of populations (for a review, see Wu, 2002). The progressive drop in MMR during progressive hypoxia subsequently reduces an animal’s aerobic scope (AS MMR–SMR) to a point where MMR is only sufficient to maintain stable SMR (and AS is equal to zero), a point referred to as the critical oxygen tension, or Pcrit. If aquatic PO2 drops toward Pcrit, the resulting hypoxia-induced drop in AS may cause important eco-physiological functions such as growth, activity and reproduction to be compromised (Chabot and Claireaux, 2008a). Major life-threatening processes are challenged at PO2
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