Reflex Cardioventilatory Responses to Hypoxia in the Flathead Gray Mullet (Mugil cephalus) and Their Behavioral Modulation by Perceived Threat of Predation and Water Turbidity

Abstract

In hypoxia, gray mullet surface to ventilate well-oxygenated water in contact with air, an adaptive response known as aquatic surface respiration (ASR). Reflex control of ASR and its behavioral modulation by perceived threat of aerial predation and turbid water were studied on mullet in a partly sheltered aquarium with free surface access. Injections of sodium cyanide (NaCN) into either the bloodstream (internal) or ventilatory water stream (external) revealed that ASR, hypoxic bradycardia, and branchial hyperventilation were stimulated by chemoreceptors sensitive to both systemic and water O2 levels. Sight of a model avian predator elicited bradycardia and hypoventilation, a fear response that inhibited reflex hyperventilation following external NaCN. The time lag to initiation of ASR following NaCN increased, but response intensity (number of events, time at the surface) was unchanged. Mullet, however, modified their behavior to surface under shelter or near the aquarium edges. Turbid water abolished the fear response and effects of the predator on gill ventilation and timing of ASR following external NaCN, presumably because of reduced visibility. However, in turbidity, mullet consistently performed ASR under shelter or near the aquarium edges. These adaptive modulations of ASR behavior would allow mullet to retain advantages of the chemoreflex when threatened by avian predators or when unable to perceive potential threats in turbidity.

To boldly gulp: standard metabolic rate and boldness have context-dependent influences on risk-taking to breathe air in a catfish

“…Catfish were fasted for 24 h prior to measurements in either condition, then individuals were transferred gently to respirometers without air exposure, to minimise effects of handling. There were four respirometer chambers immersed in well-aerated water in an outer bath (1 m 2 surface area, 21 cm water depth) with the entire setup screened behind tarpaulin so that routine air-breathing behaviour was not inhibited by fear of human presence (McKenzie et al, 1991(McKenzie et al, , 2007Shingles et al, 2005). Fish were placed in the respirometer in the evening between 18:00 h and 20:00 h and fish were then allowed 16 h to recover from handling.…”

“…Context has profound effects on air breathing in bimodal fishes. A reduced availability of resources -aquatic hypoxia -is a profound stimulant of air breathing (Chapman and McKenzie, 2009;Graham, 1997;Johansen, 1968;Lefevre et al, 2014a,b) but surfacing is inhibited if fishes perceive a risk of predation (Chapman and McKenzie, 2009;Shingles et al, 2005;Smith and Kramer, 1986).…”

Increase in Anoxia in Lake Victoria and Its Effects on the Fishery

“…ASR places fish at significantly greater risk from aerial predation by birds . It has been demonstrated that if fish perceived a risk of predation they modulated the behavioral component of the ASR chemoreflex (Shingles et al, 2005). In Lake Victoria, Barbus neumayeri Fischer decreases the cost of ASR by having a more efficient oxygen-uptake method that reduces time at the surface and lower the threshold for ASR (Olowo & Chapman, 1996).…”

“…In Lake Victoria, Barbus neumayeri Fischer decreases the cost of ASR by having a more efficient oxygen-uptake method that reduces time at the surface and lower the threshold for ASR (Olowo & Chapman, 1996). Shingles et al (2005) found that exposure of flathead grey mullet to a model avian predator delayed the onset of ASR in hypoxia or in response to direct chemoreceptor stimulation with Sodium cyanide (NaCN). Additionally, the fish surfaced preferentially under a sheltered area in their experimental chamber or close to the walls.…”

“…Fish have also been shown to reduce the risk of surfacing by selecting less risky habitats or at less risky times of the diel cycle (Shingles et al, 2005).…”