Green, L., & Jutfelt, F. (2014). Elevated carbon dioxide alters the plasma composition and behaviour of a shark. Biology Letters, 10(9), 20140538.
Ocean acidification and fish behavior
As the concentration of carbon-dioxide in Earth’s atmosphere increases, the concentration of carbon-dioxide in oceans also increases. This shifts the carbonate ion balance in oceans and effectively lowers the pH in a process known as ocean acidification. While we commonly consider the effects of ocean acidification on calcifying organisms (plankton species, corals, crustaceans, molluscs), it has also been shown to illicit a wide range of behavioral anomalies in bony fish including activity, boldness, and altered sensory cues. One study found that larval clown fish in acidified seawater (pH 7.8) were attracted to sensory cues they would normally avoid (Munday et al. 2009).
Elasmobranchs (a class of fish with cartilaginous skeletons) including sharks, rays, and skates have not been studied for the effects of ocean acidification. This taxonomic group contains many species which heavily depend on olfaction (smelling) and electroreception (ability to sense electricity) for navigating their environment. Both sensory methods may be affected by changing ion concentrations in seawater with the progression of ocean acidification. Researchers from the University of Gothenburg in Sweden set out to investigate whether increased concentrations of carbon dioxide in seawater would affect the behavior and physiology of an elasmobranch, the small-spotted catfish shark.
Twenty small-spotted catfish sharks (borrowed from a public aquarium) were exposed to either normal carbon dioxide concentrations or high concentrations – based on conservative estimates for the year 2100 – for one month prior to testing. Shark behavior was tested through lateralization experiments based on methods in another study lead by one of the coauthors of this study (Jutfelt et al. 2013). The lateralization test essentially tests how behavioral symmetry (turn right/left) changes between the control and high CO2 groups by exposing sharks to a chamber with turning choices (Figure 1). Oxygen consumption was measured at resting rate and maximum metabolic rate. Blood pH and plasma concentrations of common ions were also measured. Skin samples from the pectoral fins were also assessed for morphology and abnormalities.
Results and significance
Blood pH, oxygen consumption, and denticle (scale-like but made of dentine, similar to human teeth!) structure did not differ significantly between the control and high CO2 treatment sharks (Figure 2). The blood plasma concentrations of HCO3- (bicarbonate ion) increased and Na+ (sodium ion) decreased in the high CO2 sharks. Like many other fish species, sharks use their gills to exchange H+/Na+ and Cl-/HCO3- with seawater. Altered Na+ and HCO3- in the blood plasma suggests that the gill mechanism may also counterbalance the effects of ocean acidification in the future. However, this study was short and may only indicate temporary acclimation.
High CO2 sharks had significantly more symmetrical lateralization than the normal CO2 sharks. This is different from other experiments with bony fish species which found decreased lateralization. The high CO2 sharks also had fewer swimming events but much longer swimming duration (Figure 3). Hyperactivity has been observed in some other studies with fish in high CO2 environments. This response may be related to altered ion concentrations in the blood plasma affecting portions of the central nervous system.
In summary, blood plasma ion concentrations, activity and lateralization were affected by acidified seawater. Does this mean other, larger elasmobranchs will also exhibit increased activity by the year 2100? Potentially. Perhaps the rate of human induced climate could exacerbate the intensity of sharks within tornados (a.ka. Sharknados)…