Does something smell (pH)ishy?- Potential effects of ocean acidification on reef fishes

In aquatic environments, CO₂ can naturally vary across the local and global scale due to biological activity, upwelling, winds, and currents¹. However, with the onset of the industrial revolution and urbanization, the average CO₂ levels in the oceans have dramatically increased¹. In the last 200 years, it is expected that at least 30% of the CO₂ from anthropogenic activities has been absorbed by our oceans². When CO₂ is hydrated in water it forms carbonic acid (H₂CO₃)¹. Thus, causing a rapid decline of the pH in these ecosystems, a process often known as ocean acidification¹. A lot of research has focused on its effect on plankton, corals, mollusks, and echinoderms due to their risk of calcification³. Yet, the ability for marine fishes to persist is widely unknown as many initially believed them to be safe under these conditions due to their specialized homeostatic mechanisms (e.g. Acid-Base balance) that allow for regulation of pH and CO₂ in their blood and tissues¹.

Figure 1: Ocean acidification process https://www.masteringbiologyquiz.com/ocean-acidification/

However, over recent years there has been an increasing body of evidence that suggests that increased CO₂ and changes in pH levels have a number of sublethal effects on fishes such as otolith overgrowth, alterations to metabolic rate, shifts in behavior, detection/interpretation of critical sensory cues, and impaired learning^3,4. Olfaction, in particular, is especially important as it may affect the fish’s homing abilities, mate preference, and predator detection^2,3,5. Therefore, posing a great threat to an individual’s fitness (i.e. survival and reproduction).

The potential underlying cause of behavioral changes and sensory disruption may be due to alterations in neurotransmitter function. In acidified waters, the concentration of CO₂ in the blood will increase, known as hypercapnia¹. In order to restore homeostasis, the fish will utilize their Acid-Base balance mechanisms by upregulating their excretion of hydrogen (H⁺) and accumulating HCO₃¹. This also causes a decrease in plasma chloride (Cl^–). The shift in normal exchange of these ions has been found to reverse the normal function of GABA-A receptors (i.e. widespread neurotransmitter in the central nervous system)¹. GABA-A receptors become excitatory instead of inhibitory, therefore altering behavior, stress response, and sensory system function^1,4.

Many marine fish species exist in a pelagic larval phase as they transition into benthic adulthood. During this time, they use their learned and innate olfactory senses for predator recognition and avoidance³. Olfaction is vital in finding suitable habitat and avoid predation during settlement, as they typically settle at night when vision is limited³. However, various studies have demonstrated that ocean acidification impairs the olfactory function thus making it difficult for fish to detect, discriminate, and avoid predation risk.

Dixson et al. (2010) tested this theory in a population of newly hatched orange clownfish (Amphiprion percula). Clownfish has an innate ability to differentiate between the chemical cues of predators vs. non-predators. However, by simulating present and predicted CO₂/pH levels Dixson et al. were able to determine that the fish’s ability to do so was lost in acidified seawater. In fact, the larvae shifted from complete avoidance to attraction to the predator cues. This fatal attraction may have a number of implications on the overall population structure of marine fish by limiting population replenishment and sustainability.

Figure 2: Orange Clownfish (Amphiprion percula) https://www.maxpixel.net/Fish-Aquarium-Clown-Fish-Anemone-Fish-Amphiprion-1496866

Munday et al. (2010) performed a similar experiment to determine if damselfish larvae (Pomacentrus wardi) are also affected. Larvae exposed to acidified water engaged in riskier behavior as they spent more time active, were less responsive to predator cues, and moved farther away from places of refuge. Additionally, at high levels of CO₂, the larvae demonstrated a preference for the predator cue. Therefore, indicating that they lose their ability to detect or differentiate the risk of the cues. These shifts in behavior have the potential to expose the larvae to increased rates of predation and alter energy allocation/metabolic rates.

Ward’s Damselfish (Pomacentrus wardi) http://fishesofaustralia.net.au/home/species/2355

Based on these studies, it is suggested that 700 ppm CO2 may be the highest level at which reef fishes are able to somewhat adapt. Although, mortality is still likely to increase at that concentration. If emissions continue on their projected trajectory it is likely that atmospheric CO₂ could reach 850 ppm by 2100⁵. Further research is needed to clearly determine the various physiological processes and potential effects across taxa⁵. However, it is evident based on these studies that there is potential that ocean acidification will have implications for various populations of marine fish.

REFERENCES:

1. Tresguerres M, Hamilton TJ (2017) Acid-base physiology, neurobiology, and behaviour in relation to CO₂-induced ocean acidification. Journal of Experimental Biology 220:2136-2148.
2. Munday PL, Dixson DL, Donelson JM, Jones GP, Pratchett MS, Devitsina GV, Døving KB (2008) Ocean acidification impairs olfactory discrimination and homing ability of marine fish. PNAS 106:1848-1852
3. Dixson DL, Munday PL, Jones GP (2012) Ocean acidification disrupts the innate ability of fish to detect predator olfactory cues. Ecology Letters 13:68-75.
4. Hamilton TJ, Holcombe A, Tresguerres M (2013) CO₂-induced ocean acidification increases anxiety in Rockfish via alteration of GABA_A receptor functioning
5. Munday PL, Dixson DL, McCormick MI, Meekan M, Ferrari MCO, Chivers DP (2010) Replenishment of fish populations is threatened by ocean acidification. PNAS 107:12930-12934.