How your everyday life could be making sharks less hydrodynamic - by Former Intern Kylie Bostick

Ocean Acidification’s Effect on Dermal Denticles

Picture a normal morning: you wake up, turn on your lights, brew a cup of coffee, make yourself breakfast (say, eggs and bacon) and then drive your car to work. What seem like monotonous everyday tasks, could actually be (indirectly) impacting the hydrodynamics and overall body condition of cute little sharks hanging out on the seafloor in South Africa.

Atmospheric carbon dioxide (CO2) levels are currently increasing at a faster rate than at any point in the last 650,000 years, largely due to the CO2 emissions from electricity and heat production, transportation, and agriculture. In order to counteract increased atmospheric CO2 concentrations, more CO2 must be absorbed by global carbon sinks, the largest of which is the ocean. The issue with this is, that as the ocean absorbs CO2 it undergoes a series of chemical reactions which result in the production of free hydrogen ions (H+), which increase the acidity of the water. This process of converting CO2 to H+is known as ocean acidification. Since the 19th century, the average oceanic pH has dropped 30%, from a level of 8.21 to 8.10, and is expected to continue to drop as low as 7.3 by the year 2300.

Most ocean acidification studies have focused on the effects of increasing acidity on calcifying organisms, such as corals, crustaceans, and mollusks, as acidification directly impacts their ability to form their calcium carbonate shells and skeletons. However, low pH has been observed to affect fish species as well. In elasmobranchs specifically, acidification studies have yielded mixed results based on species and methods used. Some have demonstrated that acidity can alter swimming patterns, increase metabolic energy expenditure, impair body condition, weaken olfactory ability, reduce hunting behavior, and cause denticle damage.

One such study by Dziergwa et al. in 2019, set out to find if 7.3, the predicted global oceanic pH by 2300, would be acidic enough to cause denticle damage in puffadder shysharks (Haploblepharus edwardsii), a small (max total length of 60cm!) benthic catshark endemic to shallow, temperate waters of South Africa. Denticles are the “scales'' that cover a shark’s skin and act to improve hydrodynamics and protect against minor abrasions suffered during hunting or mating. Denticles are made up of calcium fluoro phosphate and calcium hydroxyl phosphate, both of which are weakly soluble when exposed to high enough acidity. Dziergwa et al. found that puffadder shysharks kept at a pH of 7.3 for 63 days experienced denticle damage in around 25% of all of their denticles, compared to only 9% in the control group kept at a pH of 8.1. Healthy denticles increase swimming speed by up to 12%, so sharks with damaged denticles would likely have to spend more energy swimming to make up for the decrease in hydrodynamics. This extra energy cost would have to come from somewhere, and Dziergwa et al. hypothesized it would come at the expense of overall growth, though were not able to test this due to the length of their study.

[Figure from Dziergwa et al 2019]

The results from Dziergwa et al. raise many other questions about how ocean acidification may affect sharks in the future. Their study looked at the puffadder shyshark, which lives on the seafloor and doesn’t need to be swimming in order to breathe; however, what about sharks that do? More active sharks, such as our beloved reef sharks, great hammerheads, and bull sharks here in Bimini, use ram-ventilation to breathe, meaning they have to be actively swimming to pull oxygen from the water through their gills. If these sharks are exposed to higher levels of acidity and suffer denticle damage, in turn reducing their hydrodynamics and requiring more energy to swim, how will their breathing be affected? Dermal denticles also have the same chemical makeup as teeth, so theoretically, a shark’s denticles and teeth should erode away at the same rate under acidic conditions- how could this impact a shark’s ability to feed?

Ocean acidification is an ongoing phenomenon that will continue for decades to come unless drastic measures are taken to massively reduce anthropogenic CO2 emissions. More research will need to be done to understand its impacts on marine organisms and ecosystems to be able to predict the future of our oceans and maintain the biodiversity we are so lucky to see, not only just here in Bimini, but in oceans all across the globe.

References

Dziergwa, J., Singh, S., Bridges, C. R., Kerwath, S. E., Enax, J., & Auerswald, L. (2019). Acid base adjustments and first evidence of denticle corrosion caused by ocean acidification conditions in a demersal shark species. Scientific Reports, 9(1). doi: 10.1038/s41598-019- 54795-7

Di Santo, V. (2019). Ocean acidification and warming affect skeletal mineralization in a marine fish. Proceedings of the Royal Society B, 286(1894), 20182187.

Eissa, A. E., & Zaki, M. M. (2011). The impact of global climatic changes on the aquatic environment. Procedia Environmental Sciences, 4, 251–259. doi: 10.1016/j.proenv.2011.03.030