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Physiology

Small but mighty: Will the epaulette shark survive ocean acidification?

Johnson MS, Kraver DW, Renshaw GMC, Rummer JL (2016) Will ocean acidification affect the early ontogeny of a tropical oviparous elasmobranch (Hemiscyllium ocellatum)? Conserv Physiol 4(1): cow003; doi:10.1093/conphys/cow003.

 

Introduction

Epaulette Shark

The epaulette shark (Hemiscyllium ocellatum) is a relatively unknown species, but surprisingly, this little guy has some really amazing biological features. Back in the 1990’s, scientists discovered that the epaulette shark, a small reef dwelling shark found on the Great Barrier Reef, can actually survive without oxygen for at least three hours. I repeat, three hours. Considering the average human can only live for a few minutes without oxygen, the epaulette shark’s ability is an incredible feat. This performance is due to a peculiar evolutionary trait where the little shark drops its blood pressure and widens its blood vessels to help deliver more blood to important organs like the heart and brain. This adaptation is not only interesting, but important for their role in the ecosystem.

As if this breath holding ability wasn’t enough, the epaulette shark has another interesting trait: it can walk. They have a unique ability to “walk” on their pectoral fins, allowing them to come out of the water, move over coral exposed at low tide, and slither into tide pools. These tide pools tend to have depleted oxygen levels at low tide due to lack of water flushing in and out. Therefore, the epaulette shark uses its walking ability to first access the pools, and then remain in place to feed even with low oxygen levels due its fantastic low oxygen adaptations. These interesting traits have made this little shark the focus of many scientific studies. Epaulette sharks are small, easy to keep in captivity, and lay eggs, making them a great proxy for other sharks scientists cannot study as easily (i.e. the big ones with the sharp teeth). Predictions and extrapolations can therefore be made for other species using epaulette sharks as a model.

Before we get to the meat of the paper, let’s quickly recap ocean acidification.  Due to human-caused climate change, the oceans are absorbing high amounts of carbon dioxide. Long story short, the pH of the oceans is dropping because of this absorption, making the water more acidic. However, sharks evolved in a time period ~420 million years ago when the amount of carbon dioxide in the atmosphere was 8-10x greater than it is currently. So that makes scientist ask, will sharks even be affected by the increasing acidity? In a recent published paper by Dr. Jodie Rummer’s group at James Cook University in Australia, they set out to see if increasing ocean acidification levels would affect epaulette sharks within the egg case as embryos and after hatching.

What did they do?

First, egg cases from epaulette sharks were raised in two different treatments: one where carbon dioxide levels were maintained at current ocean levels (8.14 pH) or an increased acidity level (7.88 pH). Over the course of the baby shark’s (called a pup) development within the egg case, a slew of measurements were taken to observe growth and development. Similar to a chicken egg, shark embryos are attached to a yolk sac that they feed off of throughout gestation. To measure growth, a light can be shone through the egg case, in a process called candling, and measurements of the length of the pup and the size of the yolk sac were taken (Figure 1). Additionally, scientists took videos of each egg case to measure ventilation rates (how often the pups are pumping water over their gills) and tail oscillation rates (how often the pups are moving their tails to circulate new water into the egg case). The length of gestation was recorded for each embryo from time of fertilization to hatching. Finally, newly hatched epaulette shark pups where raised for 30-days to examine survival rates after emerging from the egg case.

Figure 1. Embryos during the candling process. Note the control embryos on the top row and the elevated carbon dioxide on the bottom over their gestation (days post fertilization).

Figure 1. Embryos during the candling process. Note the control embryos are on the top row and the elevated carbon dioxide on the bottom over their gestation (days post fertilization). Yellow arrows indicate the embryo next to the round yolk sac.

What did they find?

            In terms of growth and development, the size of embryos and yolk consumption rate over time, did not differ between the current and future predicted carbon dioxide conditions (Figure 2A & 2B). Similarly, there was no significant difference in tail oscillation rates and ventilation rates between the treatments. However the authors did find that the egg hatching was 43% higher in current carbon dioxide conditions over elevated carbon dioxide levels. Similarly, after the sharks hatched, there was a 23% higher percent of survival in the current carbon dioxide conditions in comparison to elevated carbon dioxide.

Figure 2. (A). The growth rate of the embryos within the egg case throughout gestation (days post fertilization). (B). The amount of yolk remaining (in percentage) throughout gestation. Notice the similarity of the lines in the control and elevated CO2 treatments in both plots.

Figure 2. (A). The growth rate of the embryos within the egg case throughout gestation (days post fertilization). (B). The amount of yolk remaining (in percentage) throughout gestation. Notice the similarity of the lines in the control and elevated CO2 treatments in both plots.

So what?

            Overall, epaulette shark eggs were not hugely affected by increased . However, the authors did note that they may not have studied the proper measurements to observe some of the differences between treatment groups. Overall however, the results are a very good sign for the species, because the ability of offspring to survive will help maintain the population of this shark throughout the west coast of Australia and beyond. It is important to note that increasing carbon dioxide is not the only change happening in the ocean. Temperatures are expected to warm in this area by 2-3ᵒC. Therefore, the eggs and new hatchlings could be compromised by other factors like temperature alone, or in combination with increased carbon dioxide. Additionally, as mentioned earlier, epaulette sharks are a very tolerant species that can handle a large range of environmental conditions. Therefore, this little shark may be able to cope with changing ocean conditions from climate change, but other sharks may not be so fortunate. Epaulette sharks provide a good starting point for scientists to begin to understand how growth and development may change alongside the changing ocean conditions.

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