The world's oceans are a sink for carbon dioxide, absorbing about 25 percent of all the additional CO2. With rising levels of greenhouse gases, oceans have experienced an average 0.1-unit reduction in pH since preindustrial times. Sharks have such a superior sense of smell -- they’ve been called “swimming noses” -- but changes in seawater chemistry projected for the end of this century could affect their feeding, according to findings published in Global Change Biology this week.
To find prey in the expansive ocean, large predators rely on odor tracking; chemical signals can be transported much farther in the marine environment than visual, mechanical, or electrical signals. According to a “business as usual” scenario, projected CO2 levels for the year 2100 are expected to exceed 900 ppm, while the pH of ocean waters will drop an additional 0.3 to 0.4 units.
Previous studies have shown CO2-rich acidic water impairs sensory functions of reef fishes, Science reports, making them less capable of smelling predators. To see if sharks are similarly affected by ocean acidification, a team led by Danielle Dixson from Georgia Institute of Technology held adult smooth dogfish (Mustelus canis) for five days in three kinds of pool settings: present-day (control) CO2, moderate CO2, and high CO2 levels.
After the five days, the small sharks were released into a pool where prey odor (concentrated “squid juice”) was released in a stream, and their tracking and attack behaviors were monitored. The sharks were able to freely choose between two sides of the flume (or channel): One contained a plume of food odor, the other contained a plume of only flume water. Pictured to the right is a shark in one of these “choice flumes” where the plumes are dyed.
The team looked at three things: the percentage of time spent in the odor plume side compared to the ambient seawater side of the flume, the percentage of time spent near the odor source where the concentration was highest (compared to the downstream end ), and the attack level displayed toward the odor source. To measure attack behavior, a "food" brick was placed at the source of the odor.
Both control and mid CO2-treated sharks maintained normal odor tracking behavior, but high CO2-exposed sharks significantly avoided the odor cues indicative of food. Control sharks spent more than 60 percent of their time in the water stream containing the food stimulus. This value fell below 15 percent in high CO2-treated sharks. Furthermore, sharks in the mid and high CO2 conditions showed reduced attack behavior compared to the controls, which would bite on the brick or bump it with their snouts much more frequently.
One-third of oceanic shark species are threatened by extinction, with 64 of them on the ICUN Red List. Sharks have adapted to acidifying oceans in their evolutionary history before, the team say, but they’ve never had to adapt as quickly as the changes are occurring today.
Images: Elizabeth Roberts via Wikimedia (top), D.L. Dixson et al., 2014 (middle)