How sharks survived a drastic spike in global temperatures

The sharks we recognize today as apex predators of the open ocean evolved from stout, bottom-dwelling ancestors. The transformation of sharks has been linked to a period of intense global warming 93 million years ago – a significant volcanic event that drastically increased carbon dioxide levels and ocean temperatures.

Changes in fin shape helped sharks adapt

UC Riverside researchers, through extensive measurements of both living and fossilized sharks, discovered that in response to this climate shift, some sharks developed elongated pectoral fins. This adaptation facilitated their transition from benthic zones to open waters, improving their swimming efficiency and speed.

“The pectoral fins are a critical structure, comparable to our arms,” said lead author Phillip Sternes, a UCR biology doctoral student. “What we saw upon review of a massive data set, was that these fins changed shape as sharks expanded their habitat from the bottom to the open ocean.”

Longer pectoral fins made shark movements more efficient. “Their fins are comparable to the wings of commercial airplanes, long and narrow, to minimize the amount of energy needed for movement,” Sternes added.

Greater swimming speeds 

The experts also found that open-water sharks developed greater speed compared to their bottom-dwelling counterparts. 

“Shark muscle is very sensitive to temperature,” explained co-author Tim Higham, a professor of biology at the same university. “The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds.”

Shifting into open waters

Currently, most sharks are bottom dwellers, residing in the benthic zone. These sharks are typically slimmer, flatter, and medium-sized predators. Only about 13% of modern sharks are agile, open-water predators. 

The researchers speculate that declining oxygen levels at the sea bottom during the Cretaceous period may have driven ancient sharks to evolve and inhabit the open waters.

Temperature rise and shark evolution 

The average sea surface temperature today is around 68 degrees Fahrenheit, but during the Cretaceous period, it was much higher, averaging about 83 degrees. This temperature rise and the subsequent evolutionary changes in sharks occurred over a span of one to two million years.

“We had pretty warm open-sea surface temperatures throughout the era, and then a distinct spike that took place over a one- or two-million-year period,” said co-author Lars Schmitz, an associate professor of biology at Claremont McKenna College.

Predicting how sharks will adapt

While historical global warming drove evolutionary changes in some species, it led to the extinction of others. The current rapid pace of global warming makes it difficult to predict how sharks and other marine life will adapt. 

Some sharks, like tropical tiger and bull sharks, are beginning to migrate farther north, but it remains uncertain whether threatened species will adapt quickly enough to survive.

“The temperature is going up so fast now, there is nothing in the geologic record I am aware of that we can use for a true comparison,” Sternes concluded.

How modern sharks adapt to temperature rise

Sharks are adapting to rising ocean temperatures in various ways due to their ectothermic nature, which makes them reliant on environmental temperatures for body heat regulation. 

Shifting habitats

Many species are migrating poleward or to deeper, cooler waters, seeking more suitable climates. This shift in habitat can disrupt local ecosystems, given sharks’ roles as apex predators or significant ecological players.

Reproduction 

The reproductive and growth cycles of sharks are also being affected, with some species experiencing accelerated growth but reduced fertility and offspring survival under warmer conditions. 

Shark behavior 

Behaviorally, sharks might adjust their activity patterns, including hunting at different times, possibly becoming more nocturnal to escape the heat of the day.

Immune stress

Physiological stress is another significant impact, as continuous exposure to higher temperatures can weaken sharks’ immune systems, increasing susceptibility to diseases. 

Food availability 

Additionally, changes in the distribution and abundance of prey due to temperature shifts can lead to food shortages, forcing sharks to adapt their diets and potentially impacting their overall survival. 

The study is published in the journal Current Biology.

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