Shark denticles reveal human impacts over centuries

Human impacts now kill about 100 million sharks per year. An international team of researchers led by ecologist Erin Dillon recently focused a study on shark denticles to determine how reef communities have changed as a result of human activities. 

Sharks have microscopic scales called dermal denticles thar provide them with protection from abrasive surfaces. The team compared the abundance and variety of denticles from a Panamanian coral reef 7,000 years ago to those in reef sediments today.

Dillon began the research as an intern at the Smithsonian Tropical Research Institute (STRI), and is currently finishing her doctoral work at UC Santa Barbara.

“If you have ever petted a shark in an aquarium touch tank, denticles are the reason why shark skin is rough like sandpaper if you rub it in one direction yet smooth in the other direction. Sharks are essentially covered by millions of tiny teeth,” explained Dillon.

Just like humans shed dry skin and dandruff, sharks shed their denticles, and they accumulate in marine sediments. The oldest shark denticles ever found – discovered in the Harding Sandstone of Colorado – are about 455 million years old.

STRI paleobiologist Aaron O’Dea uses data obtained from fossil- and modern coral reefs to reconstruct baseline conditions before human colonization. These reconstructions also help scientists understand how evolutionary processes change through time.

“Placoderms in the Paleozoic, and then marine reptiles in the Mesozoic, were larger and ate sharks,” said O’Dea. “Placoderms ruled the oceans for around 70 million years and marine reptiles of the Mesozoic ruled for more than 100 million years.”

“Sharks are only top predators now because extinction events preferentially took out other groups but allowed sharks to survive. Sharks seem to have remarkable evolutionary resilience and I was fascinated to work on a technique that would help us explore how sharks have fared more recently when humans step into the picture.”

The team sampled material from a 7000-year-old fossilized reef in Bocas del Toro, Panama and from nearby modern Caribbean reefs. O’Dea asked Dillon to analyze the samples and look for shark denticles.

“What started as a three-month-long internship turned into a two-year stay in Panama and then expanded into part of my PhD thesis,” said Dillon. “I’ve grown with this project as my role shifted from first exploring and processing the samples as an intern to leading the project, analyzing and interpreting the data, and spearheading the writing.”

One week’s worth of samples took about a year of lab work to recover and identify the denticles. 

“Finding the first denticles was thrilling,” said Dillon. “They were beautifully preserved and abundant enough to provide insights into millennia-old shark communities.”

Ultimately, Dillon found that denticle accumulation rates – which represent shark abundances – were over three times higher before humans began using marine resources in the region. While all types of denticles had declined over time, those found on commercially valuable sharks declined the most. 

Based on evidence from archaeological studies and historical records, Dillon determined that the steepest decline in shark abundance occurred in the late 20th century. 

“When the Spanish arrived in the Americas, they wrote fantastic accounts of seas swarming with sharks,” said Dillon. “But these days we see very few sharks: we are lucky to see the occasional nurse shark.”

“Our data show that sharks in Bocas del Toro have been depleted both by long-term harvesting, which accelerated in the second half of the 20th century, and by habitat degradation, which began even earlier with the expansion of banana cultivation and coastal development. There is so much land-based runoff from the coast today that in some locations it’s like swimming through limeade.”

“Taken together, Erin’s body of work shows that denticle assemblages can be used with care to help reconstruct past shark communities through time,” said O’Dea. “Sharks are an integral part of ocean health and play important roles in the great diversity and functioning of coral reefs. Having empirical estimates of past shark abundances and community composition helps us frame our understanding of what is natural in the seas.”

According to Dillon, the researchers hope to extend this method to other locations to examine broader geographic patterns of change in reef shark communities over long ecological timescales. 

“For example, we’re currently reconstructing trends in denticle accumulation along Panama’s Pacific coast over the last several thousand years using sediment cores. These baseline data will help us explore the causes and consequences of changes in shark abundance and functional diversity. Our work can also help tailor shark management goals to this local region.”

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Chrissy Sexton, Earth.com Staff Writer