Shark-like fishes like sharks have been a predator in the oceans for 400 million years. But now, humans kill 100,000,000 sharks each year, disrupting the food chain. Smithsonian scientists have revealed the evolution of sharks over the past 7000 years. They used microscopic shark scales from coral reefs in Caribbean Panama to examine the role of sharks before and after they were hunted. This new use of dermal denticles could provide context for innovative conservation strategies for reefs.
The skin denticles, microscopic scales that cover a shark's body (dermal denticles) reduce drag and protect them from abrasion by hard substrates and attachments.
Erin Dillon started this research as an intern at Smithsonian Tropical Research Institute, and is currently finishing her doctoral work at University of California, Santa Barbara. "Denticulles are the reason shark skin is rough, like sandpaper, if you rub in one direction but smooth the other," she said. Sharks have millions of tiny teeth.
As humans shed their dry skin and dandruff just as sharks shed denticles which build up in marine sediments. The Harding Sandstone in Colorado has the oldest known denticles, which are approximately 455 million years old.
Aaron O'Dea is a STRI paleobiologist. He uses clues from both fossil- and modern corals reefs to reconstruct baseline conditions prior to human colonization and to understand how ecological as well as evolutionary processes change over time.
"Placoderms of the Paleozoic and marine reptiles of the Mesozoic were larger and ate sharks. O'Dea explained. O'Dea explained that Placodroms ruled the oceans for approximately 70 million years, while marine reptiles from the Mesozoic ruled over the oceans for more than 100 millions years. Because sharks are now the top predators, extinction events have favored other groups and allowed sharks to survive. The evolutionary resilience of sharks is remarkable and I was intrigued to develop a technique to help us understand how they have fared in recent times when humans enter the picture.
His team collected material from a fossilized reef that was 7000 years old in Bocas del Toro (Panama) and other nearby reefs. Erin Dillon was asked by O'Dea to check if there were shark denticles within the samples.
Dillon stated that what started out as a three-month-long internship became a two-year-long stay in Panama, which was then extended into my PhD thesis. As an intern, I was responsible for processing samples and then became the leader of the project. This included analyzing the data and writing.
It took approximately a year to identify and recover the denticles after collecting samples from fossil reefs, modern coral rubble, and other sources. Dillon and his colleagues had to sort through 300 kg of reef sediments. This was enough to fill two bathtubs. They first used acetic acid for dissolving the chalky sediments. Then they used a paintbrush to separate the denticles under a microscope.
Dillon exclaimed, "Finding the first dental cells was exciting." They were well preserved and plentiful enough to give insight into the shark communities of millennia.
Dillon was interested in understanding the relationship between shark denticles and shark numbers. She also wanted to know how reef sediments fossilize denticles and what sharks have which denticles. She was able to publish these studies and return to the fossil record.
She discovered that shark abundances and denticle accumulation were three times higher than they were before humans started using the marine resources in the area. The most significant declines in denticles were those from commercially valuable species. The oldest specimens contained more denticles of fast-swimming, pelagic sharks such as hammerheads or requiems. Dillon, however, found that nurse sharks have a higher frequency today than in the past.
Dillon also studied historical narratives and archaeological studies to add to the fossil record. According to historical records, the greatest decline in shark abundance was seen in the latter part of the 20th century. This coincided with the establishment of a Panama shark fishery that targeted only pelagic sharks. However, the decline of nurse sharks' denticles suggests that other factors, such as the loss coral reef habitat and prey items, are also responsible.
Dillon stated that "When the Spanish arrived to the Americas, the Spanish wrote amazing accounts of shark-swarming waters." "But we now see very few sharks. We are fortunate to see the occasional nurse shark. Our data shows that the habitat of sharks in Bocas del Toro has been reduced by both long-term harvesting which has accelerated in second half of 20 century and habitat degradation which started earlier with expansion of banana cultivation and coastal development. It's almost like swimming in limeade because of the amount of land-based runoff that is being generated by the coast.
O'Dea stated that "all of Erin's work shows that denticle assemblies can be used with care in order to reconstruct past shark communities through the passage of time." "Sharks play an important role in coral reef health and diversity. We can use empirical estimates of shark abundances and communities from the past to help us understand what is natural in our seas.
Dillon stated that "we hope to expand this method to other places to examine broader geographical patterns of change in reef Shark communities over long ecological timesscales." We are currently reconstructing trends of denticle accumulation along Panama’s Pacific coast over several thousand years with sediment cores. These baseline data will allow us to explore the causes and effects of changes in shark abundance, and functional diversity. Our research can help us tailor our shark management goals to the local area.
This study was possible thanks to the generous support of Panama's Secretariat for Science and Technology, SNI, STRI and the Association of Marine Laboratories of the Caribbean (SENACYT), the Save Our Seas Foundation and the Alfred P. Sloan Foundation.
The Smithsonian Tropical Research Institute is a part of the Smithsonian Institution. It is located in Panama City, Panama. The institute promotes conservation and furthers knowledge of tropical biodiversity. It also trains students to conduct research in tropics. Promo video.