A team of scientists led by Erin Dillon from UC Santa Barbara recently used fossil shark scales to reconstruct current shark communities. Scientists have also applied the technique to more recent past, showing how this method can be useful for understanding species interactions over time and space.
Scientists are trying to determine how shark populations have changed over the years. They used a special technique called ichthyofaunal analysis, which is: comparing fossilized scales from ancient times (7000 yrs ago) with those found today on reefs in Panama. The worldwide plummet of sharks started as early as records began-which was about mid 20th century and humans were blamed for this decline because they had been using marine resources since then that included fishing and other activities such as deep sea mining or even pollution due to nuclear testing at Bikini Atoll during WWII caused huge amounts of radioactive fallout levels higher than what occurred near Chernobyl after Ukrainian President Leonid Kuchma’s visit there when it happened in 1986.
The study found that the number of sharks in Caribbean Panama decreased by two-thirds over the last few centuries. The speedier species were hit harder and have lessened enough to be reflected in records from only a century ago.
“These results give us new insight into what a ‘healthy’ shark community might look like on a coral reef before human exploitation,” said Dillon, a doctoral student in the Department of Ecology, Evolution, and Marine Biology. “And they can help us set more appropriate and location-specific baselines for management and conservation.”
Sharks are one of the most misunderstood creatures. Unlike their cartilaginous skeleton, shark teeth do not readily fossilize and often all that remains is their hard tooth; however with careful analysis of sediments many microscopic scales can still be found which were believed to only exist on skin. The two structures seemed similar in nature but different locations within a body—just like us humans who house lungs deep inside our chest cavity while we keep our heart right outside it!
With tough-skinned sharks’ skeletons resistant to preservation, they usually leave behind just sharpened teeth as signs from an ancient era (1). However, under special conditions when examining sedimentary layers around these fossils researchers have been able to find hundreds – or even thousands –of tiny samples.
Scientists often rely on microfossils to reconstruct ancient ecosystems. Items like scales, pollen grains and plankton shells can provide a wealth of information about the conditions and denizens of past ecosystems that aren’t preserved in large fossils. What’s more, sharks shed an average of 10 million teeth during their lifetime while they also lose 20-25 kilograms worth or dermal denticles annually which are made up out mostly cartilage so scientists find it much easier to study these microscopic remains rather than relying solely on shark teeth for evidence concerning our planet’s history as well as its current inhabitants because there is no need to use destructive methods such as extracting them from hard surfaces where they may be embedded
Dillon and her team were fortunate to have access to a fossil reef in Bocas del Toro, on Panama’s Caribbean coast. Normally, ancient reefs are entombed under the living coral, but construction had exposed the site, enabling the scientists to collect samples over several years before it was filled in.
The researchers collected sediments from the natural fossil reef. Sediment built up in between branching coral fingers and was preserved for millions of years, so they were able to find a time capsule piece of history when it accreted.
The team used radiometric dating to estimate the age of a reef. Corals incorporate trace amounts of uranium into their skeletons as they grow, so scientists can use methods like radioactivity decay rates and carbon isotopes in dissolved organic matter from coral samples to determine how old it is.
After a long day’s work, Dillon reveled in the acetic acid laced with her sediments. A simple joy when she had to separate 300 kg of sand from around 400 grams worth of denticles each time and then find them under a microscope for hours on end, but it was necessary if she wanted to create more beautiful works like this one that hung in front of me now.
Researchers from the University of Miami study how shark teeth and scales work in order to make our own devices more effective. For example, understanding which denticle shapes correspond with different functions can help us design better surfer boards that are less prone to drag but still protect against abrasion. Sharks like great hammerheads and silky sharks swim fast so their thin scales have points or ridges for reducing drag while nurse sharks spend most of its time near tough substrates necessitating thick plates for protection when moving around these substrate surfaces
Different denticle shapes correspond with different functions such as speed reduction on certain species or increased strength needed by other types based off where they live; some examples would be ridge spacing if an animal needs a strong burst speed versus protecting itself.
Sharks have different sized teeth and shapes in their mouths, so it’s hard to match the shape of a tooth with one species. Sharks differ from each other when you look at scales too because they are found all over its body instead of just on its mouth like humans’ teeth.
The team’s painstaking analysis ultimately paid off.
“We showed that tiny shark scales can be well-preserved and found in high enough abundances to reconstruct shark baselines over long ecological timescales,” Dillon said, “and we found about a 71% decrease in total shark abundance between the mid-Holocene—before major human impact in our study region—and now.” These prehistorical reefs would have had similar environmental conditions to those of today, she added, with the primary difference being that they predate the earliest evidence of human occupation in this part of Panama.
The authors also discovered that the types of sharks found on these reefs shifted between prehistoric times and today. These shifts were dependent on what type of reef the shark was living in as well. Midwater swimmers, like requiem and hammerheads, declined more than demersal species, like nurse sharks.
“If you went snorkeling on these reefs a couple thousand years ago, not only would sharks have been a more common sight but there would have been relatively more fast-swimming pelagic sharks,” she said.
Dillon was struck by the fact that sharks of all types declined over this time period. “If fishing were the only driver, then we wouldn’t expect to see such a big drop in nurse sharks over time because they have low commercial value and are rarely targeted by fisheries in the region,” she said. “But we did.” This suggests that the observed shark declines weren’t simply the result of direct impacts on the animals, like overfishing, but might also have stemmed from indirect factors like the loss of reef habitat or available prey.
Dillon and her co-authors also looked at historical accounts of shark abundance through time. “We found that the biggest decline in shark abundance, according to these records, occurred in the latter half of the 20th century,” she said. Between these accounts and the results from the fossil record, the evidence suggests that most of the shark declines in this location happened within the past 100 years.
New research has revealed that sharks have been on the decline since as early as 1900. The species are being forced out of their habitats due to global warming, overfishing and pollution in our oceans today. These findings show us what damage we can do if left unchecked so it’s crucial for governments around the world to work together now before they’re gone forever.