Oceans Before Humans: An Investigation Into the Decline of Shark Populations

In my last blog post, “Oceans Before Humans: What Can Sponges Tell Us?”, I walked you through Smithsonian Tropical Research Institute (STRI) Intern Mike Hynes’ project examining how and why sponge communities in the Caribbean have shifted over time, and how this information is helping to develop better, more informed conservation measures. Today I’m going to focus on a different project by Erin Dillon, a Stanford University grad with a degree in Marine Biology. Working on a collaborative project here in Panama with Aaron O’Dea (STRI), Dick Norris (Scripps Institution of Oceanography) and Katie Kramer (Scripps Institution of Oceanography), Erin is examining what shark populations might have looked like before humans began to harvest them. By examining shark dermal denticles – essentially “teeth” on the skin of sharks – Erin hopes to add one more piece to the puzzle of what a “pristine” ecosystem looked like.

Unlike bony fish, sharks are cartilaginous, meaning their bodies are predominantly made of a flexible connective tissue called cartilage. Instead of being covered in traditional fish scales, the bodies of sharks are covered with these dermal denticles. These rigid scales are the reason the skin of sharks feels rough to the touch if you stroke towards the sharks head as opposed to in the direction of their tail — sort of like rubbing sand paper.

Dreaming of Denticles

denticles_on_sharkThe purpose of dermal denticles is two-fold: 1) to reduce drag and turbulence, enabling sharks to swim more efficiently, and 2) to provide protection. Both of these functions are accomplished by the composition of the denticles; a layer of dentine (like our teeth) that is covered by an enamel-like substance called vitrodentine, which adds further structure and protection to the denticle. While they stop growing in size after a certain point, denticles are constantly being produced as a shark grows. These denticles grow around the body of the shark, producing outer coverage that could be compared to wearing a chainmail suit; this arrangement combined with the strength of the denticles provides protection from large predators such as other sharks, down to small parasites (http://www.elasmo-research.org/education/white_shark/scales.htm).

Denticles come in a wide variety of morphologies that correspond to various functions characteristic of certain species of sharks. Previous studies have organized them into five different functional groups:

denticles

  1. Generalized – more ancestral form found in most sharks, particularly those living closer to the bottom – these play a variety of roles.
  2. Abrasion strength – thicker denticles which protect demersal sharks from the rough environments they inhabit.
  3. Defensive – deter the settlement of small organisms and parasites on the skin of demersal and schooling sharks.
  4. Drag reduction – interfere with the boundary layer created by water moving past the shark as it swims to improve its hydrodynamic properties.
  5. Bioluminescent – permit bioluminescent sharks to have photophores and for the light to shine through – most common in mesopelagic sharks.

Ranging in size from 100µm to 1.2mm in size (a size range from the diameter of a human hair to the diameter of a pin), finding the denticles requires a high-powered microscope. While one might assume that because these dermal denticles sound so unique, they must be easy to spot – this couldn’t be farther from the truth! In fact, on average, Erin says she usually finds a measly four denticles per kilogram (a little over 2 pounds) of sediment!

From Top Predator to Top Conservation Priority

Because these denticles are made of such hardy materials, they are preserved surprisingly well in sediment and can therefore be used as a proxy for the abundance and taxonomic diversity of sharks in the past. With the help of these miniscule and elusive dermal denticles, Erin is seeking to reconstruct what shark populations looked like before human impact in the Caribbean, specifically in Bocas del Toro, Panama and the Dominican Republic. Using dermal denticles that have been preserved in sediment from both living and dead modern reefs in addition to those from similar 8,000-6,000 year old fossil reef sites, Erin is able to compare both the size and taxonomic composition of shark assemblages over time starting with when their populations were in a “pristine” condition, and progressing to current stock sizes that have been heavily impacted by human interactions.

sharkchange

Preliminary data suggests that the functional community composition of sharks may have shifted over time, particularly in terms of the relative abundance of demersal and predatory sharks, which play important and diverse roles in maintaining coral reef health and resilience. By discovering pre-human baselines and understanding how shark communities have changed since that time period, Erin and her fellow researchers hope that more accurate conservation goals can be developed and met.

What’s next for this dermal denticle diva? For the moment, Erin’s project is focused solely on Caribbean reefs, but she has her sights set on expanding the study to the Pacific. Such data would facilitate a comparison of how the modern-day spatial diversity and natural abundance of sharks in the Caribbean vs. the Pacific is reflected in dermal denticle assemblages. This is particularly interesting considering how different these habitats are, in addition to the fact that Caribbean underwent a massive extinction 2 million years ago!

Shark-Population-Map

If you would like to learn a little more about shark conservation, here are a couple links to provide you with some great foundational information: 1) Ocean Health Index: Global State of Sharks, Rays, and Chimeras 2) Shark Trust

Also please check out Madison Stewart, a truly awe-inspiring shark conservationist who proves daily that sharks are stupendous creatures that deserve not only our protection, but our love as well.

Tune in next time to hear a little about my own project here at STRI working with Caribbean corals!

Images Courtesy of: 1) Shark Decline Map

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