Sharks

A sediment core inspired road trip

US GEOLOGICAL SURVEY MENLO PARK CAMPUS
MENLO PARK, CA

As coral reefs accrete upward, their histories become buried beneath the surface. One way to unravel this history is to visit areas where the older reef has been exposed by excavations, road cuts, or storm channels. This is what we did in Bocas del Toro, Panamá and in the Dominican Republic’s Enriquillo Basin. In most places, however, living reef or mangroves grow on top of the Holocene reef. To access the underlying history, we can insert several-meter-long sediment cores into the substrate. While this technique samples a sliver of the surface area of a reef, it has the unique advantage of looking back in time. Depending on the rate of reef accretion and sedimentation, sediment cores can travel back several hundred to several thousand years with high temporal resolution. The remnants of the animals and plants captured in these cores help paint a picture of the reef in the past and how it has changed over time.

Today, I had the good fortune of sampling a gravity core extracted from Palmyra Atoll’s lagoon by a team of USGS scientists. In addition to being home to numerous sharks, Palmyra has an interesting human history despite never being settled. The atoll was originally discovered in 1802 when an American ship was blown off course in a storm. In 1862, Palmyra was claimed by the Kingdom of Hawaii, and it later became a U.S. territory following Hawaii’s annexation and subsequent statehood. At that time, it was privately owned by the Fullard-Leo family. During WWII, the U.S. Navy took over and drastically altered the atoll for use as a naval air facility. Afterwards, it was occasionally frequented by yachters despite its remote location – a nearly 1000-nautical-mile sail southwest of Hawaii. From 2000-2001 onwards, the atoll has been under the protection of the Nature Conservancy and U.S. Fish and Wildlife Service as a National Wildlife Refuge, and a handful of scientists and donors visit each year.

The 3.2m-long sediment core that I sampled covers nearly 650 years of history on the atoll, capturing its pristine state prior to discovery in addition to the last 200 years of sparse human occupation. We will use the core to begin investigating whether shark baselines on the atoll are naturally dynamic and whether the abundance and diversity of sharks changed after the arrival of humans. Palmyra has been the location of many shark tagging and survey studies over the last 12 years since the creation of the Palmyra Atoll Research Consortium (PARC), and it is considered to be a relatively untouched, healthy reef ecosystem. This new historical time series will help us understand whether its condition has fluctuated over time before these monitoring efforts began.

This opportunity was certainly worth the 10-hour round-trip road trip from Santa Barbara to Menlo Park, which is located about 40 minutes southeast of San Francisco. In addition to picking up the core from the sample storage refrigerator on the USGS campus, my secondary aim was to use their facilities to slice it into 64 5cm-long segments, each representing about 10 years of reef history. The core was composed of very fine, silty material, so cutting through it was not difficult with the help of some metal utensils and a meter stick. Furthermore, the challenging task of cutting the core in half through the PVC piping had already been accomplished. I carefully measured and cut each section and placed them into individual labeled plastic bags. After several hours of meticulous slicing and record keeping, I had amassed quite the pile of samples and was caked in a thin layer of dry mud.

What’s in a sample?

Monday was a good day. We met our boat exactly where and when planned (a first!) and motered south to Caracasbaai, past the colorful buildings of Willemstad, a towering cruise ship, and a fleet of tiny sailboats that looked remarkably like the schools—flocks? — of flying fish that also skimmed the waves. Erin easily located a potential denticle site in a short snorkel foray, and the dive team quickly got to work filling the sample bags. While they worked, the boat captain tossed in a fishing line baited with what he’d caught on the trip down. By the time Felix and I were winching the samples up to the surface, more fresh fish sizzled on the boat’s tiny stove. It is immensely satisfying to snack on fresh fish and simultaneously admire a pile of fresh samples. Like I said, it was a good day. We traveled back to Piscadera with tired bodies, satisfied stomachs and 20 bags full of denticles.

…Well, not quite. The sample bags are filled with fine-grained reef sediment, which may or may not contain denticles. So, why are we collecting bags and bags of sand anyway? It may seem like a strange way to determine the relative health of a coral reef ecosystem, much less shark abundance. However, hidden within the reef sediment is a time-averaged sample of some components of the reef ecosystem. It’s similar to a school’s lost-and-found: over time, the accumulated detritus preserves a sampling of the year’s fashion trends and fads. Grabbing a sample of the lost-and-found might better represent the school’s generalized clothing choices than an hour’s observation during gym. Similarly, observing a reef during the day (when humans are most active) may not accurately represent the diversity of the reef’s inhabitants, or provide a good way to compare a declining reef to a “pristine” reef less impacted by, or familiar to, human presence. The reef sediment is analogous to the lost-and-found: the denticles, shells, teeth, bones, and other hard parts are the cumulative signal of the reef’s inhabitants. While not every part of the reef is preserved, what is preserved can be compared between sites and across time to assess key differences in reef diversity. The sediment is a way to look at several years’ worth of diversity rather than assessing the reef based on a couple days of highly-variable observation.

At present, it appears that sharks are uncommon in the waters surrounding Curacao. We haven’t seen any in our time here. In contrast, anecdotal evidence suggests that sharks were very common in the past. The samples collected on this trip will hopefully address this disparity. Furthermore, other members of the O’Dea lab will look at otoliths (fish ear bones that preserve growth history much like tree rings do), molluscs, bryozoans and other components of the sediments, trying to piece together a broader picture of the reef’s health. Hopefully, the sediment collected in this field trip will show how the reef we see today—the reef which supported the delicious, though small, fish we snacked on—compares to the reef’s recent years, and how that compares the reef before humans arrived. Just as the bulk sample bags preserve a time-averaged history of the reef’s inhabitants, they also preserve the time-averaged impacts of industry, tourism and fishing on the reef’s health. That’s pretty good for a bag of sand!

New expedition: Curaçao February 20th

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The Baseline Caribbean team is gearing up for another expedition, this time to the sparkling, cerulean seas of the Netherlands Antilles in the Southern Caribbean.

Reefs on these Dutch Islands are in relatively good shape when compared to the rest of the Caribbean. But what were they like in the past?

Anecdotal evidence suggests that sharks were once abundant around these islands, yet empirical data are desperately needed to guide management. Reef fish communities are overfished today, but by how much? When did coral communities begin to deteriorate, and did it depend on their proximity to historical settlements?

To get at these questions and more, we plan to collect nearly a ton of sediment from modern and fossil reefs along the southwest coast of Curaçao. We also hope to get the chance to explore Klein Curaçao — a 1.7-square-kilometer uninhabited island just southeast of its big sister and namesake.

Instead of reading about the results in a stale journal in two years’ time, experience science in action. Beginning February the 20th, join us with daily posts, photos, and short videos from the field on the Baseline Caribbean science blog.

This expedition builds on our previous exploits in Panama, Belize and the Dominican Republic. We see familiar faces return: Erin Dillon (who recently hightailed it to the McCauley Lab), faithful malacologist Felix Rodriguez and ring-leader Aaron O’Dea. We are also joined by some fresh blood in the form of fish ecologist and evolutionary biologist Michele Pierotti and STRI videographer extraordinaire  Ana Endara.

A huge Thank You to our supporters who will make it possible: The Caribbean Research and Management of Biodiversity field station (CARMABI) who kindly gave us a Research Prize, the Smithsonian Tropical Research Institute (STRI), and YOU! – the generous donors who contributed to our crowd-funding campaign. Stay tuned…!

 

 

 

 

Denticle side view

Some of the denticles that we find in the sediments are exceptionally well-preserved. Here is an example of a denticle that still possesses the basal plate that once held it to the underlying skin. In many cases, though, we only find the crown (the top part). This side view taken via scanning electron microscopy (SEM) can give you an idea of the three-dimensionality of a fully intact denticle. It almost looks like some strange sort of hat (which perhaps I will 3D-print, patent, and sell online if our funding runs out prematurely or is frozen by the government… just kidding).

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Some things you may notice about this denticle:

  • It has a very thick crown proportional to the crown length. While many fast swimming shark species have denticles that are thin and light, this denticle means business. Its thickness lends durability, forming a protective ‘armor’ around the shark. This type of denticle is generally found on sharks that live in close-association with the benthos.
  • The crown is positioned at a slight angle relative to the base. This angle can alter the way in which water flows over the denticle, thereby changing the hydrodynamics of the shark on a micro level. Some of the fastest swimming species are thought to be able to vary this angle through adjustments to the basal plate and skin tension (Raschi & Tabit 1992).

 

Literature cited

Raschi W, Tabit C (1992) Functional aspects of placoid scales: a review and update. Aust J Mar Freshw Res 43:123–147

Evidence of a nighttime visitor

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Scanning electron microscope image of a denticle extracted from the fossil reef in Bocas del Toro, Panama. 283x magnification.

Here is a denticle that I was not expecting to find. It belongs to a shark that I only on second thought included in my denticle reference collection, one that ought not belong on a coral reef. This appears to be the denticle of a crocodile shark (Pseudocarcharias kamoharai). While denticles can normally only be identified at the family level, this one fails to resemble any other denticle in my reference collection… except one. It looks almost identical to those that I isolated from the skin of a verified crocodile shark at the Smithsonian National Museum of Natural History (image below), except that it is a little smaller and much more weathered.

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Scanning electron microscope image of a denticle isolated from the body of a crocodile shark for inclusion in our reference collection. 185x magnification.

Not only are crocodile sharks unassociated with coral reefs – in fact they generally only ascend from the depths at night to feed – but this shark has not been documented in the region in which this denticle was found. What’s more amazing is that this denticle is not even modern. It was extracted from our  6,600-year-old fossil reef!

While you may have never heard of a crocodile shark, they are listed as ‘Near Threatened’ on the the International Union for Conservation of Nature (IUCN) Red List. We obviously need to learn more about this species of shark in order to better protect it.

Our finding suggests that crocodile sharks did occasionally venture onto the lagoonal reefs of Bocas del Toro, Panama in the past (and perhaps even in the present day as well). More importantly, to me, this denticle epitomizes the beauty of the technique. Extracting and analyzing dermal denticle assemblages can reveal the rare, cryptic, or ephemeral elasmobranch visitors on reefs, an otherwise very challenging task to accomplish.

Crocodile shark. Source: PIRO-NOAA Observer Program. Wikimedia commons.

 

The next installment of shark denticle photos is here!

Over the next several weeks, I’ll be showcasing scanning electron microscope (SEM) images of some of the denticles that we extracted from the modern and ~7,000-year-old fossil coral reefs in Bocas del Toro, Panama. While the SEM images that I released previously were from known species of sharks in my reference collection, the denticles that I’m about to show you came from sediment samples we collected and processed. Our job now is to become sleuths and figure out what types of shark shed them.

Like a portrait, these denticles can paint a picture of the sharks they came from. They are the bards of the sharks of lore, lost in the sands of time… Ok, that’s one massive cliché (and I might be a bit too obsessed with denticles), but you get the point. Denticle morphology can provide us with useful insight into the historical ecology of sharks. For example, a denticle’s thickness can reveal whether its owner lived in the crevices of a reef or up in the water column. Its ridges can tell us whether or not it was a fast swimmer, speed which it may have used to catch nimble prey or swim long distances.

Some denticles are better storytellers than others, however, so we need to figure out who is who. This is one of the more unusual denticles that I’ve found so far. What type of shark do you think it may have come from? Stay tuned for my interpretation.

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The third iteration of sediment processing

The sediment processing workshop will now resume in a new location: UC Santa Barbara. First things first, I had to transform the lab. Since the size of each sample was smaller, I could also downsize the set-up a bit and move everything indoors. This meant no more massive outdoor ‘drying bubbles’ or tent-covered mazes of 5-gal buckets for digesting sediments… at least for now. We did, however, order 100 gallons of acetic acid. What a lot of vinegar!

The funnest challenge was building the new drying oven from an old storage cabinet, several power strips, and a box full of incandescent light bulbs. Drilling through the metal cabinet to feed the cords through was by far the hardest part. Standard drill bits didn’t cut it, so I had to upgrade to a step drill bit. That worked like a charm, but it still took several hours of drilling. Once everything was installed, I turned on the power strips and basked in the glow for a moment. It was toasty – exactly as intended! When I did a test run the next morning, I was pleased to find that the sediment dried in under a day.

Now, we are just about ready to rock and roll…

Funded!

There are 15 hours left in our crowdfunding campaign and we are 101% funded. If you want to make a last minute donation to show your support, there is still time remaining. Thanks  so much to everybody who backed our project during the campaign! YOU are making our work possible.

Stay tuned in February as we travel to the reefs of Curaçao and share our experiences from the field.

First stop before Jimani (1 of 3)

If you love sharks… three days left

$3000 to go with just three days remaining! If you think sharks are cool, consider donating just $5 to support our cause. Also, please share our project campaign across your social media and friend networks. We’ll need to reach our goal to receive the funds, and our field work won’t be possible without your support. We can make this happen!

You can support our work and learn more about the project here.

Denticle SEMs colored

Lab note #1: preliminary observations from Curaçao

A new lab note has been posted for our crowdfunding campaign, which shares some preliminary observations from Curaçao made during our trip last year. You can check it out here!

We’re 25% funded with 19 days to go! Thanks to all of our backers so far. This field work will not be possible unless we reach our funding goal.

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Denticle of the day: Scanning electron microscope image of a lemon shark (Negaprion brevirostris) denticle at 245x magnification. Scale = 100 micrometers.

 

 

 

Week 1 crowdfunding campaign update

Thanks to everybody who donated during the first week of our crowdfunding campaign! We really appreciate your support. Every little bit counts, and we’ll need to reach our goal of $4,000 in the next three weeks to receive the funds for our next field work mission. Again, please check out our project at experiment.com/sharkskin and share it with your friends and colleagues. This campaign will help us uncover the history of sharks on reefs in Curaçao and keep the ‘Baseline Caribbean’ blog posts rolling.

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Coral reef challenge crowdfunding campaign: starting August 23

Yesterday evening, Aaron and I launched a crowdfunding campaign as part of Experiment.com’s Coral Reef Challenge. We are raising money to support our upcoming field work in Curaçao, which will supplement an award we received at the Association of Marine Laboratories of the Caribbean scientific meeting last year. You can check out our campaign’s page and learn more about what we’re planning to do at experiment.com/sharkskin. The campaign participating in this challenge with the most donors by September 13 will win an additional $1000, and our campaign will run for a total of 30 days. We must raise at least our goal of $4000 to receive the funds. We’ll need your support to reach this goal and keep the blog posts flowing as we collect more samples for the Baseline Caribbean project. Donors will receive a shoutout on our Baseline Caribbean blog as we report live from the field, so keep an eye out in February. Thank you in advance for your support!

-Erin

Progress report

How much? We processed 61 large bags of sediment for dermal denticles, totaling 534kg (1177lbs). This is equal to 6 baby elephants at birth or a small mature great white shark.

How long? Processing this massive amount of sediment took about three months total, although we’re still picking out some of the denticles. Fortunately, many of the processing steps could be done simultaneously:

~2 months washing and sieving the samples

~3 months digesting the carbonate with acid, split into six rounds of digestions. This ate up nearly 265L (70 gallons) of acetic acid.

~1.5-2 months picking for denticles

Now how much does it weigh? The acid digestions reduced our load of sediment to around 6kg total. That’s about a 99% reduction in weight. Now that’s much easier to sort through!

How many denticles have we found so far? We’ve currently found about 150 denticles of various forms, but we still have more to pick!