Sharks fear killer whales. How does this impact the ecosystems they share?
- A new study finds that sharks will flee areas they met orcas in for up to a year.
- Killer whales are known to eat sharks, but it is unknown if the sharks are fleeing because they know that too.
- The discovery will change our understanding of how marine ecosystems evolve.
Orcas, also known as "killer whales," are pretty cool. They're usually friendly despite their nickname, and are in an elite club of animals with no natural predators. With a range that spans the world and a coloring reminiscent of an equally popular but much less capable land animal, their image permeates pop culture.
But a new study published in Scientific Reports offers another reason to be impressed by these majestic creatures; they are so intimidating to ocean life that even great white sharks flee in terror before them.
The true apex predator
The study, titled "Killer whales redistribute white shark foraging pressure on seals," results from years of investigations into the movements and behavior of 165 tagged great white sharks, observations and records of killer whale movements, and information on seal populations off the coast of California. They also looked to previous descriptions of shark and whale interactions to give context to their findings.
The sharks immediately turned tail and fled in every time they crossed paths with orcas. They'd also stay away from that place long afterward. Only one observed shark dared venture back to where it had just encountered the whales, and it didn't stick around. Most of the sharks merely fled a bit further up the coastline, while others went much further out to sea to avoid the whales.
Why are they doing this?
Orcas have been known to eat great whites. The remains of the sharks are a grotesque sight to behold and are always missing their livers, no matter how much else remains or is missing. If the orcas have discovered a source of Chianti to pair with them or not remains unknown at this time.
However, we don't currently know if the sharks are fleeing because they understand that risk, because they knew the orcas would fight them for the same food supply, because whales look big and scary to them, or some combination of the three.
Before this gets too frightening, there are no known cases of wild orcas killing humans, and only a few examples of injuries being caused by these interactions. Orcas kept in tiny boxes for long periods can be a bit more violent, but that's another story.
Anything that makes sharks flee in terror will have an impact on the ecosystem. In this case, elephant seals benefit.
Observations of seal populations show a decline in predation events after orcas that can last for an entire year. While orcas occasionally snack on elephant seals, they stick to fish most of the time. It's a boon for the seals in areas the sharks leave, though the seals in the places they flee to might not see it that way. The findings of this study will inform our understanding of seal population fluctuations.
As lead author Salvador Jorgensen explains, the study also demonstrates that "food chains are not always linear. So-called lateral interactions between top predators are fairly well known on land but are much harder to document in the ocean. And because this one happens so infrequently, it may take us a while longer to fully understand the dynamics."
For those wondering how much longer it may take to reach that understanding, this study relied on decades of data on shark, whale, and seal populations in addition to the recently collected information. While demonstrating the value of long-term datasets and the long-term importance of minor interactions is great for science, the impatient may be disappointed at the slow pace of progress.
But the most important take away of this study might be the obvious one:
Never made an orca angry, unless you're tougher than a great white shark.
Nuclear weapons, whale sharks, and how to use both to make eco-tourism more sustainable.
- Scientists have finally determined the age of whale sharks using radioactive elements from bomb tests.
- Using the new data, the age range of the animals' bones has now been determined.
- The findings will help conservationists better maintain whale shark populations.
Majestic whale sharks, the gentle giants of the shark family.
Weighing in at 9 tons (20,000 pounds) and typically growing to around 10 meters (32 feet) long, the whale shark is the largest living species of fish. Despite the name, it is not a whale, though it is the size of one. Like many kinds of whales, it filter feeds on plankton.
Many things about the whale shark have remained unknown to science; how long they can live, their mortality rate, and how exactly to determine the age of a specimen from its remains was chief among them. However, these questions are now a little closer to being settled. In a study recently published in Frontiers in Marine Science, scientists explain how they were able to date the bones of two whale sharks who met their fate earlier than they may have expected.
Like trees, whale sharks' bones have growth rings. Scientists have known about these rings for a while, but how quickly the rings grow has been unknown. It is difficult to use them to estimate the age of a shark if you aren't sure how much time each ring represents.
A whale shark vertebra from Pakistan, in cross section, showing 50 growth bands
Image: © Paul Fanning, Pakistan node of the UN Food and Agricultural Organisation
This is where carbon-14 comes in. As a result of nuclear bomb tests during the Cold War, large quantities of carbon-14 were put into the oceans. The isotope slowly made its way up the food web and into the bodies of larger animals. Knowing the yearly changes in the amount of carbon-14 in the oceans due to bomb testing, scientists merely had to compare that data with the changes seen in the sharks' bones.
"We found that one growth ring was definitely deposited every year," said Dr. Mark Meekan of the Australian Institute of Marine Science in Perth, a co-lead on the study. "This is very important, because if you over- or under-estimate growth rates you will inevitably end up with a management strategy that doesn't work, and you'll see the population crash." This means the sharks used in this study were around 35 and 50 years old at the time of their deaths.
Working forward from there, the scientists were able conclude that the animals may have an age range of 100-150 years. "Earlier modelling studies have suggested that the largest whale sharks may live as long as 100 years," Dr. Meekan explained in a statement. "However, although our understanding of the movements, behaviour, connectivity and distribution of whale sharks have improved dramatically over the last 10 years, basic life history traits such as age, longevity and mortality remain largely unknown. Our study shows that adult sharks can indeed attain great age and that long lifespans are probably a feature of the species. Now we have another piece of the jigsaw added."
Whale sharks are an interesting species that many eco-tourists want to see. Conservation efforts for them rely on having accurate data on their longevity, mortality rate, and the age of specific animals. This information will help those managing ocean preserves keep the population stable for future generations to enjoy.
The relatively quick evolution of nine unusual shark species has scientists intrigued.
- Living off Australia and New Guinea are at least nine species of walking sharks.
- Using fins as legs, they prowl coral reefs at low tide.
- The sharks are small, don't be frightened.
Natural selection takes time. According to the fossil record, sharks, for example, have been essentially the same for hundreds of millions of years. But something's up lately, and by "lately" we mean the last nine million years. Sharks off of Australia have learned to walk. Not Great Whites, fortunately. Small sharks that feed on coral reefs. Cute sharks, actually.
Scientists have known for some time that five such shark species exist, but new research nearly doubles that number to nine. The new information comes from a 12-year study from an an international team of scientists from University of Queensland (UQ), Conservation International, CSIRO, the Florida Museum of Natural History, and the Indonesian Institute of Sciences and Indonesian Ministry of Marine Affairs and Fisheries published in Marine and Freshwater Research.
Don't mess with success
Over the last 400 million years, only about 1,200 shark species have emerged. "We see animals from 180 million years ago with exactly the same teeth," Gavin Naylor of the Florida Program for Shark Research at the University of Florida tells National Geographic. While it's true they're not the most prolific reproducers, and have a long life span, that's still plenty of time for useful mutations to arise. On the other hand, if it ain't broke, don't fix it — Earth and the oceans may change, but as predators, sharks do just fine as they are. Even if, as Naylor says of sixgill sharks, they "seem stuck back in time."
Walking to dinner
The walking sharks, or "epaulette sharks," live in coastal waters off northern Australia and the island of New Guinea. They prowl coral reefs when the tide goes out, walking through shallow water on their pectoral fins in the front and pelvic fins in the back, on the hunt for crabs, shrimp, small fish. They're adept at wriggling their way into tight nooks to find food, too. "At less than a meter long on average," says Christine Dudgeon of UQ, "walking sharks present no threat to people, but their ability to withstand low oxygen environments and walk on their fins gives them a remarkable edge over their prey of small crustaceans and mollusks." Says Dudgeon, "During low tides, they became the top predator on the reef."
The abilities of the small sharks — they're less than three feet in length — definitely put them in a class of their own, says Dudgeon: "These unique features are not shared with their closest relatives the bamboo sharks or more distant relatives in the carpet shark order including wobbegongs and whale sharks."
Though the five epaulette species don't look much alike, varying in markings and color, their DNA identified them as family. Says Dudgeon, "We estimated the connection between the species based on comparisons between their mitochondrial DNA which is passed down through the maternal lineage. This DNA codes for the mitochondria which are the parts of cells that transform oxygen and nutrients from food into energy for cells."
What's the hurry?
The researchers theorize that a few factors may have accelerated the epaulets' evolution. First off, they keep to themselves in their own separate region, with extensive inbreeding perhaps speeding up the rate of mutation. "Data suggests the new species evolved after the sharks moved away from their original population, became genetically isolated in new areas and developed into new species," explains Dudgeon. "They may have moved by swimming or walking on their fins, but it's also possible they 'hitched' a ride on reefs moving westward across the top of New Guinea, about two million years ago."
Another possible factor are the ever-changing reefs themselves. They're continually in flux as oceans change and as corals live and die, with rising and falling sea levels, as well as changing currents and temperatures. The epaulettes' success depends on adapting quickly to a very dynamic environment, about which Naylor says, "It's the shark equivalent of the Galápagos, where you can see shark evolution in action."
Beachgoers needn't fear for their tootsies just yet, but just wait another few million years, and who knows?