Baikal seals are fans of bite portions, and this dietary peculiarity may be the reason seals thrive.
Found in Russia’s vast Lake Baikal, Siberian mammals devour small marine crustaceans, probably using fillable teeth in a manner similar to how whale whales feed, according to a new study.
Research suggests that Baikal seals (Pusa sibirica) use a combination of special teeth, speed and ability to swallow dozens of inch critters called amphipods in a single dive, scientists reported Nov. 16 in the Proceedings of the National Academy of Sciences.
Typically, seals eat fish and shellfish, although some southern seals, such as crab seals (Lobodon carcinophaga), are eaters of krill, another type of small crustacean. For Baikal seals, there can be great benefits to hunting amphipods. Crustaceans "are very predictable," says marine biologist Yuuki Watanabe at Tokyo's National Institute of Polar Research. "They form large aggregations and come to the surface at night."
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Exploiting such a reliable food source in the food chain, says Watanabe, can make Baikal seals more resistant than other seals to human-driven environmental impacts such as warming temperatures (SN: 5/1/17 ).
Up to 115,000 P. sibirica seals populate Lake Baikal, and the species is listed as “the least worrying” by the International Union for Conservation of Nature. This is much more abundant than seals in similar lake habitats, such as the ringed seals of Lake Ladoga in northwestern Russia and Lake Saimaa in Finland, which gather a few thousand.
Watanabe has been studying Baikal seals since 2003. He then had tests of depth-mounted measuring devices on seals that showed they reliably changed their diving depths at night, suggesting that animals could follow a particular food source. .
And previous records of the stomach contents of seals have shown that animals ate amphipods at least occasionally, performing daily migrations from the depths to shallow and back surfaces. Thus, in June 2018, Watanabe returned to Lake Baikal, the largest and oldest freshwater lake in the world, to see if it could gather direct evidence that seals fed on swarms of amphipods.
Watanabe and his colleagues grabbed eight seals and fixed cameras and accelerometers on their backs, recording what the seals ate, how fast they swam, and their diving depths. The team found that seals quickly caught individual amphipods in their nocturnal dives, up to 154 on a descent, and caught one amphipod every 2.5 seconds.
Over the course of a single day, it is estimated that seals make thousands of catches. All those snacks add up. Based on estimates by Watanabe and colleagues, Baikal seals can get about 20 percent of their daily calorie needs from amphipods alone.
When Watanabe examined the Baikal seal skulls in museum collections and compared the skulls with those of other seals, he noticed that the teeth on the cheeks of Baikal animals have bent margins that give the teeth a similar shape with longer and more developed tips. another species of northern seal. . Baikal spotlights can use these teeth to efficiently sift their plankton premium from the lake, expelling excess water with each sip, the researchers say.
Mia Wege, a marine ecologist at the University of Pretoria in South Africa, was surprised that such small prey could form an important part of a seal’s diet.
The size of the freshwater amphipods that eat Baikal seals, she said, "is much smaller than other krill species or amphipods consumed by seals." Wege says that makes sense because Siberian seals are among the smallest seal species, with bodies that can run on less fuel.
In the future, Watanabe wants to conduct feeding experiments to confirm how seals are using their simple compasses. He also wants to study the winter diet of animals, as he thinks amphipods can flock under the winter ice of the lake, potentially offering a dense, reliable feast for hungry seals. Determining how seasonal ice cover influences seal food sources is increasingly important, says Watanabe, as the lake’s winter ice decreases due to climate change.