Freshwater seals manage to make a meal of micro-shrimp
Not only is Siberia's Lake Baikal the world's largest freshwater lake, it's also the only home of the Baikal freshwater seal. A new study indicates that the animals subsist largely on tiny crustaceans, which it was formerly thought were not worth the effort of eating.
Scientifically known as Macrohectopus branickii, the shrimp-like crustaceans are the world's only freshwater amphipod with a planktonic (free-floating) lifestyle. Each one gets no longer than 38 millimeters – in fact, most are under 10 mm in length, weighing less than a tenth of a gram.
Ordinarily, for an animal as large as a seal, the small amount of energy that could be obtained by eating these amphipods would be less than the amount that was expended by chasing them down. For this reason it had been assumed that the Baikal seals fed almost entirely on fish, as do other seals.
Recently, however, scientists from Japan's National Institute of Polar Research attached accelerometers and video cameras to some of the seals, to observe their underwater feeding habits. Surprisingly, it was found that the animals consume thousands of the amphipods each day, averaging 57 per dive.
And unlike baleen whales, which swallow huge numbers of planktonic animals in one gulp, the seals capture the amphipods individually. In fact, the seals exhibited "the highest consumption rate ever recorded of any aquatic mammal that feeds on single prey one at a time rather than scooping up lots of different types of prey all at once."
It was determined that part of the rationale for this behaviour lies in the simple fact that the Baikal seals are one of the smallest types of seal. This means that they derive a substantial net energy gain from going after prey that wouldn't sustain larger seals.
More interestingly, however, analysis of museum skull specimens revealed that the seals have unique comb-like teeth along the sides of their mouth. These allow them to expel water from their mouth while retaining prey inside, meaning they don't have to swallow a mouthful of water every time they swallow an amphipod. As a result, they're able to hunt the amphipods much more quickly and efficiently than would otherwise be the case.
The scientists now believe that the Baikal seals' unique feeding strategy is the main thing that's allowing them to thrive, while other seal populations are succumbing to human-caused factors such as overfishing and habitat loss.
A paper on the research, which is being led by Assoc. Prof. Yuuki Watanabe, was recently published in the journal Proceedings of the National Academy of Sciences.