Largest sea-cow

The largest sea-cow or sirenian was Steller's sea-cow Hydrodamalis gigas, named after Russian explorer Georg Steller, who discovered it in 1741 off what is now named Bering Island (the largest of Russia's Commander Islands) in the Bering Sea, between Russia and Alaska, during the Great Northern Expedition captained by Vitus Bering. When fully adult, it was 8–9 m long and weighed 8–10 tonnes, far larger than any other modern-day species of sirenian, but due to excessive hunting by European sailors for its meat, fat, and hide this huge yet inoffensive, slow-moving sea mammal had become extinct just 27 years later, in 1768.

More closely related to the dugong (but three times as long) than to the manatees among present-day sirenians, Steller's sea-cow is also known from Pleistocene fossil remains, which reveal that it formerly possessed a much wider distribution than the shallow seas off the Commander Islands to which it was confined in Steller's day, fossils having been found as far-distant as Monterey in California, USA, and Honshu, Japan. Consequently, it was already much-reduced in range when formally discovered during the mid-18th century, so its combination of tasty flesh, useful fat and hide, sluggish nature, and slow-moving behaviour would tragically soon prove fatal for this spectacular marine mammal when confronted by humans. Having said that, sightings of alleged specimens were reported from the Bering Sea during the 1800s and even once as recently as the 1960s, but these have never been confirmed.