They sing at such a low frequency, Dr. Cerchio said, that when he was diving in their habitat, he felt rather than heard their distinct, rhythmic patterns of song, and typically had to speed up the recordings to actually hear them.

Dr. Cerchio said many whale experts and amateurs thought they had seen Bryde’s whales, a slightly larger species of baleen whale, when they were really watching Omura’s whales. It’s like the difference between coyotes and wolves, if you didn’t know they were two species, he said.

Japanese researchers first identified Omura’s whales in 2003, based on a 1998 stranding in Japan and tissue from eight animals killed during Japanese scientific whaling operations in the 1970s. The Omura’s whales have relatively small bodies, distinct genetics and unusually shaped skulls , leading researchers to conclude that the new species had split off from its genetic cousins 17 million years earlier.

Omura’s whales are baleen whales, meaning they are filter feeders, and they can be identified by their asymmetric coloration. The right side of their jaws are white, with a swirling, smoky splash of light coloration and four bisecting dark stripes on the right side of their heads, and their backs are decorated with asymmetrical chevrons. They favor tropical environments more than than most whales and don’t migrate, Dr. Cerchio said.

After publishing his 2015 paper, in which he described more than 40 whales seen in the wild and expanded their range beyond the Indo-Pacific, Dr. Cerchio said people sent him pictures of similar looking whales.

“Little by little it became clear that there were a lot more out there that could be researched and tallied,” he said.

At the urging of Bob Brownell, the paper’s senior author, Dr. Cerchio counted images he received, those he’d stumbled across on the internet, as well as sound recordings and historical sightings dating back to a 1955 magazine article from Hong Kong University that misidentified an Omura’s whal e as an immature fin whale.