Humpback Whale, Sulphur Bottom Whale
Whales and Whaling pictures

The Humpback of the Pacific. Megaptera versabilis, Cope.
The Sulphur Bottom of the Pacific. Sibbaldius sulfureus, Cope.

The sulphur bottom whale is another name for the blue whale. As these animals pass through tropical waters, a "bloom" or form of algae grows on their underside, the green algae on a bluish background giving a yellow tinge. This earned them the alternative name. On leaving tropical waters for more temperate or polar seas, so the algae cannot grow any more in the colder waters and so dies off leaving the whale as it was originally.

For many years it was not recognized by all that the blue whale and the sulphur bottom were in fact the same animal and so they were given different Latin and common names.