Hitchhiker. A family tree of cyamids, the tiny white crustaceans (below), reveals the natural history of their hosts, the right whales (above).

Listening to Lice

John is a Science contributing correspondent.

If you want the full story, the saying goes, it's best to be a fly on the wall. Or, as a genetic study reveals, a parasite on the whale. Researchers are filling in the evolutionary history of right whales with the help of the crustaceans they carry.

The trouble with whales, as geneticists see it, is that they have small populations and long lifespans that make it difficult to build a family tree. Take the right whale, a plankton-loving giant of which three populations exist: one in the northern Atlantic, another in the northern Pacific, and one in the Southern Ocean, which includes the southern parts of the Atlantic, Pacific and Indian Oceans. Most scientists believed the three groups are actually different species; but figuring out when they split up has been tricky because the whales haven't racked up enough telltale DNA mutations to sort that out.

Enter a species of tiny crustaceans called cyamids--dubbed whale lice--teeming on the whales' skin, where they nibble on callouses. Because cyamids have no free-swimming stage of life and are passed from mother to calf and during other close contacts, their genetic history is linked to that of their hosts. By scraping off cyamids from the bodies of stranded right whales around the world and sequencing one of their mitochondrial genes, a team led by evolutionary biologist Jon Seger of the University of Utah in Salt Lake City built up a cyamid family tree.

It tells a whale of a story. Right whales diverged from a single species between 5 million and 6 million years ago, when the isthmus of Panama formed, the team reports in the October issue of Molecular Ecology; and at least one whale crossed over from the southern to the northern Pacific within the last 1 million to 2 million years. The equator is a major barrier to the blubber-clad whales because they can overheat. But the data show that there has been a recent visit, and the fact that cyamids were transferred suggests that the southern visitor was welcomed into the pods of the northern Pacific whales.

"The neat thing about this work," says marine biologist Roger Payne of the Ocean Alliance in Lincoln, Massachusetts, is that the millions of years of separation "confirms that there really are three separate species of right whale"--a claim Payne had until now been "hesitant to accept."

Related sites
Jon Seger's site
The Ocean Alliance

Posted in Biology, Evolution