Updated: September 16, 2005
Secrets Of The Whale Riders: Crablike 'Whale Lice' Show How Endangered Cetaceans Evolved
Science Daily University of Utah biologists studied the genetics of "whale lice" -- small crustaceans that are parasites on endangered "right whales" -- and showed the giant whales split into three species 5 million to 6 million years ago, and that all three species probably were equally abundant before whaling reduced their numbers.
The five-year study, published in the October 2005 issue of the journal Molecular Ecology, is the first in which the genes of whale lice were used to track the genetic evolution of whales. They are not true lice, but instead are tiny crustaceans named cyamids. The harmless parasites ride the whales their entire lives, so their evolution reflects the evolution of the whales.
"These are the original whale riders," says Jon Seger, a University of Utah biology professor and the study's principal author. "It's a case of the whale riders telling us what the whales did."
["Whale Rider" was a 2002 Oscar-nominated film about the legend of an ancient Maori chief who crossed the sea on the backs of whales, and a modern Maori girl in New Zealand who aspired to become the new chief.]
"Whale researchers have dreamed for years of being able to ride with the whales and see the world they experience," says study co-author Vicky Rowntree, a University of Utah research professor of biology and senior scientist at the Ocean Alliance/Whale Conservation Institute in Lincoln, Mass. "Whale lice have been doing it for millions of years, and can tell us things about the whales we can't learn any other way."
By studying certain genes in the whale lice, the researchers showed:
A single right whale species diverged into three species -- North Atlantic, North Pacific and southern ocean right whales -- 5 million to 6 million years ago when movements of Earth's tectonic plates formed the Isthmus of Panama, separating the Pacific and Atlantic. That set up warm equatorial currents that kept most blubber-laden right whales from crossing the equator because they could not tolerate the warm waters.
One southern right whale crossed the equator 1 million to 2 million years ago, leaving evidence of the trip when its whale lice bred with those on North Pacific right whales.
The highly endangered North Atlantic and North Pacific right whales once may have been as abundant as southern right whales, suggesting their reduced genetic diversity is due to whaling in recent centuries, not some longer-term, deadlier cause. That raises hope for the ultimate recovery of the North Atlantic and North Pacific species
Seger and Rowntree, who are spouses, conducted the study with Zofia"Ada" Kaliszewska, who graduated the University of Utah and now is aHarvard University graduate student; Wendy Smith, a Utah biologygraduate student; and 14 co-authors who collected whale lice or whalegenetic material from beached whales around the world.
How Whale-Riding 'Lice' Tell Tales about Whales
Cyamids were nicknamed "whale lice" by earlywhalers, who often were infested with real head and body lice. Whalelice are related to crabs and shrimp, and cling to the whales' raised,callus-like patches of skin -- named callosities -- the grooves andpits between callosities, and also to skin in slits that cover mammaryglands and genitals. The white whale lice covering the callositiescreate distinct markings that stand out against the whales' dark skin,making it possible for scientists to distinguish individual whales.
"Their lifestyle is analogous to that of lice in that theylive on the skin of the host and they eat sloughed skin," Seger says.Each whale louse looks vaguely like a miniature crab, ranges fromone-fifth to three-fifths of an inch in length, and has five pairs ofsharp, hooked claws. About 7,500 whale lice live on a single whale.
Whale lice infest only whales, just as bird lice infest only birds andhuman lice infest only people. Recent genetic studies of head and bodylice have revealed details of human evolution. Whale lice hang onto thewhales throughout their lives, so they share a common ecological andevolutionary history with the whales. Genes from whale lice actuallymay reveal more about the whales than the whales' own genes do, becausethe parasites are much more abundant and reproduce more often thanwhales. As a result, the parasites have much greater genetic diversityand scientists have more mutations to track.
The Utah research focused on genes found inmitochondria -- the power plants of cells -- and that mutate at a highrate, acting like a clock to reveal when evolutionary events happened.The scientists calibrated the clock by comparing genes from whale licewith related snapping shrimp.
The Utah biologists extracted DNA from whale lice,determined the sequence of the genetic components in a particularmitochondrial gene, and then built family trees to show therelationships among whale lice.
The same three whale louse species -- Cyamus ovalis, Cyamus gracilisand Cyamus erraticus -- were thought to infest each of the threedifferent species of right whale. But the new study revealed that likethe whales, each whale louse species also split into three species, soNorth Pacific, North Atlantic and southern ocean species of rightwhales each are infested by three distinct species of cyamid. Thattripled -- from three to nine -- the number of cyamid species infestingright whales.
The Right Whale -- for Extinction?
Right whales --which can reach 60 feet and 70 tons -- were named because "they werethe 'right' whale to kill, the first whale to be commercially hunted"1,000 years ago off Spain; their blubber made their carcasses float foreasy recovery, Rowntree says.
Two of the three species of right whales are in danger ofextinction. A recent study estimated only 350 remain in the NorthAtlantic. Rowntree says about 200 survive in the North Pacific, whilethe Southern Hemisphere population of 8,000 to 10,000 whales is growing7 percent per year and recovering from whaling.
When whaling began in the Southern Hemisphere in the 1700s,there were an estimated 40,000 to 150,000 right whales there, Rowntreesays.
North Atlantic right whales have lower geneticdiversity than southern ocean right whales. But the new study showed"the genetic diversity of whale lice is virtually as great for theNorth Atlantic right whale as for the southern right whale, suggestingthat historically (but before whaling) the North Atlantic right whalepopulation was comparable in size to that in the Southern Hemisphere,"Seger says. "This suggests that the reduced genetic diversity of NorthAtlantic right whales happened recently, possibly due to whaling, notbecause the whale population was small even before whaling."
Whale louse populations correlate with population sizes ofright whales, so if North Atlantic right whales had small populationsbefore whaling, the diversity of their whale lice would not be as greatas those on the southern right whales.
Limited data from North Pacific whale lice suggest right whales alsowere abundant there before whaling began, in line with early whalingrecords, Rowntree says.
Small population size can be harmful because it isimpossible to avoid inbreeding and an increased risk of geneticdisease. The study raises hope for endangered Northern Hemisphere rightwhales by suggesting that their reduced genetic diversity is arelatively recent phenomenon and perhaps not as severe overall as itappears to be in the particular genes that were studied, Seger says.
One Whale Species Became Three as Oceans Separated
Some20 million years ago, North and South America were separated by deepseas, but 18 million years ago, undersea volcanism slowly began forminga volcanic island chain. By 3 million years ago, the chain formed solidland, the Isthmus of Panama, linking the two continents. By 5 millionor 6 million years ago, the sea between the two continents was soshallow that whales could not swim between the North Pacific and NorthAtlantic, Rowntree says. Changing circulation patterns established warmcurrents that discouraged right whales from moving between southern andnorthern oceans.
"Right whales have such thick blubber theycan't cross the equator," Rowntree says. "The waters are too warm. Theycan't shed heat."
Seger says: "The genetics of whale lice show conclusivelythat the three species of right whales have been isolated in the NorthPacific, North Atlantic and Southern Hemisphere for about 5 million to6 million years," with a possible range of error from 3.6 million to9.9 million years.
That is consistent with previous studies of the right whales' genes.One limited study suggested North Atlantic right whales (Eubalaenaglacialis) and southern (Eubalaena australis) right whales becamedistinct species 3 million to 12 million years ago. A more completestudy suggested North Pacific right whales (Eubalaena japonica)diverged from southern right whales 4.4 million years ago, give or take2.5 million years.
The new study revealed a fascinating detail. About 1 million to 2million years ago, at least one southern right whale -- and no morethan a few -- managed to cross the equator and join other right whalesin the North Pacific. The biologists reached that conclusion becausethey found that Cyamus ovalis whale lice in the North Pacific are muchmore closely related genetically to southern Cyamus ovalis than eitherpopulation is to those in the North Atlantic. Another species, Cyamuserraticus, does not show such a pattern; those whale lice in all threeoceans are all more distantly related.
The simplest explanation is that a single southern right whale crossedthe equator and mingled with North Pacific right whales. Some of theabundant Cyamus ovalis whale lice jumped from the southern whale tonorthern whales, but the less common Cyamus erraticus, which livesmainly in genital areas, did not.
Despite the barrier posed by warm water, climate has changed over theages and "probably there were times when the equatorial waters weren'tas hot as they usually are, and some adventurous juvenile male crossedthe equator," Rowntree says.
Note: This story has been adapted from a news release issued by University of Utah.
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