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Thursday, April 4, 2002
Sea lampreys excited by eau de liver bile
By ROWAN HOOPER
The sea lamprey is a parasitic, eel-like fish with a fearsome, tooth-covered "oral disk" instead of a regular mouth. When attacking, the lamprey rears its head, and clamps its oral disk onto the skin of other fish. With its grasping tongue, it feeds on blood and body fluids for an average of 76 hours, leaving a bloody scar on the fish's body and often killing it.
Sadistic ancient Romans would throw slaves into a pool of lampreys, condemning them to drawn-out, agonizing deaths for minor misdemeanors and trivial accidents. Pliny the Elder reported that Vedius Pollio, the Roman who devised the lamprey torture, did so "because he took pleasure in beholding a man, torn and plucked in pieces all at once." Pollio evidently enjoyed the "pleasant sight" of death observed close-up. As well as watching the lampreys eat slaves, the Romans liked to eat the fish. Lamprey milt -- the sperm and reproductive organs of male fish -- was considered a delicacy at Roman feasts.
These days humans aren't fed to lampreys, but the fish are nevertheless widely loathed, being held responsible for the devastation of salmon, bass and trout stocks in the Great Lakes of North America. Lampreys are native to the North Atlantic, but probably found their way into the Great Lakes via shipping channels. Despite the parasitic lifestyle, however, there is one thing that loves a lamprey: a female lamprey.
Sex pheromones are used by many species of insects when females want to attract males, but their use by males and by vertebrates is less well understood. In vertebrate species known to use pheromones, the chemicals are only effective over a short range, but this week in the journal Science, biologists from the United States and Britain describe how male lampreys attract females from up to 65 meters away -- using excretions of bile acid from their livers. The biologists painstakingly isolated the bile acid and determined that it functioned as a sex pheromone.
"The identification has taken a long time," said Weiming Li, fisheries biologist at Michigan State University and lead author of the Science paper. "It was a difficult task, but we have finally found a way to isolate and identify this pheromone that has been suspected, but never documented. It has given us a new way to look at pheromone communication."
To find the chemical, Li's team spent two years condensing a ton of water to about 30 mg of pure compound. They then performed bioassays to trace the key chemicals. The team washed candidate compounds through a lamprey's nose and used electro-olfactograms to measure the neurological response.
Behavioral tests confirmed that female lampreys are influenced by something the male releases. In choice-chamber tests in the lab, females preferred to spend time in water that male lampreys had been in. And in the field, radio-transmitter-tagged females were able to locate natural spawning sites far upstream.
A whole battery of further tests revealed that the chemical is bile acid, synthesized in the liver and excreted by special cells in the gills. The finding puts a new spin on fish courtship.
"People have thought that it was the sexually mature male that goes after a female," said Li, "but now we know that in the sea lamprey, it is the male that is sending the active [chemical] signal, instead of just being spied by the female. This is important."
Male sea lampreys build horseshoe-shaped nests, release their bile into the water and wait for females. A female wanting to spawn swims toward the bile trace and anchors herself to a nearby stone using her oral disk. The male then wraps himself around her and brings his genital pore close to hers. After mating, the female spawns around 60,000 eggs, some 14 percent of which will be safely deposited in the nest.
The authors speculate that the lamprey evolved the use of bile acid as a pheromone, instead of the steroids more common in other vertebrates, because bile acids are more water-soluble and can be produced in larger quantities.
The work, done in collaboration with Alexander Scott of the Centre for Environment, Fisheries and Aquatic Sciences in Weymouth, England, will have two main applications on top of the general insight into fish pheromones.
First, the authors write that "interference with this pheromone system offers an attractive target for selective and environmentally benign control of the sea lamprey, whose invasion of the Great Lakes represents arguably the worst ecological disaster ever to befall a large watershed."
The second consequence will be of interest only to those with culinary tastes similar to the Romans: Control of the pheromone system can also help boost lamprey populations in places where they are considered delicacies.
You can e-mail Rowan Hooper at email@example.com