The love life of the sea lamprey, a primitive, bloodsucking jawless fish resembling an eel, begins with a male luring females to his rocky nest using pheromones. The male then caresses a female’s abdomen with a swollen hump, known as “rope tissue,” located in front of his anterior dorsal fin. If the female responds, the male attaches onto her head using his suction cup–like mouth and wraps around her body, bringing their urogenital pores into proximity. Eventually, both partners flex vigorously, simultaneously releasing eggs and sperm into the water. While observing the mating routine, neurobiologist Yu-Wen Chung-Davidson and colleagues at Michigan State University wondered if there’s more to male rope tissue than meets the eye.
They examined dorsal tissue samples from male and female sea lampreys, Petromyzon marinus, which, as adults, return to rivers to spawn. In juveniles of both sexes, rope tissue was characterized by white fat cells. But by the time lampreys hit adulthood, their rope tissue had diverged dramatically. In adult females, it showed abundant collagen fibers and small interstitial cells, while in males there were remarkably large fat cells, and the interstitial cells were larger. In males the cells were rich in oil droplets and mitochondria—the minute organs that produce energy inside cells—and the team noticed they were suspiciously similar to heat-producing brown fat cells, which are unique to mammals. Could cold-blooded lampreys produce heat in their rope tissue? To find out, the researchers hooked up miniature temperature probes into several mature male lampreys and then observed their behavior. When ovulating females came near, mature males’ humps heated up by as much as half a Fahrenheit degree—apparently a turn-on for potential mates.
The metabolism of cells in male lampreys’ rope tissue resembled that of brown fat cells, but was distinct. First, their genes express a protein related to, yet different from, the one that enables brown fat to generate heat. Second, although they have the same types of fatty acids as brown fat cells—in even higher quantities— the lampreys’ cells have additional ones of their own. Finally, in mammals, both young and old, brown fat tissue is distributed in various parts of the body and produces heat in response to cold. In male lampreys the highly localized tissue produces heat solely in response to sexually mature female lampreys. (Journal of Experimental Biology)