The shape of the discharge is the fish's "face," says Carlson. "It's how they recognize one another."
The sensory pathway that detects and analyzes the electric discharges in the Mormyridae had been well studied, but only in two or three species, Carlson says, and the family has more than 200. Given its diversity Carlson asked whether changes in electrical communication might have influenced rates of speciation.
Three anatomical advances underlie the ability to send and receive diverse electrical signals: cells able to produce different discharges, a global distribution of the sensors that detect the discharges' shape, and a more complex signal-processing area of the brain to analyze them.
In 2008 the National Science Foundation awarded Carlson a grant to travel to Gabon (where many mormyrid species are found) to study the mormyrid brain, and how brain anatomy maps onto the evolutionary tree of the fishes.
His team found that changes in brain anatomy and the resulting ability to fully exploit electric signal space did indeed lead to rapid speciation, a result published in the April 29 issue of Science.
The electric organ
Each pop is one discharge of an electric organ located at the base of a fish's tail. The organs consist of stacks of disk-like cells called electrocytes, "pretty much like watch batteries in series," says Carlson.
The electrocytes all fire action potentials simultaneously, and so their tiny action potentials sum to produce a discharge that is typically about a few volts.
"These signals don't propagate as electromagnetic waves," Carlson explains. "Instead they exist as an electrostatic field, just like you'd get by sticking a battery in the water.
"That's why these fish are so good at recognizing pulses with different shapes," he says. Waves are distorted during transmission, so that their fi
|Contact: Diana Lutz|
Washington University in St. Louis