When Matt Arnegard joined Carl Hopkins' electric fish lab in 1998, Hopkins thought that two of the fish types he was working with (type I and type II)formed two different species. Arnegard explains that Hopkins drew this conclusion because African electric fish of the same species communicate with a single kind of electric pulse, called an Electric Organ Discharge (EOD), and appear only to mate with fish that emit the same EOD. Both of the fish types that Hopkins was working with had distinctive EOD signatures. But when Arnegard began looking closely at genetic markers from each of the fish, it was clear that the relationship between the two groups was more complex. Despite their apparent signal differences, the fish were genetically indistinguishable. What was going on? Were the type I and type II fish the founding fathers of two new, as yet undescribed, species, or were they both members of the same species, with one group on the verge of evolutionarily diverging from the other? Puzzled, the team decided that they needed to know more about the enigmatic fish. Arnegard decided to test how the two fish types responded to electric discharge `calls' from their own, and the other group(p. 2182).
Arnegard and Hopkins left their Cornell lab and set off for the fishes'home in the Ivindo River Basin, deep in the heart of Gabon's rain forest. After a 14 hour drive over rainy-season dirt tracks to the Institut de Recherche en Écologie Tropicale, Arnegard and a team of local fishermen were ready to begin trapping both fish types to see how they responded to each other's electric discharges. Arnegard recalls that trapping the fish was relatively straightforward. By tracking the animals in the river with an electrode attached to an oscilloscope, Arnegard could distinguish whether he was catching type I or type II males.
Recording the males' responses to a choice of type I and type II female EODs, Arnegard realised that the type II males chose to ignore the type I female's EODs, but vigorously attacked the electrode simulating a type II female's EOD, replying with their own electric chirrup; type II males were well attuned to EODs from their own females. But the type I males' responses were less clear-cut. They seemed unable to distinguish between type I and type II female EODs, responding enthusiastically to both. Type II males seemed to be more selective in their EOD responses than type I males.
More surprisingly, Arnegard realised that the type I males were significantly larger than the type II males. He explains that collecting fish during the mating season allowed him to clearly differentiate youngsters from mature males, and when he compared adult males from both groups, it was clear that type I males were on average 20% longer than the type II males. The differences between the two fish types were more than EOD deep.
Returning to Cornell, Arnegard teamed up with neurophysiologist, Scott Jackson, to test how the fishes' Knollenorgan EOD receptors responded to the individual EOD signatures. Recording Knollenorgan electrical activity as he played EODs to the fish, Arnegard found that the receptors responded to both type I and type II EODs, but generated different response patterns to each signal, allowing the fish to discriminate EODs of their own type from other's EODs.
Having found that type II fish are smaller than type I fish, and respond most strongly to EODs of their own type, Arnegard explains that it is possible the type I and type II fish could be on the brink of diverging into distinct species, but he is anxious to test this possibility further. He says that`this could be a very interesting evolutionary snapshot', adding `whatever evolutionary pressures are behind the size difference are probably linked to the signal difference'.