Anglerfish earned the name for their unique evolutionary hunting adaptation—a front dorsal fin known as an illicium that functions as a lure. To humans, an illicium may resemble a fishing rod, but to potential prey, they mimic aquatic worms. In the case of anglerfish’s frogfish subgroup, for example, they camouflage into their surroundings while bobbing their front fin to attract their next meal. Once close enough, a frogfish then stops moving the fin to gobble up their target.
Marine biologists have long wondered about the identity and exact location of the neurons that control the dorsal fin’s critical motor function. Thanks to recent research at Japan’s Nagoya University, the mystery appears to be solved—and it may help better our understanding of vertebrate evolutionary history.
To identify and catalogue frogfish motor neurons, a team led by bioagricultural professor Naoyuki Yamamoto first injected tracers into frogfish spinal cords—specifically the ventral horn, which controls and regulates swimming movements. Once in place, researchers could visualize and observe which motor neurons lit up during what activities, such as bobbing the illicium to attract prey. Afterwards, they also conducted similar tracer injection experiments on white-spotted pygmy filefish to compare neurological activity between the two species. The results, documented in the team’s recent study published in the Journal of Comparative Neurology, surprised them.
“This is an extremely rare case,” Yamamoto said in a statement.
Researchers discovered that motor neurons responsible for a frogfish’s illicium reside in its upper back, also known as dorsolateral zone. Interestingly, however, those neurons are completely separate from the ones which issue commands to the fish’s other three dorsal fins, located in the lower side of its ventral horn, called the ventrolateral zone. In contrast, all of a filefish’s dorsal fin motor neurons are located solely in its ventrolateral zone.
“Their location was shifted [during evolution] to serve a role completely different from their original function,” Yamamoto said, adding that the “unprecedented” find may have implications that potentially reach far beyond frogfish.
“While we, as land animals, do not have fins, our forelimbs and hindlimbs are similar to the pectoral and ventral fins in the light of their distribution in the spinal ventral horn, and our ancestors also once had dorsal fins,” he said, explaining that motor neuron group organizations are similar across vertebrate animals. Because of this, unique neuron arrangements may exist in several other species that exhibit specialized behaviors.
“Our study provides a new point of view on motor neurons, and we hope it prompts similar studies in other species that lead scientists to understand the rules that govern their organization,” said Yamamoto.
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