Researchers have discovered a unique group of motor neurons in anglerfish that control the first dorsal fin, which is utilized for hunting. As this fin transformed from a tool for swimming and buoyancy into a specialized weapon for catching prey, the motor neurons relocated within the central nervous system. These findings could enhance our understanding of evolutionary processes in vertebrates, including humans.
Anglerfish are remarkable for their adaptations to extreme environments and their use of lures to catch prey. A team from Nagoya University in Japan has discovered a distinct population of motor neurons in frogfish, a subgroup of anglerfish, that are dedicated to the “fishing” function of the first dorsal fin. As the fin evolved from aiding in swimming to hunting, the position of these motor neurons in the central nervous system changed. Gaining insight into how motor neurons migrate as their roles evolve can deepen our comprehension of vertebrate evolution, including our own species. This research appears in the Journal of Comparative Neurology.
Frogfish are known for their remarkable camouflage, which allows them to ambush prey like small fish and crustaceans. They have four dorsal fins along their backs that are crucial in their life cycle. The middle dorsal fins are employed for threatening behavior, while the fin at the back serves both for stability and for propulsion during swimming. The foremost dorsal fin, called the illicium, is distinctive with its rod-like shape and a lure at the tip that resembles a clam worm.
Frogfish utilize the illicium as if it were bait, enticing prey fish to approach under the false impression that it is food. As soon as the prey strikes, the frogfish swiftly captures it in a single gulp.
A research team led by Professor Naoyuki Yamamoto from the Graduate School of Bioagricultural Sciences at Nagoya University aimed to investigate the neurons involved in this extraordinary hunting technique. They identified a group of motor neurons in frogfish responsible for moving the illicium, which they termed ‘fishing motor neurons’, and compared these with motor neurons from the other dorsal fins.
The research group utilized tracer injections to study this region. Fish coordinate their swimming movements via the ventral horn of their spinal cord. By using a tracer, researchers can effectively visualize the motor neurons located in this part of the spine.
Yamamoto and his colleagues found that the motor neurons for the illicium are situated in the dorsolateral zone (upper back) of the spinal cord, whereas the motor neurons for the second, third, and fourth dorsal fins are located in the ventrolateral zone (lower side).
“It’s quite rare to see motor neurons associated with the illicium that started as dorsal fin motor neurons but relocated to fulfill a different role,” Yamamoto noted.
In their research, they also compared the motor neurons of frogfish with those of white-spotted pygmy filefish to highlight species-specific differences. Unlike frogfish, filefish use their first dorsal fin primarily for threatening behavior towards competitors and predators. Their motor neurons were found in the ventrolateral zone of the ventral horn, similar to the other dorsal fins in frogfish.
“This comparison indicates that motor neurons may have migrated during the evolution of their roles,” Yamamoto stated. “The motor neurons responsible for fishing behavior originated as dorsal fin neurons but transitioned to a new location in the central nervous system. This is an unprecedented finding, and we are eager about its broader implications.”
Yamamoto also suggests that these insights could relate to human evolution. “Although land animals like us do not possess fins, our forelimbs and hindlimbs exhibit similarities to the pectoral and ventral fins regarding their distribution in the spinal ventral horn, and our ancestors once had dorsal fins,” he explained. “Motor neuron organization is consistently comparable among various vertebrate groups. Many vertebrate species display highly specialized behaviors, and our study offers a fresh perspective on motor neurons, potentially inspiring similar research in other species to help unravel the principles governing their organization.”
