Unique motor control system of frogfish's specialized 'fishing rod' discovered

Characterized by their unique adaptations to extreme environments, anglerfish are known for using lures to attract prey. Researchers at Nagoya University in Japan have discovered in frogfish (a subgroup of anglerfish) a specialized motor neuron population in the first dorsal fin used for this “fishing” behavior. When the first dorsal fin evolved from a swimming and buoyancy aid to a hunting tool, motor neurons shifted their position in the central nervous system. Understanding the way motor neurons change location as their function changes has implications for our understanding of the evolution of vertebrates, including ourselves. The study was published in the Journal of Comparative Neurology. 

Frogfish, a subgroup of anglerfish, are known for their ability to blend seamlessly into their surroundings, helping them ambush prey such as small fish and crustaceans. Frogfish have four fins located on their back, known as the dorsal fins, that play important roles in the fish’s lifecycle. The dorsal fins in the middle of the fish’s body are used for engaging in threatening behavior, whereas the fin at the back is not only engaged in threatening behavior but also used for providing stability and propulsion when swimming. The front dorsal fin (known as the illicium) stands out because it has a rod-like shape with a lure (called eska) on the distal tip, which looks like a clam worm. 

Frogfish use the illicium like an angler waving bait. The prey fish is deceived into approaching the eska believing it to be food. When it bites it, the frogfish strikes and engulfs the prey fish in a single gulp.  

A research group led by Professor Naoyuki Yamamoto of the Graduate School of Bioagricultural Sciences at Nagoya University was interested in the neurons that enable this unusual behavior. They identified motor neurons in frogfish that move the illicium and enable its fishing behavior, which they named 'fishing motor neurons', and compared them with those of the other dorsal fins.  

To study this area, the researchers used tracer injections. Fish rely on the ventral horn of their spinal cord to regulate their swimming movements. By using a tracer, researchers can visualize motor neurons in the ventral horn of the spine. 

Yamamoto and his team discovered that the illicium's motor neurons reside in the dorsolateral zone (upper back), separate from the second, third, and fourth dorsal fin motor neurons, which are located in the ventrolateral zone (lower side) of the ventral horn.  

“This is an extremely rare case in which motor neurons for the illicium were originally dorsal fin motor neurons, but their location was shifted to serve a role completely different from their original function,” Yamamoto explained. 

Researchers compared the motor neurons of frogfish to those of white-spotted pygmy filefish to explore differences between species. Unlike frogfish, the filefish uses its first dorsal fin to threaten other individuals and predators. Motor neurons of the filefish were found in the ventrolateral zone of the ventral horn, not in the dorsolateral zone, similar to the second to fourth dorsal fins of the frogfish. 

“This comparison with other species suggests that motor neurons migrated during the evolution of their function,” Yamamoto said. “The motor neurons that perform fishing behavior were originally dorsal fin motor neurons but moved to an unusual location in the central nervous system. This is an unprecedented discovery, and we are excited about its implications.”  

Yamamoto believes that his findings can also inform human evolution. “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 explained. “The organization of different groups of motor neuron groups is similar among vertebrates. In vertebrates, there are several species with highly specialized behavior. 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.” 

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