Unraveling the Mystery of Anglerfish’s Deadly Fishing Trick

Anglerfish are noted for their unusual adaptations to severe settings and use of lures to attract prey. Researchers from Nagoya University in Japan revealed a specialized motor neuron population in the first dorsal fin employed for “fishing” activity in frogfish (a subgroup of anglerfish).

Motor neurons moved to a new location in the central nervous system as the first dorsal fin changed from being a swimming and buoyancy help to a hunting gear. Knowing how motor neurons shift positions in response to changes in their functions has consequences for our knowledge of vertebrate evolution. The Journal of Comparative Neurology published the study.

Frogfish, a subgroup of anglerfish, are noted for their ability to blend in with their surroundings, allowing them to ambush prey like small fish and crustaceans. Frogfish have four fins on their back called dorsal fins, which play a vital part in the fish’s life cycle. The dorsal fins in the center of the fish’s body are utilized for threatening behavior, while the fin at the back is used for stability and propulsion while swimming.

The front dorsal fin (known as the illicium) is distinctive because it is rod-shaped and bears a lure (named eska) on the distal tip that resembles a clam worm.

Frogfish use the illicium in the same way that an angler would wave bait. The victim fish is tricked into approaching the eska, assuming it is food. When it bites, the frogfish strikes and consumes the prey fish in one gulp.

Professor Naoyuki Yamamoto of Nagoya University’s Graduate School of Bioagricultural Sciences led a study team that examined the neurons that allow for this unusual behavior. They identified motor neurons in frogfish that move the illicium and enable fishing activity, called ‘fishing motor neurons,’ and compared them to those in other dorsal fins.

The researchers employed tracer injections to investigate this topic. Fish use the ventral horn of their spinal cord to control their swimming movements. Researchers visualize motor neurons in the ventral horn of the spine using a tracer.

Yamamoto and his team revealed that the illicium’s motor neurons are positioned in the dorsolateral zone (upper back), as opposed to the second, third, and fourth dorsal fin motor neurons, which are placed in the ventrolateral zone (bottom 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.”

Naoyuki Yamamoto, Professor, Nagoya University

To investigate species differences, researchers studied motor neurons in frogfish and white-spotted pygmy filefish. Unlike frogfish, the filefish uses its first dorsal fin to frighten other fish and predators. The motor neurons of the filefish were discovered in the ventrolateral zone of the ventral horn, not the dorsolateral zone, which is identical to the frogfish's second to fourth dorsal fins.

Yamamoto added, “This comparison with other species suggests that motor neurons migrated during the evolution of their function. 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 his research can help inform human evolution as well.

“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. 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,” Yamamoto concluded.