In most bony fish, or teleosts, fin motor neurons are found on the sides (ventrolateral zone) of the underside (ventral horn) of the spinal cord. The motor neurons that control the illicium of frogfish are in their own cluster and are located in the dorsolateral zone. This is unusual in fish.
“The peculiar location of motor neurons in fish is undoubtedly related to the specialization of the illicium that favors fishing behavior,” the team said in a study recently published in the Journal of Comparative Neurology.
Fishing for answers
So what does this have to do with evolution? The white-spotted pygmy filefish may look nothing like a frogfish and have no built-in lures, but it is still a related species and may have something to tell us.
Although the filefish's first dorsal fin doesn't actually move (its main purpose is thought to be to deter predators by looking threatening), there are still motor neurons controlling it. Motor neurons for the first dorsal fin of filefish were found in the same location as motor neurons for the second, third and fourth dorsal fins of frogfish. In frogfish, these fins do not move much while swimming, but can appear threatening to a predator.
If the same types of motor neurons control the non-moving fins in both species, the frogfish has something extra when it comes to the function and location of motor neurons that control the illicium.
Yamamoto thinks that the unique group of fishing motor neurons in frogfish indicates that, as a result of evolution, “the motor neurons for the illicium [became] separated from other motor neurons” to end up in their own separate cluster, away from motor neurons that control other fins, as he said in the study.
What exactly caused the functional and location shift of motor neurons that give the frogfish illicium its function is still a mystery. How the brain influences their fishing behavior is another area to be explored.
Although Yamamoto and his team speculate that specific brain regions send messages to the fishermen's motor neurons, they do not yet know which regions are involved. They say more research needs to be done on other fish species and the groups of motor neurons. neurons that power each of their dorsal fins.
In the meantime, the frogfish continues to be its crazy self.
Journal of Comparative Neurology, 2024. DOI: 10.1002/cne.25674
