Fish Feels Its Way With Fins


Cartoon animators have got it right — fish really do use their fins for more than just swimming. Or at least bluegill sunfish (Lepomis machrochirus) do. They use their pectoral fins — those are the ones on the sides — to reach out and touch obstacles, helping them to navigate. That’s the finding from a new study from two Harvard biologists published in the Journal of Experimental Biology.

Bluegill sunfish may be familiar because they’re the a popular sportfish in the eastern United States. Throw out a hook with a worm into the local lake, and you might just catch yourself a bluegill supper.

These fish live in the littoral zone, close to shore, an area that can be full of vegetation that the fish have to navigate through. The fish have decent vision, and they also have help from their lateral line, a special organ that helps detects movement and vibration in the water.

But maybe the fish have another way to help them spot obstacles in the water, the Harvard team thought. The researchers began by creating an obstacle course for the fish, with evenly spaced acrylic tubes in a big tank. Then they filmed fish swimming through the tank. To cut off the fish’s vision, they turned out the lights. To disable its lateral line, they treated the fish with cobalt chloride.

The scientists had hypothesized that only when its others senses were disabled would the fish reach out with their fins. To their surprise, however, the fish used their fins to touch the acrylic tubes even when they could see and/or use their lateral line. The bluegill sunfish would use their pectoral fins to propel themselves forward between posts, but they would also wrap those same fins around the obstacles.

“Fish did not appear to push off of the posts to change heading or move forward. Forward motion did not initiate until the beat following the tapping contact with the obstacle posts,” the researchers write.

The pectoral fins act a bit like our arms while we’re swimming, propelling and steering a fish through the water and helping it to feel its way through the weeds.

Image from U.S. Fish and Wildlife Service, via Wikimedia

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