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

How Scientists Figured Out Salmon Use Magnetism To Navigate

salmonToday I wrote about salmon navigation on NPR’s food blog The Salt:

After hatching in a freshwater stream, young salmon make a break for the ocean, where they hang out for years, covering thousands of miles before deciding its time to settle down and lay eggs in the same stream they were born.

So how do these fish find their way back to their home river?

According to one theory, it’s all about magnetism. When salmon are young, the theory goes, they imprint on the pattern of the Earth’s magnetic field at the mouth of their native river. Years later, when the salmon head back home to spawn, they home in on that pattern. In a study published Thursday in Current Biology, the scientists behind that theory now say they have evidence that’s exactly how the fish are navigating.

What got cut from the story is how the scientists made the connection:

The researchers determined that the fish must be using magnetism to navigate by studying sockeye salmon from the Fraser River in Canada. The mouth of that river is blocked from the ocean by Vancouver Island, meaning that the fish can’t directly swim back into the river from the Pacific; they have to choose a route either north or south of the island to get home.

That set up a natural experiment: If the salmon are navigating by the earth’s magnetic field, they should pick the route entrance that has a magnetic field pattern that most closely resembles the one at the mouth of the Fraser.

But it gets a little complicated because the earth’s magnetic field varies by strength and direction of the magnetic lines, and that field wiggles ever so slightly year to year — as evidenced by the changing location of the magnetic north pole.

So over time, the northern route is sometimes a closer match for the salmon, and vice versa. And it turns out that “fish migratory routes are following that wiggling” in the magnetic field, says lead author Nathan Putman of Oregon State University in Corvallis, which means they’re probably using it to find the exit off the Pacific highway and the way to their home river.

Image credit: Current Biology, Putman et al.

Why A Dung Beetle Wouldn’t Like City Living

51924_webOne of the downsides of living in a city is that they aren’t the greatest spots for stargazing. That’s kind of sad for urban dwellers, but it would be a big problem if we were dung beetles.

That’s because dung beetles of one species, Scarabaeus satyrus, navigate using the light of the Milky Way, say researchers from Sweden and South Africa in the journal Current Biology.

Dung beetles got their name because they feed on feces (they definitely count as one of the world’s more disgusting critters). Some species, like S. satyrus, collect their dung into a ball that they roll away from the dung pile, so they can save it for later. The direction that they roll their ball in isn’t too important, but a beetle does want to make sure it rolls it away from the pile and away from potential competitors.

During the day, the insects can use the sun for navigation, but at night it can get a bit more complicated. There’s the moon, of course, but the moon isn’t always out. And that’s when a secondary light source, like the Milky Way, becomes a beacon in the sky.

Image credit: Emily Baird/Courtesy of the University of Witwatersrand