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Learning About Lateral Line Sensitivity from the Lowly Toadfish

Although slandered as “some of the ugliest and laziest fish known to inhabit the waters of the northeast*,” oyster toadfish have made significant sacrifices on behalf of humanity. Because their sense of balance functions like our own, toadfish are used in studies of motion sickness, dizziness, and diseases that affect equilibrium. These hardy fish even orbited Earth with astronauts as illustrious as former Sen. John Glenn, for studies of how balance adjusts to changes in gravity.

Graphic showing position of lateral lines on a Toadfish.

The lateral line has two components: 1) The lines on the head forming the anterior lateral line system. 2) The lines on the body and tail forming the posterior lateral line system.

Toadfish (Opsanus tau, a.k.a. the ugly toad) have also been integral to understanding a sensitivity that no human possesses — the ability to detect subtle changes in water pressure through a lateral line system (see image). Fish use their lateral lines to detect prey along with their eyes, noses, and inner ears.

Sea Grant-funded researcher Allen Mensinger, associate professor with the University of Minnesota Duluth Department of Biology, and his colleagues recently discovered that the nerves of the anterior lateral line on the toadfish’s head begin firing messages to the brain when prey approaches within half a body length. This is a much shorter distance than conventional wisdom and previous research suggest it should be.

Their findings, published in the Journal of Experimental Biology, are of interest to scientists and anglers alike.

“Older studies using different methods led us to think that the lateral line system senses prey swimming within a few body lengths,” said Mensinger. “Our research, which expanded our ability to monitor fish in more natural circumstances, certainly reduced the range of the lateral line as we knew it.”

Languorous bottom-feeders though they may be, oyster toadfish have become the fishy equivalent of a lab rat at the Marine Biological Laboratory in Woods Hole, Mass., where Mensinger and his colleagues conducted their work. Oyster toadfish have a simpler nervous system than mammals. Also, their large flat heads allow easier access to sensory end organs.

Mensinger didn’t intend to focus on toadfish, but the experimental advantages of the saltwater species were too great a lure.

“Species like walleye and steelhead are more difficult to work with,” said Mensinger. “But as we continue to refine our electrode and telemetry equipment, we’re anticipating opportunities to include fish from Lake Superior’s waters in our research.”

For their experiments, Mensinger, Lucy Palmer (a Sea Grant graduate assistant), and their colleagues broke through barriers of traditional methods for studying lateral line activity. Instead of drugging a fish and noting its response to vibrating spheres, they developed methods to watch nerve activity in an alert, free-swimming fish. This was accomplished through an implantable, rechargeable microwire electrode and telemetry tag.

“Having the ability to observe pulses through the nervous system in free-swimming fish opens new avenues for understanding fish behavior such as home stream migration and schooling,” said Mensinger.

Diagram of the arena used to conduct studies using the Toadfish.

The anterior lateral line of a toadfish begins responding to the presence of a killifish at about half a body length. A killifish needs to swim closer than about a third of a body length to fully engage the nerves of this system.

To stage predator-prey encounters, the researchers built an arena and included a PVC pipe for habitat. Electronics were integrated into both the pipe (where the predatory toadfish often lurked) and arena to allow for monitoring and recharging of the toadfish’s telemetry tag. A killifish served as prey. By monitoring the activity of microwired nerve fibers via computer and video, the researchers could determine when the toadfish detected the killifish.

Results suggest that the wave of water produced when a prey fans its fins is like a whisper… one that can only be sensed when it is generated within less than 4.7 inches, and only really understood at a range closer than 3.2 inches.

In a related article, Palmer and Mensinger established that tricaine (MS-222), a common anesthetic used on aquatic organisms for over 80 years, temporarily deadens a toadfish’s anterior lateral line nerves. After the anesthesia is withdrawn, the nerves recover within about 90 minutes. However, light anesthesia does not have a significant effect, suggesting that lateral line studies conducted with lightly sedated fish reflect normal responses. Their study, published in the Journal of Neurophysiology, is the first to quantify triacaine’s effects on neural sensitivity.

Mensinger is currently collaborating with Peter Sorensen, professor with the University of Minnesota Department of Fisheries, Wildlife, and Conservation Biology, on Sea Grant research to apply the neuro-technology to steelhead trout. They are interested in how this migratory variety of rainbow trout responds to odors that might be attracting them to their native streams.

To order free reprints of Palmer and Mensinger’s journal articles, look for JR 512 and JR 500 at Sea Grant’s journal reprints order page.

Low-Down on Lateral Lines

Fish and some amphibians have a sensory organ — the lateral line system — that other species have lost. Sometimes referred to as the “sense of distant touch,” lateral lines convert subtle changes in water pressure into electrical pulses similar to the way our inner ear responds to sound waves. Running lengthwise down each side of the body and over the head, these pressure-sensing organs help their owners avoid collisions, participate in schooling behavior, orient to water currents, elude predators, and detect prey.

Lateral lines are composed of neuromasts (hair cells surrounded by a protruding jelly-like cupula) that usually lie at the bottom of a visible pit or groove. These hair cells — the same sensory cells found in all vertebrate ears — convert mechanical energy into electrical energy when moved. Presumably, auditory and lateral line pathways evolved in close association since they share many features.

*Marine Biological Laboratory. (1998, October 16) More Toadfish in Space: NASA will Study Balance in Two Woods Hole Toadfish, a Senator and Six Astronauts in Upcoming Shuttle Mission. [News Release]

By Sharon Moen
April 2006

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