Thursday, March 19, 2015

19/03/2015: How fish feel

In a ground-floor laboratory just off Washington Square Park, in New York, a shoal of black polyurethane fish sat in an open cardboard box, Nicola Twilley writes for The New Yorker

According to their keeper, Leif Ristroph, an assistant professor of mathematics at New York University, the fish—each about five inches long and with the slight bug eyes of a juvenile—had been cast by a taxidermist in Florida from a mold of a "freshly euthanized" rainbow trout. Hardly sporting-trophy material to begin with, they had gotten somewhat dinged up while in Ristroph’s custody. 

Still clearly visible along their flanks and around their snouts, however, was the reason for their existence: a series of bumps called the lateral line. Biologists have known for years that these bumps, which are sensitive to water pressure, allow fish to orient themselves in their environments, navigating turbulent waters and chasing prey. But, until earlier this year, when Ristroph and two of his colleagues published a paper on the lateral line in the journal Physical Review Letters, one question remained unanswered: Why, in all but a very few species of fish, are the bumps arranged in more or less the same way?
The lateral line was first described in 1666, by the Danish scientist Nicholas Steno. While on a visit to the court of the Grand Duke of Tuscany, Steno was called on to dissect a large shark that had been caught near the town of Livorno. The episode is best known now for the epiphany that Steno had regarding “tongue stones”—that is, fossilized teeth, which, he realized, looking at the lifeless jaws before him, must once have belonged to living animals—but he also noted a series of small raised structures that ran lengthwise along the shark’s body. 

“What might be the function of so perforated a passage, I shall not yet venture to decide,” he wrote. 

“I have found it empty, without even a trace of slimy fluid.” 

If Steno had been able to look closer, he may have found his slimy fluid; by the start of the twentieth century, biologists had worked out that each bump in the lateral line contains a gelatinous blob that is embedded with tiny hair cells. The hairs wobble in response to changes in water pressure, and their movement is translated into an electrical impulse in a nearby nerve. (This system is the aquatic analogue of the human inner ear, whose hair cells allow us both to hear and to maintain our balance.)

What interested Ristroph was less the workings of the lateral line than its placement. For their study, he and his co-authors, James Liao and Jun Zhang, examined the fluid dynamics of a swimming rainbow trout. Since, as Liao pointed out, “you can’t put pressure sensors on a fish and have it behave normally,” they used the polyurethane stand-ins, mounting them one at a time in a water-filled mechanical loop surrounded by cameras. 

They added hundreds of small glass particles to the water, then covered the whole contraption with a large cardboard box, which provided a dark background for (and some eye protection from) a powerful green laser situated underneath. As water slipped past the stationary fish, the laser light bounced off the moving glass particles, turning them into tracers.

What the team found, after adjusting various settings in the tank and calculating the speed and trajectory of the tracers, is that the bumps in the lateral line correspond to the points on a fish’s body where the surrounding water exerts its greatest hydrodynamic pressure. In other words, fish have sprouted flow-sensing antennae exactly where they get the best signal, no matter the conditions. 

“They have figured out how to be as lazy as possible,” Ristroph said. The finding that an animal’s sensory apparatus is perfectly suited to its environment may seem like underwhelming news, but it is likely to have effects beyond biology and mathematics. 

“Usually, when something is the same across species, there’s a reason for it,” he said. 

“Presumably they have lots of other tricks that we don’t know.”

 Read the full article HERE.

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