Robot fish shows how the deepest vertebrate in the sea takes the pressure | Science News

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Robot fish shows how the deepest vertebrate in the sea takes the pressure

Goo inside this snailfish keeps its flimsy body from being squashed

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7:00am, January 3, 2018

ROBO-FISH  Tests with this robot snailfish, whose shape came from photographs of a real fish snapped from different angles, reveal why the deep-sea fish has a mysterious goo in its body.

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It’s like having “an elephant stand on your thumb.”

That’s how deep-sea physiologist and ecologist Mackenzie Gerringer describes the pressure squeezing down on the deepest known living fish, some 8 kilometers down. What may help these small, pale Mariana snailfish survive elephantine squashing, says Gerringer of the University of Washington’s Friday Harbor Labs, is a body bulked up, especially at the rump, with a watery goo.

The snailfish family gets its nickname from the way some shallow-water species in thundering tides grip a rock with a little suction cup on the belly and curl up. “Quite cute,” Gerringer says, and maybe, if you squint, somewhat like a snail.

She and colleagues discovered the deepest fish in 2014 in the western Pacific Ocean’s Mariana Trench and described the newly named Psuedoliparis swirei November 28 in Zootaxa. To catch specimens, Gerringer and colleagues turned to extreme trapping. They weighted a boxy, mesh-sided trap with steel plates to sink it. It took about four hours to fall to the bottom.

The scientists baited traps with mackerel, which snailfish don’t eat. But the fish do eat the underwater amphipods that mob a mackerel feast. Remotely related to harmless garden pill bugs, trench amphipods clean mackerel to the bones, Gerringer says. “I certainly wouldn’t swallow a live amphipod after seeing what they can do.” A snailfish, however, has a second set of jaws at the back of its throat that render crustaceans safe to swallow.  

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For animals that live in such extreme pressures and temperatures (1° or 2° Celsius), snailfish don’t “look very robust … or very armored,” she says. “You can actually see the brain through the skull.”

Skimping on dense muscles and bones may improve snailfish buoyancy and save energy. These fish also lack air pockets that give a little lift to some other fishes, but that would get squashed to nothing so far down. Instead, the snailfish have inner deposits of a watery goo, more buoyant than muscles and bones and less compressible than air.

The goo also may aid swimming efficiency by offering a cheap shape improvement, Gerringer and colleagues proposed December 6 in Royal Society Open Science. To test the idea, she 3-D printed and motorized a robo-snailfish. Easier than catching a real one, Gerringer says.

A latex sleeve around the robot tail let her add or subtract water as an approximation of the gelatinous tissue. With an empty sleeve, the wide fish body pinches in to a thin tail, inviting vortices that cause drag. With this abrupt narrowing, robo-swimming proved a struggle. Filling the tail-sleeve to create a tapering rear let the robo-snailfish swim faster.

This goo is cheap tissue to grow, Gerringer says. It’s mostly water, one thing a fish living underneath eight kilometers of ocean has in abundance.

Citations

M.E. Gerringer et al. Distribution, composition and functions of gelatinous tissues in deep-sea fishes. Royal Society Open Science. Published online December 6, 2017. doi 10.1098/rsos.171063.

M. E. Gerringer et al Pseudoliparis swirei sp. nov.: A newly-discovered hadal snailfish (Scorpaeniformes Gerringer: Liparidae) from the Mariana Trench. Zootaxa. Vol. 4358, November 28, 2017, p. 161.   doi 10.11646/zootaxa.4358.1.7.

Further Reading

P. Kollipara. Microbes flourish at deepest ocean site. Science News. Vol. 183, Mary 4, 2013, p.19.

A. Witze. Descending to the Challenger Deep.  Science News Online. December 11, 2012.

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