WEST PALM BEACH, FLORIDA—Pinkish and transparent, hadal snailfish (Notoliparis kermadecensis, video above) look nothing like most of their fellow deep-sea dwellers, which tend to be dark, with big, sharp teeth and odd body shapes. What’s even more puzzling is that under their skin, the snailfish—which live in the world’s deepest trenches—have either a layer or pouch of gelatinous material that pours out if the skin gets punctured. Researchers had thought that perhaps the fish stored nutrients there, but in the eight snailfish species analyzed, the fluid is almost 98% water, researchers reported here this week at the annual meeting of the Society for Integrative and Comparative Biology. Instead, they noticed that with the fluid sac deflated, 17-centimeter-long snailfish looked more like tadpoles than fish, a body type that’s not very efficient in water. So the team used a 3D printer to make a snailfish model and outfitted it with a silicone tail, batteries, and a program to make it swim. Without the fluid-filled sac at the base of the tail, the robot could barely swim forward, but it did just fine with the sac attached, as the sac helped fill out and streamline the body so it tapered smoothly to the tail, they report. Food is scarce at 7000 meters where hadal snailfish live, so the researchers suspect that they evolved the fluid-filled sacs to replace otherwise energetically costly muscle that typically gives fish their svelte look.

(Video credit: Alan Jamieson/University of Aberdeen, U.K.)