Schools of small fish follow them for company or shade, but sharks, less friendly, have on occasion chomped on their elongated bodies. These new arrivals to the underwater world are small submersible robots known as gliders because they thrust themselves through the water not with propellers but by simply changing their buoyancy. Thanks to this neat trick, gliders consume just a trickle of power and can remain at sea for several months at a time, surfacing only to get a GPS fix and beam data to satellites. A mission using a conventional autonomous underwater vehicle (AUV) lasts only hours.

Now it seems gliders have caught the attention of some other big fish. This month, the U.S. Navy plans to announce the winner of a contract for 154 gliders, plus spare parts, launch-and-recovery equipment, and monitoring systems. The order, valued at tens of millions of dollars, sent defense contractors racing to gobble up the leading glider technologies.

In June, iRobot, in Bedford, Mass., best known for its Roomba vacuum cleaner and bomb-disposal PackBot, became the exclusive licensee of Seaglider, developed at the University of Washington, in Seattle. In July, defense industry giant Teledyne Technologies acquired Webb Research, in East Falmouth, Mass., creator of the Slocum glider. General Dynamics had earlier subcontracted Bluefin Robotics, in Cambridge, Mass., a licensee of the Spray, a glider jointly developed by the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution.

The Navy solicitation, part of a larger program called Littoral Battlespace Sensing, Fusion, and Integration, involves using fleets of gliders to gather data on ocean currents and on acoustic properties that may affect military sonar systems. The Navy deal ”got a lot of attention because right now people are buying gliders in ones and twos and threes, so it's a big increment,” says Russ Davis, an oceanographer at Scripps, which is part of the University of California, San Diego. Gliders are enticing, he says, because of their relatively low cost: a fully equipped unit sells for about US $100 000 and can go for six months; a data-gathering mission using a ship can cost over $30 000 a day.