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newsJim Capaldi — license licensed under the CreativeCommons Attribution 2.0 Generic licenseWhy and how didgills really evolve?Freshwater stingrays usewater as a “tool” in problem-solvingtests, scientistshave demonstratated for thefirst time. Using a plastic pipewith one end sealed and containinghidden food, researchersobserved how the fish overcamethe challenge of gettingthe meal from the containerStingrays use toolsto solve problemsStingray tool use showcasesits cognitive abilitiesIn a recent study just publishedin the in the scientific journalAnimal Cognition a team ofresearchers tested the ability ofcaptive South American stingraysPotamotrygon castexi tosolve problems, by setting thema series of underwater tasks. Allfive test subjects quickly learnedto use jets of water as a tool toextract a meal of hidden foodfrom a plastic pipe. It revealsthat the fish, once thought a“simple reflex animal”, has cognitiveabilities to rival birds, reptilesand mammals, scientists say.The experimental protocol,which gave the animals the opportunityof correcting a wrongvisual cue decision, resulted infour out of five subjects correctingan error rather than makingan initial right choice. One of fivesubjects reached 100 percentcorrect trials in the visual discriminationtask.The ability to use water as anagent to extract food from thetesting apparatus is a first indicationof tool use in batoid fishes.Freshwater stingrays, found inmany tropical waters such asthe Amazon River, are related toocean stingrays. Like sharks, theyhave skeletons made of cartilage,rather than bony skeletonsof less closely related teleostfish. In the past, scientists haveassumed that such cartilaginousfish have limited cognitive abilities,in part because they havebeen difficult to study, says DrMichael Kuba from the HebrewUniversity of Jerusalem in Israelwho undertook the latest study.The stingrays not only performedthe tasks well but also demonstrateda range of problem-solvingstrategies.“Tool use in fish is far from anythingseen in birds or mammals,“explained Kuba. He said thatthe definition of tool use, usingan agent to achieve a goal,was set by cognitive scientist DrBenjamin Beck in 1980.The stingrays meet this definitionby using water as a tool,manipulating their bodies to createa flow of waterthat moves foodtowards them. ■Biologists have cast doubton the long-held theorythat fish gills evolved primarilyfor the purpose ofbreathing.A new experiment suggests thatit is likely that fish evolved gills forthe primary purpose of regulatingthe chemicals in their bodies.Gills usually consist of thin filamentsof tissue that have a highlyfolded surface to increase surfacearea. A high surface areais crucial to the gas exchangeof aquatic organisms, as watercontains only 1/20 parts dissolvedoxygen compared to air. The filamentscontain blood by whichgases are exchanged throughthe thin walls. Oxygen is carriedby the blood to other parts ofthe body. Carbon dioxide passesfrom the blood through the thingill tissue into the water.Clarice Fu, a zoologist from theUniversity of British Columbia inCanada, and colleagues whostudied the development of gillsin rainbow trout larvae found, asthe larvae matured, theirgills developed the abilityto regulate thechemicals in theirblood earlier thanthey began to takeup oxygen.The team measuredthe uptakeof ions across thegills. Ions, which arecharged chemicalparticles such assodium, are necessaryfor the body’scells to function, butthey become toxicif their levels in theblood become toohigh. To maintain thisdelicate ion balancein their blood, fishexchange these ionswith the surroundingwater through their gills“In freshwater fish, like rainbowtrout, they tend to lose ions fromtheir blood to the water, becausethe ion concentration in blood isgreater than that of freshwater,”Fu told the BBC.“When the gills are still immature,a significant portion of ionuptake occurs at the skin. As thefish get older and the gills mature,[this] cangradually shiftto... thegills,” Fuadded.“We found that ion uptake shiftedfrom the skin to the gills earlierthan oxygen uptake. This led us topropose that the gills are neededfor ion regulation earlier than theyare needed for oxygen uptake.”■As the larvae of rainbow troutdeveloped, the pressures drove ionexchange to the gills before the animalsstarted to breathe through theirgills, so the same thing mayhave happenedas the fishevolved6 X-RAY MAG : 34 : 2010 EDITORIAL FEATURES TRAVEL NEWS EQUIPMENT BOOKS SCIENCE & ECOLOGY EDUCATION PROFILES PORTFOLIO CLASSIFIED