While exploring Australia's Great Barrier Reef, professional diver Scott Gardner heard an odd cracking sound and swam over to investigate. What he found was a footlong blackspot tuskfish (Choerodon schoenleinii) holding a clam in its mouth and whacking it against a rock. Soon the shell gave way, and the fish gobbled up the bivalve, spat out the shell fragments, and swam off. Fortunately, Gardner had a camera handy and snapped what seem to be the first photographs of a wild fish using a tool.
Tool use, once thought to be the distinctive hallmark of human intelligence, has been identified in a wide variety of animals in recent decades. Although other creatures don't have anything quite like a circular saw or a juice machine, capuchin monkeys select "hammer" rocks of an appropriate material and weight to crack open seeds, fruits, or nuts on larger "anvil" rocks, and New Caledonian crows probe branches with grass, twigs, and leaf strips to extract insects. In addition to primates and birds, many animals, including dolphins, elephants, naked mole rats, and even octopuses, have shown forms of the behavior.
Tool-using fish have been few and far between, however, particularly in the wild. Archerfish target jets of water at terrestrial prey, but whether this constitutes tool use has been contentious. There have also been a handful of reports of fish cracking open hard-shelled prey, such as bivalves and sea urchins, by banging them on rocks or coral, but there's no photo or video evidence to back it up, according to Culum Brown, a behavioral ecologist at Macquarie University in Sydney, Australia, and a co-author of the present paper, to be published in a forthcoming issue of Coral Reefs.
The tuskfish caught on camera was clearly quite skilled at its task, "landing absolutely pinpoint blows" with the shell, Brown says. A scattering of crushed shells around its anvil rock suggests that Gardner didn't just stumble upon the fish during its original eureka moment. In fact, numerous such shell middens are visible around the reef. Blackspot tuskfish, members of the wrasse family, are popular food fish, so it's surprising that its shell-smashing behavior has remained unknown, Brown says. "My feeling is that when we go out and really look for it, it'll turn out to be common."
"I absolutely loved it," says ethologist Michael Kuba of the Hebrew University of Jerusalem of the finding. Last year, Kuba and two colleagues documented stingrays in a laboratory forming jets of water with their bodies to flush food out of a pipe. But solid external objects like rocks are harder to dismiss as tools than water jets, Kuba says, and examples from the wild avoid concerns about whether a behavior elicited in the lab is "natural."
Primatologist Elisabetta Visalberghi of the Institute of Cognitive Sciences and Technologies in Rome is less convinced. Visalberghi, who documented the hammer-wielding monkeys, adheres to a stricter definition of tool use that requires the animal to hold or carry the tool itself, in this case the rock. "The form of tool use described [in tuskfish] is cognitively little demanding and present in a variety of species. Often it has been labeled as proto-tool use because the object used to open the shell is still, fixated to the sea bottom, and not portable as stone tools used to crack open nuts by chimpanzees or capuchin monkeys are," she writes in an e-mail. Seagulls dropping shellfish onto hard surfaces to crack them or lab rats pushing levers to get rewards would join tuskfish in the category of proto-tool—but not true tool—users.
Brown acknowledges that exactly what constitutes tool use is controversial. But he argues that it's not logical to apply the same rules to fish as to primates or birds. For one thing, fish don't have anything but their mouths to manipulate tools with, and for another, water poses different physical limitations than air. "One of the problems with the definition of tool use as it currently stands is it's totally written for primates," he says. "You cannot swing a hammer effectively underwater."