The Nutcracker Suite: Fish that Use Tools to Crack Open Shells

In a paper we recently published in the journal Coral Reefs, we reported the use of tools by three species of fish in the family of wrasses. The paper describes how all three species use corals as anvils to break open and eat juicy sea urchins.

Wait…what? Fish using tools?

Let’s back up a bit.

There is a general sense that the ability to use tools is an exclusively human domain. Our ability to use tools separates us from other species — we are graspers, manipulators and engineers, and primarily because of our opposable thumbs, we have gone from wielding clubs to building spaceships in the blink of an evolutionary eye. Tools are now integral to our lives and identities, to the extent that some have argued that we are already cyborgs — it is difficult to tell where we end and our tools begin.

Impressive as this is, we do not have a monopoly on tool use. Chimpanzees use twigs to draw out termites from their mounds and throw rocks to defend themselves. Similarly, gorillas have been seen using sticks to measure water depth before venturing in. It has long been known that many primates are quite adept at using tools. In some ways, this is understandable since these are our close cousins, who are termed “reasonably intelligent” and have opposable thumbs. However, if we look further, we can find animals using tools even without grasping digits. For instance, sea otters routinely use rocks as hammers to crack open shellfish, and mongooses break eggs by hurling them at rocks and hard structures.

The real question to ask then is what exactly is tool use? In all the examples above, animals use tools in ways similar to humans — using a hand which picks up and wields an external object and uses it to attain some immediate goal. Most common definitions of tool use are heavily primate-centric, based on how humans and higher apes typically use tools. Does this mean that animals lacking arms or similar grasping appendages are incapable of tool use? Not quite. It turns out hands aren’t necessary at all! Vultures throw rocks onto eggs to break them open, crows fashion leaves to extract insects from crevices, and dolphins tear up sponges to cover their rostrum for protection while foraging. All with their beaks.

What is common to all these examples is that tool-using animals are generally regarded as “higher” or more intelligent species. It seems natural to link the ability to use tools with an animal’s cognitive abilities, and it is easier to accept that mammals and birds, evolutionarily closer to us, having higher cognition and intellect, are capable of using tools. However, there are plenty of examples of tool use in animals that aren’t conventionally considered “higher” animals. Octopuses are known to carry stones around to smash open otherwise inaccessible clams. And while octopuses are celebrities in the invertebrate world for their displays of ingenuity, several other invertebrates also show tool use. Observers have seen wasps pounding a pebble on the ground to aid in fashioning a flattened opening to their nest, and ants are known to drop grains of sand onto guard bees to help them raid beehives for pollen. It seems that wherever we look on land, animals are industriously and inventively finding things in their environment to use as tools.

Things change when you get to the water. Water is a viscous medium, and objects behave very differently in water because of their buoyancy. Without gravity to assist you as on land, dropping objects onto things will not work to break open stuff. Yet aquatic animals find ingenious ways to manipulate their environment. Archerfish, for example, use the water itself as a tool by squirting it from their mouths to dislodge insects and other prey above the water’s surface. To do this, they must account for the change in refractive indices between different aquatic environments and calculate the angle at which they must squirt the water. They get pretty good at this. They are also capable of shooting at moving prey. While archerfish are well-known, examples of tool use among fish are relatively rare. One reason is that they are more difficult to observe. And the general perception of fish as less intelligent has also biased tool-use studies.

Coming to our observations, we did not go looking for tool-using fish. We were interested in studying a small burrowing sea urchin (Echinostrephus molaris), whose numbers are high in the Kavaratti atoll in Lakshadweep, India. At such high densities, the urchin can cause significant erosion on reefs, and we were keen to know if this species had any natural predators that keep its numbers in check. For this, we placed multiple underwater cameras at different locations in the reef with an urchin in view. Although there were plenty of potential predators on the reef, we observed only a few that were interested in the urchin. The difficulty was navigating the urchin’s formidable defences, and most unsuspecting small fish would get a mouthful of spines in attempting to eat it. When we examined our videos, we were amazed to see a vibrant fish called the checkerboard wrasse (Halichoeres hortulanus) gingerly upturn the spiny urchin with its mouth to lift the animal safely in its jaws. It then swam with its prize to a nearby coral boulder and flung the urchin against the hard surface with perfect aim and angle to break off its spines. It repeated this action to break the urchin shell to get to the good stuff inside. This fish clearly used the coral as an anvil to clear the spines and break open the shell (test) to reach the soft interior.

If we thought this behaviour was unique to the checkerboard wrasse, we were in for further surprises. In subsequent videos, we observed two other wrasses, Jansen’s wrasse (Thalassoma jansenii) and crescent wrasse (T. lunare), doing the same thing — using anvils to break into urchins to eat them. This was exciting, given that this behaviour had never been scientifically reported for checkerboard and Jansen’s wrasses before. And while there was a report of tool use in a crescent wrasse, it had never been recorded using tools in the wild.

Despite there being over 27,000 fish species, the use of anvils has been previously reported in only about 18, and they all belong to the same family of wrasses (Labridae). The prey ranges from invertebrates like crabs, clams, and urchins to baby sea turtles. With our observations, this number increases to 20 species. What about this family of fishes makes them so adept at tool use? Is it brain size? In many primates and birds, tool use correlates with the relative sizes of different brain lobes. Is that true with fish as well? Using tools does not necessarily imply higher intelligence. However, the fact that our species were able to adapt their feeding strategies so quickly when provided with an entirely new food source is intriguing. It is difficult to shake off the conviction that these are highly intelligent fish. While we would like to believe this, we cannot help feeling that these behaviours are not more widely reported because we just haven’t looked enough. We observed these behaviours within a few days of putting out our videos. With more careful looking, we may well find tool use to be commonplace on the reef.

For now, there is a lot we do not understand about tool use by animals in the wild. But this little piece of serendipity has taught us the value of paying attention. Natural history is a dying tradition, and as scientists, we often find it difficult to lay down our measuring instruments and merely observe. The natural world still has a lot of surprises left to offer. The next time we put on snorkels and masks, we must look more closely. If fish can use tools adeptly, who knows what else they may get up to when we aren’t watching?