Three fish species that live in the Laccadive Sea, off the southwest corner of the Indian coast, are capable of using tools, we recently found.
All three species used live or dead coral structures as anvils to break the hard shells of sea urchins so they could get to the edible bits inside. The Jansen’s wrasse (Thalassoma jansenii) and the checkerboard wrasse (Halichoeres hortulanus) have never been reported using tools before, and this is the first documented instance of the moon wrasse (Thalassoma lunare) using tools in the wild.
Our study was published in the journal Coral Reefs in February. It adds to the growing number of reports of tool-use in animals. While the sophistication of human tool use, from flints to computers, distinguishes us from other animals, the use of tools itself is more common than scientists first imagined.
Reports of animals like chimpanzees using straws to draw water and crows fashioning leaves to extract insects from crevices are well known whereas reports of aquatic animals using tools have been far less common. This is partly because they are much more difficult to observe and because of the perception that species like fish are ‘less intelligent’. So scientists have thought they are less likely to be capable of complex cognitive tasks.
Why do fish need tools?
As with many scientific discoveries, our observations were accidental. We were diving off the western coast of Kavaratti, Lakshadweep, to study the burrowing sea urchin (Echinostrephus molaris). These small urchins are an important agent of reef erosion. They dig deep burrows into the structure of a reef, weakening its overall framework. And they are abundant in Kavaratti.
We were interested in identifying if the urchin had any natural predators that could help regulate its population. To identify these predators, we placed multiple underwater cameras at different points in the reef to observe them undisturbed.
As we expected, few fish were able to prey on the urchin. E. molaris like most sea urchins is covered in an armour of spines. It also has a hard skeleton called a test that protects its internal organs, making it difficult for fish with unspecialised mouth parts to prey on it.
Yet in multiple instances, we found the Jansen’s, checkerboard, and moon wrasses were using makeshift tools around them to break open urchin tests, like people might break open a walnut shell. The cameras recorded most of these scenes when we weren’t actively watching them, although we were able to observe the last few in person.
How did the wrasses use tools?
Unlike primates, birds, otters, octopuses, and many other animals that scientists know are capable of wielding tools, fish have no hands, claws, or tentacles. This presents an obvious challenge for tool use, since being able to grasp and manipulate objects are key requirements for using tools.
Some fish have found innovative ways of using water itself as a tool. The archer fish, for example, spouts jets of water through its specialised mouth to shoot down prey above the surface, on land. To achieve this, especially with moving prey, the fish needs to have a sense of the angle and force of the jet and maintain perfect timing.
The three wrasses in our study don’t have specialised mouthparts like the archer fish. They make do instead with a series of specialised behaviours.
Eating a burrowing sea urchin presents two difficulties. The first is the pincushion of long, sharp spines that a fish has to navigate if it doesn’t want to get a mouth full of venomous jabs. The second is the hard test itself. In instance after instance, we observed how the three species of wrasses tackled these challenges.
Upon finding an urchin, a wrasse would skilfully approach it from the side and use its snout to gingerly turn the urchin over with a series of pushes and jabs. The underside of the urchin has fewer and softer spines, allowing the wrasse to safely pick it up in its jaws. With the prize in its mouth, the wrasse would swim to a nearby hard coral and strike the urchin against it, dusting off the prickly spines. Even more strikes would break open the test as well.
Once the shell was cracked, the wrasse could eat the soft bits inside.
In this task, the wrasses used the same coral reef as a tool to achieve two different purposes.
Why does finding matter?
Scientists have previously observed anvil use in only about 18 fish species around the world, all belonging to the same family of wrasses, Labridae. The three wrasses in our story also belong to this family. Their prey includes invertebrates like crabs, clams, and urchins, and vertebrates like baby sea turtles. The fish’s sizes, methods of approach, handling of prey, and geographical locations vary, however.
These commonalities and differences raise the question: why is such tool-use so (relatively) common only in the Labridae family of wrasses? As with primates, does this ability depend on brain size? Or is it simply the case that we don’t have enough observations from other families of fish?
Our study raises but doesn’t answer these questions. What it does, however, is reinforce the need for more rigorous observations of the natural world. As naturalists spend more time in the water, they are observing more tool-use in fish. Often these observations are incidental, yet documenting them carefully is critical so we can ask larger questions about habitat use by fish, the development of animal intelligence, and predator-prey interactions, among others.
Rohan Arthur is a senior scientist working with the Oceans and Coasts Programme of the Nature Conservation Foundation. Siddhi Jaishankar is a research assistant at the same programme, working on coral reef erosion by sea urchins in Lakshadweep, and other projects involving seagrasses and dugongs in the Andaman Islands.