2019 | Îlot Maître, New Caledonia

Graphic tuskfish (Choerodon graphicus) anvil striking

Life’s tough. Especially the external bits. Whether it’s a lobster’s exoskeleton, a coconut’s husk, or rhinoceros dermal plates, life goes to great pains to keep internal parts protected from the harsh extremes of the outer world, and especially from things that might want to eat them.

In response, predators have stepped up their game. Usually this involves building greater strength in jaw or claw, but more rarely animals enlist rugged parts of their environment to lend a hand. This post examines one such response: various tropical species of tuskfish smash open tough-shelled molluscs on rocks or coral.

A quick aside: it’s debatable whether this behaviour is strictly ‘tool-use’, because the fish holds onto its prey, not a separate piece of rock or coral. That’s not an issue I’ll go into here, and you can see my post on tool use definitions if you want to pull on that thread. But to give the tuskfish its due, it’s a fish—there’s not a lot of ways it can hold onto anything.

Violence in paradise

The waters off the coast of New Caledonia are warm, shallow and clear. Protective coral reefs ring the shore and nearby islands, including Îlot Maître, some 5km from the capital, Noumea. (My own fieldwork history on New Caledonia involved wild crow tool use, not fish, but I can attest to the serenity of a quick post-work dip in these crystal seas.)

In the November 2020 issue of the Journal of Fish Biology, Kimberley Pryor gives an account of her search for tuskfish mollusc-smashing. Her initial approach was archaeological—swimming some 100m offshore from Îlot Maître, she surveyed for isolated rocks or coral formations that were surrounded by scattered and broken shells. She found two in particular that met her criteria, both rocks about 20cm in size, protruding from the sandy seabed and only a few metres apart:

The discarded shell fragments were a clue that these were regular venues for the kind of violence Pryor was seeking. She named these anvils Rock 1 and Rock 2 (a and b in the image), and then she waited.

Over the course of five days, she witnessed 16 tuskfish come to these rocks, each with a shelled mollusc clutched in its slightly-protruding teeth (hence the name). The fish collect their prey from hiding places in the sand around the anvil site, using their pectoral fins and mouth to clear away the overlying debris.

Once the prey is captured, the tuskfish makes a direct line for its favoured anvil. It positions itself alongside the rock, then makes a sudden, full-body twisting motion that rapidly turns its head towards the anvil. It releases the mollusc at just the right time to propel it against the rock, chipping the shell surface. The fish can do this on either their right or left sides—they are ambidextrous.

The graphic tuskfish (Choerodon graphicus) is not a particularly small fish: those observed by Pryor were 40-60cm in length. They are able to generate considerable force. Still, it takes on average 7 strikes over almost a minute and a half before the shell is sufficiently damaged that the fish can get at the mollusc meat inside.

Pryor recorded the same behaviour in 2018, as you can see in her video below. The energy of each strike is clear from the sharp sounds captured by her underwater camera:

It doesn’t seem at this stage that the anvil-striking behaviour is learned by one fish watching another, the way that humans typically learn. Instead, there appears to be an innate or genetic component to it.

The genetic argument gains support if we look at the few other known cases of fish that strike molluscs against their underwater environment. For example, in 2006, researchers watched a black spot tuskfish (Choerodon schoenleinii) break a cockle shell against a rock in the Keppel region of Australia’s Great Barrier Reef. And in 2009 an orange-dotted tuskfish (Choerodon anchorago) smashed a bivalve against a rock in Palau, having first extracted it from the sand by fanning away debris with its pectoral fin.

In each case, and despite living thousands of kilometres apart, these related species perform essentially the same movements as they crack open their prey. Just compare this video from the Palau report with the New Caledonian one above:

There’s even evidence that this activity can extend beyond shelled molluscs, with tuskfish subjecting other prey to the same harsh treatment. By chance in 2014, a team from the University of Queensland caught a blue tuskfish (Choerodon cyanodus) on one of their remote underwater cameras in the shallows off Heron Island, again on the Great Barrier Reef.

Instead of a shell, the fish looms into view carrying a young green turtle (Chelonia mydas). For those who have greater sympathy for turtles than they do for molluscs, I’d suggest not watching the film itself, which as you can imagine isn’t kind to the prey. For the curious, you can see it and the brief report here.

And to give the general idea, here are predator and prey in a still image from the video:

A break-out star

People worldwide film animal behaviour, but there’s one clear signal that a critter has really hit the big time: it appears on a David Attenborough-narrated wildlife documentary. And that’s what Percy the orange-dotted tuskfish managed in the first episode of the spectacular 2017 program Blue Planet II.

I covered this for Nature Ecology & Evolution at the time, but the short version is that Percy had a favourite coral outcrop that the show’s producers nicknamed ‘the castle’. Just like around the New Caledonian rocks, the interior of this raised, ringed coral was covered in shell fragments resulting from past destruction.

Here’s Percy with its next victim:

Instead of a rock, Percy used a particularly tough section of the coral wall as its target. But everything else again revealed that genus-typical behaviour: using its pectoral fins to excavate and clear away sand, as well as picking up and moving pieces of coral that might hide a tasty mollusc beneath.

To finish this story then, I’ll leave you in the safe hands of the greatest television naturalist, Sir David Attenborough. Remember to keep an eye out for Percy’s search behaviour, the repetitive nature of its strikes (played for a few comic effects by the BBC team), and the tell-tale debris field that marks Percy’s castle as an arena of mollusc despair:

Sources: Pryor, K. (2020) Frequent visits to various anvils during tool use by the graphic tuskfish Choerodon graphicus (Labridae). Journal of Fish Biology 97: 1564-1568. || Pryor, K. & A. Milton (2019) Tool use by the graphic tuskfish Choerodon graphicus. Journal of Fish Biology 95: 663-667. || Jones, A. et al. (2011) Tool use in the tuskfish Choerodon schoenleinii? Coral Reefs 30: 865. || Bernardi, G. (2012) The use of tools by wrasses (Labridae). Coral Reefs 31: 39. || Harborne, A. & B. Tholan (2016) Tool use by Choerodon cyanodus when handling vertebrate prey. Coral Reefs 35:1069.

Main image credit: Pryor et al. (2020) Fig. S1 || Video 1 credit: Pryor & Milton (2019) || Video 2 credit: Bernardi (2012) || Second image credit: Harborne & Tholan (2016) || Percy image credit: @LucyHockingsBBC via Twitter.