Cooperative Hunting in Moray Eels and Grouper Fish

Read time: 5 minutes

Symbiotic relationships are a fascinating occurrence in the animal kingdom, and can be a sight to behold in species deemed the most unlikely of allies. For most of us, when we think of mutualistic inter-species relationships we conjure images of remora fish hitchhiking on sharks, oxpecker birds stationed atop hippos or perhaps bees buzzing between flowers. In short, interactions whereby one species services another for the promise of a meal. You might even remember exciting renditions of terrestrial mammalian carnivores forming inter-species hunting groups, like the fabled tales of wolf and hyena coalition, whereby both species set aside the usual enmity to shake down prey together in the middle east.

Mutualism is the most common symbiosis, where both or all species benefit and is often done by exchanging skill sets. Cooperative hunting is a form of this, and is an ongoing source of fascination for researchers. The division of labour in a cooperative hunting context requires impressive social organisation skills, and we see it among well-known social predators like lions, chimpanzees, hyaenas, grey wolves and African wild dogs. Every group member must know their role and anticipate the actions of their teammates. Pretty impressive when you consider the degree of self-awareness needed to conduct such organised hunts.

But what about fish? Do they possess the cognitive processes necessary for such complex co-ordination? Well, apparently so. And even more astonishing, with other species: grouper fish and giant moray eels work together to flush out prey! This inter-species relationship was first discovered by Redouan Bshary, a researcher from Switzerland who was studying cleaner wrasses in the coral reefs of the Red Sea, and published in 2006. Since then, footage of this unlikely partnership has been shared widely among scientists across the globe – leading to further discoveries on cross-species cooperation between groupers and other underwater predators. One such revelation was even featured in Blue Planet II, when cameraman Alex Vail went searching for grouper-moray interactions, but was astounded to instead find grouper fish cooperatively hunting with octopus: reminding us that marine life still holds so many secrets we have yet to uncover.

We all know what a pack of lions hunting together looks like, but a fish and an eel make for rather odd bedfellows. So what exactly does this underwater relationship involve? Luckily, we are able to answer this question through extensive video and field data. Most importantly, the grouper fish are the instigators: visiting the eels at their resting places and signalling that it is ‘time to hunt’, inviting them to a joint search. They do this in the strangest of ways – by shaking their head vigorously a few cm away from the moray’s own head (about 3 – 6 shakes per second!) and erecting their dorsal fin. They also sometimes find a resting eel and simply swim past repeatedly, or rest on the sand nearby – but eels are less likely to be roused by these techniques, and head shaking gets the best response. Groupers may well be the brains of the operation, as moray eels have never been observed to signal to groupers or initiate the hunts. In 58% of observations, the moray eels responded by leaving their crevice to swim off with the grouper fish through the reef, searching for prey together. Apparently, it is important that the grouper finds the right eel as not all are willing to help (or understand the invitation).

Marine life still holds so many secrets we have yet to uncover

Groupers also ask for help after failed solo hunts, when prey hides somewhere they cannot follow. They do this in 2 ways; firstly, by swimming directly to a giant moray eel that’s within 15m of the hiding prey, signalling and attempting to guide the eel back to the prey location. And secondly, by lingering above the hidden prey and giving wide head shakes, at the same time performing a head stand! This behaviour grabs the attention of not only nearby eels but also other predators, and in the original study 2 other predators (napolean wrasse and yellowlip emperor) approached and explored the crevice alongside moray eels. This behaviour is not so unheard of as within the animal kingdom many creatures signal to other predators where the food is, in the hopes of scavenging the remains – an interesting example being greater honeyguides, wild birds who deliberately lead humans to bee colonies. However, it’s a valuable discovery nonetheless as it adds to existing evidence that fish can hold cognitive abilities as impressive as those we often reserve for terrestrial animals – and reminds us ‘intelligence’ is not one single property but varies between species depending on their individual ecological challenges.

Groupers and moray eels have very different hunting styles; groupers are semi-benthic piscivors who hunt in open water, whereas moray eels slither through crevices in the reef and corner their prey there. Together, the two predators have complimentary hunting skills, and a coordinated hunt creates a multipredator attack hard to avoid: eels get the prey hiding in cracks and crevices where groupers cannot follow, and groupers get the prey that dart for open water. Interestingly, the animals do not share prey. Unlike lions at a carcass, when either the eel or the grouper catch a prey animal they swallow it whole – there is no divvying up of who gets what. However, this may not be a bad thing as researchers did not observe any overt aggression from the unsuccessful partner: in fact, it may even be positive, as it avoids antagonistic monopolisation of carcasses – in some unspoken understanding, they work together to flush out prey and whoever gets it, gets it. Either way, the relationship is definitely good news for the groupers who catch around 5 times as much prey when hunting together with the moray eel than when alone! Direct comparison with moray eels has not been possible as research was unable to conduct appropriate statistical analysis for solitary eels – as solitary eels simply remained within crevices. When hunting with groupers however (thus going in and out of crevices in coordination with grouper presence) moray eels were observed to have a successful catch rate roughly equivalent to their hunting partners. So this cooperative hunting relationship appears to be mutually beneficial to both animals!

Read more:

https://www.jstor.org/stable/10.1086/667653?seq=7#metadata_info_tab_contents
https://www.nationalgeographic.com/science/phenomena/2013/04/24/groupers-use-gestures-to-recruit-morays-for-hunting-team-ups/
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0040431
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4633842/
https://www.bbc.co.uk/mediacentre/mediapacks/blue-planet-ii/coral-reefs
https://www.cam.ac.uk/research/news/fish-as-good-as-chimpanzees-at-choosing-the-best-partner-for-a-task
https://www.ias.ac.in/article/fulltext/reso/024/05/0547-0559

Frustrated some papers aren’t free?This blog definitely doesn’t advise copying & pasting the paper’s DOI link (usually found at the top under the title) into SciHub. SciHub is technically an illegal ‘pirate’ site (equivalent to movie streaming sites except without the pop-up ads or viruses) which believes access to science should be available for everyone. University lecturers and students definitely never use this site to access free scientific papers… The SciHub domain changes frequently but the current one is Sci-Hub.tw. Don’t do it.

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