Move over intense iguana pursuit, there's a new interspecies chase bonanza hot off the BBC showreel. And just like its popular predecessor, this clip is filled with interesting behaviour that can only be witnessed at the land-sea interface. Crabs "walking" on water? Eels "walking" on land? The end is nigh!


While filming for their new series "Blue Planet II", BBC camera crews encountered a group of lightfoot crabs (Grapsus grapsus) that must run the intertidal gauntlet each day to feast on tasty algae. For the crimson crustaceans, the path to a full belly involves passing a series of rock pools (that's "tide pools" for you Americans), where chain moray eels, common octopuses and other hungry predators await. 

It's worth noting that the BBC clip features multiple crabs (it does not show one continuous chase), a fact that Sir David Attenborough has been careful to point out. The broadcaster took some heat online earlier this year after a producer from "Planet Earth II" confirmed that some hunting scenes in the series were stitched together from several takes.

We've seen the ocean's eight-armed overlords taking to land to hunt crabs before – most notably in Western Australia – but it came as a surprise to witness the eels following suit in the BBC footage. The elongate animals don't attempt the fish-out-of-water stunt quite as often, but it turns out they're well adapted to the task. A protective mucous coating helps morays avoid injury as they slink over sharp rocks, and the animals can stay out of water for some time without risk. 

Over in Hawaii, green moray eels have been seen covering ground during both active hunts and scavenging missions:

Cristian Dimitrius, a director of photography who has worked on films like "Brazil: A Natural History", has also encountered the behaviour. In fact, he's even seen chain morays take this tactic to new heights.

During a film shoot in Fernando de Noronha, a group of islands along Brazil's Atlantic coast, Dimitrius managed to film the first sequence of the eels "jumping" from pool to pool in pursuit of lightfoot crabs.

"While the octopuses chase [crabs] underwater, moray eels catch them above water," Dimitrius wrote in a field journal update. "They observe the crabs' movements, and the moment they come within reach, the moray eels jump!"

Photographer and marine biologist Dr João Paulo Krajewski, who first noted the eels' impressive hunting displays during fieldwork in the region, led the Fernando de Noronha expedition. According to Krajewski, it took 30 days of careful waiting to nail down the shots. (Given the similarity of species featured in the recent BBC clip, we'd wager that the same location was involved.) 

"Tide pools' harsh and dynamic environment is usually very hot, wet and salty, and you never know where the animals might be hiding," says Dimitrius. "The action we wanted to film only happens when the tide is falling and the crabs are migrating to the edge of the pool. It was very difficult to be in the right place at the right time."

It's possible that the unique feeding strategy of chain moray eels is an example of coevolution at work: lightfoot crabs are known for their hopping prowess, and the predators may have learned to leap for their lunch out of necessity, over time.

Scientists are still learning the mechanics and function of the lightfoot crabs' vertical jumping. A 2014 study found that crabs of all ages and both sexes can accomplish the feat. Similar behaviour has also been seen in lightfoot crabs in the Galapagos, as well as in several other species that live near stone-speckled rivers.

All of these populations inhabit rocky, wet habitats where predators exist in close proximity – and while jumping is clearly an effective escape manoeuvre, it seems to help crabs in other scenarios, too. A high spring, for example, can allow access to foraging grounds that sit atop rocks with otherwise unscalable faces. And initial work also suggests the ocean-dwelling jumpers use their abilities to avoid being swept away by waves. 



Top header image: zaufob/Flickr