If you're struggling to find food, you might try looking in a new place or perhaps setting a trap. You probably wouldn't tie your body into a knot.

But that's what one species of moray eel has been filmed doing.

Shanta Barley of the University of Western Australia in Perth and her colleagues were studying moray eels on the Scott Reefs off the north-west coast of Australia.

They lowered two video cameras on a metal frame into the water. The frame also carried a plastic mesh bag containing 1kg of pilchards, to lure meat-eating fish.

A fimbriated moray eel approached the bag and tried to eat the pilchards. After struggling with it for a little while, it grabbed the bag with its teeth and tied its "tail" region into a loose knot.

Next the eel pushed the knot along its body so that it struck the bait bag. It did this repeatedly.

Barley says it was probably trying to dislodge the pilchards with the force of the impacts, and that tying its body into a knot allowed it to concentrate all its force on one spot.

The findings have been published in the journal Marine Biodiversity.

Several other species of moray eel have been observed to tie themselves in knots, but for different reasons. In some cases they may be trying to squash pieces of food that would otherwise be too large to swallow.

Barley's team also saw a second species, a honeycomb moray eel, take an unusual approach to the bag of food.

The moray grabbed the bag with its mouth, then used its tail as a paddle to push water towards the bag. This seems to have provided it with extra leverage, twisting its body hard enough to rip the bag open.

Barley says behaviours like these may be common in long thin fish, such as eels and hagfish.

These fish are much more flexible than other species, which makes it easier for them to perform contortions.

"Our observations suggest that having an eel-like body shape opens the door to unusual and useful feeding techniques unavailable to conventionally-shaped fish," says Barley.

These tricks may mean the morays can have a bigger impact on their ecosystems than a predator of their size would normally manage.

That's because they can eat prey that are larger than their mouths, which most fish can't. In other species, "you can largely predict the size of the prey based on the size of the fish's mouth," says Barley. "Morays appear to have cunningly circumvented this annoying biological rule of thumb."

Top predators like sharks have a powerful effect on the ecosystems they live in. Often they allow a greater range of species to flourish, by keeping the populations of medium-sized species in check.

It could be that the morays are playing a similarly crucial role. "If morays are using all of these fancy tricks to eat lots of large prey, they could be having a really key impact on coral reef food webs, and therefore ultimately the health of the reef itself," says Barley.