The massive mouth of the Silver arowana helps it to capture prey above the surface, says new research.
The Silver arowana, Osteoglossum bicirrhosum, generally feeds on fish living in the rivers and streams of the Amazon's massive flooded forest, but it's also an expert aerial feeder.
Large specimens can leap several feet out of the water to take terrestrial and arboreal prey including large insects, spiders and even birds.
Now a team of scientists from the University of South Florida, the University of California, the US Fish and Wildlife Service and Eckerd College have studied the mechanisms through which Silver arowana feed on items above the water surface.
Their findings are reported in the latest issue of the journal Environmental Biology of Fishes.
To see how their mode of feeding differed, the scientists presented the arowanas with food both above and below the surface, then filmed them with sophisticated high-speed digital cameras.
By analysing the footage, the team were able to observe how the kinematics of the jaws, pectoral fins and cranium differed, and found that movements were generally much slower underwater.
When arowana feed on items above the water surface, their feeding mode is characterised by faster, larger movements and they open their jaw much later than they do when trying to engulf their prey under the water, presumably to stop them from scooping up an enormous mouthful of water as they leave the surface.
"The delayed opening of the mouth may serve to reduce the effects of drag..."
The team reckons that the massively oversized mouth of the arowana might be an advantage for aerial feeding, too:
"The comparatively large gape during leaping may facilitate prey capture by overcoming variability in the apparent position of the prey due to refraction, while the delayed opening of the mouth may serve to reduce the effects of drag.
"This distinctive leaping behaviour allows exploitation of the terrestrial prey base, especially during seasonal inundation of the Amazon river basin when the aquatic food base is widely dispersed."
For more details on how arowanas feed, see the paper: Lowry, D., Wintzer, AP., Matott, MP., Whitenack, LB., Huber, DR., Dean, M and PJ Motta (2005) - Aerial and aquatic feeding in the silver arawana, Osteoglossum bicirrhosum. Environmental Biology of Fishes, Vol. 73., No. 4. pp 453-462.