Monday, November 30, 2009

Hammerhead Sharks, 360° Vision - and More

"Scalloped Hammerhead and Bonnethead Sharks Have 360 Degree Vision"
DiscoveryNews (November 30, 2009)

"The old idiom "eyes in the back of your head" holds somewhat true for scalloped hammerhead and bonnethead sharks, according to new research that found these sharks possess a 360 degree field of vision. Virtually nothing can escape their view.

"The study, published in the Journal of Experimental Biology, also helps to explain why hammerhead sharks have such unusual shaped heads...."

The researchers used pickups that detected electrical activity in the shark's eyes. And detected activity in a full 360° range. Those critters can, literally, see what's directly behind their heads.

I was glad to see that the article said "helps to explain why hammerhead sharks have such unusual shaped heads...." [emphasis mine]

I don't doubt that having full-circle vision helps explain their odd head shape, but there's probably more to the function these odd-looking fish's heads. It's not entirely unreasonable to think that they've got 'stretched' heads to help them pick up variations in Earth's magnetic field.

The excerpt from a learned periodical is about as accurate as I've come to expect. The "unique sensory capabilities of the hammerhead" are shared by the platypus and other critters, but never mind that.

The point is that hammerhead sharks are one of the creatures that are able to sense electric fields, and apparently use this for navigation - and quite possibly other purposes.

"...Knowing the unique sensory capabilities of the hammerhead (it detects faint bioelectric fields, which help it find hidden prey), we later surveyed the geomagnetic field around the seamount and discovered that the paths of the sharks coincided with magnetic valleys (intensity minimums) and ridges (intensity maximums) leading away from El Bajo Espíritu Santo like spokes from a hub. These magnetic traces were likely produced by the flow of magnetite-impregnated basalts from the seamount during volcanic eruptions. Taken together, the evidence suggests that this predator, unlike some others, does not congregate at the seamount because it is a biotic hotspot, but because the area provides a network of 'paths' that enables it to find a constellation of productive feeding grounds in the surrounding waters..."

From "Figure 8 / Magnetic_Field_guides_sharks.pdf"
From an article in the January-February 2005 issue of American Scientist,
excerpt hosted by Department of Geoscience, The University of Montana:

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