Natures wonder: cuttle fish survival mechanism (Introduction)

by David Turell @, Wednesday, December 02, 2015, 06:19 (1473 days ago) @ David Turell

Reduce the electric current you produce and sharks won't sense it:

http://phys.org/news/2015-12-camouflaged-cuttlefish-electrical-stealth.html

One of the cuttlefish's major predators, the shark, has eyes on the side of its head, making it effectively blind straight ahead and near the front of the mouth. So the shark relies instead on a snout studded with sensitive detectors of faint electrical fields to get the meat in the maw.

Consequently, the common cuttlefish (Sepia officinalis) has figured out a stealth technology to protect itself in the electrical spectrum, according to Christine Bedore, an assistant professor of biology at Georgia Southern University who studied the phenomenon as a post-doctoral researcher in the Sönke Johnsen lab at Duke University.

Sharks can sense a faint current emanating from the tube-like siphons on either side of the cuttlefish's head, the vent where it excretes, and the gap around its mantle.

These "bioelectric fields" aren't anything like the 500 volts an electric eel produces. They're just a tiny electrical artifact of the ion exchanges caused by the animal's metabolic processes, like respiration. Still, Bedore's experiments showed the sharks will bite when they sense these subtle fields.

A common cuttlefish at rest has a bioelectric potential of 10-30 microvolts, Bedore found. That's about 75,000 times weaker than an AAA battery. But when the animal freezes in place, slows its ventilation, throws its arms around to cover the siphons and clamps down on its mantle, the current drops to about 6 microvolts.

***

Bedore measured these tiny electrical fields as captive-reared cuttlefish rested comfortably in a tank, and then as they responded to videos from an iPad next to the tank that depicted the dark and growing silhouettes of an approaching grouper, shark or crab.

For the fish and the shark, the cuttlefish froze, covered body openings with their arms and slowing breathing. The crab silhouette inspired no such response.

***

Comment: IT the cuttle fish really 'knew' what attracted the shark, that would explain the response, but did the cuttle fish really 'know' that. Either the cuttle fish figured it out, or if they used trial and error they shouldn't be a species any more. dhw?


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