Friday mystery object #80 answer

On Friday I gave you this sectioned bit of a critter as the mystery object:

A slightly trickier one than usual, so I wasn’t surprised at the range of suggestions – ranging from a vertebra to a Narwhal tusk. Jack Ashby got in first with a tentative stab at the right answer when he said ‘sawfish maybe?‘ a suggestion supported by Carlos Grau. It is indeed part of the rostrum (beak or nose) of a Sawfish, probably the Freshwater or Largetooth Sawfish, Pristis microdon Latham, 1794. Here’s a look at the section of rostrum from the underside:

These rays are getting very rare due to fishing – both as accidental bycatch and for their fins, oil and their bizarre rostrum. They live in tropical waters and they feed by gliding over the sea bed and detecting the tiny electromagnetic signals given off by muscular contractions of fish and crustaceans, which they then cripple by threshing around with their ‘toothy’ saw to make them easy to catch. This ability to detect electromagnetic impulses is a sixth sense conferred by the Ampullae of Lorenzini, which are pores connected to canals full of a jelly that transmit electrical impulses from the surrounding water to electrosensitive nerves in the skin. This is what they look like on the surface of the skin:

Pretty impressive stuff. This kind of electroreceptor is commonplace amongst the sharks and rays, but the various electrosensitive bony fish like Ictalurus nebulosus use a different method of electroreception possibly associated with the lateral line.

I’ll leave you with an image of Pristis microdon in its living, big-nosed and smily-faced glory:

Largetooth sawfish swimming in the tunnel at the Georgia aquarium, Atlanta, Georgia - by istolethetv

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