But enough about that, I want to discuss the pores around the head. They sometimes venturing further back but they are concentrated around the head. This pores lead to electroreception organs. The pores are jelly-filled channels leading down to the organs. Within each organ are several sensory receptors called ampullae of Lorenzini*. Leading out the back of the cluster of ampullae cells are the nerves leading to the brain.
Though short, the distance that sharks can detect electrical gradients in impressive. These signals degrade quickly yet skates and rays will swim over a sandy bottom and are able to detect prey under the sand. This fact is so super cool I don’t even know what else to say about it.
The pores are arranged symmetrically around the head of the sharks and rays and this allows them to hone in on prey. For an impressive look at the arrangement of pore we should look at one of the family of hammerhead sharks.
Filter feeding sharks, like the Whale Shark, also have ampullae. Do they use them to detect when lots of small fish are available and they open their mouths and start swimming through them? Or do they aid orientation in earth magnetic field? That second has been suggested but I’m not sure there is much evidence of it.
Not unique to sharks, ampullae are also found on some ancient fish, sturgeon and lungfish for example. Another ‘living fossil’ type of fish, the Paddlefish Polyodon spathula, has a similar array of ampullae. Paddlefish are freshwater, muddy-river dwellers, they hardly have eyes (tiny little things) but are able to find individual small arthropods, like Daphnia, in the Mighty Mo (the slang term for the Missouri River). You can fish for Paddlefish. In these muddy waters if you have the permit (which allows a single fish to be taken). You must leave the rostrum behind in a bucket so the natural resource agency can keep track
How about freshwater sharks, or to be more correct freshwater elasmobranchs, because many freshwater denizens related to sharks are actually rays. Do these animals have a different form of ampullae? It turns out that three types/sizes of ampullae have been described: Macro-, micro-, and mini-. In the rays that are restricted (obligate) to freshwater, the mini-ampullae are found. These mini-ampullae have short canals, which means they shouldn’t be very effective at detecting electric gradients, unless they have something else going on. I couldn't find an answer to how well freshwater rays detect electric gradients, an area ready for some more detailed research?
That's where I'll leave it, except to ask, why don’t humans have electroreception?Tis would be advantageous for electricians. I have a proposal, once we master CRISPR**, the mechanisms stolen from bacteria that biotech folks are trying to use to alter DNA sequences, we need to add electroreception to our repertoire.
End notes:
*The array of sensory cells was first described by Stefano Lorenzini (thus the name for these sensory cells) way back in 1678. The anatomy of the cells was looked at but the function was not determined for hundreds of years after the discovery.
**CRISPIR, stands for; clustered regularly interspaced short palindromic repeats. This is a system in bacteria for remembering viruses that have previously attacked them. It makes repealing a second attack faster/easier. Very similar, at least superficially, to our secondary immune response.
Sources and Readings
If you’re interested in sharks in general nearly any shark book has more information, but let me suggest you try the old but good; Moss SA. 1984 Sharks: An introduction for the amateur naturalist. Prentice-hall. You can find it cheap, it’s a great introduction and will spark your interest.
More detailed stuff about Ampullae and electroreception:
Josberger EE, Hassanzadeh P, Deng Y, Sohn J, Rego MJ, Amemiya CT, and Rolandi M. 2016. Proton conductivity in ampullae of Lorenzini jelly. Science Advances 2: doi: 10.1126/sciadv.1600112
Marzullo TA, Wueringer BE, Jnr LS, Collin SP. 2011 Description of the mechanoreceptive lateral line and electroreceptive ampullary systems in the freshwater whipray, Himantura dalyensis. Marine and Freshwater Research 62: 771-779.
Murray RW (1974): The ampullae of Lorenzini; in Fessard A (ed): Electroreceptors and Other Specialized Receptors in Lower Vertebrates. Berlin, Springer, vol III/3, pp 125– 146
Wilkens LA, Hofmann MH, Wojtenek W. 2002. The electric sense of the paddlefish: A passive system for the detection and capture of zooplankton prey. Journal of Physiology 96: 363–377.
Wueringer BE, Peverell SC, Seymour J, Squire Jr L, Kajiura SM, Collin SP. 2011. Sensory Systems in Sawfishes. 1. The Ampullae of Lorenzini. Brain Behavior and Evolution.