Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes (sharks, rays, and chimaeras); however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been reported to have them. Teleosts have re-evolved a different type of electroreceptors. They were first described by Stefano Lorenzini in 1678.
These sensory organs help fish to sense electric fields in the water. Each ampulla consists of a jelly-filled canal opening to the surface by a pore in the skin and ending blindly in a cluster of small pockets full of special jelly. The ampullae are mostly clustered into groups inside the body, each cluster having ampullae connecting with different parts of the skin, but preserving a left-right symmetry. The canal lengths vary from animal to animal, but the distribution of the pores is generally specific to each species. The ampullae pores are plainly visible as dark spots in the skin. They provide fish with a sixth sense capable of detecting electromagnetic fields as well as temperature gradients.
Electromagnetic field sensing ability 
The ampullae detect electric fields in the water, or more precisely the difference between the voltage at the skin pore and the voltage at the base of the electroreceptor cells. A positive pore stimulus would decrease the rate of nerve activity coming from the electroreceptor cells, and a negative pore stimulus would increase the rate of nerve activity coming from the electroreceptor cells.
Sharks may be more sensitive to electric fields than any other animal, with a threshold of sensitivity as low as 5 nV/cm. That is 5/1,000,000,000 of a volt measured in a centimeter-long ampulla. Since all living creatures produce an electrical field by muscle contractions, it is easy to imagine that a shark may pick up weak electrical stimuli from the muscle contractions of animals, particularly prey. On the other hand, the electrochemical fields generated by paralyzed prey were sufficient to elicit a feeding attack from sharks and rays in experimental tanks; therefore muscle contractions are not necessary to attract the animals. Sharks and rays can locate prey buried in the sand, or DC electric dipoles simulating the main feature of the electric field of a prey buried in the sand.
The electric fields produced by oceanic currents moving in the Earth's magnetic field are of the same order of magnitude as the electric fields that sharks and rays are capable of sensing. This could mean that sharks and rays can orient to the electric fields of oceanic currents, and use other sources of electric fields in the ocean for local orientation. Additionally, the electric field they induce in their bodies when swimming in the magnetic field of the Earth may enable them to sense their magnetic heading.
Behavioral studies have also provided evidence that sharks can detect changes in the geomagnetic field. In one experiment, sandbar sharks and scalloped hammerhead sharks were conditioned to associate a food reward with an artificial magnetic field. When the food reward was removed, the sharks continued to show a marked difference in behavior when the magnetic field was turned on versus when it was off.
Temperature sensing ability 
Early in the 20th century, the purpose of the ampullae was not clearly understood, and electrophysiological experiments suggested a sensibility to temperature, mechanical pressure and possibly salinity. It was not until 1960 that the ampullae were clearly identified as specialized receptor organs for sensing electric fields.   The ampullae may also allow the shark to detect changes in water temperature. Each ampulla is a bundle of sensory cells containing multiple nerve fibres. These fibres are enclosed in a gel-filled tubule which has a direct opening to the surface through a pore. The gel is a glycoprotein based substance with the same resistivity as seawater, and it has electrical properties similar to a semiconductor. This has been suggested as a mechanism by which temperature changes are transduced into an electrical signal that the shark may use to detect temperature gradients, although it is a subject of debate in the scientific literature. 
See also 
- Roth A, Tscharntke H. (1976). Ultrastructure of the ampullary electroreceptors in lungfish and Brachiopterygii. Cell Tissue Res. Oct 1;173(1):95–108. PMID 991235
- Gibbs MA, Northcutt RG. (2004). Development of the lateral line system in the shovelnose sturgeon. Brain Behav Evol. ;64(2):70–84. PMID 15205543
- Meyer, Carl G.; Holland, Kim N. and Papastamatiou, Yannis P. (2005). "Sharks can detect changes in the geomagnetic field". Journal of the Royal Society Interface (2): 129–130.
- Murray RW (1960). "The Response of the Ampullae of Lorenzini of Elasmobranchs to Mechanical Stimulation". J Exp Biol 37: 417–424.
- Murray RW (1960). "Electrical sensitivity of the ampullae of Lorenzini". Nature 187 (4741): 957. doi:10.1038/187957a0.
- Brown BR (2003). "Sensing temperature without ion channels". Nature 421 (6922): 495. doi:10.1038/421495a. PMID 12556879.
- Fields, RD, Fields, KD, Fields, MC (2007). "Semiconductor gel in shark sense organs?". Neurosci. Lett. 426 (3): 166–170. doi:10.1016/j.neulet.2007.08.064. PMC 2211453. PMID 17904741.
- Brown BR (2010). "Temperature response in electrosensors and thermal voltages in electrolytes". J Biol Phys 36 (2): 121–134. doi:10.1007/s10867-009-9174-8.