Scientists in the US have discovered that a jelly-like material found in the skin of sharks and some other fish has the highest proton conductivity ever measured in a biological material. The jelly occurs in special pores that help the creatures to hunt by sensing the extremely weak electric fields created by prey. As well as providing important clues about how this poorly understood sensory system works, the team believes that the discovery could also lead to the development of new types of electrical sensors.

Some fish, including rays, skates and sharks, can sense electric fields as small as 5 nV/cm, which allows them to detect muscle contractions and other physiological activity in potential prey. They sense the fields using arrays of electrosensory organs in their skin called ampullae of Lorenzi (AoL). An individual AoL comprises a canal that is filled with a jelly-like substance that comes into contact with the external environment via a pore in the creature’s skin. The inner end of the canal terminates in a sac (or “alveolus”) containing cells that transmit electrical signals from the jelly to the fish’s nervous system.

However, scientists do not have a good understanding of how tiny electrical signals are conducted along the AoL from the external environment to the alveolus. Now, Marco Rolandi of the University of California, Santa Cruz, and colleagues have discovered that AoL jelly is an extremely good conductor of protons – something that could help to explain how it can transmit weak electric fields.

Jelly sandwich

The team tested samples of jelly from three different fish: the bonnethead shark; the longnose skate and the big skate. The jelly was sandwiched between two electrodes made of palladium, which can absorb and release large numbers of protons. A voltage is applied across the electrodes, causing protons from the positive electrode to enter the jelly and travel to the negative electrode. The proton current is easily measured because it is identical to the electrical current that flows through the voltage supply.

While these measurements showed that the jelly is a good proton conductor, they do not provide an accurate value of the conductivity. This is because the protons must overcome contact resistance when they exit and enter the palladium electrodes, which affects the conductivity measurement. To get around this problem, measurements were also made using two gold electrodes that were placed in the jelly between the two palladium electrodes. Measuring the voltage across the gold electrodes and the current through the palladium electrodes allowed the team to make an accurate calculation of the conductivity of the jelly.

The tests revealed that the proton conductivity of the jelly is as high as 2 mS/cm, which is the highest value ever measured in a biological material. “The first time I measured the proton conductivity of the jelly, I was really surprised,” says team member Erik Josberger of the University of Washington, adding: “The conductivity was only 40 times smaller than Nafion.” Nafion is a solid plastic material that is designed specifically to have extremely high proton conductivity for use in fuel cells and other commercial applications.

Water chains

The researchers believe that the high proton conductivity in the AoL jelly is related to the presence of a chain-like molecule called keratan sulphate. This is an acid, which means that each keratin-sulphate molecule can provide a free proton for conduction. The team believes that these protons are able to travel through the jelly along chains of water molecules that form around keratan sulphate.

“The observation of high proton conductivity in the jelly is very exciting,” says Rolandi. “We hope that our findings may contribute to future studies of the electrosensing function of the ampullae of Lorenzini and of the organ overall, which is itself rather exceptional.”

The measurements are reported in Science Advances.