Article Citation: Caprio, J., Shimohara, M., Marui, T., Harada, S., & Kiyohara, S. (2014). Marine teleost locates live prey through pH sensing. Science, 344(6188), 1154–1156. doi:10.1126/science.1252697
Animals use all kinds of sensory systems to find their prey in the water – sometimes they see it, or if the water is murky, they hear it. Some animals, like sharks, have a 6th sense called electroreception, where they feel vibrations of their prey in the water with specialized sensory cells. This new study, though, has found that sea catfish have another sense – pH reception.
Sea catfish live in murky water and hunt at night, so they can’t rely on their sight to catch their main prey item: polychaete worms that burrow into the sediment. As these worms breathe in and out through their skin, they release carbon dioxide (CO2) into the water. Releasing CO2 into the water lowers the pH slightly in the area around the worm. If the sea catfish can sense those pH changes, it will be more likely to find and eat the worm in its burrow.
This experiment was done in three parts. First, the researchers needed to figure out whether or not the worm was capable of producing enough of a pH change to be detected by the sea catfish. To do that, they put a worm in a U-shaped tube (as an artificial burrow) and recorded the pH change at different distances from the worm. They found that the worm’s metabolism lowered the pH and was detectable from 5 mm away but not from 15 mm away. That means the sea catfish would have to be close to the worm’s burrow to sense the drop in pH.
Next, they needed to know if the sea catfish would be attracted to the pH change produced by the worm. In one aquarium, they put a worm in a U-shaped tube with a sea catfish. In the other, they just put the U-shaped tube (no worm) with the fish. Both were kept in the dark for a month and analyzed. They found that the sea catfish was more likely to spend time near the burrow if there was a worm inside than if there was not.
Finally, they had to make sure that the fish was sensing the pH change and not just smelling the worm or sensing it another way. The researchers kept the burrows in the same place in the aquariums but this time, there were no worms. Instead, they pumped seawater of varying pH into the tank – seawater with a lower pH to simulate the worm being there and seawater with a normal pH. They found again that the sea catfish was more likely to spend time near the tube emitting the water with a lower pH, which means the sea catfish sensed the worms using the pH difference.
These findings clearly demonstrate that catfish have a seventh sense – being able to sense pH changes in the water caused by the metabolism of their prey. As of yet, the researchers don’t know how the catfish can do this, but they assume that there are special pH sensing cells located on the barbel (the “whiskers” of the catfish).
The researchers also found that the catfish wasn’t able to detect the pH changes from the worm as well when the pH of the water was low (8.0 vs 8.3). That means that the catfish’s way of sensing its food may be compromised in the future due to ocean acidification. The oceans have seen a 30% increase in acidity since industrialization as a result of more CO2 in the atmosphere. As ocean pH continues to drop, sensory systems of many animals could be disrupted. When that happens, animals are stressed and they might be less able to survive. Fishery populations could decrease, meaning that people around the world may have less to eat. In this case, the study directly showed a negative effect of ocean acidification: that sea catfish are less likely to find their prey when seawater has a decreased pH.