by Adam Denish, DVM
My home aquarium contains 350 gallons of saltwater and extends about 6 feet in length and 2 feet in height. It is tastefully decorated with rock formations and it is inhabited by an assortment of fish with beautiful colors and shapes.
It came as a surprise to learn that studies suggest that just as I enjoy peering into their underwater world, the fish are enjoying their view of my living room as well. What must my fish be thinking as they watch my family members sitting on the couch, moving about searching for lost keys or stealing a cookie? Reports from experimental lab studies give insights into what fish are able to detect in their surroundings and how our interactions with our pet fish might need a second look.
Fish are not typically given credit for being especially smart or for possessing a good memory. They don’t have a large brain capacity and most of their time they spend searching for food. But perhaps we have underestimated the IQ of fish. Studies conducted with captive blind Mexican cave fish reveal that these fish can identify changes made to the arrangement of objects in their aquarium.
These fish seem to have a sense similar to bats that gives them the ability to detect obstacles in their path. Further, the fish make a mental map of their surroundings and commit it to memory for future use. So no need to put off a Saturday afternoon of renovating the home aquarium because you fear the fish will get confused, they will learn their way around quicker than you would.
Another study investigated whether fish of the same species could recognize fellow individuals. The study evaluated the ability of Ambon damselfish to identify a fish they have seen previously by swimming up to a computer image with a choice of two damselfish. The study found that ultraviolet facial patterns on the damselfish were key to the test subjects’ ability to identify individuals. These UV patterns, which are not able to be detected by humans with the unaided eye, act like name tags for the Ambon damselfish.
The study went further by presenting manipulated facial images and the damselfish were still able to recognize the familiar face. So if Jack and Jill have been tank-mates for some time and Jack dies, prompting you to replace him with Chad thinking Jill will never notice, think again.
While mastering who’s who inside of the aquarium makes sense, what is most amazing is the ability of fish to see what’s happening outside of the aquarium.
The largest fish I own is a Vlamingi tang measuring roughly 9 inches in length, with eyes about the size of a dime. “Big Guy” greets me by moving to the front of the tank and looking me over with those big eyes. I would like to believe he knows it’s me and maybe I’m right. An impressive study using archer-fish gives some support to that idea.
Archerfish are found in brackish waters around Southeast Asia. They have the ability to hunt for insects along the river banks by forcefully ejecting a stream of water from their mouth, causing the insect to fall to the water and be eaten. This ability to use their mouth like a water pistol is the means by which trained archerfish were able to make selections between two images of human faces. The trained archerfish were able to correctly select the familiar face 81% of the time. The scientists then made the images more consistent by leveling the brightness and color of the images and the archerfish improved their score to 86%.
Collective studies indicate that being as smart as a fish is a pretty nice compliment. What’s interesting is that while we normally give credit to mammals for their ability to recognize their owners, land animals have the added benefit of detecting scent and hearing vocal sounds, factors that do not play a part in these studies.
We know that fish can sense time by swimming around the top of the tank close to feeding and they may perform for their keepers as they associate a human presence with food. The fact that fish can discern between different faces raises the question of why they have this ability.
The value of knowing the identity of others in your own species is essential to social animals, but to be able to discern the identity of those in another species is a higher order skill. It seems unnecessary from an ecological standpoint as humans and fish do not share the same living space. This information may provide biologists with more insights into our evolutionary past.
The fact that our pet fish have a conscious understanding of their surroundings and may even be able to recognize their owners may inspire us to take more pride in arranging the aquarium and choosing tank mates. It could also give us reason to spend more moments looking at the fish to give them the opportunity to memorize our faces.
On the other hand, the fact that my fish could identify my face in a line-up may be a good reason for me to put down that stolen cookie.