Fish Smarts

The evolutionary biologist Stephen Jay Gould once suggested that observant aquarium owners might very well know as much if not more about the behavior of tropical fish than do ichthyologists. Indeed, if you have ever owned an aquarium and have wiled away a few pleasant hours watching your fishy tenants go about their fishy business, then you likely understand how right Gould probably was. Among people who keep aquaria, it is no secret that fish exhibit individual personalities, forge useful alliances, and engage in the occasional round of fisticuffs. They also exhibit feats of intelligence that go well beyond what their traditional reputation might warrant.

Toward the end of my undergraduate days, I was fortunate enough to spend a few months babysitting a 20 gallon aquarium that was home to an eight-inch tiger oscar (Astronotus ocellatus) and a five-inch bristlenose plecostomus (Ancistrus dolichopterus). One of my favorite things was to shut off all the lights in my bedroom, except for the one in the aquarium, and lie in bed watching my charges until I fell asleep. Over time, I discovered that the oscar and the pleco had a fascinating relationship.

(Bristlenose Plecostomus)

Tiger oscars are members of the family Cichlidae and, like all cichlids, are aggressively territorial. They are also accomplished predators, and aren’t fussy about what they eat, making meals out of pretty much anything that will fit in their mouths. Most of the time I fed Larry (I called the oscar Larry) cichlid pellets from a pet store, but I must confess that I felt a bit of morbid pleasure in feeding Larry live goldfish. Years later I would learn that giving oscars feeder goldfish is a really terrible thing to do – goldfish can cause vitamin deficiencies in oscars due to the presence of thiaminase in their flesh. A better option would’ve been guppies, but at this point that’s neither here nor there.

Some predatory fish chase down their prey, and others hunt by ambush, but tiger oscars do their thing with an air of simple nonchalance that is wonderful to watch. When a feeder fish landed in the water, Larry became very still, opening and closing his gills and making slight motions with his fins, as if saying to the goldfish, “Pay no attention to me, yonder snack. I am no threat to you whatsoever.”

(Tiger Oscar, On the Prowl)

He would continue moving his fins, primarily his caudal and pectorals, by small degrees, and after a few moments you would realize that he had propelled himself all the way to the other end of the tank, right up close to the goldfish. Then, acting on some internal queue, Larry’s mouth would abruptly spring open and snap shut, quicker than a hiccup, inhaling the goldfish. A moment or two would pass and Larry would fan his gills, expand his mouth, and expel a swirl of orange scales. It was really quite beautiful in its potent simplicity; a small evolutionary poem.

Tiger oscars capture virtually all of their prey through this suction method. When they pop their mouths open they suck in a great deal of water, along with the object of their hunger. Like many other piscine predators, cichlids have two sets of teeth, one in their jaws, used to trap food, and another in their throats, called pharyngeal teeth, that are used to pulverize food before it passes on into the remainder of the digestive tract. This system allows the fish to expel difficult to digest items like scales and so on.

Larry shared his tank with a bristlenose plecostomus called William Shakespeare, though I couldn't tell you why. Plecos are suckermouth catfish (the word plecostomus means “folded mouth”), of which there are approximately 150 species living throughout the Amazon Basin and other areas in South and Central America. They are bottom feeders, subsisting on diets of vegetable matter and very tiny crustaceans. They are popular additions to aquaria, where they are often called “janitor fish” for their ability to clean algae off glass. Shakespeare spent the majority of his time behind a convex hunk of coral that was propped in one corner of the tank, where it created a sort of cave, though he would venture out from time to time, often at night.

(Pleco. Dining.)

At about five inches in length, Shakespeare was far too large for Larry to eat. This did not mean, however, that Larry was happy about sharing his tank. He didn't like Shakespeare at all, often chasing the pleco around the tank, nipping at his tail and dorsal fin. Being so harassed did not seem to bother Shakespeare a great deal. When Larry started up, the pleco would rather nonchalantly go behind his piece of coral and stay there until the cichlid storm blew over. If the coral cave had not been there I think Larry probably could’ve killed Shakespeare, had the oscar gotten it into his fishy brain to do so, but it’s hard to say.

Part of me thinks – and I have not one shred of evidence with which to back this up – that Larry was actually a wee bit frightened of Shakespeare. Whenever he would chase the pleco behind the coral, he would go screaming back across the tank as if he was afraid Shakespeare might be hot on his tail looking for blood.

(Oscar. Fleeing.)

And then one evening I witnessed something astonishing. I was lying in bed with the lights off, as was my habit, watching Larry nibble the small chunks of apple I had dropped in for him (cichlids need some fruit in their diets). Shakespeare appeared from the cave and moved slowly along the front wall of the tank, into Larry’s territory, pausing once or twice to slurp up any tasty things he found to eat down in the gravel. All at once Larry went after him, sending the pleco back into his hidey-hole. Shakespeare tried again to leave, and then a third time, with the same results. Larry, though, had evidently reached his Popeye point, and was able to stands no more.

A quick description of the decorative stuff in the tank is in order here, beginning with the coral cave. There were three ways to get in and out: through a gap in the top, through a gap a couple of inches up one side, or through a gap at the bottom, the largest of the three. Generally speaking, Shakespeare came and went through the bottom space. The floor of the tank was covered in two or three inches of red and purple gravel, with seashells scattered on top.

After Shakespeare went into hiding again, after his third attempt to visit the world outside the coral cave, Larry leapt into action. He picked up one of the seashells in his mouth, carried it across the tank, and dropped it in front of the cave's bottom opening. And then he did it again. In all, Larry deposited seven shells in front of the opening, effectively blocking it off.  Then he swam back to his end of the tank and resumed eating his bits of apple.

(Looks A Lot Like Larry)

You can accuse me of being high at the time and I would not disagree, because in all likelihood I was. But that is also probably why watching one fish attempt to wall off another fish by erecting a barrier of shells absolutely set my brain on fire. I had no idea at the time that fish came with problem-solving skills, or that they used tools. For what else could the seashells have been in Larry’s mind if not tools? He had a hole that wanted fixing and, making use of the materials at hand, what about fixing it.

Over the years I’ve shared this story with other aquarium owners. None of them were been surprised. They all had tales of “stupid” fish doing things that would beggar the abilities of a two-year-old human. It is my opinion that Stephen Jay Gould was right in his assessment of aquarium owners and their observational skills. They know a great deal about the behavior of their charges, an idea borne out by the fact that the majority of the literature available on a wide array of tropical fish species comes in the form of manuals for aquarium owners.

In any event, next time you have the opportunity to study aquarium fish, bear in mind that there might be a hell of a lot more going on in the tank than meets the eye.