The coveted gold statuettes that will be handed out tonight at the Academy Awards were manufactured on the Northwest Side of Chicago. While most of us will never see one of those Oscars, a school of the large cichlids also known as oscars are on view every day in Shedd’s Amazon Rising exhibit. And we think they’re at least as impressive.
The 13½-inch Oscar is made of a pewter-like alloy that’s coated in copper, nickel silver and finally 24-karat gold. Our oscars (Astronotus ocellatus), which can grow to 16 inches, have a mottled metallic pattern, from pewter to silver, with black markings outlined in glittering copper-orange. And they have much more expressive faces than the Art Deco knight’s minimalist countenance. In fact, to predators, this fish might look two-headed.
At one end, oscar cichlids have prominent dark, orange-rimmed eyes (with excellent vision) and large, thick-lipped mouths—adaptations that help them spot and grab the tiny insect larvae, crustaceans and small fishes they prey on.
At the other end, they have a prominent black, orange-rimmed eyespot, or ocellus, on either side of the caudal peduncle, the place where tail and body meet. In addition, the densely scaled and overlapping dorsal, tail and anal fins fit together to look like a large, big-mouthed head. The alignment of the eye and eyespot at the same level on the fish’s side completes the perfect front- and back-end symmetry. You can’t tell if this slow-moving fish is coming or going.
And that’s the point of what scientists call head mimicry, an adaptation also found among reef fishes and some butterflies and moths. In the case of the oscars, researchers suggest that the eyespots deter predation by piranhas that feed by fin nipping, such as the red-bellied seen in Amazon Rising. Oscars and several fin-nipping piranha species share many of the same Amazon basin rivers.
Fin scales are a convenient and abundant source of protein for piranhas, and the damage to the other fish is usually temporary. But extensive fin nipping leaves the prey fish open to bacterial and fungal infections. Ragged fins also make it more difficult to swim, which can then hinder a fish’s ability to pursue its own prey or escape predators.
Field studies indicate that oscars suffer less from fin nipping than do other, plain-tailed cichlid species in the same range. The piranhas can’t seem to make head or tail of the oscars, dissuading them from going in for a nip.
Oscars can also rapidly lighten or darken their color, and even go from mottled to a dark solid, making the boundaries of their bodies less distinct to predators in shadowy waters. Both the eyespots and color modulation also seem to figure in communication among oscars, especially during courtship and competitive displays.
Oscar cichlids are widely distributed throughout the Amazon basin as well as the Orinoco River basin. They prefer slow-moving, shallow tributaries with mud or sand bottoms, and they frequently take shelter under submerged branches and logs—a good place to look for them in Amazon Rising’s River Channel habitat. You’ll also find them in the Floodplain House habitat, perhaps passively staring down the red-bellied piranhas with those eyespots.
—Karen Furnweger, web editor