Thayeria boehlkei, the Penguin Tetra

Thayeria boehlkei (Penguin Tetra). Nathaniel Thayer, for whom the genus is named, was the underwriter of the Amazon expedition of 1865-66, which had for its geologist-biologist the famous Louis Agassiz. Agassiz was more than an ichthyologist; in his youth he made his public reputation expounding the fresh idea that there had been an "Ice Age," and in time Agassiz lived to become the last widely-respected professional biologist to resist the concept of evolution. The species name is for J.E. Böhlke, an ichthyologist at Philadelphia's Academy of Sciences.
 
This species is from Rio Araguaia and also from the upper Amazon basin in Peru, but the fish you'll buy have been shipped from Thailand or Singapore. When this tetra came on the U.S. market it was confused with another, T. sanctæ-mariæ, and the mistaken identity persisted for years.
 
When it's at rest, this fish has an unusual up-slanted posture that's emphasized by the black lower lobe of its caudal fin. Perhaps the slanting black stripe and blot make its outline read as less fishlike to a predator. These bold fish are competitive with each other, but in all the mutual chasing I've yet to see a fin torn. If you think of tetras as pure carnivores, like mini-Piranhas, you'll be as surprised as I was to see Penguin Tetras tear into a floating blanched spinach leaf.
 
This is an easy tetra to spawn. The females regularly fill with eggs. Pre-nuptial broadside displays, and rushing and feinting get more excited over a couple of days until in an evening fishes start to pair up and make wild dashes through the plants, with eggs blown in every direction.
 
Thayeria boehlkei at FishBase. Thayeria boehlkei at Wikipedia.