The Maryland Darter (Etheostoma sellare) is the rarest fish in the world. It is thought to be so rare because of its extremely specific habitat requirements. This freshwater fish thrives only in the part of a stream where water tumbles out of hills onto relatively flat coastal plains, and prefers rocks and crevices in clean, well-oxygenated water. It lives in the last fast-flowing areas (riffles) before water reaches the flatlands. This species is found in Maryland and nowhere else. The Maryland Darter is 2-3 inches long and its spawning period is thought to be in late April or early May. Little is known about this fish, but it is believed to live for around 3 years. It is a carnivorous species with a diet consisting of arthropods and mollusks.