Girella nigricans, commonly known as the opaleye, is a fish species found along the Pacific coast of North America. It thrives in rocky reef environments and tidepools, identifiable by its olive to dark green color with a distinctive white or pale spot near the dorsal fin. The opaleye is a herbivore, primarily feeding on algae, and is known for its social schooling behavior.

Girella nigricans, commonly known as the opaleye, is native to the eastern Pacific Ocean. Its geographical distribution includes the coastal regions of California, USA, and extends southward to Baja California, Mexico. The species is also found around the offshore islands such as the Channel Islands.

The opaleye typically inhabits rocky shorelines, kelp forests, and coastal reefs. These environments provide abundant algae, which is a primary component of their diet. The species thrives in temperate marine waters, often in areas with moderate to strong wave action and clear waters. The temperature range in these habitats is generally between 15°C to 24°C (59°F to 75°F).

Girella nigricans occupies the ecological niche of a herbivorous fish within the marine ecosystem. It primarily feeds on a variety of algae, which it scrapes off rocks and kelp. Its role involves maintaining algal growth within its habitat and serving as prey for larger marine predators, thus contributing to the food web dynamics of its native marine ecosystems.