Ophiocomella alexandri

Ophiocomella alexandri, also called Alexander's spiny brittle star or the banded brittle star, is a marine brittle star species. Theodore Lyman first scientifically described it in 1860, naming it for his friend Alexander E. R. Agassiz, who was the son of Lyman's mentor, Louis Agassiz.

This animal has five arms radiating from a central disk, giving it pentaradial symmetry. Its body is dark brown, with lighter gray bands on the upper arm surfaces that have hints of yellow. These colors shift daily, possibly as camouflage against predatory fish, similar to the banding pattern. Males and females look identical.

The central disk is rounded, with small bumps on top, and can reach up to 25.2 millimeters (0.99 inches) across. It houses the mouth, digestive system, and gonads. The arms are highly flexible, and when stretched straight, the brittle star can span up to 450 millimeters (18 inches). As the common name suggests, the arms are very spiny, with five to seven long spines on each lateral plate.

The species lives from the intertidal zone down to 70 meters (230 feet) deep. It is benthic, dwelling on the seafloor, and prefers rocky bottoms and coral reefs. Its range covers the eastern Pacific Ocean from southern California to Colombia, including the Gulf of California, and it is also found in the Galapagos Islands.

Alexander's brittle stars reproduce by broadcast spawning. They are gonochoric, with separate male and female individuals in roughly equal numbers.

Males release sperm and females release eggs into the water, where fertilization occurs. The resulting zygotes develop into free-swimming pluteus larvae. This species feeds on detritus, and there is evidence that it catches and eats small fish in aquariums.