Ophioplocus esmarki, commonly known as Esmark's brittle star or the smooth brittle star, is a species of brittle star in the family Hemieuryalidae. It can be found from the low intertidal to a depth of 70 m, from Northern California to Baja California. This is one of the few brittle star species that is visible during daylight hours.
Esmark's brittle star is typically a gray-brown color. It has a round central disc, up to 22 mm in diameter, and five arms, up to 60 mm, each with very short spines.
Ophioplocus esmarki follows pattern of abbreviated development in which non-feeding larvae called vitellaria spend only a few days as free-swimming plankton before metamorphosing into a juveniles. Adults brood their embryos, vitellaria, and juveniles inside their genital bursae (pouches) located in the central disc. One study estimates that it takes 9 to 11 years for this species to reach 50% of its final body size.