Brittle Stars and Basket Stars

Theres no question as to how these creatures got their common names brittle stars and basket stars. Brittle stars have very fragile-looking, worm-like arms and basket stars have a series of branching arms resembling a basket. Both are echinoderms that belong to the Class Ophiuroidea, which contains thousands of species. Due to this classification, these animals are sometimes referred to as ophiuroids. The mouthful of a name Ophiuroidea comes from the Greek words ophis for snake and oura, meaning  tail - words that presumably refer to the animals snake-like arms. There are thought to be over 2,000 species of Ophiuroids.   A brittle star was the first deep-sea animal to be discovered. This occurred in 1818  when Sir John Ross dredged up a brittle star from Baffin Bay off Greenland.   Description These marine invertebrates are not true sea stars, but have a similar body plan, with 5 or more arms arranged around a central disc. The central disk of brittle stars and basket stars is very obvious, since the arms attach to the disc, rather than joining to each other at the base like they do in true sea stars. Brittle stars usually have 5, but may have up to 10 arms.   Basket stars have 5 arms that branch into many slender, highly mobile arms. The arms are covered with calcite plates or thick skin. The central disk of brittle stars and basket stars is usually relatively small, under one inch, and the whole organism itself may be under an inch in size. The arms of some species can be quite long, though, with some basket stars measuring over 3 feet across when their arms are extended. These very flexible animals can curl themselves into a tight ball when they are threatened or disturbed. The mouth is located on the animals underside (oral side).   These animals have a relatively simple digestive system that is made up of a short esophagus and a sac-like stomach.   Ophiuroids do not have an anus, so waste is eliminated through their mouth. Classification Kingdom: Animalia Phylum: Echinodermata Class: Ophiuroidea Feeding Depending on the species, basket stars and brittle stars may be predators, actively feeding on small organisms, or may filter-feed by filtering organisms from the ocean water. They may feed on detritus and small oceanic organisms such as plankton and small mollusks. To move around, ophiuroids wriggle using their arms, rather than using the controlled movement of tube feet like true sea stars. Although ophiuroids have tube feet, the feet dont have suction cups. They are used more for smelling or sticking to small prey, than for locomotion.   Reproduction In most ophiuroid species, animals are separate sexes, although some species are hermaphroditic.   Brittle stars and basket stars reproduce sexually, by releasing eggs and sperm into the water, or asexually, through division and regeneration. A brittle star may purposely release an arm if it is being threatened by a predator - as long as a portion of the brittle stars central disc remains, it can regenerate a new arm fairly quickly. The stars gonads are located in the central disk in most species, but in some, they are located near the base of the arms.   Habitat and Distribution Ophiuroids occupy a wide range of habitats, from shallow  tide pools  to the  deep sea.  Many ophiuroids live on the ocean bottom or buried in mud. They may also live in crevices and holes or on host species such as corals, sea urchins, crinoids, sponges or even jellyfish. They are even found at hydrothermal vents.   Wherever they are, there are usually a lot of them, as they can live in dense concentrations.   They can be found in most oceans, even in the Arctic and Antarctic regions.   However, in terms of numbers of species, the Indo-Pacific region has the highest, with over 800 species. The Western Atlantic was second-highest, with over 300 species.   References and Further Information: Dubinsky, Z. and N. Stambler. 2010. Coral Reefs: An Ecosystem in Transition. Springer Science Business Media. 552pp.Mah, C. 2009.   The Basics: How to Tell Sea Stars (Asteroids) from Brittle Stars (Ophiuroids).   The Echinoblog. Accessed April 28, 2016.Paterson, G.L.J. 1985. The deep-sea Ophiuroidea of the North Atlantic Ocean. Bulletin of the British Museum (Natural History) Zoology 49(1): 1-162.Stà ¶hr, S., O’Hara, T. Thuy, B. (Eds) 2016.  World Ophiuroidea Database. Accessed April 26, 2016.Stà ¶hr, S, OHara T.D.,, Thuy, B. 2012. Global Diversity of Brittle Stars (Echinodermata: Ophiuroidea). PLoS ONE 7(3): e31940. doi:10.1371/journal.pone.0031940University of California Museum of Paleontology.   Introduction to the Ophiuroidea. Accessed April 28, 2016.