With over 2000 described species, Ophiuroidea is the largest class of living echinoderms, found in all oceans from intertidal to the greatest depths. It includes brittle stars and basket stars (species with branching arms) as well as snake stars (non-branching arms). The name Ophiuroidea is derived from the Greek words ophis, meaning snake, and oura, meaning tail, describing the often thin, snail-like winding or coiling arms.
Brittle stars are a quirkier, more slender version of sea stars. They are composed of a central disk and (usually) five slender arms that radiate out like skinny ribbons or strings. The central disk is made up of a skeleton of calcium carbonate and contains all the internal organs. The digestive and reproductive organs never enter the arms, like they do in sea stars. Brittle stars don't have brains or eyes, but they do have a large stomach, genitals, muscles, and a mouth surrounded by five jaws. Additionally, they have several types of nerve endings in their skin that are touch-sensitive and can sense chemicals in the water, as well as the presence or absence of light. The nervous system consists of a main nerve ring which runs around the central disk. At the base of each arm, the ring attaches to a radial nerve which runs to the end of the limb. They have tube feet on their underside, like sea stars, but the feet do not have suction cups at the end and are not used for locomotion. Rather, they are used for feeding and to help the brittle star sense its environment. The tube feet at the ends of the arms may also sense light and odors, allowing the brittle star to detect and retreat into crevices.
Like sea stars, brittle stars have a vascular system that uses water to control locomotion, respiration, and food/waste transportation, and their tube feet are filled with water. The madreporite, a trap door on the brittle star's underside, controls the movement of water in and out of the brittle star's body. It acts like a pressure-equalizing valve. A brittle star's arms are supported by calcium carbonate plates that function as ball and socket joints (like our shoulders) to give the brittle star's arms flexibility. The plates are moved by a type of connective tissue called mutable collagenous tissue (MCT), which is controlled by the vascular system. So, unlike a sea star, whose arms are relatively inflexible, the brittle star's arms have a snakelike quality which allows the creature to move relatively quickly and squeeze into tight spaces. In most species, the joints between the endoskeleton and superficial plates allow the arm to bend to the side, but not upwards. (However, in the basket stars, the arms are flexible in all directions.)
Even though their bodies are radially symmetrical, they tend to move like a bilaterally symmetrical animal. One lead arm points the way forward, and the arms on the left and right of the pointer arm coordinate the rest of the brittle star's movements in a "rowing" motion so that the star moves forward. This rowing motion looks similar to the way a sea turtle moves with its flippers. When the brittle star “turns,” instead of rotating its whole body, it just picks a new pointer arm to lead the way. Sometimes, instead of rowing, they will simply twist and coil along the sea floor, the arms reminiscent of squirming snakes. The arms give the brittle star its common name. When a predator grabs hold of an arm, the arm easily snaps off, allowing the creature to escape. This process is known as autotomy or self-amputation. When the star is threatened, the nervous system tells the MCT near the base of the arm to disintegrate. The wound heals, and then the arm regrows, a process which can take weeks to months, depending upon the species. As long as its central disk remains, the brittle star will continue to function. Brittle stars are measured by the diameter of their central disc and the length of their arms. Brittle star discs range in size from 0.1 to 3 inches; their arm length is typically a function of their disc size, between two to three times the diameter, in general (although some have lengths up to 20 or more times). Though the typical brittle star body plan has a pentagonal to round central disc that is offset clearly from the five arms, a considerable number of species depart from this generalized shape. Species with six, seven, and up to ten arms are known. A few of the brittle star species found in our waters are Microphiopholis atra, Hemipholis elongata (banded brittle star), and Ophiolepis elegans (rosette-scaled brittle star).
Microphiopholis atra has a tan or gray, round disk, 5 arms, with 3 spines on each side of its arm segments. They reach a maximum of 6 inches. The banded brittle star, Hemipholis elongata, has a mottled brown or gray disk with 5 lobes, 5 arms, and also 3 spines on each side of its arm segments. The arms sport dark banding and the tube feet are red. (Another brittle star has red tube feet, but it has 6 arms instead of 5.) They reach a maximum size of 4 inches. The rosette-scaled brittle star, Ophiolepis elegans, has a round disk that is brown on top and beige underneath, 5 banded arms with 4-6 spines on each side. These brittle stars are easy to distinguish from other brittle stars by the rosette pattern plate formation on their disk. They reach a maximum size of 1 inch disk width, 7.5 inch arm span.
Brittle stars occupy a variety of benthic habitats in all oceans of the world, from the deep sea to intertidal zones, salty and brackish waters, from polar climes to tropical waters. There are an estimated 73 brittle star species in the Arctic alone. The Indo-Pacific region sports an impressive 825 species. In one deep water region discovered off Antarctica several years ago, dubbed "Brittle Star City,” researchers found tens of millions of brittle stars crammed together. Off the coast of Southern California, millions of them can cover large areas of the sea floor in layers up to an inch thick. In the Gulf and bays, they inhabit shell, sand, or muddy bottoms. The first deep-sea animal ever to be reported on was supposedly the brittle star, Gorgonocephalus caputmedusae, accidentally dredged up by Sir John Ross in 1818 while sounding the bottom of Baffin Bay in his attempt to find the North West passage.
Brittle stars are bottom dwellers, so they mostly scavenge off the ocean floor, feeling around for leftovers, and using their tube-feet to sweep the particles along grooves on their arms toward the mouth, located on their underside. They feed on detritus and small oceanic organisms such as plankton, mollusks, worms, small, suspended organisms if available, and occasionally fish. In crowded areas, brittle stars eat suspended matter from prevailing seafloor currents by lifting up their arms to trap tiny particles and algae ("marine snow") using the mucous strands on their tube feet. The mouth has five jaws around it, and crunched-up food particles are transported from the mouth, then through a short esophagus to the stomach, which takes up much of the brittle star's central disk. There are 10 pouches, or infolds, in the stomach where the prey is digested. Brittle stars don't have an anus, so any wastes must come out through the mouth. Brittle stars are often eaten by their larger relatives, the common starfish and the spiny starfish. They hide in cracks and crevices to avoid being eaten, but they can also detect a type of chemical produced by their starfish predators which gives them a head start to escaping. Other predators include gastropods, fish, and crabs. They are also vulnerable to attacks by parasites, including protozoans, nematodes, and algae. Brittle stars play an important role in the food web. They are known as “seafloor ecosystem engineers,” meaning they reshape the seafloor sediment and influence the distribution of other seafloor species.
The sexes are separate in most species, although the sex is not obvious without looking at the brittle star’s genitals, which are located inside its central disk. A few species are hermaphroditic (both male and female in a single individual) or protandric (sequentially hermaphroditic: the individual changes its sex at some point in its life - male to female). The gonads are located in the disk, and open into thin, fluid-filled pouches between the arms, called genital bursae. Fertilization is external in some species, with the eggs and sperm being released into the surrounding water. This results in a free-swimming larva called an ophiopluteus. These larvae have four pairs of rigid arms lined with cilia. They develop directly into an adult, without the attachment stage found in most starfish larvae. Many species brood developing larvae in the bursae. In this case, eggs are held near the bursae, and then fertilized by sperm that has been released into the water. The embryos develop inside these pockets and eventually crawl out. A few species are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. Some brittle stars reproduce asexually through a process called fission, which occurs when the star splits its central disk in half. Regrowth of both the lost part of the disk and the arms occur in both halves, yielding two stars. Brittle stars reach sexual maturity at about 2 years of age, become full grown by 3 or 4 years of age, and live up to 5 years.
Brittle stars are rarely harvested directly by humans, although some species are sold as marine aquarium specimens. However, as they are a dominant component of seafloor faunas, they can be impacted by other human activities such as mining or trawling. Though some species have blunt spines, no brittle star is known to be dangerous or venomous. The brittle star’s only means of defense is escaping or discarding an arm. Scientifically, brittle stars have emerged as a key group for macroecological studies, because they occur in all marine habitats, have a range of feeding and life history strategies, and have just the right amount of diversity and abundance to statistically analyze without becoming a major taxonomic headache. (Macroecology deals with relationships between organisms and their environment on large geographical scales to characterize and explain statistical patterns of abundance, distribution, and diversity.)
Where I learned about brittle stars, and you can too!
Texas Marine Species
World Ophiuroidea Database
Science & the Sea
National Center for Biotechnology Information
Marine Education Society of Australasia
The Wildlife Trusts