Brittle stars are sea creatures with five limbs and no brain. Found on sea floors around the world, they are echinoderms along with sea stars (starfish), sea urchins and sea cucumbers but are different from sea stars. There are over 2,000 species of brittle stars. They are so named because their arms break off so easily. The limbs quickly grow back. Some species change their colors from night to day.
Brittle stars are pentaradially symmetrical (divisible into five equal parts). they use five multi-jointed limbs to locomote along the seafloor. They can move pretty fast for a starfish-like creature. Unlike other echinoderms they can move quickly and escape rapidly into crevices and rocks where they hide.
Brittle stars catch live fish, squid and crabs with their highly flexible arms. They have several ecological roles. They prey on a variety of fish, crabs, and starfish. Their large beds can create shelter for other animals such as bivalves, which feed upon diatoms and phytoplankton, removing particulates from the water column. Bivalves are also detritivores and help recycle nutrients on the ocean floor. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
Difference Between Brittle Stars and Sea Stars
Within the echinoderm phylum, sea stars are in the Asteroidea class, while brittle stars are in the Ophiuroidea class, which also includes basket stars. Erin Spencer of the Ocean Conservancy wrote: Their fundamental structure is different, especially when you look at where the arms connect to the center of the body. Sea stars have thicker, triangular-shaped arms that are typically their widest at the point of connection to the central of the body. Brittle stars, on the other hand, have much thinner arms that appear more “whip-like” than those of sea stars. The arms connect to a central disk, and it’s relatively easy to tell where the arm ends and the disk begins. [Source: Erin Spencer, Ocean Conservancy, August 13, 2019]
Next, look at how they move. Sea stars rely on their water vascular system to move. The water vascular system includes a number of small tube feet that become stiff when water is pushed into them, allowing the sea star to move on a conveyor belt-like rotation of feet.
Although brittle stars also have a water vascular system, they twist and bend their long arms to move, instead. This means that they can move much more quickly than sea stars, especially when trying to escape a predator.
If you’re still not sure, look for the location of the madreporite, also known as the sieve plate. This round, specialized plate allows water to enter the water vascular system. In sea stars, it’s located on the “top” (or aboral, meaning opposite of the side of the mouth). In brittle stars, it’s located on the “bottom” (or oral side). But, let’s be honest—it’s way easier to tell them apart by looking at their arms (see above).
Brittle Stars — Animals with No Obvious Front
Unlike humans and most other animals, brittle stars have no obvious front,. A study published in May 2012 in the Journal of Experimental Biology reported that these brainless creatures nonetheless are able to move in a coordinated way, by designating one limb as the “front-facing” limb, and using two others to propel forward. “They are pushing forward with the front two limbs, like a turtle,” Henry Astley, the study’s author and an evolutionary biologist at Brown University, told the New York Times . “The back limbs aren’t highly involved.” [Source: Sindya N. Bhanoo, New York Times, May 14, 2012]
According to the New York Times: Most animals, including humans, are bilaterally symmetrical. In other words, drawing a line down the center results in symmetrical halves. A few animals, brittle stars included, are radially symmetrical: They can be sliced in many different ways and still be symmetrical, giving them no clear “front.” But while moving, the brittle star is able to designate a front, and act as if it is bilaterally symmetrical, Mr. Astley said. The brittle star is also able to switch its front-facing limb as needed, and this enables it to swiftly change direction, Mr. Astley said. “If we turn when we’re walking, we have to change our direction,” he said. “They just decide another direction is front, and they’re off.”
Scientists have long thought that bilateral symmetry confers an evolutionary advantage, because it allows for directed movement when searching for food or avoiding predators. But the new study puts a twist on this notion, Mr. Astley said. “You can get the benefits of bilateral symmetry without being bilaterally symmetrical,” he said. “You can become behaviorally bilaterally symmetrical.”
Brittle Star Locomotion
Most animals are bilaterally symmetrical meaning both sides of the animal are the same. This trait is thought have occurred during evolution to make locomotion easier. Brittle stars are different. They are radially symmetrical around a central axis. Unlike starfish they don’t rely on tube feet to get around but instead use five long, flexible, multi–jointed limbs.
It was once thought that brittle stars moved their limbs in coordinated movements similar to tetrapod vertebrates (four-legged animals). In the abstract to the study mentioned above, Henry C. Astley of Brown University wrote: It is uncertain whether the ring-shaped nervous system, which lacks an anatomically defined anterior, is capable of generating rhythmic coordinated movements of multiple limbs. This study tested whether brittle stars possess distinct locomotor modes with strong inter-limb coordination as seen in limbed animals in other phyla (such as tetrapods and arthropods), or instead move each limb independently according to local sensory feedback. [Source: Henry C. Astley, “Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star, Ophiocoma echinata”, Journal of Experimental Biology, June 2012]
Limb tips and the body disk were digitized for 56 cycles from 13 individuals moving across sand. Despite their pentaradial anatomy, all individuals were functionally bilateral, moving along the axis of a central limb via synchronous motions of contralateral limbs. Two locomotor modes were observed, distinguishable mainly by whether the central limb was directed forwards or backwards. Turning was accomplished without rotation of the body disk by defining a different limb as the center limb and shifting other limb identities correspondingly, and then continuing locomotion in the direction of the newly defined anterior. These observations support the hypothesis that, in spite of their radial body plan, brittle stars employ coordinated, bilaterally symmetrical locomotion.
Common Brittle Stars
Common brittle stars (Scientific name: Ophiothrix fragilis) are commonly found in the eastern Atlantic Ocean from the North Sea and the British Isles in the north to South Africa and the Cape of Good Hope in the south. They have also been observed around the Azorean Islands. They live 10 years in the wild on average.
Common brittle stars are typically found at depths of zero to 350 meters (1150 feet). They often live in large groups offshore. As many as 2000 individuals per meter have been recorded in brittle star beds. Within the intertidal zone, they are most often found individually. They typically prefer hard sand and shell bottoms and are often found under shells or rocks, but can be found on rocky reefs and soft substrata as well. They favor areas with strong currents but steer clear of areas with high sedimentation because high levels of sedimentation can prevent them from feeding and can disrupt respiration. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
Common brittle stars have not been evaluated for the International Union for Conservation of Nature (IUCN) Red List. They have no special status according to the Convention on the International Trade in Endangered Species (CITES). They have no known economic importance to humans but dense aggregations of them can be a nuisance to fisherman.
Brittle Star Characteristics, Behavior and Senses
Brittle Stars are ectothermic (use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment) and have radial symmetry (symmetry around a central axis). The central body discs of common brittle stars ranges in size two to 20 millimeters (0.08 to 0.79 inches). When their arms are stretched out, some species are over a meter wide. Sexual Dimorphism (differences between males and females) is minimal: Both sexes are roughly equal in size and look similar. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
Common brittle stars are usually brown, red or white. Some are also yellow, spotted or banded. They have cryptic coloration that disguise their appearance to blend in with their surroundings. This form of camouflage helps them avoid predation.
The five legs of the common brittle stars are very long and slim (typically five times the diameter of their body in length) and are segmented, with tube feet and seven serrated spines per segment. The central body is covered in spines as well. Brittle stars have skeletal plates that function as microscopic lenses and serve as primitive eyes by focusing light unto nerve cells below. Scientist are study the lens for insights into improving switching stations for optical fibre networks. /=\
Common brittle stars are nocturnal (active at night), motile (move around as opposed to being stationary), sedentary (remain in the same area), solitary and colonial (living together in groups or in close proximity to each other). They sometimes form dense aggregations with 340 to over 2000 individuals per square meter. Often they hide under rocks and in crevices during the day and come out to feed at night. Territory size varies depending on the population. Scientists have found that there is often an ophiuroid-free "halo" separating brittle stars from nearby rocky reefs where there may be predators.
Common brittle stars sense and communicate using touch and chemicals usually detected with smell. They also employ pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species), particularly when breeding. These brittle stars form their beds based on interaction with other brittle stars, not due to environmental cues.. /=\
Brittle Star Feeding and Predators
Common brittle stars are nocturnal passive suspension feeders. They feed mainly on detritus, diatoms, phytoplankton and carrion. To feed, they lift up their arms through crevices in rocks, catching passing food particles and pass them on their its mouth where two pairs of tube feet in the mouth, a vertical row of teeth and group of tooth papillae are used to consume the food. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
Common brittle stars are prey to many species of fish, portunid crabs, and some species of starfish. Their main predators are cuckoo wrasses, common dragonets, ballan wrasses, velvet crabs, spiny starfish, and common starfish. Brittle stars have two main defense mechanisms against predators: their have cryptic, camouflage coloring and behaviors such as feeding at night and hiding during the day,. They also have the ability to lose their arms, which later grow back.
Brittle Star Mating, Reproduction and Offspring
Brittle stars are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They: 1) engage in external reproduction in which sperm from the male fertilizes the female’s egg outside her body; 2) employ sexual induced ovulation (release of a mature egg from the ovary); and 3) use broadcast (group) spawning, the main mode of reproduction in the sea. It involves the release of both eggs and sperm into the water and contact between sperm and egg and fertilization occur externally. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
Common brittle stars engage in seasonal breeding once a year. The breeding season is May to July although some have been reported to breed year-round and others breed as late as October in others, They are polygynandrous (promiscuous), with both males and females having multiple partners. Individuals usually spawn once per breeding season. Adults help juveniles to feed. The juveniles they aid, however, may not be their own as larvae develop in the water column and ocean currents move the larvae around.
Common brittle stars mate by releasing sperm and eggs into the water, with the release of sperm by the males triggering females to release eggs. The eggs are fertilized and develop in the water column. Sex organs are typically most developed from May to July. It is believed that common brittle stars reach sexual maturity when body disc is approximately one centimeters in diameter. though sex organ tissue has been found in individuals as small as three millimeters.
Brittle Star Development
The life cycle of brittle stars is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. Common brittle stars have three major life stages: 1) larva, 2) juvenile, and 3) adult. Development from the larval to the adult stage takes 20 to 30 days, depending on food supply availability, while development to full maturity may take 6 to 10 months. [Source: Patricia Holland, Animal Diversity Web (ADW) /=]
According to Animal Diversity Web: Studies of larvae indicate that they travel passively in the water column to new locations. Common brittle star eggs are 0.1 millimeters in diameter. Approximately 12 hours after fertilization, the oval-shaped larvae leave their egg membranes and form a vacuolated crest (a crest with a fluid-filled cavity in the center). A day later they form posterolateral arms. The beginning of the exotrophic larval period is determined by the formation of the mouth and anus (typically during the third day). During the next 10 days, common brittle stars develop their anterolateral, postoral, and postdorsal pairs of arms.
Metamorphosis begins around the 15th day of development, signaled by the split of the hydrocoele (water vascular system) into five lobes and subsequent wrapping around the esophagus. Next, brittle stars enter the endotrophic period. Their larval arms (not including the posterolaterals) regress, followed by the regression of the esophagus and intestine. During this time, larvae begin to develop their five adult tentacles as well as mouths and, on the undersides of the arms, tube feet (podia). Finally, juveniles lose their larval posterolateral arms and develop hooks on the ends of their arms which are used to latch on to adults. Juveniles depend on adults to help them feed until they are capable feeding on their own.
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated May 2023