Brittle Stars: Secrets of the Ophiuroidea

Although brittle stars are not main attraction in most reefs, they do make an interesting side show.

By Daniel Knop
Web Bonus Content from the September/October 2013 Issue of CORAL Magazine
Additional Bonus Brittle Star Articles

Brittle stars have a peculiar body plan. A round central body disc carries five, six, or even seven slender, radiating arms that move like snakes. The arms gave these animals not only their other common name, serpent stars, and their scientific name, Ophiuroidea (Greek: ophis = snake), but these appendages also repel some hobbyists —not everyone likes snakes.

The arms of brittle stars, unlike those of starfishes, do not seamlessly attach to the central body disc but are sharply delineated. In some species, the arms also form numerous branches. Another difference between brittle stars and starfishes is that brittle stars are able to move their arms sideways, which allows for rapid locomotion. The arms consist of individual calcite vertebral ossicles that are hinged together. Ambulacral feet (tube feet) are located on the bottom of the animal. As in other echinoderms, the feet are controlled and moved by a hydraulic canal system inside the body. They lack the suction cup extensions that starfishes use to attach to the substrate, but they do have some adhesiveness, allowing many species to dab up food particles with them. Brittle stars use their feet mainly for olfactory sensations.

The mouth is the only opening of the digestive tract; there is no anal opening. Actually, the term “tract” is a misnomer here, since it is only a stomach cavity in which the food is digested to recover the nutrients. Afterwards, the waste is eliminated again via the mouth. In principle, the brittle star’s food intake is similar to that of the starfish. These two classes of animals also share many other similarities. However, besides the differences described above, most brittle star species have spines on their arms and the madreporite, which connects the ambulacral system that controls the hydraulic feet with the surrounding water, is not located on the top side, as it is in starfishes, but on the bottom.

The gonads (bursae), in which the germ cells are formed, are located in the respiratory cavities on both sides of the armpits. The previous article describes the localization and function of the bursae in detail.

Brittle stars are known from fossils since the Early Ordovician, a geological period that began 485 million years ago, but their scientific classification has not been particularly well studied. Because of their delicate bodies, these echinoderms only rarely formed continuous fossils that could provide scientific information about the relationships of extinct species. Therefore, we know much more about the evolutionary history of other animal groups. Usually, the arms of brittle stars disintegrate quickly into their individual components. Such fossil pieces are found often, but they are not very specific, and even experts usually have difficulty with their taxonomic classification.

Habitat

Today, brittle stars are almost omnipresent in the world’s oceans. They are found in virtually all habitats, from the intertidal zone down to depths of more than 23,000 feet (7,000 m). In coral reefs and adjacent habitats, they are often found in large densities, for example in sediments or the intertidal zone. Many species live in community with corals or other invertebrates, particularly dwarf species that remain below 0.8 inch (2 cm). They form veritable colonies that cling with their thin arms to the branches of soft or horn corals during the day and stretch the arms out at night to catch plankton in the open water.

Beyond the reefs, brittle stars will occupy any ecological niche that is available to them. Excellent examples are underwater mountains, ecosystems that have been known and explored for only a few years. These mountains rise from the seabed but do not reach the surface. They are usually formed by submarine volcanoes. Their isolated location makes them ideal for the development of marine communities, which consist of sponges, corals, and other invertebrates, and sometimes also of crustaceans and fishes. Larvae drift there with the currents and can establish themselves, but the distances to coral reefs are so great that predators of any kind can hardly reach them.

These deep-water reef communities are fed by planktonic organisms carried by ocean currents. On one such underwater mountain south of New Zealand, whose summit is located about 295 feet (90 m) below the sea surface, tens of millions of brittle stars have been found in massive accumulations—in some places more than 100 per 11 square feet (1 m²).

Evolutionary history

Brittle stars are related to sea lilies (crinoids)—sessile animals with simplified bodies and reduced structures. They gave up their mobility and environmental awareness in order to colonize new ecological niches. The vast majority of their representatives have been extinct for a long time; only a few deep-sea species still exist. According to today’s understanding, the precursor forms of the crinoids developed the five-segment radial (pentaradial) symmetry from the original two-sided body shape (bilateral symmetry) that we find today in echinoderms such as sea urchins, starfishes, and brittle stars. The original two-sided symmetry is still visible in the eight-armed ophiopluteus larvae from which brittle stars emerge.

Later, when changes in environmental demands again favored mobility as beneficial or even essential for survival, the descendants of the sessile crinoids gave up their sedentary lifestyle and apparently developed a new mobility. This included the ability to perceive and respond to environmental stimuli. However, these species were restricted by their current anatomical structures; body structures they had abandoned in the course of evolution were lost. For example, it was not possible to develop the complex eyes that other animal groups had. So echinoderms had to find their own solutions for these challenges of their environment.

One example of this is the peculiar visual organs that have recently been discovered in brittle stars. Although they do not have eyes in the conventional sense, they are able to distinguish between light and dark—for instance, they can detect a shadow cast by an approaching predator and escape into a rock crack. Their development took a very unusual path, as was first demonstrated by Ophiocoma wendtii (Addadi, et al., 2001). From this research, we know that O. wendtii controls the formation of crystalline structures using specific proteins. These crystals are arranged in the skeletal elements in spherical groups. They focus the light without scattering like a tiny lens and concentrate it approximately 50 times in the focal point. Passing the light on to photoreceptors, the nervous system thereby receives a corresponding stimulus. In this way, brittle stars register brightness without eyes.

Aquarium care

Today the trade offers numerous species, not just the few large ones found in the 1980s and 1990s (e.g. Ophiarachna incrassata, Ophiolepis superba, and the bright red Caribbean species Ophioderma squamosissimus). Very often, you will find small brittle star species that reproduce spontaneously and form very dense populations under appropriate conditions. Mostly, however, they inadvertently get into the tank along with corals. One example is Ophiocoma pumila, which shows a characteristic red striation on the arms and reaches about 2.4 inches (6 cm), but other smaller species are also common.

Most brittle star species are unproblematic in the aquarium, but there are some exceptions. Feeding is very easy, because they are usually carnivorous and also feed on dead animals. Fine suspended particles attach directly to the tube feet, while larger chunks are grabbed with the tip of the arm and transported to the mouth opening by rolling up the entire arm. Many brittle stars can open wide and deform their oral discs in order to swallow large chunks.

When adding a specimen to the aquarium, one should always proceed with caution. As with all echinoderms, slow adaptation to the aquarium water is important. Brittle stars also tend to sacrifice individual arms when threatened. A severed arm can go on moving for hours. This appears to distract the attention of a predator from the fleeing brittle star. Although a lost arm soon regenerates under good nutritional conditions, such a loss should still be avoided.

Especially the small species are fascinating aquarium guests, as they increase biodiversity. They are commonly found in many reef tanks. Especially if there are no specialized predators, brittle stars multiply quickly by vegetative propagation to form individual-rich populations. However, they mostly live hidden and are rarely seen—so many aquarists have no idea how many live in their tanks. A mass spawning event, when the animals compete for the best spot to release their germ cells, provides a rare opportunity to observe the entire population at the same time.

On Internet forums, people sometimes refer to brittle stars as pests. These individuals use brittle star “wedding events”—spawns—to collect the animals in large numbers and dispose of them. This is absurd and wasteful. Those who do not want these fascinating echinoderms in their aquariums should pass them to others. Their mass occurrence alone is no burden for a reef tank, as long as no visible damage can be unambiguously attributed to them. In general, these scavengers live so much in secrecy that you barely notice their presence.

In terms of diet, one of the exceptions mentioned is the quite popular and regularly offered Green Brittle Star (Ophiarachna incrassata, see below). It can even multiply in the aquarium. This species is known to attack soft corals and fishes, but only when it is underfed—if regularly fed a chunk of fish twice a week, it should not pose any problems.

Brittle star species for the aquarium

Within the animal phylum Echinodermata (sea urchins, starfishes, sea cucumbers, and others), the brittle stars belong to the class Ophiuroidea. Ophiuroidea consists of the three orders Ophiurida, Oegophiurida, and Euryalida (or Phrynophiurida) that contain 17 families with about 250 genera. A total of around 2,000 species are known, making this class of animals the most diverse of echinoderms.

Most brittle stars interesting to aquarium hobbyists belong to the first of these three orders: Ophiurida. There we find the genera Ophiarachna, Ophiocoma, Ophioderma, Ophiolepis, Ophiothrix, and Ophiomastix, the small Amphipholis, and other small representatives of the families Amphiuridae and Ophiotrichidae.

Very peculiar brittle stars are found in the third order, Euryalida. They are more primitive than any other brittle stars and are mainly characterized by their smooth, spineless arms. In addition, they live closely tied to specific invertebrate host animals. Their arms are usually much shorter in relation to their bodies than animals in the other two orders, and the whole body is covered with a slimy secretion.

These unusual brittle stars appear occasionally in the trade as members of the genus Astrobrachion, e.g. A. constrictum. However, they are an absolute rarity and so inconspicuous that they are found only through a targeted search. In addition to these small species, the order contains the family Gorgonocephalidae, large brittle stars with highly branched arms. They look particularly impressive at night, when they spread their arms into a semi-spherical basket shape to catch planktonic food.

The vast majority of brittle star species that we find in the trade belong to the order Ophiurida. The genus Ophioderma (family Ophiuridae) contains the Red Serpent Star (Ophioderma squamosissimus), one of the most beautiful species with its orange-red coloration. Its range extends from the Bahamas to Belize. The maximum span of the Red Serpent Star is 16 inches (40 cm), although it doesn’t usually grow that big in the aquarium. Its appetite is similar to that of the Green Brittle Star (see below)—it, too, is an omnivorous opportunist that must be fed well and regularly. Any replacement food, such as food tablets or chunks of food fish, will be greedily accepted. Because the Red Serpent Star is active during the day as well as at night, it is ideal for the coral reef aquarium.

The genus Ophiolepis (family Ophiodermatidae) contains the Banded Brittle Star (Ophiolepis superba), another very pretty species. It wears brown bands on a light beige background; these bands extend all over the arms, and the round body disc has a brown star. The distribution area extends over the entire Indo-Pacific, from the Red Sea and East Africa to the Pacific Islands. The span of this species can reach almost 10 inches (25 cm). With regular feeding, this normally nocturnal brittle star can get used to looking for food even during the day. It is omnivorous and searches sediments for organic leftovers. This species is robust and ideal for a coral reef aquarium.

The Green Brittle Star (Ophiarachna incrassata) also belongs to the family Ophiodermatidae. It is durable and easy to care for. It is similar to the two species mentioned above, but far more opportunistic when searching for food. If threatened by food shortages it will even graze on Xenia. The brittle star lowers its mouth disc onto a soft coral, engulfing it with its stomach cavity, and then lifts the entire body, ripping off part of the coral. If they have to dig even deeper into their bag of tricks to survive, they will even catch small, sleeping fishes between their arms at night. However, if they are sufficiently fed with a food table or piece of food fish twice a week, they will hardly cause any damage in the aquarium.

Ophiarachna incrassata has reproduced several times in the aquarium, but this seems to be coupled to an optimal diet. Dr. Jochen Lohner kept two large Green Brittle Stars in an aquarium, and when he took it apart after a few years he found 13 adult specimens.

The representatives of the genus Ophiomastix (family Ophiocomidae) are somewhat more delicate than the species listed above, and they have a very typical shape. The arms have conspicuous spines, and if you look closely you can see that many of the spines have a club shape. Many of these species have a nice color and reach a maximum size of approximately 12 inches (30 cm). They are easy to care for, harmless, and a real asset to the coral reef aquarium. They might graze on very delicate sessile invertebrates, such as small sea squirts, but they do not touch corals.

The family Amphiuridae contains numerous small species that vegetatively reproduce in the aquarium. In the absence of specialized predators they can create very individual-rich populations. They are found in sediments, cracks, and crevices, or at the bottom of stony corals, under sponges, algae, or other places where they like to rest and let some of their arms hang in the open water. When they discover suspended food in the water, they begin to wave with their arms or even come completely out of hiding. One of these genera is Amphiura.

Representatives of the genus Amphipholis represent the smallest known brittle stars and reproduce well in the aquarium. At least one of the currently 25 species appears occasionally in the trade: Amphipholis squamata. Found in tropical and subtropical seas around the world, it reaches about 12 mm (0.5 in). The body disc measures no more than about 2 mm. These tiny brittle stars live mainly in fine sediments and can generate huge populations in the aquarium. The aquarist gets to see them only when cleaning a filter or moving rock and underlying fine sediments. These brittle stars reproduce vegetatively by division, which can be seen on those specimens that have different arm lengths because they regenerate lost arms.

 

Many tropical species that are not too large are also found in the family Ophiotrichidae. However, in contrast to the Amphiuridae, they are closely tied to sessile invertebrates such as corals and sponges, on which they live as commensals. An example is the genus Ophiothrix, containing some species with characteristic appearance. The coloration is very variable and species-specific, probably in adaptation to each host. The arms are densely covered with long spines, and the size in most species is 4–8 inches (10–20 cm). Naturally, you meet these attractive brittle stars on sessile invertebrates, like sponges. They hide in their gaps or cracks and at night scour the surface thoroughly, looking for something to eat. Distribution areas, depending on the species, are the entire western Indo-Pacific or the Caribbean.

Smaller still are the representatives of the genus Ophiothela, who live primarily on sessile invertebrates such as sea fans and sea pens. The size of this animal’s mouth disc is 2–3 mm, and the arm length ranges from 0.4 to 6 inch (10–15 mm). We usually see these brittle stars clinging with their striated arms to the branches of their host corals. With food suspended in the water, they stretch their individual arms into open water and spread their tiny tube feet, which often have a feathery structure to increase the surface area. The distribution area of the six known species covers the entire Indian Ocean and Western Central Pacific, from East Africa to the islands of the South Pacific.

Unfortunately, these attractive brittle stars are almost never purposely imported. They appear in the trade only as hitchhikers on their host corals, and you have to specifically look for them. In the aquarium, they need fine suspension food and absorb even dissolved substances from the water. However, they only survive if the food is available daily for a very long time. One or two suspension feedings per day is not enough.

The 22 species of the family Ophiocomidae are usually significantly larger than the above. In the aquarium, we encounter mainly Ophiocoma pumila; it has a fairly typical appearance, with its reddish body disc and red-striped arms. If you look closely, you can see greenish elements between the red marks, and both have the same fine line pattern. The number of arms is five or six.

Ophiocoma pumila can develop very individual-rich populations in the absence of specialized predators. Their reproduction in the aquarium is vegetative through partition, and occasionally you can see specimens with only three long arms after the separation of the body disc. However, the opposite side, with three short arms, is just regenerating. Since there are always three long arms, it is conceivable that only six-armed specimens divide.

Especially the propagation of these small brittle stars makes their aquarium care fascinating. And as seen in the previous article, the normally hidden brittle stars can occasionally be observed en masse during spawning.

References

Addadi, L., J. Aizenberg, A. Tkachenko, S. Weiner, & G. Hendler. 2001. Brittlestar optics: Calcitic microlenses as part of the photoreceptor system. Nature 412: 819–822.

Knop. D. (in press): Lexikon der Meeresaquaristik. Natur und Tier Verlag, Münster, Germany.