HOW DO THEY MOVE?

Ophiuroids  also move very differently from sea stars, which primarily use their  tube feet as little legs. Ophiuroids use their very flexible arms to  "row." One  arm is held out in front of the animal, the two arms on either side  "row," and two arms trail behind. Like  other echinoderms, ophiuroids show no  preference for any arm directing the movement. Any arm of the animal can be the leading arm  as there is no sign of cephalization (or a "head" region).

WHAT’S  INSIDE?

The answer  is fairly brief: not much. Most of the body mass of an ophiuroid is  from the calcite plates that compose it. There are the endoskeletal  elements of the disk and arms, and the fairly massive arm ossicles or  vertebral ossicles. The other major structures are the jaws. Inside  the disk, there is the large sac-like stomach. In each interradius,  there are two genital bursae. These are attached internally to the  gonads. There are a lot of muscles and connective tissues as well as  radial and circumoral nerves and water canals.

HOW DO OPHIUROIDS EAT?

Ophiuroids  exploit a number of feeding strategies. Many ophiuroids possess long  sinuous arms with long arm spines. These forms  are often filter  feeders,  as are the basketstars. The long spines or basket  shape improve the  filtering ability of the animal, or the likelihood that something  will get trapped in the spines! The tube feet then aid in the  transport of the prey to the mouth.

Other  ophiuroids are deposit  feeders,  selective or not, and ingest sediment. They digest the organic  portion and eliminate the rest. Some brittlestars are scavengers,  eating plant or animal material they come across. Many of these  species form symbiotic  associations with sponges or corals. These forms clean their hosts of debris while  getting a safe place to live in return. We are also now learning that  some ophiuroids are active predators.  Generally, small prey (like small shrimp) are captured by the  flexible arm tips. There are  videos of deeper water brittlestars  capturing large prey like squid and fish (see the tabloids section)! Ophiarachna incrassata, a large Indo-Pacifc species that is common in the salt water aquarium trade, is a known predator which traps small fish by forming a cave like structure. The brittlestar ‘stands’ on the tips of its arms, and the fish swims in between them, thinking that it has found a safe hiding place for the night. The brittlestar easily grabs a meal. Some brittlestars may consume such a large meal, that the food breaks through the disk. See “outbreak” in the Tabloids section!.

WHAT’S IN A NAME?

The name  "Ophiuroidea" means "snake-like." Watching an  ophiuroid move demonstrates why the name is so fitting! Have you ever  seen those plastic or wooden snake toys? When you hold them, they  seem to be alive, because of how the interlocking segments move with  one another (these  toys are restricted to move only in a lateral or horizontal plane). This  is very similar to how ophiuroid arms move, and in fact the  construction of these toys is very similar to what an ophiuroid arm  looks like inside! Small vertebrae or  vertebral ossicles form an interlocking series. In  the brittle stars, the movement is primarily restricted to the  lateral plane due to the type of articulation of the ossicles. In the  euryalids or basketstars, the  movement can be in both the lateral and  dorso-ventral plane as they have a different type of vertebral  articulation. These animals can generally coil their arms, which most  brittlestars can not do.

Perhaps you  are wondering, if  the animals are so flexible, why are they called "brittle  stars?" These animals  have the remarkable ability to autonomize,  or voluntarily break off body parts  (usually the arms, but sometimes  part of the central disk too) and regenerate (regrow) them! This  ability is a defensive measure, since fish and crabs are most likely  to grab an arm. The ability stems from the characteristics of a  unique echinoderm connective tissue called Mutable Collagenous Tissue.

SENSES: WHAT’S OUT THERE?

We learn about our world through seeing, smelling, tasting, hearing and touching. But how do ophiuroids sense their world? This is a major area of research, and we are learning some fascinating things!  It has been known for some time that brittle stars have tremendous chemosensory abilities. In essence, they could "smell" the world around them through their tube feet. When brittlestars are fed, they often wave their arms around in order to locate the source of the odor. Often they must rely on the water currents to determine the direction of the food. Sometimes ophiuroids can be right next to food, but if they are not "downwind" of the food, it might be difficult for them to pick up the smell.

A very exciting sense was recently proposed. Drs. Hendler and Byrne noted that some tropical brittle stars become lighter during the night and darker during the day. They proposed that ophiuroids might have a photoreceptor system which would explain their ability to rapidly respond to changes in light intensity (for example, the shadow of a predator). Recent studies of arm plates using a Scanning Electron Microscope showed hundreds of tiny lenses in the arms of some brittlestars (see image, courtesy/copyright Gordon Hendler). The idea is that light is focused onto photoreceptor cells underneath these lenses. The animals becomes darker during the day because the light could be too intense (they effectively put on sunglasses); they are lighter at night in order to pick up as much valuable light as possible.  Typically, being lighter at night would be a potential disadvantage (because predators could see the light color more easily). However, in this case, the brittle star is able to detect subtle changes in light intensity (such as the shadow of an approaching predator, or a nice crevice in which to hide). Thus, it is able to get to safety.  See links to this research on the home page.

There has been limited research on the “intelligence” of ophiuroids. Read about it in the Tabloids.

COLORATION

The coloration of ophiuroids is highly variable, but rarely conspicuous and bright. Most are patterned in some way that makes them hard to see, so as not to attract the attention of predators. Brown, black, white, gray and various mottled shades are common. Here is a picture of Ophionereis reticulata (left), showing the unusual reticulated pattern of the disk. (Photo from the book "Sea Stars, Sea Urchins and Allies: Echinoderms of Florida and the Caribbean" by Hendler, Miller, Pawson and Kier, reviewed in the Books section).  Some forms also have various shades of green, red, purple and blue! Of course, even these colors are used as camouflage! Imagine finding a greenish brittlestar in a clump of algae, or a bluish brittlestar on a coral reef in clear blue tropical water. Blue is common in species of the tropical Ophiothrix. On the right is Asteronyx longifissa displaying stunning reddish coloration. Deeper water animals are often red, because this wavelength of light is quickly absorbed by the water column. Because there is no red wavelength of light available to be reflected, the animal is, effectively, invisible to predators! (Remember that an object is a certain color, because that is the color - the wavelength of light - the object reflects: plants are green because they reflect green light but absorb other wavelengths like red light).

Stellate,  usually with five articulated arms which may be profusely branched  (in some euryalids). Central disk distinct with the arms sharply  differentiated. Mouth ventral, anus absent. Podia reduced and lack  suckers; ambulacral grooves closed; podia extend through tentacle  pores on the ventral surface of the arms. Arms with four major  plates  (though sometimes secondary plates present): 2 lateral, 1 dorsal, 1  ventral. Lateral plates often with spines. Madreporite on ventral  surface formed as modification of an oral shield, indistinct. Disk   covered with granules, spines, stumps of naked (only plates/scales  evident). Jaws, tooth structure and associated papillae are  diagnostic features.

These  are the important features used in taxonomy and classification of ophiuroids

Covering  of the disk, arm spines (shape and number), color, characteristics  of the jaws/teeth and associated papillae, radial  shields, arm combs,  oral (buccal) shields [note: arm structures are described for basal  segments unless otherwise noted (such characters may vary distally).  The number of arm spines (all groups) and tentacle pores (in Ophiomusium)  are given for one side of the arm. The same is true for the number  of oral papillae.] Ratios and comparative measures commonly noted.

PRESERVATION

The best  preservative for ophiuroids, and in fact all echinoderms, is standard  70% ethanol. Formalin is the worst preservative,  as it degrades the  calcium carbonate skeleton quickly, causing irreversible damage.  Formalin also restricts the use of the material for molecular  sequencing and thus should be avoided unless no other alternative  is  available. In such a case, the  material should be transferred to alcohol as soon as possible.  Brittlestars are commonly dried. This is acceptable,  but limits the use of specimens for ecological or  reproductive studies.

One  disadvantage of preserving ophiuroids is their tendency to autonomize their arms when stressed. Simply throwing the animal   into alcohol will most certainly result in broken arms! The animals  are typically relaxed through the addition of fresh water into the  sea water, or by slowly dissolving Epsom salt in their water. An  excellent procedure is given in the book "Sea Stars, Sea Urchins and Allies" (Click  here for more information on this book).

The most  eloquent procedure can be found in Steinbeck'sSweet  Thursday where he related Ed Ricketts' preservation of  "the good kind and sane little animals."

"Doc  got back from his collecting about four-thirty... and submerged in  sea water in his collecting buckets were hundreds of brittlestars. He  moved the brittlestars to a large, flat-bottomed glass dish and  poured some sea water on top of them. The little animals with the  snakelike arms whipped about for a moment and then settled down. When  they were quite still and resting Doc added a little fresh water to  the dish. The arms stirred nervously. He waited a  while and then  added a little more fresh water. To a sea animal, fresh water is a  poison, and if it is slowly introduced it is as subtle as morphine.  It relaxes and soothes until the little creature goes to sleep  and  dies without violence."

For more  on Steinbeck  and brittlestars, read "On the Preservation of Buttlestars" by  Carla J. Bundrick on the Culinary and  Entertainment News page!

FOSSIL RECORD

Echinoderms  are well known from the fossil record. Their calcium carbonate  skeleton facilitates their  preservation.  If you have been out fossil hunting in marine deposits, you most  certainly have collected at least a part of an echinoderm- probably  part of the 'stem' of stalked crinoids. Fossil echinoids, or sea  urchins, are also very common and may seem like rounded rocks. Brittlestars  are somewhat more rare in these deposits. Why? Well, because they  are brittle! They disarticulate quickly after death, so typically  only the plates and  arm ossicles are preserved. These are things that  only experts would likely recognize or be able to identify. Sometimes  the animal is covered with sediment very quickly. In this case, the  animal may be found intact.

Echinoderms  are a very old phylum and extend at  least to  the early Paleozoic  Era (about 500 million years ago).  The earliest ophiuroids are at  least Mississipian in age (350 million years ago). There is some difficulty  however in determining the earliest families, or deciphering the  order of evolution from fossils.   Some  groups are more likely to fossilize than others. The genus I work  on, Ophiomusium, is a relatively common fossil. But it is very  slow to disarticulate; the plates are solidly connected to one   another, and some plates are reduced leaving fewer suture points to  be degraded. Other genera have plates that are more delicate and  which, therefore, disarticulate readily. Because  of this, it is very important  to look at modern and fossil forms  when trying to decipher the fossil record.

The  classic resource for information on fossil echinoderms is the 'Treatise  on Invertebrate Paleontology,'  Edited by Raymond C. Moore, 1966. There are a number of volumes, but  the ophiuroids are covered in Part U, Echinodermata 3. This book  is probably available at university libraries. For a personal copy,  you may wish to try ABE BOOKS an  excellent resource for used books listing the inventory of dozens of booksellers.

For  additional links, see Paleontology Links.

REPRODUCTION

REGENERATION

LIFESPAN

The oldest brittlestar age record appears to be 15 years (perhaps a captive animal). Researchers have reported 5-10 years, but it varies from species to species and location to location. Part of the problem is how to figure this out. Brittles in captivity may live longer than those in the wild due to a reliable food source and low predation (or conversely, much shorter if tank conditions are not stable). It is really quite hard to tell. Many brittlestars lay down growth rings in the major plates. But the question is, “what period of time do these correspond to?” Are they annual rings, like in trees? Basically, they can live a pretty long time under proper conditions, at least many years!! Here is an interesting site that lists the life spans of many animals.