The Ophiuroids: Brittle Stars, Serpent Stars, and Basket StarsAuthor: James Fatherree
There are numerous sorts of marine creatures in the phylum Echinodermata, ranging from sea biscuits, to sea stars/starfishes, to feather stars, to sea cucumbers, all of which belong to one of five living classes within the phylum. This month we’ll take a look at the members of one of these classes, specifically the brittle stars, serpent stars, and basket stars. All of these belong to the class Ophiuroidea and are offered to hobbyists, some more often than others. Note that these may look superficially like starfishes, which are in the class Asteroidea, but the ophiuroids are a different bunch of echinoderms for a number of reasons.
To get started, let’s run through some echinoderm basics. As I said, there are all sorts of echinoderms, and many of them look nothing alike. However, if you take a closer look at them, a few physical features that are characteristic of the group become obvious. First, their bodies/body parts are arranged around a central axis. Whether or not they have arms (like a starfish), they still usually have a round or nearly round body, with parts radiating from its center. This is called radial symmetry, and it’s the same body plan that cnidarians (corals, anemones, jellyfish, etc.) are built on. Echinoderms and cnidarians both have a centrally located mouth and a round body, and many have numerous arms/tentacles radiating from the center.
However, similarities between members of the phylum Echinodermata and the phylum Cnidaria end there for the most part. An echinoderm’s radial body can be divided into five roughly equal parts, or a multiple of five, while a cnidarian’s can generally be divided into six or eight, or a multiple of six or eight. To be more specific, the echinoderms are properly said to have penta-radial symmetry rather than just radial, since their parts come in five, although you may come across a few exceptions to this five-fold plan on occasion. For reasons unknown (at least to me), there are occasional sorts of starfishes that have six/seven arms, or some other non-five number, but these are the oddballs for sure.
All echinoderms also have a unique water-vascular system, which is complex system of muscles, canals, pouches, bladders, tubes, and suckers that allows them to move about and/or to feed. If you’ve ever looked at a starfish up close and noticed the rows of little sucker-feet on its undersides, then you’ve already seen part of this system yourself. They have thousands of suction-cup-tipped “tube feet” that emerge from grooves in their undersides, which can be used for both locomotion and food collection. To the contrary, the same sorts of tube feet emerge from the arms of ophiuroids, but are used for food collection and are not used for locomotion. More on that in a moment.
Lastly, echinoderms also have a skeleton of some sort, which is composed of the mineral calcite and is covered by an epidermis (outer skin). In the case of starfishes and all of the ophiuroids, this calcitic skeleton is composed of thousands of individual plates that are held together by special connective tissues that can be made very soft or very firm. This arrangement allows them to be very flexible or extremely tough when they “stiffen” their bodies in a defensive manner. Other echinoderms, such as urchins and sand dollars, also have a skeleton made of plates, but they’re fused together to form a shell, properly called a test. If you should look closely at a dead urchin’s test you’d see that the whole thing is made up of individual plates that are melded together by the same type of joints that you’d find holding the bones of a human skull together. Still, in other types of echinoderms, like the sea cucumbers, the skeleton is rudimentary, and is nothing more that a number of tiny oddly-shaped calcitic plates, which are embedded in their thick skin of connective tissue.
So with so many similarities, why are starfishes and ophiuroids are in separate classes? Most ophiuroids may indeed look like starfishes at first, but there are actually several fundamental differences between the ophiuroids and the asteroids. First, ophiuroids have long, thin arms that are clearly distinct from their organ-containing body, which is typically rather small and somewhat flat. Conversely, a starfish’s body isn’t distinct and there’s no clear line where the body ends and the arms start. The brittle stars and serpent stars are also restricted to having only five arms, which are used for feeding and for locomotion. And, in contrast to the asteroids, ophiuroids don’t use their tube feet on the underside of their arms to slowly move around, but literally crawl/walk around using the arms themselves. This gives them considerably more speed than asteroids, and some can move their arms quickly enough to make it off the bottom and swim a bit.
Many asteroids eat by actually turning their stomach inside out and sticking it out of their mouth, which works very well for those that eat clams in particular. They only have to use their sucker-tipped tube feet to pull a clam’s shell open just a bit and then stuff their stomach inside the shell to finish the job. However, the ophiuroids lack an eversible stomach, and thus cannot eat clams (at least not in the same way), or many other items that asteroids can.
Still, many are successful scavengers and predators that eat a variety of worms, snails, and crustaceans. Some can even use their arms to hold their body off the bottom while they sit and wait for a small fish or other prey animal to swim or crawl in under it. The trap is then sprung and the arms close down and quickly move the body downward onto the prey. The victim thus ends up under the mouth, where it is consumed whole. Others are detritivores that move around over the bottom, picking up bits of fish wastes and such, while some burrow through the sediment extracting what they can. And the basket stars are quite different, as they are suspension feeders that open their feather-like arms into the current and snare anything that bumps into one of them. Anything from large plankton to small fishes can be grabbed this way, and then passed to the mouth and eaten. This certainly sets them apart from any asteroids.
Within the class Ophiuroidea, for the most part it’s pretty easy to tell the three basic types apart. Many brittle and serpent stars may initially appear to be very similar, but the outwardly visible difference in the two is the general lack of any projections from the arms of a serpent star. The arms of brittle stars are much fancier and are typically covered with lots of spines, spikes, and/or clubs of various sorts and sizes, while those of a serpent star are relatively smooth and are typically undecorated. This division by calling some brittle stars and some serpent stars is actually a non-biological one though, and is not based on real taxonomic differences between the two groups. It’s only a division based on general appearance, as some hobbyists, divers, etc., may call various ophiuroids either brittle or serpent stars, while other folks may just call them all brittle stars regardless of what they look like. So, don’t get confused if you see or hear it some other way. The fact is, there are actually several ophiuroids that look to be something in between, with smooth discs and just a row or two of relatively small projections coming off their arms. However, basket stars are a distinct group among the ophiuroids, and they have arms that are exceptionally long and thin. These thin arms are also highly branched, and branched, and branched, and branched, thus forming an effective net to capture bits of food/prey that pass by.
Okay, let’s get to some more specific info about keeping various ophiuroids (or not keeping them) in aquariums. I’ll go ahead and start with the bad news first: basket stars are off-limits. Because they live off a diet of suspended plankton and other small critters, they are not suited for life in captivity. I don’t know of a single person that has been able to keep one alive long-term in a home aquarium.
There are about 2000 species of brittle/serpent stars (Brusca & Brusca 2003), making the ophiuroids the largest class of all the echinoderms. I can’t really go into much detail about so many kinds, as their sizes, lifestyles, diets, etc., are highly variable. However, what I can tell you is that the smaller brittle stars in general are great additions to both reef and non-reef tanks because they’re scavengers/detritivores that can usually find enough leftover fish food and wastes for themselves, while not bothering anything else. There are also several species that are especially good for reef aquariums that have deep sandbeds, as they burrow into the sand and move around, which helps keep the sand mixed up and cleaner. However, there are also some particularly big ones that will also eat small fishes and some motile invertebrates, so beware of the larger species.
Of course, you can also give most brittle stars any sort of fish food, such as clam or shrimp meat, or even flake food, and they’ll usually gobble it up quickly. I have two in my own reef tank that stay hidden in the rockwork for the most part, and they’ll catch flake food with their arms/tube-feet when it drifts close enough to them. All you see is some skinny arms flailing between some pieces of live rock near the bottom, and every once in a while they snag something. To the best of my knowledge they’ve never bothered any of my other livestock though, and neither has any other small-to-medium brittle star I’ve ever had.
While you won’t likely see them for sale, there are all sorts of relatively small brittle stars that live in sponges and/or coral heads and which typically have very thin arms that are quite fuzzy looking. So you might find one living in a specimen at some time, but these are nothing to worry about. I’ve never seen any of these do any harm to whatever they were living in.
Serpent stars, on the other hand, are generally bigger than brittle stars and can cause problems. Many are detritivores, like most brittle stars, but some are carnivores, and I’ve seen and heard about large serpents eating everything from small fishes to hermit crabs; you’ll want to keep that in mind before adding one to a tank.
I had a very large red one that would smell the fish food I put in one of my non-reef tanks and would immediately come out from underneath the coral head it called home, stand on two arms, and prop itself up on the front pane of the tank, waving the rest of its arms for food. Dropping in a few sinking shrimp pellets was all it took to keep it happy and growing. I did come up with a missing damsel every once in a while, and was left to wonder if it had caught them. I definitely wouldn’t have put it in a reef tank for fear that, if nothing else, it would bowl over anything and everything in the tank. Still, as bad as all that sounds, there are some that stay smaller in size and will scavenge up enough food to stay trouble-free.
Aside from size/compatibility issues, there are a couple of other things to know about brittle/serpent stars before I finish. The first is that you need to be very careful when acclimating any of them. For whatever reason, I’ve found that these are generally quite sensitive with respect to changing conditions, and should be given plenty of time to adjust to your tank’s water. A slow drip acclimation is likely the best way to go, and all you need is a small bucket and a piece of airline tubing. Put the specimen in the bucket with the water from the store, and then start a siphon from the tank to the bucket with the tubing. To slow down the flow, just tie a knot in the tubing. Then, let the tank water slowly mix into the store’s water until you have at least four times as much water in the bucket as you started with (roughly). Then add your specimen to the tank.
Secondly, be sure to look for any sort of white slimy-looking matter on a specimen while shopping. They tend to turn white and mushy when they are in bad health, so be on the lookout for anything that doesn’t look right. In my experience they very rarely recover once this starts to happen, so pass over a specimen showing any such signs.
However, there is no need to pass on a specimen that has lost an arm or two, as long as it appears healthy otherwise and is actively growing back the lost arm(s). Echinoderms as a whole are well known for their capacity to regenerate lost parts, as they can grow whole new body parts with ease in the event that they may be lost. So, as long as there is no sign of decay, and you can see that a new arm is growing back, rest assured that it will continue to grow and the owner will recover as long as conditions are good in its new home.
For more information, check out WetWebMedia.com or look elsewhere online for help with the species you are interested in researching.