Aquarium Invertebrates: Sea Stars -- Linckia spp.
About once or twice a month I see a question posted on one of the reef bulletin boards asking about adding a Linckia sea star to someone's reef tank. It may be a red, blue or purple Linckia, but the questions are usually pretty similar otherwise. How do I care for this animal? What is the best way to acclimate them? And, are there any differences among the different color variations of these sea stars? This month, I'll try to answer these questions, and provide some information about how best to select, introduce, and maintain one of these beautiful sea stars in the reef aquarium.
The most commonly imported "Linkia" sea star in the pet trade is Linckia laevigata, but despite its beauty and the frequency with which these animals are imported, there is very little known about the natural history of this animal. In fact, it turns out that we are not even very good at telling the different species of Linckia apart, because L. laevigata (the blue Linckia ) and L. multifora (the red Linckia ) turn out to apparently be the same species! Using molecular genetic techniques, Williams (2000) showed that L. columbiae and L. bouvieri were good species, but L. guildingi consists of two different "cryptic" species (non-interbreeding species that look so similar that they can not always be told apart by eye). Although the two species currently known under the name L. guildingi look the same to our eye, the genetic evidence shows that they have not been able to interbreed for about 1 million years! Unlike the cryptic differences in L. guildingi, the obvious differences we can see between L. laevigata and L. multifora are apparently insufficient to warrant them being classified as different species according to Williams (2000). Despite the differences between these two morphologically distinct groups of sea stars (individuals can be reliably distinguished by their color and color pattern, number of madreporites and ratio of arm length to breadth), researchers found no evidence that these sea stars were genetically isolated from one another in the same way as the two groups currently known as L. guildingi are.
In terms of the different color morphs, most people generally want to know if they all behave the same, or whether different colors of sea stars have different behaviors in the aquarium. The simple answer to that question is maybe. The problem is that there is quite a bit of natural variation in color among L. laevigata collected from different areas, and there are some regions where L. laevigata are almost always brilliant blue, while in other areas the stars may be predominantly pale, brownish, or even orange in color. Given that the taxonomy of the group is a little fuzzy still, and we're not even really sure of the species, the importance of differences between the color morphs are somewhat hard to pin down, exactly. What is known about Linckia is that the animals gets their color from a blue pigment called linckiacyanin and some accessory yellow carotenoids that give the star its rich blue color . Depending on the exact ratio and combination of pigments in the star, the colors of the starfish can range from blue, to brown or orange, but it is not clear whether there are any behavioral or ecological differences among sea stars that have these different colors. Although certain color variations are more common in some areas than others, researchers do not know whether these color differences result from some ecological or genetic differences among the sea stars.
The "purple linkia" is a different story, however. There are some purplish morphs of L. guildingi in both the Caribbean and Indo-Pacific, but they are not commonly the ones offered for sale in pet shops (in fact, I have yet to see a purple one of these stars in a pet shop). Instead, the purple star offered for sale under the name "purple linkia" is most likely to be Tamaria stria. As with the case of the true Linckia stars, however, little is known about the biology of these stars. The only reference I know of that gives a firm answer as to their feeding biology is the Baensch Marine Atlas , which claims that the star feeds by "grazing Aufwuchs from hard substrates" - the basic translation being essentially unknown scum-eater. That's still not the most specific of descriptions about the diet of these animals, though. There have been a number of reports on various bulletin boards about these stars turning out to be predatory in some reef tanks, but in this case the difficulty is knowing whether or not the animal has been correctly identified at any step in the chain of supply.
As I mentioned above, there are several species of Linckia, and not all of them are blue, red or purple, and some of the stars commonly sold in pet shops under the name of Linckia may very well belong to other genera. I have no doubt as to the authenticity of these reports - i.e., that some people have been sold a sea star that was labeled "purple linckia" and have subsequently had problems with that animal in their tanks. However, that does not mean that the "purple linckia" in their tank is the same species as the one in yours or mine. The variation reported in the behavior of these stars may represent a case of individual variation, or it could indicate that there is a lot of misidentification going on.
My reason for taking reports of voracious predation by Tamaria stria with a grain of salt is that I've had one for years without any sign of trouble in my own reef tank. As I just mentioned, there is always the possibility that individual differences may be important when different animals behave differently in different tanks; even though they belong to the same species, it is possible that some stars may be docile and harmless, while others turn out the stuff of horror movies. Another common pattern is when an animal that appears harmless in some tanks turns out to be a real trouble-maker if moved to another. These differences among individuals and different tanks are one reason to always err towards caution if you have any concern. Widespread misidentification of the stars being sold as "linkia" in the pet shops is another reason that I speculate that reports of predation by T. stria may be somewhat overblown. Various Fromia species as well as Leiaster, Ophidiaster, Echinaster and Nardoa species are occasionally offered for sale as Linckia in petshops, and I have even seen "knobby linkia" (really the predatory star, Gomophia egyptiaca) offered for sale in a prominent San Francisco fish store. Many of these misidentified "linkia" stars are predatory on colonial tunicates, sponges, or other permanently attached marine invertebrates. Given that so many stars are occasionally sold under the common name "linkia" there is little doubt in my mind that the misidentifications are not perfectly consistent, and there may be multiple species of these unknown sea stars being sold in the trade.
I should also emphasize that I do not think that the majority of dealers are making a deliberate attempt to mislead you when this sort of misidentification happens, but I would argue that very few retailers know the biology of these animals well enough to identify them or predict the potential risks of introducing them into a reef tank. If a dealer gets in a new critter that they know nothing about, they are left with either not calling it anything more specific than "starfish" (which generally doesn't sell well, because people want to know what it is) or labeling it with their best guess at identification. By default, popular and well-known common names (such as "linkia") have a way of being applied pretty broadly sometimes. Thus, as a consumer, you have to make some effort to educate yourself to make sure that you are really buying the animal that you expect, or you're likely to end up with a surprise sometimes.
OK, so even now that we're all aware that there is an important distinction between the true Linckia and other look-alike sea stars, we're still only slightly further ahead in terms of understanding exactly what they need in an aquarium. Even if we limit the discussion here to only the true Linckia stars, the problem is that there is just not a lot of good information on their biology. Despite the beauty and obvious widespread interest in these stars, surprisingly little is really known about the exact feeding behaviors or preferences of these animals in the wild, and only anecdotal accounts are available for their needs in the aquarium. That is kind of surprising, given that Linckia is one of the most common and obvious sea stars on many Indo- pacific reefs.
Based on the anecdotal evidence from aquaria, however, L. laevigata is generally thought to be primarily an opportunistic scavenger, perhaps being even saprophytic (preferring to consume dead items as they begin to decay), but as also been observed to apparently feed on algae and microbial films as a non-selective surface grazer. I say 'apparently feed' because to the best of my knowledge there has never been a single scientific study on the natural diet or nutritional requirements of these stars, and controlled feeding experiments are rarely done on animals in reef tanks at home. Therefore, all we really have are observations of what the animals seem to be sitting on and look like they might be eating - anecdotal reports of the diet and feeding preferences, but no real evidence that the animals really ingest and gain nutritional benefits from the stuff we think that they are eating in the aquarium. Having said that, however, this is the only information that we have currently on what these animals eat, so that's what we must go with. Most sea stars feed by everting their stomachs (they extend the outer section of the stomach to surround the food), secreting digestive enzymes onto the food item to break it down, and then pulling the stomach together with the digested food back into the body (see Shimek 1997, 1998 for further detail on this process). Although this process is much more obvious and easy to observe in a variety of other sea stars commonly sold in the hobby (such as the popular chocolate chip starfish, Protoreaster nodosus), it is likely to occur in the same way, albeit far less obviously, during feeding by Linckia laevigata as well. Based on their behavior in the aquarium, these sea stars appear to feed by grazing surface films off live rock or similar substrate. In his book Invertebrates: A Quick Reference Guide, Julian Sprung (2001) reports that the aquarium diet of L. laevigata and L. multifora typically include bacterial films, small encrusting sponges, and dead mollusks.
Although it has also been reported to feed on surface films like it's blue cousin, the red Linckia (still widely referred to as L. multifora), on the other hand, is thought to derive much of its nutrition from suspension feeding on organic detritus and particulates. Depending on the source, the primary feeding method of the red linckia may be either: 1) sitting with their arms raised into the water column and using secreted mucus strands to capture floating particles which they then collect and eat, or 2) everting their stomachs onto algae and microbial films in much the same manner as described for L. laevigata above. Given the recent evidence that both these groups of stars are actually the same species, I would lean towards the red and blue morphs feeding in largely the same manner. However, I would also emphasize to readers that until some scientific studies of the feeding preferences and behavior of these stars become available, any report of the feeding methods and requirements for Linckia remains essentially a matter of at best well- informed opinion.
In any case, assuming that we are right to at least some extent in our estimations of what these sea stars are feeding upon, much of the nutrition for these species probably comes from diatoms, bacteria and other filming organisms found on live rock. It is widely known that these stars often reject any attempts at artificial feeding in the aquarium, and will typically crawl off pieces of fish, shrimp, squid or prepared food that other stars (such as the popular chocolate chip starfish, Protoreaster nodosus or its more attractive cousin, the bahama or general star, Protoreaster lincki) will readily consume (e.g., see Shimek 1998). Because Linckia stars appear to derive the vast majority of their nutrition from surface films, and are known for actively rejecting attempts to feed them, they are unlikely to do well in a tank that is recently, (within 6 months) set up, or one in which there is not enough live rock for them to continually find new surface films from which to graze. Therefore, they are not really recommended for reef tanks smaller than about 50 gallons or so for the long-term. Although small stars may do well in smaller tanks for some time, they will eventually require more space. The problem is that in such small tanks, even if the star is successfully acclimated (I'll come back to this important detail in a moment), a reasonably-sized sea star (say about the size of your hand) will not have enough well-aged rock surface to continue to find food for extended periods of time. Another important consideration for tanks smaller than about 50 gallons is that Linckia laevigata can get quite large (30 cm or more across) and the amount of food they require will increase with their size -- obviously, the amount of established live rock in your tank will have to be quite large to support a grazing star which is about a foot in diameter!!
In general, there is no need to worry about a species of Linckia being compatible with other reef tank inhabitants, because these animals are not aggressive (eating slime off your live rock does not threaten many animals in a reef tank), and the stars themselves are chemically defended from many fish predators (they possess chemical defense compounds called saponins). These saponins turn out to be a highly effective way of discouraging potential predators. In fact, Linckia has a complex assortment of these defensive chemicals, some of which it shares with the highly distasteful Crown of Thorns starfish (_Acanthaster planci_), the famous sea star that eats reef corals . Unlike the chemical defenses of sea apples (_Pseudocolchirus_ spp.) and some sponges and tunicates, however, these chemical defenses do not pose a risk to tank-mates because they are not released from the skin even under duress (see Toonen 1998 for more detail on chemical defenses), and should not be a concern to anything other than predators which try to nip at the star. Despite these defenses, however, they are not safe with all fish that are commonly maintained in aquaria. For example, the dog-faced puffer (_Arothron nigropunctatus_) is a common predator of sea stars in captivity, and even if well-fed, these fish may make a snack out of any starfish added to their tank.
So, after reading all that, you decide that you can provide for the requirements of the star, and want to get one, there are a few considerations before buying one. First, make sure that the star is active, firm to the touch and without any discolored patches across the body. There is considerable color variation in L. laevigata, making it is sometimes hard to recognize discolored patches from mottled colors on some animals (Bob Fenner has some pictures of sea star discoloration in his article linked from the references below). I would say that unless you are familiar with the natural color patterns of the species, it is best to err on the side of caution and avoid buying a really mottled Linckia. Also, as I mentioned above, you'll want to make sure that you're actually getting Linckia rather than some other sea star for your reef tank. If you are reasonably confident about the identity of the star, check to see if the animal is at least partially hidden from intense illumination and actively crawls about (although they are often not really active during the day, it should at least move every night). These animals are largely nocturnal, and should be actively searching for food when the lights are dimmed or off on the aquarium. If the star is lying inactive under intense lighting, I would wait until you see one that is behaving in a more natural way before buying it.
These animals are notoriously delicate shippers, and more often than not a beautiful blue star starts to show white discoloration and begins to literally disintegrate a week or two after shipping. There are many ideas about why this happens (including osmotic or pH shock, bacterial and fungal infections, etc.), but unfortunately no one really knows what the ultimate cause of this problem is. The fact that we don't know exactly what causes this problem does not make it any less serious - it still appears to be the most common pattern of mortality in these sea stars in captivity, so take it seriously. Because we do not know the exact cause, there is also no known cure, so this is a case where a little preventative care is likely to really pay off. The best advice I can offer you is to be patient - it often takes a while to find a Linckia in really good shape at the local pet shop, but a little patience in that search will be repaid if the animal survives and thrives in your tank. There is nothing quite as disheartening as getting an exciting new animal, only to watch as it slowly disintegrates into goo because the animal was not healthy from the beginning.
Another consideration is that these stars are particularly prone to parasitic infections of the small snails (_Thyca crystallina_), and in some regions as many as 1/4 of the Linckia are infected with these pests . You should check for these snails clinging to or boring into the underside of the arms before you pay for the star. Females of these parasitic snails have a proboscis (elongated mouth) that penetrates the skin of the sea star and sucks the hemolymph (the echinoderm equivalent of blood) almost like a small, shelled mosquito. Unlike a mosquito, however, the adult snail actually burrows into and becomes permanently fused to the sea star, and should never be picked off! Because it is physically attached to the sea star, removing the snail will almost certainly result in more damage than leaving the snail attached. Besides, these parasitic snails turn out to be a relatively minor problem. Other than a slight alteration in some of the skeletal elements around the proboscis, the main effect of these snails on the star appears to be the loss of tube feet under the snail's shell (which probably has no effect on the health of a star in the aquarium over the long-term).
Although research suggests that these parasitic snails cause little ultimate harm to their hosts , their presence is an additional stress that the stars can do without when being moved to an aquarium, and their presence provides a potential vector for infection. So, if you have your choice of several healthy stars, take one without any parasitic snails first; if you don't have a choice, it probably won't matter to the star in the long-run.
Again, just in case you didn't take me seriously the first time, the single biggest problem with Linckia sea stars, even more so than other echinoderms, is that they require proper acclimation and tend to ship poorly. Their delicate nature makes it all the more important to start with a healthy specimen if you decide to introduce one of these animals to your reef tank. Therefore, it is exceptionally important to acclimate this animal carefully (of course, all animals should be acclimated carefully, but it seems to be just that much more important to the survival of these stars). If you can find a healthy star, you should make every effort to bring it home quickly (long periods of time in a bag seem particularly hard for them to handle as well), and then acclimate it to your tank water slowly to minimize the stress on the animal as it is transplanted into your aquarium. This is also a case where it turns out to be important to check the water conditions of the supplier from which you are getting the animal. If your local shop maintains a salinity that is much lower than your own tank (natural seawater is ~ 35 ppt, or roughly 1.025 SG at 80ºF), then acclimation will be more difficult and survival of the star becomes more of a gamble. If your aquarium is more than a couple of parts per thousand (ppt) different from that of your supplier, then you are more likely to have problems introducing one of these stars to your tank. In this case, you need to try some more extreme measures to introduce the star to your aquarium. There are a number of suggestions in an article archived in the reefs.org library (Various Authors 2000), but one of the options if you have a quarantine tank (and you should), is to set it up with the conditions of the water in the pet shop and slowly acclimate the star to the conditions at which you maintain your reef tank before transferring it.
If the salinity of the water at your supplier is relatively close to your tanks, then it's in the best interest of the star to be brought home quickly and acclimated to the aquarium. Personally, I usually use a drip system to acclimate the animals, but this is a hassle, and most people find the cup method to be much easier and equally as effective. This is simpler because it doesn't take anything more sophisticated than a small cup to accomplish. After the sealed bag has been floated for about 10 - 15 minutes to equalize the water temperature, you start to add about ½ cup (~125 ml) of water every five minutes until you've filled the bag. Once the bag is full, dump half of that water down the drain (not into your tank) and repeat this until the bag is full again. Fill the bag at least twice, perhaps even three times just to be sure that the animal has sufficient time to acclimate to your water conditions. Once you have completed the acclimation, remove the star from the bag (discarding the water left behind), and place the animal onto an open rock area in your tank. The star should promptly (within minutes) adhere to the rock, and then move off in search of a partially shaded area to begin feeding. Once these stars are successfully introduced to an aquarium, they seem to be quite hardy and few people report problems with a well-established animal. It appears to be largely a problem of shipping and acclimation that gives these animals their delicate reputation, so take your time and give your animal a fighting chance at surviving the move to your tank.
Like the majority of other sea stars, the sexes appear to be separate in Linckia, and the animals spawn gametes freely into the water column. Most often, they will hold onto the substrate with the tips of their arms, arch the body high into the water and spray either sperm or eggs into the water above them. If a male and female happen to spawn in close proximity to one another, the fertilized eggs develop into feeding larvae within a couple of days. These larvae spend about 28-30 days in the water column before settling onto a hard surface on the reef and metamorphosing into a tiny version of the adult star . Spawning of these stars in home aquaria is rare, and although larval culture techniques for echinoderms are well-established (see Toonen 1996 for details), the long planktonic lifespan of the larvae of Linckia makes raising them at home a difficult prospect.
Although these stars require extra care in the initial selection, once a blue Linckia is successfully introduced into a large, well established aquarium with plenty of live rock to explore, they are usually quite hardy and are certainly a beautiful addition to a reef aquarium. In closing, I just want to thank everyone at the Omaha Marine Society for some great discussions about sea stars that have helped with this article.
- Baensch Marine Atlas, H. Erhardt & H. Moosleitner. 1998. Volume 3: Invertebrates, with the collaboration of R.A. Patzner, translated and revised by G.W. Fischer & S.E. Borrer. Microcosm, Shelburne, Vermont, pp. 737 - 1326.
- Bouillon, J., and M. Jangoux. 1984. "Note on the relationship between the parasitic mollusk Thyca crystallina (Gastropoda, Prosobranchia) and the starfish Linckia laevigata (Echinodermata) on Laing Island reef (Papua New Guinea)." Annales de la Societe Royale Zoologique de Belgique 114:249-256.
- Egloff, D. A., D. T. Smouse, Jr., and J. E. Pembroke. 1988. "Penetration of the radial hemal and perihemal systems of Linckia laevigata (Asteroidea) by the proboscis of Thyca crystallina, an ectoparasitic gastropod." Veliger 30:342-346.
- Fenner, R. "Sea Stars, Class Asteroidea." WetWeb Media (http://www.wetwebmedia.com/seastars.htm)
- Minale, L., C. Pizza, R. Riccio, F. Zollo, J. Pusset, and P. Laboute. 1984. "Starfish Saponins 13. Occurrence of Nodososide in the Starfish Acanthaster planci and Linckia laevigata." Journal of Natural Products 47:558.
- Riccio, R., O. S. Greco, L. Minale, J. Pusset, and J. L. Menou. 1985. "Starfish saponins: 18. Steroidal glycoside sulfates from the starfish Linckia laevigata." Journal of Natural Products 48:97-101.
- Shimek, R. 1997. "Sea Stars." Aquarium.Net. ( http://www.aquarium.net/0797/0797_2.shtml )
- Shimek, R. 1998. Sea Stars - "The Whys, Why Nots and Wherefores." Without a Backbone, Aquarium Frontiers online. ( http://www.animalnetwork.com/fish2/aqfm/1998/april/wb/default.asp )
- Toonen, R.J. 1996. "Home breeders FAQ for marine invertebrates." ( http://biogeek.ucdavis.edu/larv_faq.html ), see also the Captive Breeding references from Reefs.UK ( http://www.reefsuk.org/CaptiveBreeding/Articles/Articles.asp ) and the Captive Breeders Column Forum from this magazine
- Toonen, R.J. 1998. "Cucumber defenses". Reefs.org online library article
- Various authors. 2000. "Starfish Acclimatization." Reefs.org online library article (http://www.reefs.org/library/article/starfish_acclimatisation.html)
- Williams, S. T. 2000. "Species boundaries in the starfish genus Linckia." Marine Biology. 136:137-148.
- Williams, S. T., and J. A. H. Benzie. 1993. "Genetic consequences of long larval life in the starfish Linckia laevigata (Echinodermata: Asteroidea) on the Great Barrier Reef." Marine Biology 117:71-77.
- Zagalsky, P. F., F. Haxo, S. Hertzberg, and S. Liaaen-Jensen. 1989. "Studies on a blue carotenoprotein, linckiacyanin, isolated from the starfish Linckia laevigata (Echinodermata: Asteroidea)." Comparative Biochemistry and Physiology B Comparative Biochemistry and Molecular Biology 93:339-354.
- Sprung, J. 2001. Invertebrates: A Quick Reference Guide Ricordea Publishing, Miami, Florida