I've been in this hobby on and off for 15 years and hardcore for about 5 now, but this is a new one to me – my local fish store got 3 feather stars in last night. They are very beautiful and interesting creatures, but I didn't want to spend $80 on something I knew nothing about. What are their requirements, are they reef safe, etc. etc.?
Feather stars are members of the Order Comatulida, and together with the Sea Lilies (Order Isocrinida) form the Class Crinoidea in the Phylum Echinodermata (sea stars, sea urchins, and sea cucumbers are the most familiar members of this group). They generally have between 10 and 200 arms attached to a central disk (called the calyx), and each arm has lateral branches giving them a feather-like appearance (hence the common name). There are about 400 living species of feather stars, and like the basket stars (Order Euryalae, Class Ophiuroidea) they are among the most beautiful of the invertebrates found on coral reefs. Like other echinoderms, they have tube feet, but unlike sea stars, sea urchins and sea cucumbers, they do not use these tube feet for locomotion. Rather, the have claw-like fingers called cirri that the animals use to grasp the substrate and crawl slowly around on the sea floor. When threatened by a predator or dislodged from their perch, these animals are capable of alternately undulating their arms to propel themselves into the water column and actually swimming for short distances. Like other echinoderms, they lack eyes, so their swimming is undirected, but remarkably graceful, and is a beautiful sight to behold.
Success rate with these beautiful animals is terribly low, and they are not recommended for even well-established reef tanks run by experienced hobbyists. Unless you plan to set up an aquarium specifically for these animals, their chance of survival in the aquarium is quite low. Like most marine invertebrates, these animals are capable of slowly digesting themselves as they starve, and can survive months without food before showing signs of starvation. The normal course of events with feather stars is that the animals look and behave normally for a couple of months, but then slowly start to disintegrate from the tips of the arms inward. Feather stars are suspension feeders, and like most animals that feed in this way, are very difficult to care for in an average aquarium. However, with feather stars, the prognosis is worse than average, because these beautiful animals are even more difficult to feed than the majority of filter feeders; they have a very specific feeding mode that requires a lot of suspended planktonic food of only a certain size range. Although the requirement for suspended particulate food of a given size range is generally true of other filter feeders as well, many filter-feeders capture everything that passes through their filtering apparatus (passive capture) that is approximately the correct size (there are no "taste" receptors on the filtering structures of many invertebrates) and transport it to the mouth where the particles are sorted, and the "tasty" ones are ingested, while the undesirable particles are ejected as "pseudofeces." Even desirable particles can be ejected this way (in balls of mucus) if the suspended particle load is too high, and the animals are capturing more than they can consume. Fortunately, most suspension-feeding invertebrates typically become less selective as they get more hungry -- so animals like feather dusters and flame “scallops” will usually end up ingesting whatever they can catch if they’re hungry, even if they would ignore these same foods in the wild (this has been demonstrated in a number of captive feeding experiments). This makes the feeding of these animals a little easier, and requires only sufficient particles of the right size and appropriate nutritional value.
Unfortunately for us (and for the feather stars) they filter particles actively, by selecting the "tasty" particles from the water before they capture them. That means that they don't even capture foods that they do not want, and even worse, their food preferences seem to be "hard-wired" on the basis of size, weight, and flavor (or more accurately, some combination of those factors). What I mean by “hard-wired” is that they only respond to the tasty food items that they would normally take in the wild, and the few species tested do not appear to show the same decreasing selectivity response when they get hungry that many other filter feeders do. That means that you need to be able to provide a natural diet for the animals in order for them to have *any* chance of surviving in your tank.
So what is the natural diet? Well, this turns out to be a big problem as well -- first, there haven't been detailed studies of a lot of species, so we're forced to assume (and hope) that the rest of the animals do the same things as the ones that are studied. Unfortunately, despite the fact that there so many species, and they can often be quite abundant in certain areas, they are poorly studied and we don't really know how general the patterns seen in the few studied cases are. With a bit of luck, though, the species in your local petshop has the same preference for very small zooplankton, invertebrate larvae, ciliates (and other protozoans), and large phytoplankton (such as diatoms and dinoflagellates) as the few species that have been studied. If the animal recognizes these same prey items, there is at least a chance of providing food for it. If the star does not respond to the presence of these food items, then your guess for providing food for it is as good as mine...
In general, studies of feeding in these amazing animals suggest that most of their energy comes from the consumption of invertebrate larvae. Invertebrate larvae are the (typically) microscopic juveniles that are released by about 80% of all marine animals and which spend some variable amount of time (from minutes to months) swimming in the ocean currents before they are capable of metamorphosing into the adult body form (like a caterpillar metamorphosing into a butterfly). There are no commercially available sources of invertebrate larvae for the average hobbyist (there *are* larvae that are commercially available -- such as oyster, abalone and Trochus snails – but they're so expensive that it would be impossible to use them as a food source). Unless you're close to the coast and have the luxury of collecting egg masses to culture for feeding or keeping a tank full of Trochus to continually produce larvae for you, most people are unlikely to have enough larvae to feed even a small feather star.
Despite all those problems, I think it is possible to keep one of these animals in an aquarium. In fact, I must admit that I have a feather star (Himerometra sp.) in one of my tanks. The animal seems to be thriving (it is regrowing all the arms damaged during shipment) in my refugium sump. The star has attached itself to the "spraybar" the delivers water from the tank to the sump, and simply spreads it's arms into the flow generated from that. This tank has a well-stocked deep sandbed in the aquarium, the refugium sump, and the in-tank refugium, all of which are stocked with detritivore kits and live sand from various suppliers. There is a high density of sand fauna and there is a constant supply of worm larvae, rotifers and protozoans in the aquarium from those sources. In addition, I feed the tank heavily with enriched brine nauplii and phytoplankton every other day, as well as rotifers, copepods and various invertebrate larval cultures when I have excess available. I have not had the star for very long (less than a year), but it has shown significant growth in the time that I have had it, so I am hopeful that it will do fine – of course, I'm prepared to start specifically target feeding it invertebrate larvae in a separate container if the animal shows signs that it is not getting enough food at some point.
The bottom line is that regardless of how beautiful they are, I can’t really suggest that you buy one of these animals from the shop. The chances of being able to keep the animal successfully is very slim, and if the shop is able to sell them to you, it is likely to encourage them to bring in more. I think that you will do the shop and the animals a favor by explaining to the shop keeper how difficult these animals are to keep, and let them make an informed decision about whether or not to order any in the future.