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Seastars are ubiquitously associated with "the seashore" and are instantly identified by neophytes as reef organisms'¦ So'¦, why are there so few kept in our marine aquariums? Solving this puzzling disconnect is the purpose of this article. Just what are Seastars? What roles do they play in natural environments? Why is it so hard to find a good one!?
The What of Asteroids:
Seastars are Echinoderms, that is, one of five living Classes (a taxonomic category) of the Phylum Echinodermata'¦ an aptly commonly labeled group called the spiny skinned animals. Along with Sea Cucumbers, Crinoids (Sea Lilies), Brittlestars (and Basketstars), and Sea Urchins (and Sand Dollars), the echinoderms are some of the most readily identified of sea creatures.
Amongst unifying characteristics of the Echinodermata are an outright or secondary radial pentaramous symmetry'¦ that is, instead of our bilateral, mirror-image appearance, theirs is a pin-wheel of five sections. Not always easy to see from the outside, dissecting a spiny skinned animal will reveal this arrangement internally. <perhaps pictured>
These animals also possess and use a type of "closed tube" pressurized system for moving about. This water vascular system/network is most easily viewed in our Seastars under discussion. The little tube-feet (pictured) one sees moving about under Stars, allow them to move about, and coming in so handy for feeding, prising open shellfish in some species. Also part of this water vascular system, is the usually prominent madreporite (pictured) on the aboral (top surface, away from the bottom) that serves as a filtered intake for water that is used in the water vascular system. A more central spot on the top middle or so is the anus. <perhaps pictured>
Echinoderms don't have bones that allow for structural integrity and attachment of organs, but either hard external (exo-) skeletons (as in the Urchins) or in the case of Seastars, integumentary (skin) elements called ossicles that interdigitate to render some solidness.
One last feature we'll mention is the presence and function of little cutting structures called pedicellariae'¦ if you've every wondered why not much grows on the outside of Seastars, it's these pedicellariae in action that snip off any would be hitchhikers, keeping the surface clean.
The (Biological) Trouble With (Not Tribbles) Asteroids:
A real root problem with our keeping Seastars in particular and Echinoderms more generally is a tacit misunderstanding of their inherent nature. Sure, they look slow (they are) and innocuous (they're not) "just sitting there" not moving much, doing anything'¦ But I assure you, this appearance is deceiving. This group of organisms has proven time and again to be "a" or the determining group of reef animals that select for what else lives on the worlds reefs and deeper marine domains. Their actions may seem slow, but they are relentless in their hunting, feeding peregrinations'¦ selecting out much larger organisms (e.g. huge algae called kelps), stony corals (e.g. the Crown of Thorns Seastar, Acanthaster planci (pictured)). These are no push over species, believe me. If you don't know by now, you will soon by reading here that some of the most popular Seastars kept are not "simple detritivores" or "algae grazers", but eaters of other animals.
The majority of Seastar species in the world are generalized predators'¦ feeding on other slow or attached invertebrates like the celebrated bivalves, to dead/dying non-vertebrates and fishes. Some species are outright coral polyp feeders (the Crown of Thorns is most notable), and there are even some planktonic filtering species. The group we want to select for are largely detritivorous'¦ feeding on settled bio-mass, interstitial fauna on or in substrates. Unfortunately, some Seastars continuously sold are not of this last category.
Want more? It's not simply because they have spiny skin that predators by and large give Asteroids a wide path. It's not for looks alone that so many of these animals are warning (bright and warm-) colored'¦ many are known to produce toxins and secondary metabolites of unpalatable and poisonous nature. Hence the re-emphasizing that you keep smaller species, or large/r ones in very large systems'¦ for dilution of possible materials, and do your best at maintaining good water quality (water changes, skimming, RedOx, chemical filtrant use, macro-algae culture'¦) to prevent toxic situations.
Other than the inherent challenges of recognizing nutritional strategies'¦ and providing for them, and keeping your Seastar happy to avoid a toxic event, what can be said of differentiating this group into more and definitely less aquarium suitable species? Actually, there is quite a bit. There is a very large accumulation of data on what constitutes historically much better and dismal history of survivability with the Seastars'¦ and even notions as to the why, wherefore and means of improvement.
As of today we can only state what the current and past situation has been, and it is this: the more popular Seastars don't have good average survival in captive care. The reasons are borne mostly of poor, very stressful collection, holding and shipping procedures, and of course to the species involved shortcomings in meeting these challenges AND adapting to aquarium conditions. Nonetheless, I am going to mention both the "good" and "bad" species and urge your understanding in undertaking to successfully house whichever you choose, WITH the foreknowledge that many just don't make it for the stated reasons above.
The Best Seastars for Captive Use:
To re-cap, these are the species that on average do better in staying alive under aquarium conditions. "Your mileage may vary" and it is certainly not impossible that the better organisms listed here won't do well for you in small trial numbers, systems'¦ or alternatively, that the "bad" ones live and live for you. Again, this splitting of good/bad is a reflection of having handled thousands of specimens over decades of time.
The Very Best: The Genus Fromia:
What a delightful genus! Small in size, beautiful in appearance, and of the highest marks in coping with aquarium conditions! Though they may be apparently sustained by feeding on detritus and its micro-organism content, these Stars should be purposely fed on high-protein tablets, and fish, and bivalve meat occasionally. Fromia are undoubtedly my favorite choices in Seastar selection. (Pictures): Some notable species within this genus:
Some Good Medium-Sized Seastars:
These are not typically kept in-stock by retailers, and for the
purposes already stated I generally discourage mail-order/etail
sourcing of these animals'¦ Better to have your LFS special
order and hold on to the specimen/s for a few to several days for
The Genus Nardoa:
Especially when started small, these species tend to get along with most all other invertebrate livestock. From the Indian to Central Pacific Oceans; Africa to the Philippines. 'Typified as having knobby tuberculations on their top-side surfaces.''
Bad to Absolutely Dismal Asteroid Selections:
Again, I proffer no apologies for the fact that so many of the organisms offered for aquarium use don't typically live well or long historically. Amongst the Seastars, one can find the following "losers" most everywhere in the wholesale and retail trade.
Linckias: Still Dying After All These Years:
There are quite a few Seastars sold as Linckia/Linckia spp. Some of other genera. And some that are not peaceful bacterial et al. deposit feeders as the "true" Linckias of many colors (and at least two species). The true Linckias are good choices where available in initially healthy condition (a rarity), and carefully acclimated, placed in established, large systems (at least a hundred gallons for a small specimen) with plenty of live rock, detritus to feed on, and not too many competitors. The genus is named in honor of J.H. Linck who wrote a monograph of seastars in 1733.ã'' Tropical Atlantic and Pacific''
Knobby Seastars By Any/All Other Names:
These are much less desirable, but very commonly offered and attractive, are the "knobbed" Sea Stars, Family Oreasteridae of the genera Protoreastor, Pentaster and Pentaceastor. They are distinguished by having dull spines, bumps or knobbles on their dorsal surfaces, with these often seen in colors that contrast with their overall body pigmentation. Sold under names like Red-Knobbed, Chocolate Chip, and other labels, these species are hardy but aggressive feeders, more than happy to mount and consume sessile'' clams, oysters and all manner of corals, soft and stony and anemones.
Some Good AND Bad Choices:
Coldwater Animals Misplaced: Pisaster Disasters, Patiria'¦
A brief mention of a despicable practice... the selling of colder (non-tropical) organisms, in this case Seastars, as tropical aquarium specimens. These animals rarely acclimate to warm water conditions, either falling apart within days or stress-starving to death in a few weeks. Avoid them unless you have a system designed (chilled) for their appropriate keeping. Two commonly-offered California, U.S.A. examples that need to be kept at 68-70 degrees F. or cooler:
Picking Out Healthy Specimens of All Species: <sidebar>
Know your species'¦ what you're looking for, and at.
Take your time'¦ Most incidental/collateral losses of these animals occur within the first few days of their arrival. Avoid "right out of the bag" sales. Give them time to rest up, recover'¦ if they're going to.
Look for blotches, mis-colorations, holes and sunken areas. These should disqualify a purchase. Most Seastars that show such markings perish soon afterwards.
Select for good behavior and de-select for bad. Observe the prospective purchase for a few minutes'¦ is it moving? Does it respond to food or appetite stimulant added to its system? It should. Are these animals "trying to climb out" of the tank? Why? Something is amiss with their world, them or both.
It may seem incongruous, but missing lengths or even entire arms are not such a big deal when selecting Stars'¦ as long as the ends are entire (i.e. not open, decaying), these "comets" will re-grow the missing bits in time if in good health and care.
Whatever, which ever Seastar species you plan to keep, they all require careful selection, acclimation, and placement in good-sized (for food, stability) systems that are well-established and not too "clean". Don't be totally dissuaded by my opinions stated here or the historical record even. Do just be aware of your likely chances with keeping these species. Again, do make sure and get a positive identification to species, and research the nutritional, and system size and type needs before purchasing any Seastar.
Calfo, Anthony. 2004. Starfish: Considerations for the common (and commonly misunderstood) varieties. Reefkeeping Magazine. Http://reefkeeping.com/issues/2004-06/ac/feature/index.htm
Mancini, Alessandro. 1991. Starfishes in tropical marine aquaria. TFH 9/91.
Rohleder, P.G. Undated. Linckia and Fromia- Two starfish for the reef aquarium. Aquarium Digest International #53.
Schlais, James F. 1981. Walking on water. FAMA 8/81.
Sprung, Julian. 2006. Sea Stars, our five-legged friends. Some sea stars are easily maintained in a relatively simple aquarium set-up, but many don't do well in captivity. AFM 6/06.
Wilkens, Peter. 1974. Stars to brighten your aquarium. Marine Aquarist 5:3,