Echinoderms in Aquaria... by Ronald L. Shimek, Ph.D.

Despite their success in nature, which is fostered in no small part by their odd organ systems and strange body structures, relatively few echinoderms are appropriate for aquaria. This lack of suitability is due, in part, to their strange internal morphology, which makes them particularly sensitive to changes in salinity. Echinoderms, in general, are animals requiring full strength seawater and they are intolerant of changes in salinity. Although many aquarists maintain their aquaria properly, often at least some tanks along the distribution and dealer network contain water that is hyposaline or have otherwise inappropriate conditions. Rapid, unnecessary and inappropriate changes in salinity during transit may result in the animals being injured and dying a slow and lingering death, often in the tank of some hobbyist who really is not to blame.

Most echinoderms are not suitable for reef tanks for an entirely different reason. With only a few exceptions, they are just too large to be kept successfully in the average aquarium. No echinoderm reaches a huge size, although the largest are sea stars may reach a diameter of about 10 feet. At the other extreme, except during their larval stages, there are no really tiny ones, either. Most of them are moderately-sized animals, with a body mass that averages about the size of a clenched fist. This tends to make them a bit too big for most reef aquaria. While many of them would do well in a tank that was 1000 gallons or larger, there are relatively few of them that are small enough to exist within the confines of a typical reef tank without acting as a significant force in tank remodeling or as a significant predator upon a tank's inhabitants.

One other aspect limits the acceptability of many tropical echinoderms as aquarium inhabitants. Their natural history is largely unknown. Temperate echinoderms have been well-studied for several centuries, and are really best known from the echinoderm-rich fauna of the Oregonian biome of the Northeastern Pacific (see, for example, D'Yakonov 1950, 1954; Paine 1966, 1974; MacGinitie and MacGinitie 1968; Mauzey et al. 1968; Birkeland 1974; Engstrom 1974; Van Veldhuizen and Phillips 1978; Austin and Hadfield 1980; Highsmith 1982; Lambert 1981; Kozloff 1983; Mladenov and Chia 1983; Cameron 1985; McEuen 1986; Anderson and Shimek 1993). A tremendous foundation of echinoderm biological acumen has been developed largely from the study of this particular fauna. Unfortunately, studies of the tropical echinoderms have been comparatively sparse; few of their tropical cousins have been studied in any great detail. While we can guess the physical conditions under which they may be kept, in most cases we know next to nothing about their natural history. In the most pragmatic sense for aquarists, this means in most cases that we don't, for example, have the slightest idea what they eat. Since obtaining nutrition is the first order of business for animals, if we are to be able to keep the animals alive, we need to know what to feed them.

Finding the appropriate foods for animals takes a great deal of study. While it might seem that all that is necessary is to go out to the real world and see what the animal is eating, this may be very difficult for any number of reasons. And even if the animal is observed feeding, each feeding event is only one datum; a lot of separate data may needed before its diet can be ascertained. Each individual observation provides only a small part of the answer. Additionally, most animals are adventitious feeders, at least to some extent, so what they eat may not be what they need or prefer, but simply what is available. Most animals also need to have some variety within their diets. In most cases, this dietary variability will not be too extreme. The true feeding generalist or omnivore, an animal that feeds like humans, is very rare in nature. Unfortunately, people let their own dietary experiences influence their judgment in what they offer or feed to the animals in their care. The philosophy, if such an outlook can be dignified with that term, seems to be: "If I can eat a jalapeño pepper, popcorn and anchovy pizza, it stands to reason that the animals in my aquarium should be able to feed on just about anything." This is hogwash, of course, but the fact that it is utterly foolish doesn't prevent aquarists from trying it. In fact, the fact that it is utterly foolish probably means most of them will try it.

During an animal's evolutionary history, one of the major driving forces of natural selection may be the avoidance of competition, and this may result in specialization upon a particular food source. If animals from two species that potentially are able to utilize a variety of foods can avoid competition for food by being able to switch to alternative foods, these potential competitors may get enough additional food to produce more offspring than if they had actively competed for a limited food resource. In an evolutionary sense, this means both species win. If they can't use some alternative food, however, they may both get insufficient food to survive or reproduce. As a result, in many situations it appears that if two species with similar dietary requirements exist in the same geographic area, the only way they may coexist is by eating different things. For organisms such as songbirds, this may mean something as simple as feeding on different sized seeds. It may mean that animals such as nudibranchs or sea stars, for example, become specialized on different types of sponge prey.

Once the specialization of a predator upon a given food resource starts to occur, a positive feedback loop may get established. For example, if a given sea star acquires, through some mutation, the ability to feed upon a type of sponge which a potential competitor can't eat, then anything that facilitates the ability of that first species to obtain increased nutrition from feeding on that prey enhances its ability to produce offspring. Such adaptations might include such physiological attributes as the development of specific enzymes allowing the detoxification of noxious chemicals produced by the prey, or the development of specific behavioral attributes that facilitate location of the prey. In a sense, once specialization on a class of dietary items occurs, the animal is generally locked onto a path of becoming more specialized. Any changes in its genes allowing the development of new enzymes or feeding structures are favored. However, given that all resources are finite, when something is gained, something else must be lost. Predators that have truly specialized upon given food sources are generally unable to eat other foods. They may lack enzymes that digest specific prey chemicals. In the case of a sea star that has acquired the ability to eat a specific variety of sponges, it may acquire the ability to detoxify some of the toxic chemicals in its new prey, but this may come at the cost of being unable to detoxify the toxic chemicals in other potential prey. Soon, its sensory systems may adjust so much to its new prey that it will not even be able to identify other potential, and possibly edible, food items. Such undetected items will not be eaten even though they are potentially highly nutritious.

The specialization upon specific foods has another potential drawback with regard to the husbandry of these animals. The special, "necessary" food may provide some sort of essential nutrient lacking in other foods, even if those other foods are acceptable and even if the animal is capable of living for extended periods eating them. This situation would be analogous to what is seen in humans who go for long periods eating foods that contain insufficient amounts of vitamin C in the development of the fatal condition called scurvy. The same types of nutritional problems may be occurring in some of the animals kept by aquarists. Unfortunately, in most cases we just don't have enough data about their natural diets to assess the relative sufficiency of their available diets in aquaria.

Worldwide, the number of different echinoderms is relatively small; only about 6,000 species, more-or-less, have been described. Most of these are not found on coral reefs, perhaps because the actual amount of oceanic habitat occupied by coral reefs is pretty small. Even though coral reefs are renowned for their diversity, the diversity of echinoderms found there is not truly exceptional. Echinoderms do not harbor zooxanthellae, thus they don't garner any specific benefit from the shallow, highly illuminated waters of the reef. Instead, coral reefs serve different "functions" for the different echinoderm groups. For the crinoids, they provide some additional habitat space. The other groups primarily forage in and on the reefs for foods, and are generally as abundant as might be expected of secondary and tertiary consumers in rich habitats. In some temperate areas, particularly in the aforementioned shallow water Northeastern Pacific, echinoderms may be significantly more diverse and abundant than they are, on average, in coral reef areas.

Echinoderms are often brightly colored and aesthetically attractive animals, and this is certainly true of reef echinoderms as well as their temperate cousins. This means they are collected for the reef hobby. Some small fraction of these collected animals, and unfortunately for some species it may be a very small fraction indeed, survive to spend the remainder of their days in a coral reef aquarium. As with all animals, echinoderms have a finite lifespan. Unlike most animals, however, echinoderms do not have a finite life expectancy and have no old age or senescence. If provided with a good environment and plenty of food, they may live a very long time, indeed. It is difficult to get age estimates from soft-bodied animals such as sea cucumbers, but with diligence and long-term research projects, some reliable estimates of the life spans of temperate sea urchins and sea stars are beginning to be made. Age estimates of some red sea urchins (Strongylocentrotus franciscanus) from the Pacific Coast of North America, based on marking and determination of actual growth over time, indicate the average animal may expect to live more than 100 years, and life spans in excess of 200 years are a distinct possibility (Ebert and Southon 2003). Long life spans are also known or hypothesized for many sea stars (Carlson and Pfister 1999). There is no reason at all to suggest that these age estimates are in any way unusual for echinoderms, in general. This means that tropical animals likely live as long as these temperate animals. If reefs continue to persist and if the appropriate research is carried out, we might expect to see similar data from coral reef areas in a few decades. As far as aquarium husbandry is concerned, I consider successful husbandry to mean persistence of the animal in the system for extended periods (several years or more). And to date, the techniques available in the hobby have failed utterly and miserably to promote long lives for many of these animals. Specimens of only a few species of echinoderms typically live longer than a year in reef aquaria, and only a few of these have a track record that indicates they are likely to live to anything approaching a normal potential. Lest the reader think that I am painting with too broad a brush here, I think it useful to examine the various types of echinoderms for patterns, of both success and failure.

In a group comprised of many beautiful animals, the crinoids, or feather stars and sea lilies, are among the most beautiful. They are also among the most difficult to keep alive in captivity for anything over a few weeks. Unlike some echinoderms, crinoids appear to be relatively hardy with regard to water conditions. This is likely a result of their reduced body cavity sizes compared to all other echinoderms, so the potential for damages due to changes in salinity is minimized. Unfortunately, this relative hardiness does nothing to confer a survival advantage in aquaria.

Figure 1. Crinoids are effectively impossible to keep in aquaria. They should not be
either imported or purchased.

There are about 600 living species of crinoids, and they are a moderately successful group. The sea lilies, or stalked crinoids, are often very abundant at depths exceeding about 300m (1000 ft). Free-living crinoids are very abundant on Indo-Pacific reefs, and are present but less obvious on Caribbean ones (Fell 1966; Messing 1997). They are all "passive" suspension-feeding animals, which means they don't generate the water currents that bring food to them (Leonard 1989). This means that not only is the type of food important in their husbandry, but also that they likely need water flow that has specific characteristics. They are not animals that are normally found in regions of turbulent flow or surge, and it is unlikely that they could ever persist in a reef aquarium whose current flow was turbulent and generated by point source generators such as power heads. Although it has not been investigated, they probably need laminar flow to be able to feed.

Crinoids are anything but passive, however, in their selection of foods, and may make active choices as to the type and sizes of foods that they eat. Additionally, their diets are unusual in that they often appear to contain large amounts of the reproductive materials of other invertebrates (Rutman and Fishelson 1969; West 1978; Meyer 1979, 1982a, 1982b; La Touche and West 1980; Smith et al. 1981; Holland et al. 1991). It appears that many crinoids may engage in what is termed glutinous feeding, essentially "pigging out" during mass reproductive events such as the synchronous spawning of reef corals. Presumably, they get the majority of their food energy during such times, but they may still need to regularly feed on other plankton. Other than the reproductive products of other invertebrates, they appear to eat ciliated protozoans, and small crustacean zooplankton.

Figure 2. Many crinoids appear to feed upon larvae and eggs
of other invertebrates.

Although there are a few scattered reports of one or two individuals persisting in aquaria, in most cases the crinoids imported for the reef hobby die within a short period. They typically die by slowly sloughing off the distal ends of their arms until all that is left is the body, which then dies. This pattern is consistent with what happens to other echinoderms during starvation, and it is likely that effectively all of the crinoids imported for the aquarium hobby die of starvation within a few weeks of being placed in a reef tank. Until such time as aquarists have access to a constant supply of eggs, embryos, or invertebrate larvae of several types, the husbandry of crinoids should not be attempted.

It has been said that the true icon of the reef aquarium hobby is the image of a pair of clownfish nestled in their host anemone. However, I think the animal most likely associated with the marine environment by most folks is a starfish. Probably for this reason, many reef aquarists wish to keep a sea star. Until they have a sea star in their tanks, these people may subconsciously feel they don't have a successful aquarium. Unfortunately, most tropical sea stars are as hard to keep as the crinoids. In most cases, the only difference is that once acclimated to reef aquaria their larger mass and energy reserves allow them to persist longer before they starve to death.

Sea stars have a rather peculiar gut and its structure contributes to the problems that aquarists encounter in keeping these animals alive and healthy. The mouth is located in the middle of the animal's bottom surface. The gut runs vertically, culminating in an anus located roughly in the middle of the upper surface. This digestive system is divided into several regions. Just inside the mouth is found a section of the gut, referred to as "the cardiac stomach," which does most of the digestion. The upper end of the cardiac stomach connects to "the pyloric stomach," a baglike region which extends out into each arm as a series of pouches called "pyloric caeca." Digested food byproducts, fats and sugars, are stored in the walls of these pouches. In most species, the upper part of the pyloric stomach continues upward as a thin-walled intestine connecting to the rectum under the anus on its upper surface. In a few species, the gut is blind-ending at the level of the pyloric stomach and lacks an intestine or anus.

Most people think that starfish feed by extending their stomach outside of the body into some prey item, such as a clam, and digesting it. This type of feeding, referred to as "cardiac stomach extension," is found in some of the most familiar sea stars such as the common intertidal bivalve-eating species Pisaster ochraceus of the North Pacific and Asterias forbesi of the North Atlantic. A different form of cardiac stomach extension occurs in such species as Pteraster tesselatus, which eats sponges and Hippasteria spinosa, which eats sea pens, or Acanthaster planci, the infamous coral-eating sea star. Here the cardiac stomach may be extruded over sessile prey or even over the substrate. Digestion occurs between the stomach's surface and the substrate, and food is absorbed into stomach tissues.

Figure 3. An example of cardiac stomach extension. Here an Asterina miniata in an aquarium is
eating a small sea cucumber. The cardiac stomach is visible enfolding the cuke.

Not all sea stars extend their stomachs out of the mouth, however; many sea stars ingest their prey and digest it internally. The first type of these "prey eaters" are the sand stars, generally species of either Luidia or Astropecten, which feed on small bivalves, sea cucumbers or worms. These animals lack an intestine or anus; upon completion of feeding, indigestible food remains are regurgitated out onto the sediment surface. The second type of prey eaters is typified by the multi-rayed sun stars, such as Solaster and Pycnopodia. These animals typically eat larger epibenthic and mobile prey. Finally, there are the sea stars such as the semi-rigid Henricia species, some of which can feed by extending mucous strands to catch prey (Mauzey et al 1967; Carlson and Pfister 1999). The astute reader will have noticed that, except for the sand sifting stars, all of the species listed above are temperate animals; this reflects the preponderance of information about the temperate species relative to the tropical ones.

Of course, for most coral reef aquarists, any information about temperate animals is only of peripheral interest, if that much. So, what do we know about tropical echinoderms and their dietary needs? For some few species, we know quite a lot. For most, however, the resource base could be best described as "slim." Actually, considering the likely ecological importance of these animals, the lack of data about them should be described as "abysmal."

A great deal of dietary information does exist about a few species of tropical sea stars; these are the animals that derive all or some of their nutrition from eating corals. Some of these "bad boys" do get imported for the aquarium hobby. The most common of these asteroids are Acanthaster planci, the infamous "Crown of Thorns;" Choriaster granulatus, called the "Kenya Star" or "Dough-boy Star;" and Culcita novaeguineae. This last species, known as the "Cushion Star" or "Biscuit Star," hardly looks like a star when fully grown, appearing rather spherical. Every now and then, you have to wonder about either aquarists or importers or both in the "naming" of these animals. In nature individuals of these species either primarily eat corals or only eat corals (Guille and Ribes 1981; Endean 1982; Glynn and Krupp 1986; Sano et al. 1987; Faure 1989; Walbran et al. 1989; Cameron et al. 1991a,b; Musso 1993; Chess et al. 1997). Nonetheless, they are still imported, put up for sale and, I suppose, purchased... In the vernacular of the day, DUH!!!

Figure 5. Coral eating sea stars, Left: Acanthaster planci, Middle: Choriaster granulatus, Right: Culcita novaeguinae. These species will probably do well in a reef aquarium if they are provided with sufficient food.

Adult individuals of these species are also quite large, in excess of a foot in diameter. Interestingly, these species will likely do pretty well in aquaria. Unlike most other tropical stars, their diet is known, and can be purchased. (!) Provided their aquaria are large enough, stocked with enough of the appropriate corals for them to feed upon and as long as the salinity is kept to that of full strength sea water, they will likely be quite hardy. With only a couple of exceptions, virtually all other sea stars have a short life span in captivity.

None of the remaining asteroids listed are known to primarily eat corals. That's the good news. The bad news is that in most cases, probably because they don't eat corals, they have been largely ignored, even by echinodermatologists. The very few data about what they do eat are sparse, often relating to only one of several species in a genus, for example. The bottom line is that there is no sure or reasonable guide about what to feed them that might provide their appropriate nutrition.

Marble stars, so-called because of the marbled pattern seen on their upper surface, are commonly found in the aquarium trade. They are relatively small five-armed stars, typically reddish or orange with contrasting, often lighter, rounded tuberculate plates on their arms. The central disk is small and is often distinctly colored. The edges of the rays often have a row of prominently rounded or bumpy plates. Numerous species are found in the Indo-Pacific, and most of them, at one time or another, probably make it into the aquarium trade. Their small size recommends them to hobbyists, and if their diets were known, they would likely be good aquarium animals. Individuals of Fromia species are found on both reef rubble and reefs. Some Fromia are considered to be sponge and tunicate predators, but the diets of most are not known. Fromia species appear to do well for awhile in established aquaria, presumably as there may be a source of sponges and small sessile animals for them to eat. They seldom persist much more than a year or so, before they "run out of gas and sputter to a stop."

Most species names in this complex should be taken with a grain of salt, probably a grain about a meter on a side. A great deal of evidence suggests that sea stars with the external morphology that hobbyists call "Linckia laevigata," may actually encompass several species. These species, however, appear to be ecologically similar and it is unlikely such differences that exist are important as far as the average hobbyist is concerned (Williams 1999). Linckia sea stars are characterized by a small, or almost absent central disk, and rays that are cylindrical and of a more-or-less uniform diameter. They look rather like five pencil or cigar-shaped legs joined together at one end. Individuals of the larger Linckia species may exceed 40 cm in diameter. At such sizes they are not suitable for most aquaria. The smaller, multicolored, Linckia multifora, is much smaller, seldom exceeding 10 cm across.

These sea stars are taxonomically grouped in the Family Goniasteridae which is characterized by animals that cannot extrude much of their stomachs. All of them are probably predatory on small sessile animals, such as sponges or soft corals growing on the substrate surfaces; however, this dietary characterization remains simple supposition. In the hobbyist "literature," they are commonly reported to eat algae and bacterial films. I was unable to confirm this in my literature review for this article. The aquarium sources that cite these as foods all seem to be quoting one another as the definitive reference and, unfortunately, none of them cites any scientific source for its supposition about diets. Nonetheless, these stars appear to be harmless to many of the animals that are kept in reef aquaria.

Individuals of Linckia species, particularly Linckia laevigata, are profoundly and seriously harmed by rapid changes in salinity; additionally, they appear to suffer "shipping" stress. As a result these animals need to be treated VERY carefully during acclimation to the home aquarium. This acclimation should be done slowly, and; acclimations of more than six to eight hours are often required. Once established in aquaria, Linckia generally appear to do well and may persist for a year or more. However, they often seem to slowly die, probably due to a lack of some specific dietary item. For large animals, they are surprisingly benign. They seldom knock over rockwork, and do not harm most other animals while they are dying.

Species from these two genera are similar in shape and are typically dull in color: brown, grey, off-white or black. Along the edges of each ray are large shield-like plates; the plates and their spines are typically much larger in Astropecten species than in Luidia species, resulting in an "armored" appearance to the sides of the arms. Spines on these plates tend to give the sides of the arms a "spiny" appearance. The spinal length, however, may vary significantly from species to species, and in a few species they are only a few millimeters long. Individuals of Luidia species often have limper, more flexible, arms than do Astropecten individuals. Although most of these stars have five rays, Luidia species may have more. They vary a lot in size; Luidia superba, reaching diameters of 1.1 m, is one of the largest sea stars, but most species in this group are smaller, reaching maximum sizes of less than 30 cm. All of these stars tend to exhibit similar behavior. They move across the surface of sediments until they find an area that seems promising, after which they burrow down into the sediment, often rather deeply. While burrowing, any potential food items, and that may be effectively ALL animals, that they can catch are transported to the mouth, ingested and digested. When they are through cleaning the specific area of food, the stars surface to move to a new spot. They will generally not scavenge excess food remaining on the surface, needing instead to collect from below the sediment's surface. These stars need a significant variety of food for good health, and require a lot of food. The amount of animals in a rich sand fauna of a few square meters will support a 10 cm diameter star for no more than few months. Putting one of these animals into a tank with less than several square meters of sediment surface is condemning it to a slow death by starvation.

Found in shallow sandy or sea grass areas, individuals of this species are sometimes offered for sale in the reef hobby. They have five short, indistinct arms which merge into a large central disk. Covered with short blunt spines arranged in a reticulated or net-like pattern, this species is typically orange, tan, or brown, with the spines being darker. Adults are relatively large and impressive animals reaching up to about 30 cm across, and weighing at least a kilogram. In nature, Oreaster reticulatus individuals eat sponges found living on the sand or on sea grass (Wulff 1995). The stars live on sand, and as such are really not suitable for marine reef tanks dominated by rockwork. They lack the flexibility needed to crawl on rockwork, and in hobbyists' systems there are few sand-dwelling or sea grass-dwelling sponges. Consequently, these stars will not do well in hobbyists' tanks. Nonetheless, if they are given a supply of an acceptable sponge, they may last for a few months before they succumb.

Figure 6. Oreaster reticulatus. In nature this species is found on sand
substrata and eats sponges.

Found throughout the Indo-Pacific, these species have five relatively stiff rays. They have a smooth, almost featureless epidermis from which rises a series of large, visible spines. Protoreaster nodosus grows to about 12 cm across, while Protoreaster lincki may be larger, up to 30 cm or so. These species are sand- or seagrass-bed dwellers, and as such are not really adapted for a reef tank. In nature both appear to be obligate sponge predators, although they will eat some other items, such as sea anemones or soft corals, in reef tanks. Nevertheless, most of them kept in reef tanks appear to eventually die of malnutrition. They may be able to survive for a while in a tank with a lot of sponges, provided they can get to the food. Their stiff, spinous bodies tend to prevent them from getting into rockwork.

One, or perhaps more, species of small sea stars in the family Asterinidae is the only sea star that can be said to thrive in some reef aquaria at the present time. The species is indeterminate; its geographical origin is uncertain, and there are numerous similar described species. These are small brown, tan or grey animals, generally not more than about half an inch (13 mm) in diameter. Flattened from top to bottom, their three to seven rays and central disk merge into one another. They reproduce asexually by fission and if there is sufficient food almost all the stars in a population will be regenerating rays or other body parts. They are substrate feeders and move around with their stomachs extruded over the substrate digesting who knows what. It is a pity that these animals are so drab. In some systems, they are quite prolific and even if they don't thrive, they appear to be able to survive in most others.

Figure 7. Asteriniid stars common in aquaria. Left: Oral View. Right: Aboral View. The taxonomic
identity of these stars remains uncertain.

Occasionally, some populations of these asterinids have been reported by aquarists to eat either soft corals or stony corals. These coral-eating forms, perhaps different species, seem to be quite uncommon, constituting less than five percent of the various populations.

The Ophiruoidea, the brittle or serpent stars, as a group contains some of the better aquarium success stories among the echinoderms. This is probably due to the fact that, as a group, they appear to be dietary generalists on various types of food, mostly of animal origin. Most of them appear to be able to feed in several different ways. Some of the varied ways of getting food that have been described include using their tube feet to capture food, looping their arms around larger food items which are then "rolled" up to the mouth, several different kinds of suspension feeding including feeding by electrostatic means (LaBarbera 1977). Finally, many of them are fully predatory and will ingest slow moving or sessile prey simply by moving over it and shoving it into their mouths. Ophiuroids lack a complete gut. Most of the volume of the central disk is taken up by the large and capacious stomach. At the end of a feeding cycle, indigestible remains are burped out and the animal goes in search of more food.

Many species of larger ophiuroids are sold to hobbyists, generally, and mistakenly, described as being "scavengers." In nature, these animals are almost always predatory, and while some of them adapt to the feeding regimens of a reef aquarium and become scavengers, others may not. The lack of adaptation may be manifested in one of two ways: either they starve to death or they remain predatory. This latter manifestation can have some interesting consequences. Probably the most potentially destructive animals that may be introduced into aquaria are not the mantis shrimps of novice hobbyist nightmares, but the large green brittle stars of the species Ophiarachna incrassata.

This species of brittle star is, without a doubt, one of the most voracious predators that may be put into a reef tank. Ophiarachna incrassata have been documented to eat several species of aquarium fish, including firefish, various and sundry damsel fishes, mandarin fish, blennies, small gobies, and cleaner wrasses. Additionally, they have been observed tearing other brittle stars apart to eat their gut's contents, and they may have the same habit with sea anemones and corals. And, if that weren't enough, they have also been observed to eat cleaner shrimp and other crustaceans.

They are beautiful animals; their base color is light green to olive, and they have a fine patterning of light, white or yellow spots and dark, black or dark green bands on the surface. The disk may be large, up to five cm across in a large animal, and relatively thick. When ingesting a large meal, the disk may assume the proportions of a large marble or golf ball. These are large brittle stars, potentially measuring up to about 50 cm across the arms. Animals about half this size are often seen for sale. The arms are relatively stout and highly muscular, for a brittle star, and there are rows of evident spines running down the length of each arm. The good news is that they are harmless to aquarists. The bad news is that they appear able and willing to eat just about anything else in their tanks. Finally, a true echinoderm success story for aquarists!

Actually, the true echinoderm aquarium success story concerns the small (about one cm in diameter) brittle stars that seem to populate almost all reef tanks. These are specimens that have been identified as Amphipholis squamata. This is a "species" of small brittle star that has a morphology consistent with the original description of Amphipholis squamata and is quite literally found from the Arctic to the Antarctic, and in many shallow water rocky environments in between. This probably is a group of very similar species with a successful and consistent morphology. It is inconceivable that a single species would be so plastic as to be able to occupy all of the different habitats from which this one species is reported. Similar species "swarms" have been found in other groups of marine animals and it is likely that a reasonable and thorough investigation will show that this is a similar situation. The taxonomy doesn't matter, of course. What matters is that these small brittle stars are some of the most successful at occupying marine aquaria.

Sea urchins or echinoids are another group with which aquarists have mixed success. Some of them are quite beneficial and desirable in aquaria, while others range from being impossible to keep to downright deadly. I dealt with them in some detail in a column about a year ago, and refer the reader there for more specific information.

The final group of echinoderms to be discussed is the sea cucumbers, or holothuroids, affectionately known as "cukes." This is another group with which aquarists have been largely unsuccessful at long-term maintenance. The types of cukes available to aquarists include a rather diverse taxonomic array; their husbandry, however, is rather straightforward. In general, regardless of their taxonomy, are two functional types of sea cucumbers are found in hobbyists' tanks. These are the filter-feeding types, such as the infamous sea apples, but also including a number of others, and the bottom moppers, such as the tiger tail cukes and several other species. There appears to be no real insurmountable problem with maintaining these animals other than giving them sufficient amounts of food.

Suspension-feeding sea cucumbers, or those with highly branched feeding tentacles at their oral end, require a lot of plankton per unit of body mass. Put another way, they need a lot of food. Small ones such as the brilliant yellow Colochirus robustus often appear to do reasonably well in tanks where sufficient phytoplankton is regularly added to the tank. The larger ones generally seem to persist for a while, and then die. Often they just seem to "fade away," which is a classic sign of malnutrition. In some cases, this can result in disastrous consequences as many holothuroids contain toxic chemicals in their body's walls, which are liberated during their death throes and subsequent decomposition. Given enough planktonic particulate material, however, they do seem to survive indefinitely. That particulate material should include at least several types of phytoplankton, and perhaps small zooplankton as well.

Bottom mopping sea cucumbers, such as the various species of Holothuria or Stichopus often kept in tanks, are harder to keep alive over the long term. These animals use short feeding tentacles to sweep or mop the substrate to collect various types of detritus. True detritus is defined as being of algal and plant origin and these animals often seem to be specialized feeders on such material. Animal-based foods, and occasionally small animals, will often pass through their guts undigested. The problem with detritus as a food is that it is of very low nutritional quality, both in nature and in aquaria. Consequently, these animals tend to need several square feet of substrate to forage over to get their daily square meals, and the bigger the cuke, the more square footage it needs. Large ones need a lot of sandy substrate! Generally, when added to reef tanks, these animals often slowly, but surely fade away. Given enough food, however, they may grow, and some of them may reproduce by fission.

Figure 8. Holothuria edulis, a bottom-mopping sea cucumber found in the Indo-Pacific. It lives on
or around corals. These cukes eat sediment or detritus from the sediment by mopping it up with
short-branched tentacles that surround the mouth.

With care and foresight, some echinoderms may be successfully kept in aquaria. Unfortunately, with few exceptions, the animals with the best track records of survival in reef aquaria tend to be those that are not terribly attractive. A few species of small cucumbers, a number of brittle star species, a few small sea urchins and one or two sea star species seem to run the gamut of echinoderms that are being maintained successfully. Successful husbandry of echinoderms depends upon maintaining the proper physical factors, primarily salinity but, probably more than that, it depends upon offering them the appropriate foods. This is particularly a problem with the sea stars. Aquarists tend to treat all animals as if they were generalized feeders and, here, as with the species in most large animal groups, that is not the case. Until more research into the diets of these beautiful and wonderful animals is done, aquarists will not know what to feed them or even if they can successfully feed them. Unfortunately, however, they will still be imported and purchased. Many echinoderms are ecologically very important, and alterations of their populations often has unforeseen and devastating effects on the communities in which they are found. The removal of large numbers of sea stars and sea urchins for the aquarium hobby is likely to be having significant effects on the reefs from which they are taken.