Suitability for Aquaria?
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.
The Problem of Food
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
The Criteria for Success
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
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.
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
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"
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
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.
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
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.
Figure 4. Ingestion of prey. The Solaster
dawsoni (top=eater) is ingesting its prey, Solaster
stimpsoni (bottom=eatee). This process in these
particular stars takes about four days. The predator
was about 30 cm in diameter and was smaller than the
star it ate.
Tropical Sea Stars of Interest:
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."
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
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."
laevigata - Blue Linckia,
multifora - Multicolored Linckia,
speciosus - the "so-called" Red "Linckia."
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
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.
Caribbean Cushion Star
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.
nodosus - Chocolate Chip Star,
lincki - Red Spined Star
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
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
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
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
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.
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
or around corals. These cukes eat sediment or detritus from
the sediment by mopping it up with
short-branched tentacles that surround the mouth.
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.