Provided you are not in one of the human
rabbit warrens that could be described by the phrase, "concrete
jungle," and you take a look at your surroundings, one
of the first things you might notice is that most of the living
things you can see are plants. A primary characteristic of
most terrestrial ecosystems is that the vast majority of the
visible living material, or biomass, is comprised of plants.
Plants are organisms adapted to catch the energy in sunlight
and convert it into structural and metabolic materials. All
other life in the terrestrial environment depends upon plants
either directly or indirectly. Animals, terrestrial bacteria,
and fungi, either eat plants or use plant products for their
In those areas of the aquatic environments
where light penetrates, the same sort of relationship holds,
except that the oceanic photosynthetic organisms that everybody
else is dependent upon are not plants. In one of the odder
peculiarities of nature, true plants are represented in the
marine environment by relatively few species, mostly grasses
and mangroves. The photosynthetic organisms in the oceans
are algae, and these algae are a diverse group of organisms.
Back in the halcyon days of my misbegotten youth, budding
biologists learned that all living things could be classified
into the animal and the plant kingdoms. Later, we learned
that bacteria were sufficiently different from other life
to be put into a kingdom of their own. Over the last couple
of decades, biologists have learned a great deal more about
the genetic and structural information about all the life
on the planet than we knew twenty- five or thirty years ago.
As result, we have learned that in addition to the animal
and plant kingdoms, there are now about twenty comparable
kingdoms of life. Most of these contain different types of
algae. Although aquarists often lump all algae together, these
algal groups are very distantly related. For example, green
algae and diatoms, are really no more closely related to each
other than they are to the aquarists observing them. The many
types of algae are all specialized to live in various marine
environments and capture different types of ambient light
energy. They are similar to plants, however, in that they
convert that captured energy to build sugars and structural
Algae dominate all shallow water ecosystems,
and this is nowhere more apparent than in shallow-water temperate
and boreal rocky marine areas, such as the along the shores
of both the Northeastern and Northwestern North America. In
these areas are found extensive and lush areas rich with the
growth of many algae. If any of the readers of this piece
live near a rocky sea shore in these areas, and go to visit
it, the shore will be covered in algae, often in great profusion.
Offshore in deeper waters, particularly in the Pacific, they
may find kelp beds; really forests of huge algae. Kelp are
not plants, but they get as large or larger than most plants,
and they are amazing organisms. Growth rates of many types
of kelp may exceed a yard in length per day.
Figure 1. Red sea urchins, Strongylocentrotus franciscanus,
from the Northeastern
Pacific, eating the bull kelp, Nereocystis
Well then, why, when see pictures of thriving
coral reefs do we not see huge extensive beds of kelp, algae
or other seaweeds? Interestingly, the algae are present in
such areas, they are just not visibly apparent. In one of
the first truly modern quantitative examinations of coral
reefs, Odum and Odum in the 1955 paper listed in the references
showed that about 80 percent of the non-bacterial biomass
found on a coral reef was comprised of various algae. The
corals of the "so-called" coral reef amounted to
about 10 percent of the biomass. Algae were found growing
on the reef surface, within the corals as zooxanthellae, within
the coral skeletons, within the sediments and within the very
rock of the reef itself.
So when we think of a coral reef, why don't
we think of an algal dominated system?
The answer is pretty simple. On healthy,
normal coral reefs, we don't see them. The algae are either
small or they are cryptic. The word, "cryptic" has
picked up some unfortunate and inaccurate meanings in some
of the self-styled aquarium literature, but as far as biologists
are concerned, cryptic means "hidden, often buried out
of sight." Consider that crypt comes from the same root
word as cryptic, and discussions of algae on the reef are
truly "Tales From The Crypt."
Many algae are cryptic for one significant
reason, to avoid being eaten. One of the ways that has evolved
in a couple of algal groups as a way to be protected from
predation is the incorporation of the tissue mass of the algae
within a calcareous substrate. Calcareous algae are found
in several algal groups, the green algae, (e.g. Halimeda
species), the brown algae (e.g Padina species) and
most importantly, the coralline red algae (e.g Amphiroa
sp.). These crunchy or rocky algae are really safe from many
herbivores. For example, grazing snails such as Astraea
or Trochus species, cannot eat them. Many herbivorous
fish also leave them alone. Although some of these algae are
quite evident and visible, others are truly cryptic as they
look like nothing more than rock. Visually oriented herbivores
will often leave such algae alone; either they are too hard
to eat, or they don't look like food. Similarly, many frondose
and otherwise highly visible macroalgae have evolved another
form of defense; the production of chemicals that discourage
However, there is one group of invertebrates
that excels at the consumption of all types of algae, and
which can graze quite easily on the calcareous algae. That
group of invertebrates is the class of animals referred to
as echinoids, or sea urchins.
Figure 2. Mespilia globulus, the blue tuxedo
urchin. These animals eat mostly
coralline algae, but will
also eat green filamentous algae.
Anatomy and Biology
The animal group that scientists refer
to as the "Phylum Echinodermata" contains sea stars,
sea cucumbers, sea lilies, sea urchins and brittle stars.
Although most of these animals are at least passably recognizable
to most aquarists, judging from the questions that I get asked
in the "Ask Dr. Ron" forum, the basic biology of
these animals remains absolutely unknown to the vast majority
of hobbyists. This is a shame, for all the echinoderms, and
sea urchins in particular, are some of the most interesting
and truly bizarre animals found in the sea. There is nothing
like them in either terrestrial or freshwater habitats, and
they seem about as far from humanity as one can get in the
animal kingdom. Nonetheless, they are relatively closely related
to us, as witnessed by the fact that much of what we know
about the very early stages of human embryology was demonstrated
initially and studied most thoroughly using sea urchin embryos.
All echinoderms are characterized by having
calcareous plates called "ossicles" in their body
wall. Unlike corals, the calcium carbonate in these plates
is arranged into calcitic crystals rather than aragonitic
ones. In most echinoderms the ossicles are not fastened tightly
together, resulting in the animals having flexible bodies.
In contrast, in most of the shallow-water sea urchins, or
echinoids, the ossicles are fused together to form a rigid
internal skeleton. Zoologists call such a skeleton, a
"test," a word derived from the Latin word, "testa,"
meaning shell, brick or tile; so these animals, indeed, are
Regular sea urchins are strongly pentamerous;,
that is, they have a five-fold symmetry. When viewed from
either the mouth (or oral) or anal (or aboral) ends, the spines
will be seen to be arranged in five or ten rows spaced regularly
and evenly around the body. Two types of structures occur
in these rows. The first and most evident are the spines.
These spines are made from additional ossicles found in the
body wall; instead of being fused with the ossicles of the
test, these additional ossicles are elongated and articulate
bumps called "bosses." There is a complex musculature
around the base of the spines which allows the spines to be
rapidly moved. Depending upon the sea urchin they may have
a limited range of motion, or they may be quite mobile. Perhaps,
the most mobile and rapidly moving spines are seen in the
long spined urchins in the genus Diadema. These spines
give both the sea urchins and the Echinoderms, their names.
The Greek word, echinos, refers to both the spiny European
hedgehog and to sea urchins. From this word, the names Echinodermata
(or spiny-skinned animals), and Echinoidea (or sea urchins),
have been derived.
These spines are so characteristic of the
sea urchins that they really need to be discussed in more
detail. One of the fundamental distinctions found within the
taxonomic class Echinoidea, is between the primitive and advanced
sea urchins. The primitive sea urchins are known as Cidarid
urchins, and are represented in aquaria by the pencil urchins.
In all urchins the spines grow within the body
wall and are covered by an epidermis. This epidermis is
lost after the spines mature in the pencil urchins and their
kindred. These are the only echinoderms where the spines are
not wholly covered by tissues, and even in them the base of
the spines has a tissue covering.
Figure 3. A deep-sea cidarid sea urchin from the Bahamas.
Note that the largest spines are
white and not covered with
colored epidermal tissue, but that the smaller spines are
covered. The biggest spines here were about one quarter inch
The fact that the spines are covered, in
whole or in part, with living epidermis in most urchins has
an important implication for their care, by the way. If you
have a sea urchin, and it starts to lose or "drop"
spines, that animal is in very serious and probably non-recoverable
trouble. Each dropped or lost spine represents an open wound
on the surface of the animal. While one or two spines may
be lost from time to time, the wholesale loss of spines results
from malnutrition or disease and leaves the surface of the
body open for massive infection. Such an occurrence generally
represents a terminal condition. The urchin typically dies
within a few days of when it starts to drop spines.
Incidentally, we definitely know which
urchins are primitive ones. In the fossil record there are
many records of mass extinctions, such as the event that killed
the dinosaurs 65 million years ago. As drastic as that event
was, it was not the most impressive of these extinction events.
The most significant mass extinction occurred about 225 million
years ago and defines the end of the Paleozoic Era on Earth.
Roughly 95% of all species of marine organisms died at this
time. Most animal groups living at the time, such as the trilobites,
went wholly extinct at the end of the geological period referred
to as the Permian, and that includes most echinoderm groups.
Only a few cidarid
sea urchins survived. From that one group, all other sea urchins
subsequently evolved, so the cidarids are definitely the ancestral
type of sea urchin.
Figure 4. Stylocidaris lineata, another deep-sea
cidarid sea urchin from the Bahamas. With no tissue
the largest spines, those spines often accumulate quite a
growth of animals on them. Here the
largest spines are covered
Although sea urchin spines are superficially
similar, they vary significantly in structure throughout the
various sea urchin groups. In the long spined urchins, such
as Diadema, the spines are thin and fragile and designed
to break off in the wounds they create. In others, such as
the rock dwelling tropical urchins in the genus Colobocentrotus,
they are stout and rounded. In the former group, the spines
are venom tipped and used in defense, while in the latter,
they are blunt, short, and used to deflect surf and surge.
In a few sea urchins, the spines are tipped with venom sacs.
Recall that the spine is covered by tissue, and in these species
a small hollow sac will be found at the spine tip. If a predator
attacks the urchin, it may rupture the venom sac at the same
time the skin of the predator is punctured. Venom is then
introduced directly into the wound along with a portion of
the spine that continues to lacerate the tissue. In some urchins,
such as Diadema, the spines are constructed in such
a manner that they break into segments in a wound, and the
segments are effectively barbed. If these spines break within
a muscle, continued muscle contraction will force the spine
deeper and deeper into the wound. A biologist, I once knew,
carelessly stepped on a small Diadema, while walking
in shallow water in the Florida Keys. She shoved a spine deeply
into the sole of her foot. It took about three and half months,
but the spine remnant eventually was surgically removed from
the top of her foot after it had worked its way completely
through it. This kind of spine is generally an effective defense
against most predators; they get one spine in them, and remember
it for a LONG time.
Figure 5. Venom tipped spines from a deep-sea echinoid.
Note that the calcareous spine is clearly surrounded by tissue,
and the bulbous tips of these spines are venom sacs. Accidentally
one of these urchins in the laboratory was
There is one more elaboration of spines
seen in sea urchins, and that is the development of structures
Pedicellariae are constructed of several small spines which
have become modified to articulate with one another and function
as snapping jaws. Each pedicelliaria
is typically found on an elongate and extensible stalk, and
they reach out to pinch any small animals or body parts, such
as the adhesive tube feet of a predatory sea star, that threaten
the sea urchin. The most highly developed pedicellariae are
seen in the so-called "flower sea urchins," in the
genus Toxopneustes. Each of the three jaws of the Toxopneustes
pedicellariae is equipped with a venom sac and gland.
These jaws act as pincers in the event of an attack. They
generally are regarded as acutely defensive, and a nip for
even one of the pedicellaria is sufficient to cause extreme
pain. Such bites have resulted in one known human fatality.
Regular sea urchins all have pedicellariae, and often several
types, that are located in and around the spines. Thankfully
for hobbyists, most of these are not venomous.
Figure 6. A pedicellaria from a temperate sea urchin,
The structure is about
1/16th of an inch across at the widest part.
If you have a test from a dead sea urchin,
you will see that there are rows of plates
with two small holes in them. These rows run from pole to
pole. Each pair of tiny holes is the site where a single tube
foot or "podium" would be found in the living animal.
Tube feet are long, tubular, extensible structures, each tipped
with an adhesive pad. While some sea urchins move by using
their spines rather like stilts, most of them move by the
use of these tube feet. Each animal has several hundred tube
feet and they move along and are fastened to the substrate
by them. Some urchins, particularly the blue tuxedo urchins,
Mespilia globulus, use some of their tube feet to attach
and hold shells, small rocks or debris to their surface. These
attachments are thought to be "tactile" camouflage;
basically a way to hide the urchin from predators that lack
eyes, such as sea stars. If a sea star encounters the rocks
on the surface of a small sea urchin, it may not perceive
of the urchin as prey sufficiently rapidly enough, allowing
the urchin to escape its grasp. Additionally, in the region
around the mouth, most regular sea urchins have large well-
developed oral tube feet that are presumed to be chemosensory
and used to taste potential food items.
Figure 7. Mespilia globulus, the blue tuxedo
sea urchin, covered with calcareous algal skeletons.
fragments are held on to the urchin by the use of its tube
feet. Many brown tube feet are visible
extending out from
the left side of the urchin.
Sea urchins are to most shallow subtidal
marine ecosystems what grazing animals such as bison or cows
are to terrestrial communities. They are THE major
herbivores. In a very real way, we owe sea urchins, particularly
the long spined sea urchins, such as Diadema species, a debt
of gratitude, as they are keystone animals maintaining coral
reefs as coral "dominated" areas. The affect of
sea urchin grazing and their presence on the reefs was really
not clear until the mid-1980s. Until that time, while it had
been hypothesized that the urchins were important in structuring
the reef, there was little absolute evidence of this effect.
At this time, the effects of sea urchins in structuring the
near shore rocky environments in temperate areas were well
known, basically from the events on the west coast of North
America. In these areas, the decimation of the sea urchin's
major predator, sea otters, in the latter half of the nineteenth
century had lead to a population explosion of sea urchins.
At the time this event occurred, it went unnoticed; after
all there weren't any recreational scuba divers in the 1860s.
After conservationists began to introduce sea otters back
into their ancestral ranges, the changes in the community
that developed from the otter predation on the urchins was
clearly significant. Once the otters were re-established,
kelp beds re-appeared and the whole benthic community changed.
In this case, the keystone species was the sea otter, but
the agents of community change were the sea urchins.
Figure 8. The results of grazing by a dense aggregation
of the temperate red sea urchin,
Other than small patches of coralline algae and some small
solitary corals, no other living things were found on the
rock surfaces in this area. Everything else had
by the sea urchins.
In the Caribbean in the early 1980s, a
pandemic swept through the long spined sea urchin populations.
It started around Panama and soon moved throughout the Caribbean.
Within a couple of years, the vast majority of Diadema
urchins had died. The cause of this sea urchin disease is
unclear, but it is an example of an animal pandemic or epizootic.
We tend to forget that other organisms may get diseases just
as humans do, and that such diseases may have rapid and broad
consequences. Such events are natural phenomena, and this
is a good example of such events. Within very short order,
many of the Caribbean coral reefs started to become dominated
by algae. These algae were always present on the reefs, but
their abundances had been kept in check by the urchins' grazing.
These algae have changed many reefs, causing the localized
and perhaps regional extinction of corals and many other benthic
organisms. The algae simply grew over them and smothered them.
It has now been about 20 years since this catastrophe, and
we are seeing the appearance of a few, presumably disease
resistant, Diadema in various areas. Recovery is slow
because urchins must exist at relatively high densities for
spawning events to be productive and the scattered individuals
on most reefs today prevent much successful fertilization
and, thus, recovery of populations. Work is also being done
to try to re-establish Diadema in many of its previously
known ranges; however there is a long way to go, and it is
by no means certain that the changes induced by the loss of
the sea urchins are not permanent. So
if you are a scuba
diver and are swimming on a coral reef somewhere, and you
see some long spined urchins, give a nod of silent thanks
for their presence, as they are helping preserve the reef
that you are enjoying.
Here are some links to information about
the Diadema epizootic:
They can, of course do the same thing
in a reef tank. These are herbivore's herbivores. However,
the somewhat unselective nature of their diets may also result
in the grazing of small encrusting corals and coralline algae,
especially once the tank is rapidly grazed of other algae
by these potent herbivores.
The business end of the sea urchin is the
mouth which is centered under the spherical body. Sea urchins
don't have the blubbery lips of a sea cucumber, or a baby-kissing
politician. Nope, instead they have one of the most complex
and architecturally interesting feeding structures known in
the animal kingdom. Just inside the mouth is a complex arrangement
of both fused and articulating ossicles, supporting five continuously
growing teeth. There are muscles that move both the teeth
and the entire ossicles and tooth structure. This whole structure
is called the "Aristotle's lantern" for a fancied
resemblance to ancient Greek hand lantern, such as Aristotle
might have carried. The teeth are comprised of polycrystaline
calcite formed in long slivers and glued together with proteins.
They grow from the inside end around the outer edge of the
mouth cavity and meet in the center of the mouth. They meet
at an acute angle and the action of eating serves to sharpen
the teeth. These teeth are amazingly strong! The green sea
urchin found throughout the northern seas, Strongylocentrotus
droebachiensis, has been documented to eat its way through
reinforced concrete piers, eating both the concrete and
the reinforcing iron bars inside it. This species, and other
sea urchins, have also been documented to eat through the
lead encased, copper wires of undersea telephone and telegraph
Figure 9. The top of the Aristotle's lantern in a dissected
sea urchin. The esophagus would exit
from the center of the
structure, and the mouth is underneath it.
Follow this link for a diagram of an Aristotle's
Photos of the same structure in a dissected
Such a strong feeding structure developed
really for only one reason, to eat calcareous algae. Probably
the only efficient way that such algae may be eaten is by
physically abrading it, and that's what these teeth
are for. Additionally, this apparatus also will eat things
like Bryopsis and other filamentous green algae completely,
as the urchin can eat the small algal filaments that extend
into the rock. It does this by eating the rock to get out
the "goodies" on and in it.
Once eaten, the algal fragments pass internally
into a relatively simple gut. The esophagus, suspended by
fine membranes, passes vertically up from top of the Aristotle's
lantern and then makes a right angle turn to pass outward
to the inside of the test where the intestine makes one complete
loop around the equator. After this, the hindgut again passes
to the center of the animal and runs up the center vertically
to the anus on the middle of the upper surface. In long spined
sea urchins, the anus is found on a small bulbous stalk in
the middle of the top of the animal. It is often ringed with
a colored band, and looks rather like an eye, if you ignore
what is coming out of it, that is. I have been told by many
folks that the structure they see has to be an eye, and the
animal looks around with it
Sea urchins have a lot of sensory cells,
but rather few sensory structures or organs. They do have
five colored eyespots surrounding the anus at the top of each
spine row. These are not eyes, but rather sensors that determine
light or dark. Many of the urchins are nocturnally active,
and will seek darkness as a means of shelter during the day.
Sea urchin sexes are generally indistinguishable
by the casual observer, although some researchers have learned
to "read" subtle differences in shape that allows
them to predict what gender the animal that they are examining
is. They are broadcast spawners, liberating their gametes
into the water column, where eggs and sperm meet and form
a new individual. The embryonic sea urchin develops in the
water, where it becomes a feeding, swimming, larva within
a few hours to a few days, depending on the species. The larvae
may live a long time in the water, and they can become quite
complex and relatively large animals, up to a few millimeters
across in some cases. If conditions are optimal, these larvae
may actually bud off little clonal tissue blebs that develop
into more larvae as well. Eventually, the larvae will settle
to the substrate, and metamorphose into a juvenile
sea urchin. Once they settle out of the plankton, sea urchins
may live for a long time. Their ages can be determined by
counting rings in some of the ossicles, and average ages in
some populations are on the order of twenty or so years, with
many animals living quite a bit longer. As with all echinoderms,
they don't die of old age, but succumb to disease, accident
or predation. They play the odds of life, and eventually they
lose the bet.
Figure 10. A large, about ¼ inch long, sea urchin
larva, collected from the plankton of the
N. E. Pacific. The
gut is visible in the center; the mouth is found between the
arms on the left. Fine skeletal rods are visible
running down each arm. The animal
moves in the direction that the arms point.
Urchins In The Aquarium
Most of the urchins commonly offered for
sale are listed in Table 1. In general, what I have termed
"desirable" sea urchins will eat a variety of algae,
including coralline algae. When they eat coralline algae,
the pattern of eating creates a variegated patterning of the
algae that gives aquaria a natural look; such herbivory seems
to stimulate the algal growth. "Undesirable" urchins
are undesirable for a variety of different reasons, ranging
from their diets, to their size, to their potential to harm
the aquarist. Specifically among the latter, Toxopneustes
pileolus can kill from the shear shock reaction to its
pain, and should never be purchased; the danger from an inadvertent
brush against it would be too great. I have not been able
to document any fatalities concerning the fire urchins, but
contact with the spines can cause severe and debilitating
pain that lasts hours to days.
1. Some of the sea urchins commonly or occasionally
offered for sale to aquarists. Desirable species
are good herbivores, but most will eat coralline algae
as well as other less desirable algae. Follow
the links to go to an illustration of the given species.
or Common Names
good algae eaters
eats soft corals. Common on live rock
eats soft corals. Common on live rock
eats soft coral. Occasional on live rock.
are a bit dangerous. Caribbean species. Do not
purchase, allow to remain in the wild.
are a bit dangerous. Pacific species, ring around the
anus white or light blue.
are a bit dangerous. Pacific species, ring around the
big for home aquaria, otherwise fine.
big for home aquaria, otherwise fine.
big for home aquaria, otherwise fine.
carnivorous; will eat many decorative animals, also
can eat “sleeping” fish, or shrimp.
can inflict severe pain
can inflict severe pain
can inflict severe pain. In the linked image,
the pedicellariae are the small round, flower-like structures.
In general, sea urchin care is reasonably
straight forward. They need good full strength sea water,
and should be slowly acclimated to changes in the sea water.
They will do best in salts with low heavy metal or trace element
concentrations. They generally need a lot of food. In particular,
the amount of coralline algae necessary to keep a small blue
tuxedo urchin healthy is quite significant. Unless a tank
has a lot of coralline algae it is best not to buy this species.
They need temperatures in the normal coral reef range to thrive;
temperatures around 82° F would be optimal.
Sea urchins are a normal component of coral
reefs and they are valuable additions to marine reef aquaria.
Some of them are just about the only animals that can reliably
control many of the problem algae that plague many aquarists.