A Spineless Column by Ronald L. Shimek, Ph.D.

Sea Urchins, A Testy Subject


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 existence.

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 compounds.

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 luetkeana.

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 herbivory.

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 "testy organisms."

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 on raised 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 in diameter.

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
covering the largest spines, those spines often accumulate quite a growth of animals on them. Here the
largest spines are covered with zoanthids.

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 brushing against
one of these urchins in the laboratory was exceedingly unpleasant.

There is one more elaboration of spines seen in sea urchins, and that is the development of structures called "pedicellariae." 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 globiferous 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, Strongylocentrotus franciscanus.
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.
These 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,
Strongylocentrotus franciscanus. 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
been eaten 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 cables.

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 lantern:

Photos of the same structure in a dissected sea urchin:

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…. Nope…

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 two center
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.

Table 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.

Category or Common Names



Desirable Species


Generally good algae eaters


Caribbean Rock Urchins




Echinometra lacunter

Occasionally eats soft corals.  Common on live rock



Echinometra viridis

Occasionally eats soft corals. Common on live rock


Pacific Rock Urchin





Echinometra mathaei

Occasionally eats soft coral. Occasional on live rock.


Long-spined Urchins





Diadema antillarum

Spines are a bit dangerous. Caribbean species.  Do not purchase, allow to remain in the wild.



Diadema savignyi

Spines are a bit dangerous. Pacific species, ring around the anus white or light blue.



Diadema setosum

Spines are a bit dangerous. Pacific species, ring around the anus orange.


Tuxedo urchins





Mespilia globulus

Prefers coralline algae



Microcyphus rousseaui

Prefers coralline algae





Undesirable Species





Lytechinus williamsi

Too big for home aquaria, otherwise fine.



Lytechinus variegatus

Too big for home aquaria, otherwise fine.



Tripneustes ventricosus

Too big for home aquaria, otherwise fine.


Pencil Urchins





Eucidaris tribuloides (Caribbean)

Largely carnivorous; will eat many decorative animals, also can eat  “sleeping” fish, or shrimp.



Eucidaris metularia (Pacific)

Largely carnivorous.


Fire Urchins





Asthenosoma varium

Spines can inflict severe pain



Asthenosoma ijimai

Spines can inflict severe pain


Flower Urchins





Toxopneustes pileolus

Pedicellaria 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.

If you have any questions about this article, please visit my author forum on Reef Central.


Carpenter, R. C. 1986. Partitioning herbivory and its effects on coral reef algal communities. Ecological Monographs. 56:345-363.

Ghyoot, M., P. Dubois and M. Jangoux. 1994. The venom apparatus of the globiferous pedicellariae of the toxopneustid Sphaerechinus granularis (Echinodermata, Echinoida): Fine structure and mechanism of venom discharge. Zoomorphology. 114:73-82.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Lares, M. T. and J. B. McClintock. 1991. The effects of temperature on the survival, organismal activity, nutrition, growth and reproduction of the carnivorous tropical sea urchin Eucidaris tribuloides. Marine Behavior and Physiology. 19:75-96.

McGehee, M. A. 1992. Distribution and abundance of two species of Echinometra (Echinoidea) on coral reefs near Puerto Rico. Caribbean Journal of Science. 28:173-183.

Odum, H.T. and E.P. Odum. 1955. Trophic structure and productivity of a windward coral reef community on Eniwetok Atoll. Ecological Monographs 25(3):291-320.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008

Sea Urchins, A Testy Subject by Ronald L. Shimek, Ph.D. - Reefkeeping.com