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Aquarium.Net April 97

Ron Shimek discusses different urchins we might use in our aquariums, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

There is No Reason to Be Spineless

By Ronald L. Shimek

The algal problem...

One of the most common problems in the care of marine reef aquaria is the development and extensive growth of various types of algae. Numerous methods can be utilized to control algal growth.

An Indo-Pacific pencil urchin, probably Eucidaris metularia .

These range from altering the light intensity to significant manipulation of water chemistry with the aim of denying the "problem" algae essential nutrients, primarily silica, nitrates, or phosphates. Probably the most universally tried method for algal control, however, is the introduction of animals that feed on algae.

Algal herbivores are found in a number of animal groups, but the ones most commonly utilized in marine aquaria are restricted to three major phyla, the mollusks, the arthropods, and the chordates. Molluscan herbivores are primarily grazing snails, such as various species of Astraea , Trochus , or Turbo although some aquarists also maintain populations of chitons which can also graze on the benthic algae, and I have personally found that abalone do very well in this task. An arthropod that appears to be gaining some acceptance as an herbivore is the "blue-legged hermit crab" which is a species of Paguristes found in shallow waters in the Caribbean subtropics (Schiemer, 1994). Some other crustaceans are occasionally used a herbivores. Chordate herbivores are reef fishes that eat algae, these are such fishes as some of the tangs and blennies as well as other fishes.

All of these animals and methods have advantages and disadvantages, but one of the most profound of these disadvantages is that they don't eat all the algae, and even with significant populations of herbivores and even with significant chemical manipulations, there is often a persistent problem of unwanted algae.

A deep water Atlantic pencil urchin, Cidaris blakei , collected at -575 m (- 1898 ft) photographed out of the water. Note the unusual spines. Some spines are covered in zooanthids.

To realize why this is so requires a brief foray into evolutionary ecology. In an evolutionary sense, animals respond most rapidly to those properties of their environment that cause them the most significant mortality. One obvious response is retreat away from the mortality effect. For example, many marine animals, such as sponges, have mobile larvae that select where the immobile or sessile adults live. Once the larvae metamorphose, the animal is there for the remainder of its life. Many marine animals are killed by exposure to air, so for these animals there is significant natural selective "pressure" to create a method to avoid settlement in the intertidal zone. If these animals can develop a means of restricting their larvae so that the larvae will not metamorphose in the intertidal, they have an advantage that can confer a measure of success on their offspring.

In a similar manner, algae have been undergoing natural selection by their predators for a long time. Any alga that has some property that allows it to avoid being eaten by an herbivore has an advantage over its neighbor without such an advantage. In most marine ecosystems there four major types of animals that prey on plants. These four are fishes, sea urchins, snails, and crustaceans. Generally, the first two are by far and away the most important ones. Of course, selection is a two-way street, once a predator's food starts to become safe from predation, the predator is under selection to find a way to overcome this barrier or it will starve. This results in what some biologists call the co-evolutionary race between predator and prey.

Natural selective pressures have resulted in adaptations increase the survival probability of the animals, but as all energy and responses are finite, those predators that are most successful have the most defenses against them. Additionally, some of the adaptations for defense and survival will work well against several predators. Nevertheless, for the successful removal of algae, the aquarist should consider having as many of the kinds of algal predators as is possible.

Sea urchins, the ultimate algal predators...

To read about some of the effects of sea urchins on marine algal communities follow these links:


Temperate: and choose the kelp forest option

Sea urchins are the typical members of the Class Echinoidea of the Phylum Echinodermata. Sand dollars, sea biscuits, and heart urchins are also members of this class, but they are all somewhat altered from the basic sea urchin form, and I won't discuss them further here.

The names Echinoidea and Echinodermata are both derived from Greek word echinos meaning hedge-hog or sea urchin, and denoting the spiny nature of the beasts. Spines are indeed one of the definitive characteristics of the sea urchins. Basically most regular sea urchins are roughly spherical or egg-shaped animals with a skeleton composed of plates of calcium carbonate. In most shallow water sea urchins these plates are fused to create a hard shell with an opening at the top for the anus and at the bottom for the mouth. In some sea urchins, mostly those found in deeper waters, the plates are not tightly fused and the animals will collapse into a blob if removed from the water.

Five bands of spines run from the mouth end to the anal end of the animal. In between the bands of spines are rows of tube feet. With the exception of pencil urchins, the spines are covered with an epidermis, and sometimes they are tipped with a small glandular bulb which is filled with a venom. The spines sit on a small spherical ball or boss and are moved by muscles attached at their bases. Generally, most urchins have several different sizes of spines. They also have some specialized modified spines called pedicellariae.

Each pedicellaria is a stalk topped with three jaws that close together. Some tropical sea urchins in the genus Toxopneustes and their near relatives have pedicellariae with venom glands in the base. When the jaws close on a predator's flesh, the glands inject venom into the predator. Any urchin can attempt to use its pedicellariae to defend itself against predators, but the effects of these particular pedicellariae is exceptional. The venom is REALLY nasty. When the predator that the urchin is defending itself from is a human collector or an aquarist, the results can be become very interesting. When those pedicellariae close on human skin the pain can be significantly understated as "intense," or "agonizing." The handler can expect to feel intense pain, paralysis, nausea, and in rare cases, will cease to feel anything at all as they die... Occasionally one sees these urchins for sale in aquarium stores under the common name of "flower urchins" because the open pedicellariae look like little three pointed flowers. Fortunately for the aquarist, most urchin pedicellariae lack such potent weaponry (Ghyoot, et al., 1994).

An illustration of Toxopneustes is given at this link. The pink cup-like structures all over the surface of the animal are the venomous pedicellariae.

Sea urchin internal anatomy is pretty simple, the animals have a gut that runs upward from a complex jaw apparatus to an intestine which circles the animal at the equator. After one loop, the gut continues upward to exit at the anus. There are five gonads running from top to bottom under the spines on the inside of the skeleton, each of these opens by a single pore in a skeletal plate at the top of its row of spines.

These animals use hydraulically operated tube feet or their spines as their primary means of locomotion. If they use the tube feet, the feet attach to the substrate by a glandular adhesive. A second secretion causes the release of the foot from the substrate. If an aquarist pulls a sea urchin off of a rock or the wall of the aquarium, the tube feet that the animal was using to attach to the surface while often be broken off and remain attached to the substrate. This is because the second releaser substance has not been released.

These animals are at just about the lower extreme in the animal kingdom for the lack of a defined brain. In fact the whole nervous system is so reduced that it is difficult to demonstrate with microscopic examination (Maerkel and Roeser, 1991). Nevertheless, sea urchins do exhibit behavior. One echinoderm researcher explained their behavior with the lack of a defined brain by hypothesizing that the entire nervous system acts as an associative organ rather than just a small portion which is aggregated into a brain. This supposition remains unproven, however.

As far as the algally-challenged aquarist is concerned, however, the business end of a sea urchin is the complex jaw apparatus called the "Aristotle's Lantern," for its supposed similarity to an ancient lantern. The Aristotle's lantern is an arrangement of calcareous skeletal structures, muscles, connective tissue, and teeth. It is shaped like a five-sided pyramid, with pointy end directed downward. Running down the center of each side of the pyramid is a tooth. All five teeth meet at the tip and are secreted at their upper base. The teeth consist of polycrystalline calcite cemented together in an organic matrix. The Aristotle's lantern consists of musculature that moves the teeth up and down as well as musculature that moves the whole structure up and down.

Basically, the urchin biting cycle starts with the lantern apparatus lifted off of the substrate. The teeth are retracted. The lantern is then depressed to touch the substrate and the teeth are extended by muscular contraction working some of the skeletal ossicles in the Aristotle's lantern like a lever. .

Strongylocentrotus.. The green sea urchin, Strongylocentrotus droebachiensis . This animal is about 7 cm across.

This lever action can result in significant increases in the force of the bite due to simple mechanical advantages. The teeth meet slicing off a piece of food, which is held between them. The lantern is lifted off the substrate and the cycle begins over again. This rather simple process belies the tremendous power that the teeth can exert. The green sea urchin, common throughout northern seas around the world, Strongylocentrotus droebachiensis , has been known to eat its way through re-enforced concrete piers, including eating the re-enforcing iron bars. Different deep-sea urchins have been known to eat their way through lead encased telephone cable. One presumes their fecal pellets could be used as miniature bb shot.

To see the business end of a red sea urchin's Aristotle's lantern, follow this link:

More information on the ecology of the green sea urchin in British Columbia and Norway is given in the links above discussing temperate sea urchin ecology.

Such predators can certainly cut through and eat virtually any kind of algae, and indeed sea urchins can eat the most rugged types of algae encountered in reef aquariums. In environments where the cold-water green sea urchin is common, some of the algae in the genus Desmarestia have developed a rather unique defense. They secrete concentrated sulfuric acid (pH about 1), which is held in small enclosures in their tissues. When an urchin bites into the alga, the acid is released and it acts to dissolve the urchin's teeth. Where these particular algae are common, many the urchins have a gap between their teeth just the width of the algal blade. This gap makes it difficult for the urchin to eat these algae, and in laboratory experiments it has been shown that the urchins will tend to prefer to eat other algae.

Compared to some temperate areas sea urchins are relatively uncommon in the tropics, and probably because of that, such potent defenses against sea urchins are rare in those regions. Consequently urchins make exceptionally good predators against all sorts of algal pests. Many different types of urchins are available from time-to-time. Probably the most commonly seen are the long-spined urchins in the genus Diadema . Diadema have a small body surrounded by very long and sharp spines. The spines are shortest directly under the body where the animal must get close to the substrate to eat. Long sharp spines imply that something is a significant predator of on these urchins, and in point of fact several fish species will attack and eat them.

The fish will attempt to turn over the urchin and bite through the spines around the mouth where they are the shortest. Because of this kind of predation, Diadema generally remain in crevices during the day, venturing out only at night to feed. More data on the ecology of Diadema is given in the links above referring to tropical sea urchin ecology.

Diadema... The long spined urchin, Diadema antillarium .

This behavior of hiding during the day, to emerge and feed at night is, in fact, a common behavior pattern amongst most tropical sea urchins, and probably is a result of the predation by fishes. Temperate sea urchins simply don't have this behavior, as there are effectively no fish that eat adult sea urchins in temperate regions. This pattern of hiding only to re-emerge later is at the root of probably the biggest problem with sea urchins. Echinoids can be significant tank remodelers... When they move around at night, but particularly when they go into and out of their sheltering holes or crevices, they blunder into things and knock them over. This problem can be minimized during the placement of rocks during the tank set up, but it can remain a problem.

Not all sea urchins eat just algae of course, and this is the root of the other major drawback to their use. Some urchins are particularly predatory, and many do not eat algae at all. Pencil urchins, such as Eucidaris tribuloides , are meat-eating animals (Lares and McClintock, 1991).

Eucidaris.. A Caribbean pencil urchin, Eucidaris tribuloides .

Before I found this out in my own system, I watched a pencil urchin catch and eat a scarlet cleaner shrimp. These blundering animals, with no brain and few defined sense organs can be remarkably effective at catching much more mobile and alert creatures. Pencil urchins are interesting from a purely biological standpoint, as they are the most primitive of the living urchins, and their skeletons are effectively identical to those found shortly after urchins show up in the fossil record in the Ordovician Period, a few hundred million years before dinosaurs snorted their first breath (Tasch, 1973). Part of the indication of the primitive nature of these animals is the fact their spines, alone amongst the echinoderms, are not covered with tissue.

For some more information on Eucidaris, follow this link:

Some good algal croppers are found amongst both Indo-Pacific and Caribbean urchins. My personal favorite sea urchin is Mespilia globulus , aka "The Blue Tuxedo Urchin." I have found this animal living under coral rubble around Palau, but in my tanks they have generally remained in sight throughout the day.

Mespilia... The blue tuxedo urchin, Mespilia globuli s, from Palau.

They are relatively small sea urchins, seldom exceeding 3 cm in skeletal diameter. They have a beautiful blue body and fine gray almost silver spines. In my tanks they seldom did any "bulldozing" damage, and did an excellent job at cropping the algae. Unfortunately, my urchins perished when I moved to Montana from the Seattle area and I have not been able to find this species since. Mespilia has the behavior pattern of carrying pieces of gravel and other debris over its surface, presumably as either visual or tactile camouflage, and individuals will continue to do this while in aquaria. A blue tuxedo urchin in good health will be holding onto all sorts of rubble; if the animal starts to loose its cover, it is suffering from some problem.

For some more information on Mespilia , follow this link:

Another good urchin to have on algal patrol is the Caribbean urchin Echinometra lacunter (McGehee, 1992). This species is dark purple to black and is covered with fine, but relatively robust spines. It is another small species and generally doesn't do much to rearrange tanks. It primarily eats algae, but will occasionally eat a bit of plankton or other fish food.

Larger Caribbean sea urchins, such as the "sea egg," Tripneustes ventricosus , or the "variegated urchin," Lytechinus variegatus , make good algal grazers when they are small, but as they get larger they can significantly rearrange a tank. Many other urchins from the Pacific in particular, are occasionally found in dealers tanks.

Lytechinus... The Caribbean variegated urchin, Lytechinus variegatus .

In many cases they would be beautiful and useful additions to a common reef tank. For most of these species, however, we know little of their dietary requirements, and this may make them difficult animals to maintain. Even those animals that do specialize on algae, may not specialize on the types of algae we have in our systems, or they may specialize on some types, such as crustose coralline algae, that are considered to be beneficial. Many sea urchins also seem to need some sort of animal product as portion of their diet, so if you purchase an urchin species that is new to you, be aware of the potential problems.

Other than an appropriate food source, the other critical factor in sea urchin care is water quality. Echinoderms are legendary for their lack of toleration of understrength sea water, and this seems to be particularly true of some of the Pacific species such as Mespilia . Urchins should not be kept at salinities less than 35 ppt (generally a specific gravity of 1.025). Those urchins from true tropical reefs should be maintained at reef temperatures of about 82-84 degrees F, while those from the colder tropical extremes such as Florida can tolerate somewhat cooler temperatures.

References cited and suggested readings.

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.

Maerkel, K and U. Roeser. 1991.

Ultrastructure and organization of the epineural canal and the nerve cord in sea urchins (Echinodermata, Echinoida). Zoomorphology. 110:267-280.

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.

Ruppert, E. E. and R. D. Barnes. 1994.

Invertebrate Zoology . Saunders College Publishing. Philadelphia. 1056 pp.

Schiemer, G. 1994.

Hermit crabs in the reef aquarium. Aquarium Frontiers. 1994. Summer. p. 5.

Tasch, P. 1973.

Paleobiology of the Invertebrates . John Wiley and Sons. New York. 946 pp.

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Last modified 2006-11-18 19:42