Echinoderms: Unique Characteristics
Author, Marine Biotechnology Consultant
Michael Paletta,


Cortez StarfishAs the hobby of keeping marine animals has evolved, greater emphasis has been placed on keeping a wider diversity of animals. While fish and then corals were the first animals that sparked interest in the hobby, the addition of "nontraditional" animals beyond fish and corals are gaining more attention for their function and role in making a successful reef system complete.

Some of these nontraditional animals, such as clams, are added for their beauty while others, such as crabs or shrimp, are added for their interesting behaviors. But more importantly, many echinoderms are now being kept for the important biological tasks they perform as well as their contribution to the long-term success of the reef system. Their varied feeding habits and methods utilize nutrients more efficiently and completely, minimizing the accumulation of excess nutrients in closed systems.

Many echinoderms are detrivores that feed on the waste of other animals and may even sift through the sand or other substrate in search for it. Other echinoderms may sit under rocks waiting for any morsel of food to come their way while others consume organic material found on live rock, keeping the rocks clean for corals to grow on. Still others consume plankton and other material floating in the water column. Because of these attributes, echinoderms can be considered the garbage collectors of the reef and provide an important service for the overall health of a reef system.

Blue Tuxedo Pincushion UrchinIt should be noted that some of these animals have developed predatory habits and many possess toxins. Owners of these animals should understand the potential risk of keeping them and design the aquarium accordingly. Restraint should be the watchword before adding any of these animals since they can be difficult to remove once they have been released into your reef system.

There are five classes of echinoderms that are commonly kept in the aquarium hobby. These include members of the class Asteroidea (starfish), Ophiuroidea (brittle and serpent starfish), Holothuroidea (sea cucumbers), Echinoidea (sea urchins), and Crinoidea (feather stars).

Anatomical characteristics

All echinoderms share several common characteristics despite very dissimilar outward appearances differentiating them from all other members of the animal kingdom. First, they all possess five-part radial symmetry around a central disk. Second, they all possess a very unique water vascular system (vascular system based on water). These unique characteristics distinguish echinoderms from other animals in the animal kingdom.

5-part radial symmetry

Red Knob StarThis is the most obvious common trait shared by members of this phylum. This symmetry is based on a five-part system where a central structure is surrounded by five equal parts. This is especially apparent in starfish, brittle stars and serpent starfish where the five arms or a multiple thereof are arranged around a central disk. This radial symmetry is not as apparent in sea urchins or sea cucumbers, but when these animals are closely examined or dissected, it becomes readily apparent that their body is arranged based on this 5-part symmetry. Due to this particular body arrangement, these animals do not possess what we would regard as a head. They do not possess anything close to a central nervous system and lack eyes or other means of dealing with complex stimuli. Instead, they possess a very simple combination of chemo and tactile receptors. Echinoderms are able to differentiate between light and dark and have a very keen sense of chemoreception as indicated by their relatively rapid movement when food is introduced into an aquarium.

Water-based vascular system

Echinoderms are also characterized by a unique water-based vascular system possessed by no other animal. This highly specialized system not only allows them to transport food and water along the outside of their bodies, but it also allows for other nutrients and gases to be transported as well. Their hydro-vascular system has evolved to the point that it provides a means of locomotion simply by changing the water pressure within their tube feet. This system is also unique in that water within this vascular system is open to the outside, that is, it is not self-contained. This peculiar characteristic may partly account for why these animals are so sensitive to rapid changes in water chemistry and may go into osmotic shock when salinity or ions in their environment are changed rapidly. This water vascular system is controlled by a sieve plate (the madreporite), the dark colored structure on the upper surface of most sea stars.

Defensive characteristics

Spiny skin

Echinoderms possess an interesting defensive structure called the pedicellaria. These external jaw-like structures are located not only on the ends of arms but are also found along the body. These structures help keep the multitude of larvae that are constantly looking for an attachment site from settling on the body surface of echinoderms. Furthermore, the external skeleton of most echinoderms is quite formidable, usually quite thick and hard containing spine, poison, toxic mucous, or a combination of these.

Expulsion of internal organs

A defensive behavior commonly demonstrated by sea cucumbers, echinoderms possess the ability to eject some of their internal organs when they feel threatened or stressed. Several other members of this phylum also utilize this defensive mechanism but to a lesser extent.

Sea AppleWhen sea cucumbers are stressed, either by being picked on by a fish or caught in a powerhead or pump, they will contract forcing out their digestive tract and a massive amount of tubules containing holothurin. This compound dramatically reduces the surface tension of water and it is hypothesized that this compound appears to inhibit the ability of fish to take in oxygen so they rapidly die when exposed to this toxin. This behavior is an attempt to get the offending party trapped in the filaments where it will suffocate, while the cucumber crawls away to regenerate a new digestive system. This compound is present in all of the sea cucumbers of the genus Holothuria, which unfortunately make up most of the species commonly kept in aquaria. Sea Apples, a close cousin of the sea cucumber, also possess a similar toxin that can kill all of the fish in an aquarium if they are disturbed.

Regenerative abilities

Echinoderms use their regenerative abilities as a defensive mechanism, frequently utilized by starfish when caught by an arm. When this happens, these animals will just drop off an arm and move away. The unwary attacker is left with a wiggling arm while the rest of the animal moves away to regenerate a new arm. This regenerative ability allows these animals to not only grow new appendages, but for some cucumbers and sea stars, it also allows entire new animals to be formed when the animal is split in two.

Sea Stars (Asteroidea)

Orange Linkia Sea StarJust as radial symmetry is most noticeable in sea stars, sea stars are also the most well known of all echinoderms. Sea stars live in all oceans and in virtually all zones within the reef. In the Indo-Pacific alone there are over 200 species of sea stars, in 60 genera, and in all of the oceans there are over 1800 species. Despite five or a multiple of five being the most common number of arms, there are many species that have arms that are not in multiples of five, or with arms much more numerous than we normally see, as many as 50 arms. The class Asteroidea is not limited just to simple sea stars, but also includes the cushion stars, horned sea stars, and pincushion stars.

Sea stars have distinct upper and lower portions. On the underside of each sea star is the mouth along with a distinct cleft through which food is moved to the mouth. This cleft and mouth is referred to as the oral disk. Some sea stars do not have a separate opening for removing waste and as such, their "mouths" act as both the site for the import of food and the export of waste materials.

Most sea stars are rather specialized feeders, feeding on particular food items but should the need arise many can become omnivorous general feeders. The most desirable varieties to have in a reef aquarium are either the detrivores or the algae eaters. These include members of the genera Linkia and Fromia. These stars seldom bother corals, but they need to be kept in a well-established aquarium that contains lots of microfauna as well as microalgae upon which they can feed. Sea stars can be omnivorous or specialized feeders so before any are added to a reef aquarium, their feeding requirements should be researched.

There are several predatory starfish that should not be kept. The Crown-of-Thorns, infamous for consuming large portions of the Great Barrier Reef in recent years, is probably the best-known example. However, a more problematic predatory sea star is the small sea star of the genus Asterina, introduced to many reef aquariums in recent years. This tiny sea star is usually no bigger than 1 inch across and seemingly has two color varieties. The white or gray variety is usually a harmless algae eater that consumes microalgae from the glass and live rock. However, there is another similar looking sea star that is mottled red, pink or green that feeds aggressively on stony, and in some instances, soft corals. This pest has become a frequent hitchhiker on Indo-Pacific live rock and corals. They can grow rapidly to a population that can decimate stony coral colonies in a closed system. All new coral or live rock additions should be carefully examined for these sea stars and removed immediately if found.

Brittle and Serpent Starfish (Ophiuroidea)

Serpent StarfishAlthough they look similar to the sea stars, there are some major morphological differences between these two groups. Brittle and Serpent Stars do not possess the groove along the underside of their arms and their tube feet serve primarily a sensory function and are not involved in locomotion or the transportation of food to the oral disk. Their arms are much longer and much more differentiated from the central disk and their madreporite is located on the opposite side from that of the sea stars. The light sensory organs (photoreceptors) of brittle and serpent stars also differ from sea stars.

These stars are much more cryptic than their cousins, the sea stars, and do not like to be in bright light. They have specialized photoreceptors that let them know when it is time to come out to feed. They remain hidden in the live rock or substrate until dark to avoid predation, making it very difficult to determine how many brittle stars are present in an aquarium.

Almost all of these animals possess five arms, with only a few species having more than that. In addition, their arms contain joints and muscles that allow them to move in virtually any direction much more rapidly than their tube-footed cousins. This ability to move on these hinged arms allows these stars to pull themselves into virtually any space. As long as their central disk is not too big, these animals can fit themselves into crevices seemingly too small for them. These flexible arms are used for sensing as well as to latch on to prey, which can include slow moving fish. For this reason, large brittle stars should not be housed with small slow moving fish.

These animals play a crucial role in the long-term maintenance of reef aquariums since they are very active detrivores, living most of their lives hidden within or under live rock consuming any food or waste material that comes their way. Adding some of these animals provides a benefit at a relatively low price. Keep in mind that most of these animals will not become predatory as long as adequate food is provided.

Sea Cucumbers (Holothuroidea)

Striped Sea CucumbersLike the brittle stars, sea cucumbers perform a valuable function by constantly removing detritus from the substrate. More importantly, they plow through the bottom substrate like a small earthmover and pull the substrate through their tube-like bodies. They extract any organic matter found in the substrate and expel the cleaned substrate into little piles that quickly break back down into the original substrate. This activity not only removes excess organic material from the substrate but also tills and aerates the substrate. Reef flats are literally crawling with detritus feeding sea cucumbers, processing much of the waste materials produced on the reef. While these animals may appear ugly, without them most reefs would be covered with waste.

These animals live in virtually all oceans and can be found in many varied environments from soft muddy or silty areas to hard or sandy areas. They have evolved so that different species take advantage of different substrate types. Some species even live within the live rock itself and gnaw through this hard substrate to obtain food.

Regardless of which type of substrate they live in, sea cucumbers all excrete pellet-shaped fecal matter after they process the substrate. These pellets are comprised entirely of substrate since virtually all the organic matter is meticulously removed from the substrate as it passes through the intestines of the cucumber. Once these pellets are expelled, they rapidly lose their pellet shape and fall apart, returning to the substrate.

Like most echinoderms, sea cucumbers have very good regenerative powers. This is shown by their ability to regenerate internal organs including their digestive tract that may be expelled when they feel threatened. Further evidence of this regenerative ability is shown by some species being able to regenerate an entire new animal if it is cut in half.

Impressively, some cucumbers can reproduce asexually by simply dividing apart. In a couple of species, this division will manifest itself by the mother animal first developing a small appendage on its side. This appendage will gradually grow and after a few weeks it will look like a mini-replica of the mother animal. Eventually it will drop off and become a fully developed cucumber. These animals also reproduce in the usual manner releasing eggs and sperm into the open water. The larvae are free-swimming at first and then develop into juveniles that crawl along the substrate.

All sea cucumbers have the potential to poison an aquarium by ejecting their digestive tract and releasing a toxin. Though the toxin is used only for defense, extreme care should be taken when keeping these animals. If you choose to keep these animals, it is ideal to house them in a separate dedicated aquarium free of powerheads and other apparatus on which they could get caught. Other than this aspect, sea cucumbers are quite peaceful and perform a very necessary task in an aquarium.

Sea Urchins (Echinoidea)

Longspine UrchinThere are over 800 different species of sea urchins varying greatly in appearance, ranging in size from less than an inch to over a foot across in an assortment of shapes and colors. Unlike sea cucumbers, which look benign and helpless, sea urchins look quite formidable with their extensive network of spines.

Most urchins have two distinct types of spines: large primary spines and smaller secondary spines. These spines are attached to a series of calcareous plates or skeletal ossicles by a structure very similar to a ball and socket joint and move as the plates move. A sheath surrounding the sockets moves the joints and gives the spines a great deal of flexibility. As if the spines were not formidable enough, some species of urchin also possess poison that they eject through their spines. Needless to say, for the most part, these poisonous urchins should not be kept.

Like sea cucumbers, sea urchins play a vital role on the reef by keeping algae population in check. Most species are herbivores, but there are species that are detrivores or scavengers and a few predatory species that feed on corals and other sessile invertebrates. Sea urchins feed on virtually all types of algae including calcareous algae (e.g., coralline algae). Hence, if calcareous algae are desirable, the number of urchins kept in a system should be limited to allow calcareous algae to grow. Sea Urchins have a healthy appetite and their steady consumption of calcareous algae results in the production of large quantities of calcareous sand.

Though they are most active at night, urchins seem to eat constantly and are quite indiscriminate in terms of what they eat. When keeping these animals, make sure that adequate food is available since they will rapidly consume any and all algae they come across including beautiful and desirable coralline algae. If kept in too large of a number or if there is not adequate amounts of algae, they will render an aquarium completely free of any and all algae as well as any small sessile microfauna that they might come across.

Therefore, only one urchin should be kept per each 50-75 gallon aquarium. More can be added if an aquarium becomes overgrown with undesirable algae, but the extra urchins should be removed once the algae are under control. Sea urchins perform a valuable function on the reef that can be translated and put to good use in a reef aquarium. It should be noted though, that urchins have a very strong tendency to knock over anything that is not solidly in place so this should also be taken into account before adding urchins.

Feather Stars (Crinoidea)

Carpenter's CrinoidWhile sea urchins may be considered the brutes of the echinoderms, feather stars are the delicate flowers of the group. They are delicate not only in terms of appearance, but also due to the great deal of difficulty that has plagued hobbyists that have tried to keep them.

They are the most primitive of the echinoderms, so much so that they only have a single opening that functions as their "mouth" and "anus." This opening is located on the upper portion of their body. This configuration allows them to move the food, which they capture with their feathery arms, directly to their mouth and to expel waste so that the strong current in which they thrive in carries it away from their body. These animals have small "feet" called "cirri," which they use to attach themselves to the reef. Once successfully perched in a spot, they expand their arms that act like nets to catch plankton and other food as it passes across them on the reef. This behavior makes them appear like flowers or feathers blowing in the breeze.

The lack of strong surge and adequate food is probably the main reasons why these animals tend to fail in captivity. On the reef, large quantities of organic particulate matter constantly move across the reef so these animals can readily obtain adequate nutrition. Replicating these conditions in a closed environment is very difficult since large amounts of food allowed to float in the aquarium can rapidly cause conditions to deteriorate.

Furthermore, most systems do not have the water movement and surge that these animals seem to require. Evidence that these animals are not thriving is seen when these animals rapidly drop portions of their arms. In the wild, this is a natural defense mechanism and the arms rapidly grow back, but in captivity when the arms fall off, very rarely do they grow back. Eventually all of the arms fall off and the animal has no means of capturing food and slowly starves to death. This happens in even the best systems where food is specifically given to these animals. Until we determine what is lacking for the proper care of these animals, feather stars should not be kept.

They are one of the most difficult animals to keep with long-term success. Only experienced hobbyists willing to provide for their demanding needs should attempt keeping them. Even so, success is not guaranteed so most hobbyists should not try to keep them and they should only rarely be imported.

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