The Meat Of The Matter by Ronald L. Shimek, Ph.D.

There are certain things we take for granted when we think of animals. Last month, I discussed echinoderms being very strange animals because they have, among other things, adapted a secondarily derived radial symmetry. As soon as they metamorphose from a larva into a juvenile animal, they become radially symmetrical after abandoning the bilateral symmetry found in most animals. We are all familiar with the saying, "Beauty is only skin deep." Perhaps my perceived supposition about echinoderms' oddity is based upon only superficial appearances which have little to do with the actual natural history or properties of the animal. Perhaps, if they are examined in detail, they will be seen to be very much like all other animals internally or in their behavior and other life history attributes.

Not a snowball's chance! If anything, these animals are weirder on the inside than they are on the outside (Hyman, 1955; Kozloff, 1990, Ruppert et al., 2003). When we think of normal animals, such as worms, fishes, insects, snails or crustaceans, they all share a certain array of preconceived properties. The concept of "animalness" seems to be a conservative one as far as most people are concerned. When a group of animals such as the sponges or, for that matter, sea anemones or corals, deviates significantly from those preconceptions, questions about their being "true" animals may arise. This has resulted in names such as "flower animals" or "sea anemones," names that indicate a basic confusion about the type of organism that is so named. Throughout history questions have arisen as to the type of life that such atypical organisms represent. It wasn't until the late 1700's, for example, that sponges were accepted as animals, and that confirmation had to await the result of microscopic examination of their cellular structure.

This basic confusion about what it took to be considered an animal resulted from a rather loose "gut feeling" of what characteristics were necessary to separate animals from all other living things. Although it might seem easy enough to do this, deciding on what constitutes "animalness" based on the organism's general appearance is a very difficult proposition. This is simply due to the fact that there are a lot of animals, and a lot of them are odd and unfamiliar. This plethora of oddity has resulted in a rather simple "lowest common denominator" type of definition of what constitutes an animal. Although this definition has changed a bit through the years, it is rather basic and straight forward. As we now conceive of them, animals are considered to be multicellular organisms whose cells lack walls made of cellulose or chitin and which are incapable of photosynthesis. Such a definition is purposefully broad, but it works pretty well to inclusively define the animal kingdom. This definition is one that has been created and designed as much to exclude certain organisms, such as slime molds and various of the life forms that used to be called protozoans, as it was to include others. Interestingly, it also takes microscopic observation of the organism's cells to confirm the designation of "animal." Thus, the unaided eye alone can't unambiguously show that any given "strange" organism is an animal. Similar and equally specific definitions have been made for plants, fungi, and various other groups.

Nonetheless, while such definitions work pretty well for biologists, they really cannot be, and are not, used by most "normal" people. Most folks look at an organism, and if it moves and eats, it is considered to be an animal. The conservative and preconceived definition of an animal as being "a mobile and moving organism" works pretty well for most of the animals we see around us. It works passably well for most echinoderms as well. However, they still lack many of the characteristics that people associate with animals.

Using some other characteristics as yardsticks also leads to the conclusion that echinoderms are not the normal type of animals. As an example, people will often characterize animals as "warm blooded" or "cold blooded." The underlying assumption is, of course, that they have blood. Or in other words, they have a circulatory system with a heart that pumps fluid we can call "blood." This arrangement can take many different forms. For example, there are animals with one heart, such as insects, and there are other animals with many hearts, such as earthworms. There are even animals that have a good circulatory system which lack hearts altogether, such as the ribbon worms or nemerteans. There are animals such as squids or mammals whose circulatory systems keep the blood completely enclosed within vessels. In contrast, there are animals, such as copepods, whose circulatory systems have no vessels at all; their blood simply bathes the cells and tissues. There are animals with circulatory systems using iron-based respiratory pigments, there are animals whose circulatory systems have copper-based respiratory pigments, and there are animals whose circulatory systems have no respiratory pigments at all. And then there are the echinoderms who appear to lack a circulatory system altogether.

This factor alone separates echinoderms from most other animals. They are unique amongst larger animals in lacking any specific system that appears to have the function of a circulatory system. In some, structures appear to have been derived from circulatory systems, such as the so-called "hemal strand" in sea urchins. Unfortunately, this strand doesn't circulate anything. But it is red and may contain hemoglobin… In some sea cucumbers quite an elaborate system of vessels surrounds the gut and connects it to the body wall. This could be a good circulatory system, except for one minor detail: nobody has been able to demonstrate that anything circulates in it. Digested food may be transferred to the body wall though it, but even that is doubtful, and the system has no true and defined circulatory flow.

Animals have to eat. All definitions of what an animal is agree on that point. Although a few animals such as corals and their kin, and flatworms, have a gut with only one opening, the presence of a gut generally means that animals have a gut tube with a mouth at the front and an anus at the rear. To have a mouth at the front and anus at the rear, animals must have both a front and a rear. As I discussed last month, most echinoderms lack a front end or a rear end. A front end is defined for most animals as the end bearing the head and mouth. In most echinoderms, the mouth opens in the middle of one surface of the body. That surface may face down toward the substrate, as in sea stars or sea urchins, or may face upward toward the overlying water, as in the crinoids. In any case the mouth is found only on an "end" of the animal in the holothuroids or sea cucumbers.

Generally, when animals have a mouth at one end, it is found in a structure called the "head" containing the brain and major sensory structures. Additionally, the brain typically gives rise to major cords or nerves that run to some other part of the body. Not so in echinoderms; no echinoderm has anything remotely resembling a head or brain. Those echinoderms that do have a front end or a back end, and these are mostly sea cucumbers, simply have an opening for the mouth at one end, and generally the anus opens at the other end. Although there may be sensory structures around the mouth that can "taste" the prey, there are no specific and obvious concentrations of sensory structures such as eyes, sensory tentacles, or sensory structures that can detect water-borne chemicals near, or in, this front end. They just do not have anything remotely resembling a head.

Vertebrates, such as fishes, have a brain that has developed from the front end of a tube of nerves that lies above the gut. Nerves radiate from the brain and enervate the regions around the mouth. Sensory structures are also found in this region. Biologists like to say that the reason for this type of arrangement is that it is advantageous for animals to have their sensory structures located in the front end of the animal. The premise is that it is always better to see where you are going than where you have been, to avoid potential problems. Having a sensory array around the mouth also makes sense; it would seem obvious that organisms should be able to sense what they are going to eat before they start eating it. Radially symmetrical animals typically live as ocean bottom-dwelling animals that are sessile or relatively slow moving, or pelagic animals. Although they may be highly predatory, and may even have photoreceptors, they do not generally hunt their prey visually. Instead, they may follow scent trails to prey or, like most sea stars, they may simply move around randomly until they encounter an acceptable food item.

The arguments about why animals have a head at the front end are really pretty compelling. Although almost every mobile animal has such an arrangement, the details may differ. For example, the vertebrate brain arises from nerves above the gut, while the brains of most other animals arise from a nerve ring that surrounds the throat or esophagus. In these non-vertebrate animals the main nerve cords that run to the rear of the animal generally pass down the middle of the bottom of the body rather than as a tube down the back. Nevertheless, most animals have a head that contains some sort of light receptors or eyes, some sort of "long-distance" chemical sniffer, and often, tactile organs such as bristles.

Of course, all of the signals sent by such sophisticated sensory equipment as eyes and chemoreceptors need to be decoded and analyzed, which is a primary function of a brain. Without a brain to figure out what the signals are, it would be moot to sense them. So, along with the echinoderms' lack of a brain goes their lack of all of these sensory structures. Not only do they not have a brain, ANY kind of aggregations of nerve cells are rare. The lack of evidence of any neural aggregation that could be a brain, and the absence of any sort of large sensory structures, could give the impression that echinoderms blunder through a dark, odorless universe. Such an impression would be very far from the truth.

Although these animals generally lack large evident sensory or nervous organs, they are literally covered in sensory cells of various modalities, often numbering thousands per square millimeter. While they lack any specific structures that can be definitely shown to be chemosensory, it may be said that their entire body is chemosensory. Many echinoderms have pigmented spots connected to nerves in their body. This is an arrangement similar to those found in other animals with true, but primitive, non-image forming eyes. Because of this structural similarity, the spots found in echinoderms are referred to as ocelli, or eyespots, but no immediate behavioral changes can be elicited by applying varying light intensities to them. Just to make things more interesting, however, many echinoderms often respond to bright light beams directed anywhere on their body. It appears that the photoreceptors may record or signal such things as changes in length of daylight, but it also appears that the whole body may be able to sense changes in light intensity, and to trigger behavioral responses to those changes.

All of this lack of defined nervous structure not withstanding, many echinoderms have a great deal of relatively complex behavior. How that behavior is elicited and regulated without a brain is a question that has been debated for a number of years among echinodermatologists. Perhaps the best explanation, given some years ago by Dr. Richard Strathmann, a noted echinoderm researcher, is that the whole nervous system may function as an associative and regulatory structure. In other words, the animal doesn't need a specialized region to act as a brain, because in a real sense, the whole animal is a brain.

Even the basic structure of the tissues comprising the echinoderm body is unusual. Animals typically have connective tissues and connective tissue proteins that are tough and rugged. These connective tissues constitute such structures as the tendons and ligaments of vertebrates and the tough scleroproteins of invertebrates. When cooked, connective tissues of this type often have the common name "gristle," a term that brings to mind a visceral understanding of their consistency and composition. Well, once again, echinoderms have gone their own way. Not only do they not have this standard type of connective tissue, they have, in fact, a unique type of material referred to as "mutable connective tissue" or "catch connective tissue." Depending upon the ambient ionic charge within the fluid spaces of their body, this material can be rigid and tough, or about the consistency of liquid gelatin. And it can change from one state to the other in seconds. (Follow this link and examine the first image in the article for some visual evidence of this interesting tissue).

If the mutable connective tissue is in the "locked" form, such as when a sea urchin spine is held rigidly upright, it is effectively immobile. When it is triggered to enter the fluid state in some sea cucumbers, the animal literally disintegrates as its connective tissue liquefies. A person holding the animal in his hands at this time would feel the resultant goo, that a few seconds before was the sea cucumber, flow through his fingers like thick mucus.

So far in this column, I have spent some time contrasting echinoderms with other animals and have generally indicated that echinoderms don't have this organ or that structure. There is, however, a limit to the applicability of this sort of negative comparison. Without something to constitute their body, they would have to exist simply as a void in space where nothing else was. As that is patently not the case, just exactly how are these animals put together, and what unique organs or structures, if any, do they have? From the point of view of a biologist who studies comparative morphology, the echinoderms' whole design is rather peculiar. However, not too much should be made of that. Depending upon who is doing the counting and what criteria they are using, there are between 40 and 50 major animal groups called phyla, and each has its own unique body plan and internal structures. In a sense, the animals in each such phylum may be said to be "rather peculiar." Nonetheless, as the reader might begin to guess by this point, the echinoderm structure is more peculiar than most.

Animals may be said to be composed of tissues and spaces, or "voids," within those tissues. In some terrestrial organisms, some of the voids, such as the lungs of vertebrates or the tracheae of insects, are filled with air, but in most animals, the voids are filled with fluids. In most marine animals, these fluid-filled spaces constitute an important secondary component of each animal's morphology. These spaces have two decidedly different origins. The largest space in many animals is the volume inside the gut. In a very real sense, however, the gut cavity is not within the animal at all. Rather, it is an elongate section of the exterior environment surrounded by the gut lining and closed off at either end, rather like the hole in an elongated donut. The other spaces, the true body cavities, inside animals are within the actual body structure, between the external surface of the body's epidermis and the interior outer surface of the body's gut lining.

In most animals, these body spaces don't define the animal. They may be relatively large, such as the blood cavity in arthropods and the body cavity, or "coelom," of bristle worms, or small, such as the inside of the gonads of mollusks, but they all share one property: the animal's shape and the relative "functionality" of the body are determined by something else, such as the exoskeleton of arthropods, the shell of most mollusks, or the muscular body of annelids. Only in a few animal groups, such as the sipunculans, do the shape and relative composition of the body cavity exert a primary force in the animal's natural history.

Of course, by now the reader would probably expect that I would say that the echinoderms' body cavity exerts a major influence on all aspects of the animal's biology. I could say this, but it would be trite to do so. It is really impossible to single out one organ system or one functional unit of a living organism and say that this or that structure is more important than the other. Animals are a functional whole, and the whole unit and all of its constituent parts need to be there if the organism is to survive. Nonetheless, the prominence of the body cavity and its derivitives in the echinoderms, relative to all other animals, indicates that the elaborations of these cavities and the resulting structures have been one of the more interesting themes in their evolution.

Unlike most animals, which have one or two body cavities, echinoderms have derivitives of at least six. Two of those six, the body cavity surrounding the gut and the one that occupies the locomotory organs, are large, and manifestations of them may be noticeable to reef aquarists who take a close look at any echinoderms in their tanks. For example, the sea stars' dermal gills, illustrated in Figure 4 above, are thin tissue-covered extensions of the body cavity surrounding the gut, which extend through the body wall and, presumably, act as respiratory organs. The other cavities are smaller and although probably no less important to the animal, they are less apparent.

The bodies of most animals are reasonably firm. They are composed of tissues and these tissues are durable. In a real sense, such tissues are the meat and bones of the beast. This also implies that the body cavities of many animals are pretty rugged and very stable structures. The abdominal or chest cavities of mammals are good examples. Although the cavities may be exposed if they are perforated or cut open, for example during surgery, the cavities don't collapse. This is because they are surrounded by layers of strong muscle, connective tissue, or bone.

Taking these "normal" animals as examples, and using their types of body cavity to illustrate or conceive of the cavities in an echinoderm, would be very misleading. While most animal bodies are comprised of tissues with cavities within them, echinoderms seem to be built of large cavities held together and delineated by thin layers of tissue. Such layers are sufficient to maintain the integrity of the volumes or systems they surround as long as they are supported by water. These tissues, however, are thin, filmy and diaphanous, they are exceptionally easy to break, and if they break or tear, the animal will often die. Echinoderms are, by and large, found in marine environments with full strength salinity. Rapid changes in salinity can result in osmotic imbalances occurring on either side of these delicate membranes, and that, in turn, may cause them to rupture. This is why echinoderms need to be maintained at full oceanic salinity, and it is also why they need to be acclimated very slowly. Slow acclimation allows the ionic concentrations on either side of these membranes to become physiologically balanced. This balancing takes time but, if done correctly, prevents the membranes separating the various compartments of the body cavities from rupturing, and the animal will survive.

Figure 5. This is a modified photomicrograph of a cross-section of the arm of a small sea star. To make this image, a juvenile sea star was preserved and then sliced into very thin sections. The sections were mounted on microscope slides and this one was photographed through a microscope. Some structures have been labeled. The ossicles are in the body wall, and the pyloric cecae are branches off the gut. The ambulacral system is discussed below. The various parts of the body cavities are colored blue, purple and green and labeled accordingly. Note the great extent of the cavity system. This arm was about 6 mm (1/4 in) in diameter. In larger animals the body wall is proportionally very much thinner and the cavities fill even more of the animal. Perhaps more importantly, note all the areas in red. These are places where tissues have been torn during changes in the salinity that occurred while the animal was being preserved. Similar changes occur in aquaria if the animals are not acclimated very slowly, and will result in the death of the animal.

The body cavity compartment that occupies the locomotory and food gathering system of echinoderms is called the hydrovascular, or ambulacral, coelom, and the system itself is referred to by the two synonymous terms of hydrovascular or ambulacral systems. This is a complex system of relatively high-pressure hydraulics that has no analogue in any other animal. In the barest sense the ambulacral system consists of fluid-filled tubes and vessels along with muscular valves to control and isolate portions of itself, but such a description hardly does it justice.

The ambulacral system is probably best explained in the context of its anatomy and functionality in a sea star. The basic system is easy to describe. Inside the animal, and surrounding the mouth, lies a circular tube called the ring canal, a structure somewhat like an inner tube. A thin tube, the stone canal, runs from this canal to the opposite surface of the animal ending in a perforated calcareous plate called the madreporite, which is often visible on the upper surface of sea stars. A long straight tube, called a radial canal, runs from the ring canal out into each ray. Additionally, large sacs called "Polian vesicles" also connect to the ring canal; these may function as hydraulic reservoirs. The radial canals are blind-ending and terminate at the ends of the rays. All along the radial canal, pairs of smaller side canals branch off, one on each side of the radial canal. These small canals are lined with circular muscles that can contract and close. Each of these canals terminates in one of the locomotory organs of a sea star, the tube foot. The tube foot is shaped something like a pipette or eyedropper. It has a muscular bulb or ampulla at the top, and a long cylindrical tube projecting out of the animal. The cylindrical tube has several sets of muscles surrounding it and terminates in a flat pad lined with adhesive glands.

Figure 6. A simplified diagram of the plumbing involved with the basic part of the ambulacral system of sea star; all other body parts have been removed. The mouth of the star would be found in the center of the ring canal. The radial canals would extend out to the tips of the star's arms.

Locomotion is accomplished by closing down the valve isolating the tube foot from the radial canal. This isolates the tube foot as a hydraulic unit. The muscles surrounding the ampulla relax, and muscles running the length of the tube foot contract. This shortens the tube foot and pushes the internal fluid into the bulb at the top of the foot, thereby expanding it. Small muscles on the side of the foot contract on one side and relax on the other. This causes the foot to bend in the direction of the contracted muscle. At this point the muscles along the length of the foot relax and the protractor muscles surrounding the bulb contract. This contraction forces fluid into the tube foot extending it like a small water-filled, sausage-shaped balloon. As the foot is extending, the small postural muscles that had been extended along the side of the foot allowing it to flex, contract; simultaneously, their previously contracted counterparts relax. This swings the foot through an arc. At the bottom of the arc, the adhesive pad contacts the substrate and glues itself to it. As the muscle causing the swing continues to contract, it pulls the animal along over the foot. When the foot would leave the substrate at the beginning of the upswing, other chemicals are secreted from the adhesive pad and it releases. The foot now starts to swing upward and the cycle begins anew.

Think of the coordination necessary to do this with one tube foot. Then consider that a large sea star may have 40,000 tube feet, all of them working together to move the animal along. And then consider that this locomotion is all coordinated and controlled without a brain!

The hydrovascular system is a fluid-filled system, but the fluid in it is not sea water. The fluid has been actively pumped into the system through the fine tissues that constitute the walls of the tubes. This pumping is done by the cells lining the tissue. They physiologically pump potassium ions into the tubes and simultaneously pump other ions out. The pumping results in an internal water pressure that keeps the tube tightly inflated. Aquarists who forget that sea stars and other echinoderms need slow, gradual acclimation to salinity changes often wonder what is wrong with their new pet. They report that the animal seems "fine," it just doesn't move. Well, yeah. It can't move. The rapid changes in salinity have resulted in significant ionic imbalances which rupture the delicate internal plumbing of the water vascular system. The animal can't regain the pressure necessary to move, and it dies in place.

Echinoderm locomotion results almost entirely from the ambulacral system. This is another oddity of the group. Echinoderms are moderately sized animals, and most animals their size are highly muscular and move by using some sort of appendages utilizing lever action. While a few echinoderms, most notably brittle stars, move almost entirely by direct muscular action, the vast majority are moved by the muscles in the ambulacral system.

Echinoderms, like all animals, are the sums of their parts, and then some. All of the various oddities of echinoderm structure combine to create animals that are very odd, and yet, compelling to the eye. They are just so weird that they are often fascinating.

These are obviously animals like no others. It might seem that such differences would render them rare or insignificant. After all, if their evolutionary path led to important or successful animals, it might reasonably be expected that there should be a host of copycats, animals that were "almost sea stars," in the oceans. There are precedents for just such "copying," which is termed convergent evolution. For example, the extinct reptiles called ichthyosaurs, as well as living porpoises and sharks, all share the same basic body shape. Similarly, hummingbirds, sphinx moths, and hovering bats all share the same basic shape and flight pattern, one that facilitates getting nectar and pollen from deep, trumpet-shaped flowers. Alas, no animals mimic echinoderms, and there don't seem to be any close relatives of the group as a whole. Though distantly related to animals such as vertebrates on one extreme and corals on the other, they are really unlike any other group. Additionally, there appear to be no examples of convergent evolution toward an echinoderm form by any other group.

This might indicate that they are rare and unimportant. In fact, the situation is just the reverse. They are often very abundant and in most marine sea bottom communities, they are THE dominant animals. In many ecosystems, their activities structure and maintain all other animal populations. Furthermore, they have maintained this level of ecological dominance for a very long time, indeed. Echinoderms have been the dominant life forms on the ocean bottoms for at least 300,000,000 years, and there is no sign that that is about to change anytime soon (Tasch, 1973).

Next month, I will discuss the diversity of echinoderms with some brief notes on how to maintain some of the common forms in aquaria. In nature, the coral reefs we attempt to emulate in our aquaria are largely maintained in the form we are familiar with by the actions of many echinoderms, and they have a place in many aquaria as well.