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

Echinoderms, Unique and Odd

All sorts of odd creatures appear in reef aquaria from time to time. Of course, by most non-reefkeeping standards, most of the animals that aquarists keep are strange or unusual. After all, people live on land and are most familiar with the organisms that they encounter on a daily basis: dogs, cats, birds, and houseflies; not reef aquarium animals. A coral reef is a vastly different world from the one that most people are used to, and it goes without saying that many of the animals from that world are different to the point of being weird and unusual. I think the strangest of all of these animals, however, are those that biologists call echinoderms: the sea stars, sea urchins, sea cucumbers and their relatives.

Echinoderms are highly complex animals containing organ systems that are unique, unlike anything seen in any other animals. They are typically animals with no front end, no back end, no head, and no brain (Hyman, 1955; Kozloff, 1990; Harrison and Chia, 1994; Ruppert et al., 2003). They also are the ecologically dominant animals in most marine ecosystems, including many coral reefs; without their presence these environments become vastly different places from how we normally think of them (Paine, 1966; 1974; Sammarco, 1980; Paine and Levin, 1981; Birkeland and Randall. 1981; Glynn, 1985; De Ruyter van Steveninck, and Bak, 1986; Glynn and Krupp, 1986a,b; Wallace, et al., 1986; Sano et al., 1987; Faure, 1989; Walbran, et al., 1989; Andrew, 1991; Cameron, et al. 1991a.b; Coyer, et al. 1993; McClanahan and Mutere, 1994; Chess, et al, 1997; Peterson et al., 2000; Carriero and McClanahan, 2001; Edmunds and Carpenter. 2001; Knowlton, 2001; Phinney et al., 2001; Williams and Polunin, 2001; Barnes, et al., 2002). Finally, many of them show no evidence of the aging process. Unless attacked by some disease, eaten by a predator, or killed by some environmental disaster, they have the potential to live indefinitely. Old echinoderms may be very old, indeed.

Given all of the strangeness attributable to these animals, and their popularity in aquaria, I thought I would devote several columns to the echinoderms; animals I think are just about the neatest animals we can keep in our aquaria. My last column about any echinoderms was in the November, 2003, issue of Reefkeeping and dealt with sea urchins. The present series of columns will examine the group as a whole. This first column will be a discussion of some basic echinoderm properties, including their larvae. I will discuss other aspects of their biology and the adult forms starting with next month's column.

No Front, No Back, It Is All The Same To Them

When we look at the world around us, one of the things we do is automatically classify what we see. The need to pigeon-hole things appears to be deep-seated in our species. Something either belongs to a group or it doesn't; there is no halfway house in our classification schemes. There are many ways to classify the world around us, and various types of scientists are actively pursuing new and interesting ways to do this. For the average person, however, probably the most commonly used scheme of classification is exemplified by the basic question, "Is it animal, vegetable or mineral?" Using such a question presents the person with a series of choices: "Is the item, or has it been, living (animal or vegetable) or is it non-living (mineral)?" Then after that, presuming it is or was once alive, the question becomes, "Is it an animal or is it a plant?" This is pretty straightforward, although it begs the question for several groups of living things that are neither plant nor animal. Nonetheless, on a basic level, this question is pretty easily answered for most of the items we are likely to encounter in our daily lives.

If we decide that something is an animal, what exactly does that mean to us? On a fundamental scientific level, an animal is an organism made of more than one cell that cannot produce food by photosynthesis. Such a distinction would eliminate plants from consideration, but not the fungi. To distinguish fungi from animals we need to add the criterion that the animal's cells do not have rigid cell walls. Plants typically have cell walls made of cellulose, and fungi often have cell walls made of chitin, a different sugar polymer. Animal cells typically lack cell walls altogether, having just a simple membrane separating the external world from the inside of the cell. Such a series of distinctions is well and good, but is not something a non-scientist would likely think of. What matters to most people is that animals are mobile organisms that must feed to survive. Since most plants and fungi don't wander around looking for a snack, these criteria work pretty well, particularly in terrestrial environments.

When we look at the animals around us on land or in the air, some other things are obvious criteria for "animalness." The animals we are familiar with have a front and back end, and along with these structures they have a left and right side. This means that if we examine one of these organisms, it can be divided into two halves that are mirror images of one another by cutting it through the body only on a plane that runs from the center of the back to the center of the bottom surface. The property of having two halves that are symmetrical about the body midline is what is called "bilateral symmetry" and it characterizes most animals. No animal is perfectly bilaterally symmetrical; small imperfections or large deviations from such perfections are relatively common; still it is, on a gross level, a generalized characteristic that works for virtually all terrestrial or airborne animals.

In marine and some fresh-water environments, however, plenty of animals lack bilateral symmetry. Such animals simply don't have a front, back, or two distinct sides. Perhaps the best examples of such animals are sponges, many of which lack any sort of symmetry at all. Possibly this lack of defined body form was a factor preventing their recognition as animals. It wasn't until the latter half of the eighteenth century that it became widely accepted among the naturalists of the time that sponges were, indeed, animals (Hyman, 1940). Other animals such as corals, sea anemones, and their near and distant relatives have what can be called radial symmetry. They have no head or tail, but instead have a body that is fundamentally a cylinder. The mouth, surrounded by one or more rings of tentacles, is situated in the center of one end of the cylinder. This orientation means that the animal can be divided into two equal halves by cutting through it on a plane that passes along any radius of the disk that makes up the mouth, or oral, end of the animal. Radial symmetry is characteristic of organisms that either don't move or move slowly. Many plants, for example, have radial symmetry, and that tends to influence how all organisms with radial symmetry have been viewed. The fact that many people have trouble realizing that radial animals such as sea anemones are animals is reflected in their name. Anemones are plants, sea anemones are not, yet they are often called "flower animals." "Animalness," it seems to many folks, requires bilateral symmetry.

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Figure 1. Left: This handsome fellow is a specimen of the great sculpin, Myoxocephalus polyacanthocephalus. As with all fish, it is bilaterally symmetrical and can be divided into two equal halves only by a plane running vertically along the midline of the body. Right: This small, unidentified hydrozoan is almost perfectly radially symmetrical. It may be divided into two equal halves by a plane running lengthwise down the body as long as that plane divides the oral disk and tentacles evenly.

Radial symmetry is not particularly rare among animals, but it is found in only four of the more than forty major distinct animal groups called phyla. These are:

  • The Porifera, or sponges, of which many species have fundamentally cylindrical bodies;

  • The Cnidaria, or corals, sea anemones, hydroids and jellyfishes, of which virtually all species show a basic radial symmetry;

  • The Ctenophora, or comb-jellies, many species of which are relatively close to being radially symmetrical; and

  • The Echinodermata, or the sea stars, sea urchins, feather stars, and sea cucumbers, all of which show a derived and somewhat imperfect radiality resulting from the drastic metamorphosis of a bilateral larva.

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.

Similarities And Differences

I find the changes that occur in animal bodies, either as a result of evolution or as result of growth, exceptionally fascinating. In some respects this probably accounts for my love of the echinoderms, and particularly the group referred to as the Holothuroidea, or sea cucumbers, which undergo the most drastic changes of all. Recent genetic research focusing upon structural similarities in the genetic codes of animal groups has confirmed that one of the three great branches of the animal kingdom consists of the Cnidaria, the Echinodermata, the Chordata (animals such as fishes, birds, and humans) and a couple of smaller related groups (Field, et al. 1988; Smith, 1992; Turbeville, et al. 1994; Wada and Satoh, 1994; Jefferies, et al. 1996; Lacalli, 1996; Adoutte, et al. 2000; Baldauf, et al. 2000; Jenner, 2000). In these groups, the changes seen from one to the next may provide a window into the distant past, showing some of the evolutionary changes that occurred during the history of life. Additionally, the conservation of some vital genetic traits within these groups may appear as similar properties in what are now vastly different types of animals (Strathmann and Eernisse, 1994; Sprinkle and Guensburg, 1995; Daly, 1996; Williams, 1996).

Although it is hard to imagine any two animal groups that could be more dissimilar, echinoderms and chordates share some fundamental biological properties. Many of the basic biochemical pathways and properties of our cells are the same as those found in echinoderms, but are decidedly different from those found in the rest of the animal kingdom. As an example, the chemical chitin, commonly used as structural material throughout the animal kingdom, is totally lacking in the echinoderm and chordate lineage. Similarly, many basic structural similarities at the cellular level exist that link these groups. As an example of this, the photoreceptors found in echinoderms and the rod and cone cells that are the photoreceptors of the vertebrate eye are ultimately derived from the modification of the cilium of a ciliated cell. The photoreceptors throughout most of the rest of the animal kingdom do not have a cilium as their basis, being derived instead from a different cellular structure called a microvillar surface. Finally, the early embryology of chordates and echinoderms is strikingly similar and, again, unlike that found throughout most of the animal kingdom (Ruppert, et al. 2003).

The early embryology of the echinoderms has been studied in detail for more than 150 years, and it is still yielding information of value and interest. The study of animals such as sea urchins and sea cucumbers has been undertaken, in no small part, as a way to study an analogue of early human development. For at least the first few cell divisions, animals such as sea urchins and humans undergo fairly similar development, which can be studied in detail without the potential ethical problems that arise during the study of early human embryology. As a result of all this research, we have a pretty good understanding of the basic development of most types of echinoderms (Strathmann, 1987; Ruppert, et al. 2003).

How Do We Get There From Here?

One of the vexing problems of biology is, "How does a multicellular organism develop from a single-celled egg?" The control of this developmental process has been a focus of much research, both applied and basic, for most of the last century, and remains so today. Literally hundreds of millions of dollars are being spent annually on various facets of this question to elucidate its important medical aspects. From the perspective of a marine aquarist, however, probably the most interesting part of the question remains, "How does the organism develop from a fertilized egg, or zygote, into a functional, living animal that can maintain itself in the environment?"

Biologists refer to this process as "embryological development," often shortened to just "development." Once an organism has reached the stage of a small juvenile, further development into a functional adult is pretty straightforward; often it is simply growth, with the final development being the onset of sexual maturity. This is, from the organism's view, of course, a very big deal, but it is often easy to study and understand. The organism's initial development, however, requires significant and drastic changes in its morphology. These changes are often much more difficult to get a good handle on, and as such, have become much more interesting to study. What happens in these early life stages of echinoderms has been well studied, and is pretty well-known, at least on a gross scale. However, that doesn't make it any less bizarre.

Echinoderms are great animals to use to study development. They are almost all broadcast spawning animals, with fertilization occurring in the sea. Consequently, they don't have egg shells that need to be removed, and their developmental stages can be followed in a container of sea water (but see Strathmann, 1987 for details). One characteristic of the cnidarian-echinoderm-chordate branch of the animal kingdom is the production of eggs that divide radially. To go from a single-celled zygote to a multicellular animal, the one cell that is the zygote needs to subdivide itself repeatedly. This repeated cellular division results in an embryo with many cells, but with each cell smaller than the one that divided to form it. Until the developing organism can begin to feed, all the energy and raw materials for this cell division must come from materials stored in the eggs. Most of these stored materials are, of course, yolk.

When an echinoderm zygote divides for the first time, it forms two identically appearing, equally-sized, cells. At this stage, it is not a zygote anymore; it is now referred to as "an embryo at the two-celled stage." These two cells divide synchronously resulting in a four-celled embryo. All of these cells are equal in size and when viewed from above, they exhibit radial symmetry. So, at this stage of its life, the echinoderm embryo is considered to show a fundamental primary radial symmetry. It is worth noting that most of the animal kingdom does not develop in this manner. Mollusks, annelid worms, and many other animals have four-celled embryos in which each cell is different in size from all others. Arthropods and related animals have yet a third type of development characterized by incomplete cellular division of an embryo constrained within an eggshell.

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Figure 2. Early cell division in a sea urchin, Arbacia punctulata. The cells are not perfect spheres because the fertilization membrane that surrounds the developing embryo constrains its shape. Left: Undivided zygote. Middle: Two-celled stage. Right: Four-celled stage.

The third cell division is also synchronous, but the plane of cell division is oriented at right angles to the previous divisions. In the idealized echinoderm embryo, this results in eight cells arranged in two quartets with the third plane of cell division at the equator. In practice, in most embryos at this stage, four of the cells are slightly larger than the other four. Subsequent to this stage, cell division continues to be synchronous. After seven divisions, the embryo has 128 cells, and generally is in the form of a hollow sphere called a "blastula." Blastulae are often ciliated and move through the water as small rotating spheres.

At about this time the synchrony of cell division begins to break down, and the number of cells becomes difficult to calculate or count. Generally, at some time when the embryo has between 128 and 256 cells, a dimple-like depression begins to form at one end of the embryo. Through further cellular division, the dimple becomes a pit and the pit becomes a tube. The tube extends into the cavity within the embryo. This tube is the developing gut. As the gut develops, the embryo appears as a sphere with a hole in it. That hole is called a blastopore, and is the opening of the tube on the surface. At this stage of development, the developing embryo, called a "gastrula," has the same basic structure exhibited by a developing cnidarian polyp, prior to the formation of tentacles. The embryo may be visualized as a small cylindrical organism, with one tissue layer on the outside of the body, another tissue layer lining the gut, and with only one opening to the gut. This is exactly the same fundamental architecture that is found in cnidarian animals such as corals. At this stage of development the embryo is still primarily radially symmetrical, but this type of symmetry will disappear shortly (Strathmann, 1987; Young, et al., 2001; Ruppert et al., 2003).

Further cell division results in the internal gut tube growing until it contacts the far wall of the embryo. It grows to and fuses with the far wall and an opening occurs in the wall, resulting in a hollow tube extending through the embryo. This tube is, of course, the gut of the small developing animal. The second opening becomes the mouth in all echinoderms while the first opening of the gut, the blastopore, becomes the anus. Shortly after the gut becomes open at both ends, further development occurs rendering it functional. At this stage, the embryo is considered to be a larva, and is a feeding, growing, and functional animal. Further development is generally considered to be larval development, not embryonic development, although these terms are not rigidly used. Concurrently, with the development of the gut, internal structures are starting to develop, and the animal ceases to be radial, and becomes an elongated bilateral mobile consumer of phytoplankton.

All of the above processes happen quite rapidly. It is not uncommon for an echinoderm embryo to go from a single cell to a feeding animal within 48 hours. IMAGINE! What must occur to convert a single cell with a relatively featureless interior to a feeding animal in two days? The timing of what genes turn on and off and the resultant changes that occur are simply mind- boggling. It has been estimated that humans will create simple life forms in culture vessels within a decade, possibly much sooner. Such forms will mimic bacteria in their structure. It will be a very long time, however, before we can turn a single cell into a functional animal.

Swimming Sea Urchins And Other Prickly Critters

Young larvae are simple animals, but they are functional animals that must do all of the things other animals must do. They have to eat, excrete, move, sense the environment, and avoid being eaten. We used to think that the one characteristic separating larvae from adult animals was that larvae didn't reproduce. We now know better; it appears that many, if not most, echinoderm larvae may be capable of at least asexual reproduction (Vickery and McClintock, 1998; 2000; Eaves and Palmer, 2003). Given that at many times of the year, the number of larvae may exceed the number of adults by a significant number, the fact that these larvae may be reproductive rather turns the idea of species on its head for these animals. Perhaps we should consider the so-called larva as the definitive stage, with the so-called adult existing solely to produce more larvae, rather than considering the larva as a stage to disseminate adults. Although this seems like a semantics problem, it really isn't. The larvae are subject to natural selection and evolutionary pressures just as the adults are, and we really don't know the relative contributions to either stage in the overall life cycle that is the "species" in these forms.

It really is apparent that we have to consider these animals as "life cycles" rather than as a definitive final stage at any part of that cycle. While we can readily identify most adult echinoderms, this is simply a result of their being large and evident animals. Many of the larvae are equally identifiable, once we know what characteristics to look for. The whole cycle is broken if any part of it dies, and that break is no more final if the death occurs in the adult or larval phase.

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Figure 3. Some bilateral echinoderm larvae. Left: A young sea urchin larva or echinopluteus. The internal skeletal rods are clearly visible. Middle: An older sea urchin larva. This larva has been feeding on green phytoplankton which is visible in the gut. Right: A brittle star larva called an ophiopluteus. All of these larvae move through water catching food, and they move in the direction of the tips of the arms or, in other words, the apex of the triangular body is the posterior end.

All echinoderms have bilateral swimming larvae, and it appears that the most basic forms in many groups are feeding larvae. A number of groups have non-feeding larvae, but with the exception of the larvae of feather stars, these forms appear to be descendents of feeding forms (Strathmann and Eernisse, 1994; McEdward, 1995). Echinoderm larvae are often relatively large as larvae go. The definitive larval stage, the stage that will metamorphose into a juvenile, is about a millimeter or more in length and some are much larger (Strathmann, 1987; Young et al., 2001). As larvae go, these are giants.

The one feature that is constant about larvae is that they change, they grow and otherwise develop new features; consequently, it is hard to discuss a "typical" echinoderm larva. There really is no such animal. There are, however, some common attributes and structures found throughout all the larvae. While echinoderm adults are radially symmetrical, the larvae are bilaterally symmetrical. They don't have a head, but they do have a front end, and they have sides that are mirror images of one another. Both internal organs and external surfaces reflect this symmetry, at least in the early forms. Echinoderm larvae are complex. The gut is regionated. There is a mouth, esophagus, stomach, intestine and anus. There are internal body cavities and tissue structures that develop and surround the gut. There is a larval nervous system that appears to be quite complex and sophisticated. Little is known about the nervous system; it consists of exceptionally small cells that are very difficult to see and work with, but over the last few years it has been demonstrated just how complex this system is (Nakajima, et al., 1993). The larvae have various behavioral patterns and within their scale of size some of them are quite able to avoid areas of distasteful chemicals and predators while choosing to remain in areas of high food concentrations. The larvae are capable of selecting certain types of the unicellular algae they use as food while rejecting other types. Generally, the larvae have a discrete internal skeleton made of calcium carbonate rods. This skeleton may be very complex, depending upon the larvae, and the rods may even articulate with one another resulting in moveable appendages. Finally, the change from the larvae to the juveniles requires a complex metamorphosis. This is unusual in that only a portion of the larva typically undergoes the change into the juvenile form. While in most cases, the remainder of the larva is consolidated into the juvenile, in some cases, it appears the remainder may actually be able to persist, perhaps giving rise to more juveniles.

First Cousins, A Quarter Of A Billion Years, Removed

The group of animals referred to as the Phylum Echinodermata is truly ancient. These animals were successful and dominant within the seas that covered the world with the first blossoming of large animals during the Paleozoic Era that existed from about 525 to 225 million years ago. Significantly more kinds of echinoderms were living during those times than are now alive. As with all other kinds of life, the echinoderms were dramatically reduced in number and diversity during the "Great Dying" that occurred at the end of the Paleozoic period (Tasch, 1973). Over 95 percent of all marine species went extinct during the extinction event that marked the end of that period. However, the extinctions were not evenly spread over all groups. This resulted in survivors from only a few of the array of echinoderm types and this, in turn, resulted in a confusing array of larvae with no intermediate types. Many of the distinctive types of echinoderms present in the Paleozoic Era went extinct at the end of that period, and it is likely that many forms with intermediate larval forms perished. Unlike the situation seen in the crustaceans where there is an array of progressively more complicated larval types, within the echinoderms the larvae from each of the groups are complex. There are no "simple" ones.

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Figure 4. Starfish larvae. Left: An early larva prior to the development of a juvenile rudiment.
Right: A late stage larva with a developing juvenile inside of it. This larva was about an eighth of an inch long.

The larvae all tend to have some common characteristics, but these are elaborated within each group in some truly wonderful ways. In many ways, these animals would make interesting aquarium animals, if only they were large enough to see easily. Unfortunately, they are just on the edge of easy viewing without a microscope.

Some sea cucumbers and feather stars develop larvae that basically barrel shaped. They are surrounded by "barrel hoops" of cilia and these little cylinders move though the water with surprising speed. These larvae generally don't feed and some of them metamorphose directly into juveniles. Sea urchins, sea stars, and brittle stars have more elaborate larvae which often have elaborate sets of appendages. In these larvae the locomotive force is generated by ciliary bands that are arranged around the animal, often in a very elaborate pattern. These bands of cilia also collect the small unicellular algae upon which these larvae feed. Sea star larvae lack elaborate internal skeletons; however, both brittle star and sea urchin larvae have very elaborate skeletons (Strathmann, 1987; Young, et al., 2001; Ruppert, et al., 2003).

And the Story Continues…

At the end of the normal larval period, the juvenile begins to develop as a rudiment inside the larva. This juvenile rudiment may become quite large and well developed. When the rudiment is about ready to exist on its own, the larva tends to swim down near the bottom and search for an appropriate habitat. During this period, it often touches the substrate, presumably tasting the surface. If it finds an appropriate habitat, it will often settle to the surface and in some groups even attach to the surface. The juvenile rudiment will then tend to undergo a drastic shape change; in some cases, it effectively turns itself inside out. This results in a functional small sea urchin or sea star that has given up its bilateral symmetry and taken up its radially symmetrical bottom dwelling form. If the larva doesn't find the appropriate habitat the metamorphosis can be delayed for a while. If no appropriate habitat is found within a short period, however, the larva and the juvenile within it will typically perish; these animals generally do not metamorphose into unacceptable habitats. For those animals that do find the appropriate habitats, metamorphosis means changing into radially symmetrical animals far different in form from the larvae, but also far more familiar to reef aquarists. The metamorphosis also means taking up residence in or on the ocean bottom, and that is a tale that I will continue in next month's column.

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Figure 5. Late Stage Sea Urchin Larvae. Left: A well-developed echinopluteus showing the beginning of the juvenile rudiment beside the gut. Middle: The juvenile rudiment is well developed in this larva. Right: This larva is just about ready to metamorphose into a small sea urchin. The radial nature of the juvenile rudiment is evident and the juvenile locomotory organs, called tube feet, have been formed. The larval body is being absorbed into the juvenile, but it was still able to swim as this larva was collected in the plankton. All of these larvae were about one tenth of an inch long.

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