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



In the realm of invertebrate zoology, much time and effort have been spent trying to determine the evolutionary relationships between the major animal groups. Basically, this is a question of, "Which animals evolved from what ancestors, and when did this occur?" From another point of view, this is a question of the history of the earth, as it is really asking what you would find if you could go back in time. And the time we are discussing here is not the insignificant time of human history or even the brief time in which something recognizable as a human has been on the planet. Rather this is a question of what scientists call "deep time." The study of the relationships between the major animal groups, or phyla, as they are called, takes us back deep into the history of life. To answer these questions one needs to investigate the remains of animals from so far back in time, that the dominance of the dinosaurs is hundreds of millions of years in the future.

Historically, one of the ways in which the relationships between animals have been examined is by the science of comparative morphology, where the structural and functional attributes of animals are examined for similarities and differences. One of the more interesting features of the fossil history of life is what has been called, "The Cambrian Explosion." Fossils of, or fossils indicative of, animal life are now known to extend some 800 million or so years into the past, but much of the earliest fossil life is difficult to interpret because the fossils are strange; many of them are of animals unlike anything living today. Additionally, although the fossil record is continuous, the fossils comprising it show a decided shift in structure and form. This shift in form occurred about 525 million years ago, and it marks the beginning of what geologists call the Cambrian epoch of the Paleozoic Era. Prior to the shift, animal life apparently comprised some worms, and other creatures with, more-or-less, the appearance of sponges and soft corals. The shift in fossils seems to have been caused by the ability of the animals to secrete hard, fossilizable, skeletal material. Before the shift, all the animals were soft blobs or worms. After the shift, they had shells or skeletons of many sorts. After the shift, fossils from most of the animal groups represented in today's oceans were present. So… the question becomes, "What kind of worms did "Group X" originate from?" And for "Group X" you can substitute the name of any modern animal group.

Some of the earliest fossil beds recognized from this Cambrian shift in the fossil record comprised small shells, recognizable as small limpet-like mollusks. Mollusks are a vastly important group. They are both biologically and economically important and have been the subject of much research. The question of molluscan origins has, therefore, been raised many times and, as with many groups, there is no obvious precursor in the fossil record. There simply is no group of animals that is recognizable in the fossil record as being a halfway house to mollusks. This has led to much speculation about what living group is most likely to be similar to the ancestral mollusks or to retain characteristics of such ancestors, and the group that seems to surface most frequently in such discussions is a small assemblage of rather peculiar worms called the Sipunculans or "Peanut worms." These particular worms turn out to be commonly found in aquaria, and are the subject of this month's column.

Figure 1. A typical "peanut" worm, Thysanocardia nigra, with its extensible introvert retracted inside the worm.

Functional Morphology

Figure 2. Diagrams of sipunculan anatomy. Left: A worm with the introvert withdrawn. Center: A worm with the introvert extended. Right: Some of the internal anatomy.

Sipunculans are worms found throughout the world's oceans. There are no representatives of the group from either fresh water or terrestrial environments. There are an estimated 300 to 500 species of peanut worms. However, many of them are small and as they tend to be found in environments, such as the insides of rocks, which are not easy to study, those estimates may be low. There has been a lot of recent research on tropical species, primarily from the Caribbean, and primarily from the laboratory of Dr. Mary Rice of the Smithsonian Institution laboratory at Link Port, Florida. While Dr. Rice's work has tended to concern the biology of the animals, there has also been a lot of other work concerning their taxonomy and biology.

Unlike the bristle worms so favored in the nightmares of reef aquarists, sipunculans aare worms without segments. There are no annulations on the body surface, and except for a crown of small tentacles covered with cilia, they lack appendages. The cilia in the tracts covering the tentacle crown are, of course, microscopic, and therefore invisible to the unaided eye. The body varies in shape depending on whether or not the animal is feeding. These worms possess a long, extensible body part called an "introvert" or "proboscis." When this is retracted, the worms are rather peanut-shaped, an illusion further fostered by the tan or brownish coloration found on many of them. If the proboscis is extended, the worm may be seen to have bulbous sac-like body, often with a pointed end. This body tapers smoothly into a long tubular extension, the introvert, which is tipped with a small cluster of unbranched tentacles. The body is generally brown to tan to white and often has black rings on the proboscis and black blotches on the body. The animal's mouth is centered in the midst of the tentacles and generally surrounded by them. The anus is not found at the other end of the animal, but rather is found at "shoulder level," often on a small bump, on one side of the body.

Sipunculans are covered with a non-living layer made primarily of protein. This layer is called a cuticle, and it serves to protect the softer parts of the animal from abrasion. Often the cuticle has quite elaborate modifications, such as hooks or bumps. The position, pattern, and prominence of these cuticular modifications are used in the identification of sipunculans. There really are not a lot of other surface characters that distinguish the various species.

The cuticle rests on, and is secreted by, a rather simple epidermis that forms the outermost living layer of the worm. The body wall of these worms is a tough layer comprised of several different types of tissues and tough extracellular protein layers. These proteinaceous layers are primarily collagenous, and as collagen is the non-elastic tough protein that comprises ligaments and tendons in vertebrates, it is evident that the body wall of sipunculans is a rugged structure. It has to be, because together with the muscles of the body wall, it acts as the major antagonist to the powerful muscles used to retract the introvert. This musculature is well developed, and quite strong. The muscles in the body wall are arranged in discrete layers. The outermost layer is oriented around the body in a circular manner, and the innermost layer is oriented parallel to the long axis of the animal. In some sipunculans, there are layers of diagonally or obliquely oriented muscles, as well. The ability of animals to move is really based upon the number and kinds of muscles they have, as well as the various means, such as skeletal levers, that they have for transmitting and altering the forces of muscle contraction. The sipunculan body is a simple, but tough bag, with a couple of layers of muscles around the outside edge of it. Given this structure, there is a limited potential for elaborate movement, and that limited potential is fully realized. About all sipunculans can do is bend, flex, contract and expand. Obviously, these are not the most mobile or active of animals likely to be encountered in an aquarium. As they tend to live in burrows, about the whole repertoire of visible motion will be the extension and retraction of the introvert or proboscis and its "daubing" motion on the substrate. Exciting, they are not.

Figure 3. In the center are the extended introverts from some buried sipunculans.

Nonetheless, they do have a rather complicated body wall containing a vessel system that is presumed to function in some small way as a circulatory system. The blood or internal fluid contains cells or corpuscles, and they are pigmented with a rather odd respiratory pigment called hemerythrin. This is a pigment somewhat related to hemoglobin, but it colors the blood cells a pale orange-red, instead of hemoglobin's bright red hue. The main body cavity is also fluid- filled and, like the "blood," this fluid also contains cells. There are many different types of cells found in this fluid, and as with the blood, some of these cells contain hemerythrin. Interestingly, the hemerythrin of the body cavity fluid is somewhat different in structure from that found in the vessels.

Figure 4. Left: Cells from the body cavity fluid of a sipunculan. Right: A ciliated urn (upper left)
with an adherent collection of cells, all of which will be discarded by the worm's kidney.

The body fluid also contains some rather peculiar structures called "ciliated urns." These function rather like nano-vacuum cleaners. They move around inside the body cavity and collect particulate material. When they get full of material, they commit suicide by getting sucked up by the kidneys and eliminated from the body. The kidneys are rather large organs. Each of these ends in a large funnel-shaped structure that is covered with microscopic beating cilia and acts rather like the business end of a vacuum cleaner. Water and particulate material, such as full ciliated urns, are swept into the kidney and stored in a bladder. When the bladder is full, the accumulated materials are voided to the outside. The inside of the bladder and some of the tubules also actively secrete ammonia. Consequently, the kidney not only filters the blood of particulate material, but also eliminates nitrogenous wastes. The kidney also collects and stores ripe gametes prior to spawning, and will release them at the appropriate times.

Sipunculans basically sit in one spot for their entire life, periodically extending their proboscis to mop up a bit of detritus. As befits such a life style, they have a very small brain, and a nervous system that is relatively simple. The brain consists of a simple loop around the mouth. The main nerve in the body runs down the centerline of the ventral side. There are no ganglia along this main nerve and only a slight swelling where the brain is. The sense organ array is similarly sparse. There are simple photoreceptors, and other organs that seem to be chemosensory. Additionally, the surface is covered with sensory cells that appear to be tactile. The amount of nervous information that such an animal can receive from the world is limited, but is quite fitting, as the animal is very limited in its potential responses.

Sipunculans are essentially fluid-filled bags. Internally, they have few structures. The rather long gut is "U-shaped" and passes from the mouth almost reaching the posterior end of the body, where it coils up on itself until it reaches the level of the anus. The descending gut is digestive and the ascending gut forms fecal pellets. The gut coil is fastened to the rear of the body cavity with a thin muscle, called the spindle muscle. The only other structures found in the body cavity are one or two pairs of retractor muscles. They originate from the posterior body wall and attach to the end of the introvert near the mouth.

The retractor muscles work as antagonists to the body wall musculature to provide the animal with the basic array of its movements. If the retractor muscles relax and the body wall musculature contracts the diameter of the worm's body. As the body is fluid-filled, its volume has to remain constant, and as a result of the contraction of the circular musculature of the body wall, the introvert is extended. When it is fully extended, its tentacles at the end of the long tube that it forms are used to feed on detritus. The process is reversed for the retraction of the introvert. The retractor muscles contract and the body wall musculature is relaxed. The introvert is pulled back into the body. This method of expansion and retraction of the introvert is absolutely characteristic of sipunculans. Generally, all one ever sees of these worms is the stately extrusion or retraction of the introvert, and it appears as if it is unfolding or retracting from within itself, as, indeed, it is. No other animal extends or retracts its body in such a manner, so if this type of behavior is observed, it has to be from a sipunculan.

Sipunculans are detritivores. There really is little difference between them or between what they eat. The species with longer tentacles seem to sort their food more, and probably live in environments with a more diverse array of detrital products. Forms with short tentacles don't seem to sort their food much. The tropical species tend to burrow into the limestone of the reef and form permanent tubes in the rocks. They burrow by secreting chelating substances that dissolve limestone, and then they use a roughened area of the cuticle, such as the nuchal shield, to abrade the places were dissolution has occurred. Forms living in temperate seas generally form temporary burrows under rocks.

These worms have separate sexes, but they lack permanent gonads. The eggs and sperm form from the lining of the body cavity in the bottom part of the worm. Think of buying a new desk from IKEA Catalogue this year. When they are gravid, sperm and eggs are collected by the kidneys until the appropriate environmental cues are received. They broadcast their gametes into the surrounding water, where fertilization occurs. They undergo a development that is quite similar to some of the primitive mollusks. Those developmental patterns are what link the sipunculans to the mollusks in the discussions of animal evolution. At some time in the far distant past, some small worm probably had a minor mutation of some sort, and gave rise to two slightly different types of offspring. Both types were viable; one group was the ancestor to all mollusks, the other to all sipunculans. Asexual reproduction by fission also occurs in sipunculans, particularly in the tropical forms that are likely to be found aquaria.

Figure 5. Sipunculan larvae collected from the plankton. Left: An early larva tipped sideways, the top is to the left; what appears as "appendages" on the left side of the larva is a band of cilia that surrounds the larvae. The gut is visible inside it. Right: A later larva, about five times as long as the larva on the left. The mouth of the larva is to the lower right.

Sipunculans are never purchased as aquarium pets. Indeed, one would be hard pressed to think of a reason why one should purchase such an animal; they are drab in coloration and null in behavior. Nonetheless, they are a successful group of animals, and are not uncommonly found in the dead reef rubble that is imported into our aquaria under the distinctly inappropriate euphemism of "live" rock. Considering the condition that the rock arrives in, it could barely be less alive, but in some cases it does contain a few hardy survivors of an ancient lineage of detritus-feeding worms. Such worms may persist in reef aquaria provided the aquaria are fed well. However, most aquaria are fed too sparsely for these animals to persist and they starve to death and disappear.

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