Phylum Sipuncula and Phylum Annelida Aquarium.Net April 97
Reefkeeper's Guide to Invertebrate Zoology:
Phylum Sipuncula and Phylum Annelida
Up until now in these articles I have been discussing what are called the acoelomate and pseudocoelomate phyla. This month we move into our first coelomate phylum, the Sipunculans. You are now probably wondering what the term "coelomate" means. Well, it's basically a fancy term to indicate any animal which has a true body cavity (the coelom ); given that simplistic definition, I'll assume that you can figure out why other groups are called pseudo- or acoelomate.
Generally in an invertebrate zoology class, Sipunculans would be covered along with 3 other "phyla" of coelomate worms: the Echiura, Pogonophora and Vestimentifera. I put "phyla" in quotes here, because the Pogonophorans and Vestimentiferans are almost certainly Annelids (our next group). Pogonophorans and Vestimentiferans are exclusively deep-sea animals (living as shallow as 200m and as deep as 10,000 m - or about 6 miles underwater!), and because they would never be seen in any aquarium, I will not discuss these groups further. The Echiurans are a shallow-water group consisting of about 135 species, and they usually live in burrows in soft-bottomed areas such as mudflats. Some are quite large (e.g., the Japanese species, Ikeda taenioides may reach lengths in excess of 2 m), and even among smaller species (e.g., Bonellia ), the proboscis may reach lengths of 1-2 meters (Brusca & Brusca, 1990). These animals use their spoon-like proboscis for both respiration and feeding, although in the larger forms respiration takes place primarily within the hindgut which is supplied with oxygenated water through cloacal irrigation (water being pumped in and out of the anus by muscular contraction). The proboscis is generally the only portion of the animal which is not buried in the substrate. The proboscis is covered with mucus-secreting cells causing organic detritus to adhere to it. Particles are then transported along the gutter (a ciliated food groove) of the proboscis to the mouth. Although these animals are large and obvious, I have yet to hear of one being imported into anyone's aquarium, so I will again skip further discussion of this Phylum. If you are particularly interested in these animals and want more information about them, either drop me a line or check out a good introductory invertebrate zoology textbook, such as Ruppert and Barnes (1994) or Brusca and Brusca (1990).
After that aside the first group I am actually going to cover is the Phylum Sipuncula. Sipunculans are often called the peanut worms, and there are about 300 species of these entirely marine animals. They lack a respiratory and circulatory system, but do have a well-developed, complete gut. The body surface is typically covered with fine warts, bumps, tubercles or spines. Sipunculans range from less than 1 cm to over 50 cm in length, but most are approximately 5-10 cm long. The interesting thing about the digestive system of sipunculids is that the gut is U-shaped, and the anus is actually located on the animal's back close to the region where the body joins the introvert . The introvert is a highly contractile portion of the body which bears a number of delicate tentacles around the mouth. These tentacles are used in feeding, and in the aquarium the most likely sighting will simply be the introvert and tentacles probing the area around some hole in the live rock -- the body will usually be well hidden entirely within the rock structure and is rarely seen. The body itself is typically covered with tiny bumps, warts, tubercules, or spines, but these provide little protection against large predators, and so the animals are often cryptic and remain hidden for defense. Sipunculans are almost always reclusive (for lack of a better word), and usually borrow in sand, hide in rock crevices, live beneath stones, or hide in algal or bivalve holdfasts. They are common inhabitants of tropical coral and live rock communities, and most careful observers will quickly locate these animals in their reef tanks. Although most locate a suitable hole in which to set up house, some are capable of burrowing into hard calcareous substrata. They can also often be found in empty snail shells, worm tubes and any other structure that provides a safe haven from large predators. Most species are found in shallow-water or intertidal habitats, but some deep water forms have been collected from over 5,000 m (about 3 miles).
The coelom is a particularly important innovation for these animals, because they use their fluid filled bodies as a hydrostatic skeleton . These animals are basically a muscular bag of incompressible fluid that allows them to control body movement by controlling where the liquid in their coelom flows. A hydrostatic skeleton works much like a water balloon: if you want to elongate the balloon, you need only squeeze one end and the fluid inside flows to the other end, leading to an extension of that side of the balloon (assuming that it is not overfilled so that it would break when you squeezed it). If you want to retract that extension you just made by squeezing the balloon, simply release your grip on the compressed end, and fluid will flow back into that portion and the body will return to it's original shape. The balloon must be filled with an incompressible fluid (such as water) rather than a compressible one (such as air) to be efficient. If you take a long balloon full of air and squeeze one end, you need to squeeze it a lot, and the shape at the other end only changes a little because most of the energy you expended in squeezing the balloon went into compressing the fluid inside. Only once the fluid is compressed will it be displaced and result in a change in the balloon at the other end. If you use water (which is basically imcompressible under these conditions), all the energy you expend squeezing the balloon will go directly into displacement of the fluid, and that will result in less squeezing leading to a much greater change in body shape with the same amount of energy expended.
In the case of sipunculids (and other coelomate worms), they have much more control of body movement than the simple example of the water balloon above because they have muscles running every which way within both the body and the body wall (just under the dermis or "skin"). Thus, they can not only squeeze themselves at one end in a manner analogous to you grabbing one end of the balloon with your hand, but also contract a muscle which runs the length of the body in the dermis, and both extend the other end of the body and have it curl. You can do this yourself with an elongate balloon partially filled with tap water (we actually use unlubricated condoms for this demonstration in our classes), and simulate contraction of the circular muscles (encircling the body around the dorsal-ventral, or "back-belly" axis) by wrapping your hand completely around the width of the balloon. When you squeeze, the 'body' elongates and when you relax the 'body' retracts. In reality, the animals have longitudinal muscles (running the length of the body from the anterior to the posterior end) to assist in retraction, but most of the time animals have some elastic property which leads to the body returning to it's relaxed state without a lot of energy being expended. If the animal is attacked, however, they can contract their longitudinal muscles and pull the threatened end back in much more quickly. This is generally the case with the introvert of sipunculids -- the animals use hydrostatic pressure to extend the introvert when feeding. The animal uses both longitudinal muscles and the elastic properties of the body to withdraw the introvert again. The introvert looks to many like the corrugated tubing of a vacuum cleaner. The tube is turned inside-out when withdrawn into the body, and extended by being turned back right-side-in. I realize that is difficult to understand, but imagine you have a very flexible piece of vacuum tubing laid out in frront of you, and to which you attach strings (the longitudinal muscles) that run inside the tubing. The tubing is staped to the floor at your end, and you can only pull on the strings to move the other end. When you want to pull the other end of the tubing back to your body, you pull on the strings (which run through the middle of the tube), causing the tubing to curl in upon itself and the 'other end' to be pulled down the middle of the rest of the tube until it reaches you. Does that make sense?
The longitudinal muscles also allow the animal to curl or move side-to-side. Again, you can prove this to yourself with your water balloon by squeezing the back with one hand, and grabbing a tiny piece of the balloon an inch or so in front of your squeezing hand and pulling it backwards as you squeeze the balloon. This will cause the balloon to both extend and 'rear up' from the table. Although pretty anticlimatic, that is really the basic mechanism by which these animals move. Among the annelids, there is a slight variation on this basic pattern for two reasons; first, each segment is separated from the others by a peritoneal septum (explained below), and presence of septa and a cuticle (if present) provides extra stiffness and resistence to the animal during burrowing. Annelid body walls typically consist of an outer fibrous collagenous cuticle, and a glandular epidermis and connective tissue dermis of varying thickness (although many tubiculous -- tube building -- polychaetes lack a cuticle). The fibers in the cuticle are typically arranged in a cross-helical pattern (like the fibers in a common garden hose) which imparts flexibility while resisting the tendency to bulge or fold, and which often give the body an irridescent sheen under bright light. We'll talk more about Annelids in the section below, but I'll finish the sipunculids first.
Sipunculans are benign and relatively simple to care for, because they are almost exclusively detritus feeders. There are no predatory sipunculans, so you need never worry about finding one or more of these animals in your reef tank. In general, these animals use the tentacles on the introvert (which are covered in mucus) to sweep up organic particles from the substrate or occasionally capture particles from the water column. The captured particles are either transferred to the gut directly by retraction the introvert, or are moved via ciliary currents to the mouth and then to the gut. In burrowing species, such as Sipunculus nudus , the animal directly ingests the sand and silt through which it burrows. They have no specialized respiratory system, and simply exchange gases across the surface of the body, particularly on the introvert near the nuchal tentacles where the cuticle is thinnest.
Sipunculans are quite good at regeneration, and most species are able to regrow at least parts of the tentacles and introvert, if damaged. Some species can regenerate portions of the trunk and the digestive system following serious damage. At least 2 species can reproduce asexually in this manner: the animal divides into a small posterior and a large anterior section (Rice, 1970a). Both sections then regrow the missing parts, which is especially impressive for the small posterior portion, because it must regenerate most of the body, including all the feeding, reproductive and digestive organs and muscles. With a couple of exceptions ( Golfingia minuta and Themiste lageniformis ), sipunculans are gonochoristic (sexes are separate) and freely spawn gametes into the water where fertilization takes place externally. There are some species (e.g., Golfingia minuta , Phascolion cryptus , and Themiste pyroides ) that develop directly into juvenile worms which hatch from the eggs, but most species have a planktonic larval stage of some sort (Rice, 1970b). The planktonic stage can either be a short-term lecithotrophic (nonfeeding) trochophore larva (e.g., Phascolion strombus ), or metamorphose into a secondary larval form called a pelagosphera . The pelagosphera can be either a short-term lecithotrophic larva (e.g., Golfingia pugettensis ) or a long-term planktotrophic (feeding) larva (e.g., Sipunculus nudus , Phascolosoma agassizii ). If you are fortunate enough to have one of the species that produce directly-developing or short-term lecithotrophic larvae, it is likely that the animals could reproduce in your tank. If, however, the species you discover in your tank produces pelagosphera larvae, it is very unlikely that the larvae will ever successfully develop in your tank.
Well, I think that is probably more than enough detail on Sipunculans for most people. I'll move on briefly to discuss the basics of the Phylum Annelida this month, and spend most of the next article discussing the Class Polychaeta (probably better known as the "bristleworms"). Annelida (from the Greek word annulatus , meaning "ringed") is a large phylum, with about 15,000 species of segmented worms. This group consists of 3 classes: the polychaetes (the largest group, with about 10,000 described species), the oligochaetes, and the hirudineans. The number of classes can change to 2 or 4 depending on who you ask; some elevate the Myzostomida to class level or combine the Class Oligochaeta and Hirudinea (or Hirudinida depending on the source) into Class Clitellata (Brusca & Brusca, 1990), but for those who care, I will consider the Myzostomida as highly modified polychaetes and not combine the other two classes. The annelids include familiar worms like earthworms (Oligochaeta) and leeches (Hirudinea), as well as the sandcastle-, tube-, and other marine forms often collectively referred to as "bristleworms" (Polychaeta).
As I mentioned above, annelids are segmented worms. An animal in which the body is divided into a series of longitudinally repeated sections (a condition described as serial homology ), is said to be segmented . Segmentation is generally thought of as a characteristic of invertebrates such as annelids as arthropods, but also occurs in most chordates (and some other groups we did not discuss in this series). Among the annelids, segmentation is nearly 'perfect,' with appendages, muscles, nerves, blood vessels, excretory and reproductive systems all repeated in virtually every unspecialized segment. In other animals, such as the vertebrates, segmentation is really only apparent in the components of the skeleton, muscles and nerves while the major body regions are given over to extreme specialization. The development of serially repeated sections allows the animal to control one segment more-or-less independently of the others, and provided a foundation for the advent of regional specialization. Thus, it is not surprising that annelids have invaded nearly every habitat on the planet with sufficient water for them to survive. They are particularly abundant in the sea, but many species are found in freshwater and terrestrial habitats as well. Annelid species range in size from less than half a millimeter to more than 3 m (nearly 10 ft) in the giant Australian earthworm and some marine polychaetes.
Although annelids are probably the best example of segmentation, as I said above, even this example is only "nearly" perfect. The body is really only segmented in the trunk region. The head consists of the prostomium and contains the brain, whereas the pygidium is the terminal section of the body that bears the anus. In most species, the prostomium is fused to one or several of the anterior body segments to form a composite 'head' which is often specialized for sensory and locomotory coordination and lacks true homonomy (repetition of body parts in each segment). The prostomium and pygidium are not considered actual segments, because they are formed directly from regions of the larval body and lack most of the structures associated with true segments. Once a larva completes metamorphosis into the adult body form, new segments are formed directly in front of the pygidium; thus, the 'oldest' body sections are located near the head, and the 'youngest' near the tail (this is basically true of any segmented animal).
In most annelids, each 'ring' or annulus around the body represents a segment, isolated from other segments by a transferse septum (thin peritoneal layer of connective tissue composed of two layers, one from the segment 'in front of' the joint, and one from the segment 'behind' the joint). The body musculature, lateral nerves, blood vessels, and excretory organs are also repeated or elaborated within each segment. Through the spetum, a dorsal blood vessel and a ventral nerve cord runs throughout the length of the animal, and for much of the length, so does the gut. Annelids all possess a more-or-less straight through gut, running from the mouth to the anus without much elaboration or folding. The exception to the lack of elaboration is the typhlosole , a folded structure hanging in the gut (similar to the uvula -- that thing that hangs in the back of your throat) along the length to increase surface area. In each segment, there is a nephridium ("primitive kidney") attached to a nephridiopore (opening in the septum to drain the segment anterior to the nephridium) for excretion of nitrogenous wastes. Most oligochaetes and polychaetes possess setae (paired, chitinous lateral bristles -- thus the common name, 'bristleworms') on each segment to increase friction with the substratum during movement, allowing the animals to crawl and burrow with greater efficiency. Leeches (Cl. Hirudinea), however, all lack setae, and also have superficial annuli that obscure the true segments in the animal. Hirudineans have a fixed number of segments (usually 15, 30 and 34, but those numbers may vary slightly depending upon who you ask), but the rings on the outside of the body do not correspond to the body segments. They also have a highly reduced coelomic cavity, and therefore their movements do not generally resemble those of other annelids (as people who have watched them have probably noticed).
Although this has been a very superficial introduction to the annelids, I will spend more time discussing each class and their characteristics next time. I just wanted to introduce the different classes and cover some of the basics of the mechanics of locomotion in this article. The annelids are a complex and large phylum, and one about which there appears a lack of general knowledge and a lot of misinformation in the hobby. I will probably spend a couple of articles discussing these animals before moving on to the next taxon.
Literature Cited :
Brusca, R.C., & G.J. Brusca, 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass. 922 pp. Rice, M.E. 1970a. Asexual reproduction in a sipunculan worm. Science 167:1618-1620. Rice, M.E., 1970b. Observations on the development of six species of Caribbean Sipuncula with a review of development in the phylum. Proc. Intl. Symp. Biol. Sipuncula and Echiura. I. Kotor:18-25. Ruppert, E.E. & R.D. Barnes, 1994. Invertebrate Zoology, 6 th Edition . Saunders College Publishing, Harcourt Brace College Publishers, Orlando, FL. 1056 pp.
Last modified 2006-11-18 19:42