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As the Worm Turns.......Aquarium Net November 1997

As the worms turn... or vermiform, it's a way of life in all aquaria.

By Ronald L. Shimek

"Burrowing in sand and mud, lying under stones and in crevices of rocks, concealed in various types of tubular cases which are free or attached to stones or shells, crawling over the ground or seaweeds, swimming free or attached to other animals, is found in abundance a class of animals commonly known as WORMS and generally regarded as repulsive creatures unworthy of attention."
Augusta Foote Arnold, author of "The Sea Beach at Ebb Tide (New York: Century Press, 1901).

Well, perhaps even most aquarists think that way. This quote notwithstanding, worms are common inhabitants of most marine and fresh water aquaria, and they come in a wide variety forms and types. Most of the animals that aquarists purchase are reasonably recognizable by their general shape; animals such as fishes, corals, soft corals, sponges, and sea anemones. However, there is another category of animals in which the basic shape is not very useful in identification. These are the "worms." Many different animal groups have animals that have a worm-like or "vermiform" (from the Latin Vermes meaning worm) shape.

The description of the shape is straight forward. Vermiform animals are "more-or-less" cylinders whose length generally far exceeds their diameter and they generally have a head, or anterior sensory region. These are really the only consistent parts of the definition of a worm. Depending on the worm, there may or may not be a mouth. If a mouth is present, it is generally, but not always, at the front end of the worm. Worms are often small, but the largest free-living worms exceed 100 ft (30 m) in length. If the worm is cut across the longitudinal axis, the body may be round, oval or quite flattened.

The wormy body shape is well adapted to moving through a sediment substrate or viscous substrates such as tissues (parasites are often worms), as the frontal area is small and the body can provide propulsive force from all over the surface. A few worms can swim, and some, such as the leeches, can be really very strong and vigorous swimmers.

Typically worms move through substrates in one of three methods. The first way is by gliding along, propelled by the microscopic ciliary hairs located on the bottom surface. This method of moving is generally used by the smallest of the worms, and is typical of the way flatworms move. Often the tiniest microscopic worms use this method of moving to move on, over and between sand grains. The second method is burrowing by peristaltic motion. Here the worm typically contracts muscles around the front of the worm forcing the anterior end to a rather sharp point. Then waves of contraction pass down the body. These waves have regions where the body is thick in diameter alternating with areas where the body is thin. The thick areas move in a smooth continuing contraction, generally from the front to the rear forcing the anterior end forward. This way of moving is generally slow, but is a very effective way of moving through sediments, and is relatively cheap in energy consumption. The third method of locomotion is seen in some strongly muscularized worms such as the roundworms. Here the animal sets up sinusoidal waves in the body and moves the waves along body by "wiggling" through the substrate. This type of locomotion tends to be rapid, but is very energy intensive and inefficient as much energy is expended moving the worm side-to-side relative to the forward locomotion.

A diagram showing the three major locomotory methods used by worms. The direction of the worm is moving is shown by the dark arrows. Top: Gliding locomotion, indicated by a flat worm; the anterior part of the worm is lifted off the substrate. Middle and Bottom: Locomotion in sediments by peristalsis and sinusoidal wave action; the directions of the locomotory waves are shown in white.

There are worms in many animal groups, and many of these worms are commonly found in our tanks. Although there is a greater variety of worms found in reef tanks, almost all fresh and salt water aquaria have them. Here is a listing of some types of those worms, their characteristics, and their suitability for an aquarium system (Kozloff, 1990; Ruppert and Barnes, 1994).

Cnidaria Sea anemones, corals, etc.

The larvae of cnidarians are all small, generally microscopic, worms. These are common in aquaria where some cnidarians ( Aiptasia , hydroids, corals) are sexually reproducing. Generally, these small worms called planula larvae, are not visible, but occasionally they can be seen as very small, oval objects moving on the aquarium glass.

Platyhelminthes Flatworms, planaria

Flatworms are small to relatively large worms flattened from top to bottom. The name "planaria" which has been used to describe these worms comes from the old name for some fresh-water flatworms often used in introductory biology classes to illustrate flatworm properties and unfortunately, this name seems to have crept into the marine aquarium literature. The scientific name " Planaria " no longer exists, as the worm in question is now called Dugesia . Additionally, the type of flatworm that the name described was not found in marine environments. Thus the use of the term "planaria" to describe worms in reef systems, describes worms that are significantly structurally different from the original "planaria" in a significantly different environment. However, it looks like we are stuck with the term.

Some free-living flatworms are problem animals such as the red "planaria," Convolutriloba retrogemma (Delbeek and Sprung, 1994). However, most of these worms are small, and although common in reef tanks, generally go unnoticed. Occasionally though, larger ones are seen; one of my acquaintances took a look one night at his aquarium and was startled to see an 18 inch (50 cm) long, 1/4 inch (6 mm) wide flatworm on the front glass. He saw it once and never again... Additionally many internal and external parasites of fishes are flatworms; the flukes and tapeworms. These animals can be serious pests and may kill fish.

Not surprisingly, flatworms may be recognized because they are flat. Most free-living ones move by gliding on cilia, whereas the parasites use a muscular peristalsis for most of their locomotion. Often the lateral edges of the larger free-living ones "ripple" as they move. They come a wide variety of colors, and while some are quite beautiful, most are rather a dull white, tan, beige or black. They often have at least two black eyespots at the anterior end; some species may have several dozen eyespots. Free living flatworms may be either predatory or herbivorous.

A free-living flatworm collected from the live sand in one of my aquaria; it was about 1/100th of an inch (250 m) long. The head is to the right, the mouth is on the tip of the light colored structure in the midst of the dark gonadal material in the posterior part of the worm, and there are rows of eyespots running back from the front of the worm. This animal was probably predatory on other smaller animals.

Adult of the parasitic flatworms, tapeworms and flukes, may enter a reef aquarium system when new fish are introduced (relatively few of these animals parasitize invertebrates). External parasites will be visible and may be manually removed with forceps or removed with medications. Internal parasites generally remain unknown unless they are sufficiently abundant to kill their host. Occasionally intermediate or juvenile stages of some flukes that parasitize birds or marine mammals as adults will be found in aquaria, particularly if the hobbyist has collected some snails from a local beach. The juvenile flukes parasitize the snails and use the snails' tissue to create stages that infect the adult host. If the adult host is a marine mammal, it is possible for the aquarist to become infected by the parasite by immersing their hands in the aquarium as the infective stages are released from the snail.

For more information about flatworms follow these links: 1 2

Nemertea Ribbon worms

Ribbon worms are seldom purchased by aquarists, but occasionally occur in reef systems, presumably coming in on live rock or some other substrate. Most are predatory and eat other worms and crustaceans. The ribbon worms found in aquaria are generally pretty small, but one I kept in a cold water marine tank was over 50 feet (15 m) long. I collected it on a scuba dive and kept it for about six months before release. Other ribbon worms over 100 feet (30 m) long have been seen.

The front end of Tubulanus albocinctus , a large temperate nemertean or ribbon worm. The worm was over 20 feet (6 m) long. Ribbon worms are generally oval in cross-section and they lack any obvious divisions of the body into segments. Long thin worms without segments are almost always ribbon worms. They move on the surface of sediments by a combination of ciliary gliding and peristaltic muscle movement, and are often brightly colored. Their anterior end has a head that may be distinctly set off from the rest of the body by a constriction or groove. They have a extendable proboscis which may be as long as the body of the worm, and it extends out from an internal body sheath to catch and kill prey. Most are venomous, but are generally harmless to people. For more information about ribbon worms follow this link

Rotifera Rotifers

Many aquarists know of rotifers and marine aquarists, in particular, culture a few species as planktonic food for other animals. However, most rotifers are fresh-water species, and most are not planktonic. They are small; most are microscopic. Generally they crawl or swim for short periods from place to place, and most are suspension-feeding animals. They are generally quite commonly found in fresh-water aquaria where they are often introduced on decorative live plants. Small fish will feed on them, as well many small invertebrates. They are harmless or beneficial additions to both marine and fresh water aquaria.

A diagram of the basic body form of a bottom dwelling rotifer. Unless the aquarist has access to a microscope, it is basically impossible to see the non-planktonic rotifers. If a microscope is available they are visible as small worms that crawl around on the various substrata. They have a body that telescopes in and out on itself, and a ciliary organ at the front end that looks like one or two spinning wheels. For lots of good illustrations follow this link and surf through some of the links to various examples:

Nematoda Round-worms

The round worms are found in all fresh-water and marine aquaria, and in all fresh-water and marine aquarists... Many are parasitic, but there are thousands, perhaps millions, of species that are free-living as well. The free-living forms are generally small, less than half an inch (12 mm) in length. They are found in all marine habitats, and I have found them in all parts of my reef and fresh-water systems. I have no doubt they are ubiquitous in aquaria. Some are herbivores, others are predatory, and yet others are scavengers. They move with a characteristic thrashing locomotion that is particularly well-suited for moving through sediments (or tissue for the parasites).

Parasitic nematodes can cause all sorts of problems for their hosts, living either in the tissues or in the guts. Fish suspected of having nematode infections can be treated using some commercially available products.

The larger nematodes are often visible as thin white threads. If removed from the substrate and placed in a clear glass dish against a dark back ground, they move with a back-and-forth whipping or writhing action. They are generally small enough that no other detail is visible.

For more information about nematodes including some illustrations, follow these links: 1 2

Acanthocephala Spiny-headed worms

Spiny-headed worms are all parasitic, and most infest fish. They live in the guts, by burrowing into and fastening onto the gut wall and they absorb food from the gut contents through their body wall. Both fresh and salt water fish have these parasites. Generally there is no way to tell if the fish has these parasites, but often if the fish is eating well, but not growing or even wasting away these parasites are to blame. Significant infestations can kill fish by causing complete blockage of the gut.

The only time a hobbyist would see spiny-headed worms would be if they did a an autopsy of a recently dead fish. They will be found in the intestine of fish, and often are very abundant (even in healthy fish).

For a diagram of a spiny-headed worm, follow this link :

Annelida Segmented Worms

A diagram showing some of the basic features of a polychaete or bristle worm. There are three types of segmented worms. All are recognizable by having a body divided into ring-like segments or annulations. The most commonly found one in marine systems belong to the class Polychaeta and are commonly called bristle-worms or clam worms. Those found in aquaria generally range in size from microscopic animals to animals about a foot (30 cm) long. All of these worms are characterized by the presence of numerous bristles off the sides and many have small lateral projections that look like (but aren't) small legs off the side of each segment. Most of these are harmless or beneficial as they are general scavengers. A few species, primarily fireworms such as Hermodice carunculata , are known to attack and eat gorgonians. Large populations of polychaetes are found in reef tanks with healthy live sand beds and move and stir the sand. Additionally, they are scavengers eating excess food and other debris.

A predatory annelid or segmented worm, probably in the genus Eteone. The parapodia, or appendages are visible on either side of the body. This worm was about 4 inches (10 cm) long. Other commonly found segmented worms are in the class Oligochaeta. The best known oligochaetes are earthworms, and all of those found in aquaria typically look like very small versions of the common night crawler. These worms are most common in fresh water aquaria, and the best known ones are the Tubifex worms that many aquarists use as food. These can live well in fresh water systems and provide a continual source of fish food. Similar, but smaller worms may be found in marine aquarium sands as well. The last type of annelid worm occasionally found in both fresh and salt water tanks are leeches. Most of these are relatively small, but some fresh water leeches get to about 18 inches (50 cm) long. Most are ectoparasites specialized to suck fish or crustacean blood, and they can occasionally kill their host. They are uncommon in aquaria, but if found can be easily removed with forceps.

There are lots of annelids in marine systems and some of the larger ones such as feather duster worms and fire worms are easy to see. Others may be much smaller. They can be recognized at all times by the presence of segments which make the animal's body look like it is composed of many similar sections. Often there will be small paddle-like appendages off the sides of the animal, one on each side of each segment.

For some good detailed information on the annelids in general and polychaetes in particular, search through the Aquarium.Net library back issues for the excellent discussion by Rob Toonen and some articles by myself.

Echinodermata Sea stars, sea cukes, etc.

A wormy sea cucumber from Palau, possibly in the genus Opheodesoma . This animal was about 6 inches (15 cm) long. It was found crawling over the gorgonian, which was somewhat unusual. Several other specimens were found on this dive; all had the posterior part of the body in a burrow or crevice in the reef. Burrowing sea cucumbers are effectively worms. They plow through sediments generally by peristalsis. Most are fairly small and can be effective sand-stirrers in reef aquaria. Some ingest the sand as they move through it, and digest bacteria off the sand grains. Others live in a burrow and extend their crown of feeding tentacles up out of the burrow to feed on deposits or particulate material suspended in the water column. These sea cucumbers are generally about half an inch to several inches long (1 - 15 cm). The body may be smooth or have many little bumps on it. Typically there will five bands of muscle running from one end to the other visible through the body wall. Some may have tube feet present, but many lack tube feet altogether. If tentacles are present around the mouth, there will either be five or ten of them. They have no eyes or other obvious sense organs, and generally are white, clear, or various shades of brown.

Hemichordata Acorn worms

A diagram of the basic body form of a typical acorn worm. Acorn worms are rare in aquaria, but occasionally are found in the live sand shipped out by suppliers. They can reach lengths of several feet (1-2 m), but most are smaller. They are common in natural coral reef sands and burrow through the sediment eating sand and digesting the bacteria off of the particles. They can be recognized by having a body divisible into three regions, a "nose" or proboscis, a collar, and a longer trunk. The proboscis is often brightly colored, and if the worms are removed from water they smell strongly of iodine. The trunk contains gill slits which connect to the interior gut. Acorn worms are considered to be quite closely related to the chordate animals such as fishes and ourselves.

For more information and illustrations of hemichordates, follow these links: 1 2

Chordata Fishes

There are numerous fishes that are effectively worms, probably the most frequently encountered in marine aquaria are the various eels. However, there are other, even more "wormy" fishes. These are the jawless fishes, the lampreys and hagfish. I don't think any hobbyist would keep these, but they are sometimes seen at public aquaria. Lampreys are parasitic as adults and rasp a hole in the side of other fish and eat the blood and tissue fragments. Hagfish burrow into dead organisms by entering through any aperture or orifice and proceed to eat all of the tissue except the skin and skeleton.

A north Pacific hagfish, Eptatretus stouti , entering the body of salmon suspended outside of a research submersible. The body of "worm" can be seen in the mouth of the salmon. Eventually several hags were in the salmon, entering through the mouth, anus, eyes, and nostrils. The flesh of the salmon, weighing about 5 pounds (2 kg), was completely consumed in about 15 minutes. These really wormy fish, the hags and lampreys, reach lengths of about a foot (30 cm) or more. They may or may not have eyes (lampreys have 'em, hags don't). They don't have any paired fins off the sides of the body, but often have a dorsal and ventral fin. They have a row of five to seven circular gill openings running along the upper side of the body. The mouth is a circular sucking structure, and they don't have jaws. For some more information on hags, including a downloadable movie and illustrations, follow this link Worms are specialized for life in sediments or gooey media such as tissues (or decaying tissues in the case of hags). Unconsolidated sediment environments are the most widespread non-liquid environment in the world, and consequently this basic body form may be the most widespread in the animal kingdom. Additionally, internal parasites almost have to be worms, no other body form does anywhere near as well. Although I have mentioned some of the more commonly seen "worms" here, there are literally wormy animals in most animal groups from the terrestrial insects (grubs and other larvae) to many marine reef dwellers. About the only common groups of animals that seem to lack worms are the birds and mammals, and amongst the latter weasels come pretty close to being furry worms. So, even though some may consider worms as repulsive, they are successful and are found in numbers literally beyond counting. Each and everyone of us has them in our aquaria (and in ourselves, as all humans are parasitized to some extent).

References Cited

Delbeek, J. C. and J. Sprung. 1994. The reef aquarium. Ricordea Publishing. Coconut Grove, FL. 544 pp.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

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Last modified 2006-11-20 04:27