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

Polychaete Annelid Identification, or
“You Can Always Tell A Bristle Worm…

…You just can't tell him very much…"


I get a lot of questions in the "Ask Dr. Ron" forum requesting assistance in identifying various types of wormy things, and most of these questions concern some sort of bristle worm or, as biologists know them, polychaete annelids. Polychaete annelid worms are some of the most characteristic types of marine animals, and are ubiquitous in all marine habitats. This group of animals is diverse, with over 10,000 species known, and undoubtedly many thousands of species are remaining to be discovered and scientifically described. They are also successful; in many marine areas, including some coral reefs, where the biomass of polychaetes may exceed that of most other animals. Often their abundances are measured in the tens to hundreds of thousands of individuals per square meter. On some coral reefs, a 10 centimeter (roughly four inch) diameter reef rock may contain from 2,000 to 20,000 polychaete worms.

This abundance and diversity makes for some significant problems in identification; there are simply too many of them for the identifications to be easily done. In this month's column, I will describe some of the characteristics and character sets used in the identification of these animals. In next month's column, I will give illustrations of some of the common worms found in marine reef aquaria, and show how the characters described in this month's column may be used to identify the worms.

On a basic, conceptual level, bristle worms are pretty easy animals to understand. They are long, thin, cylinders. Additionally, they are subdivided along their length into small compartments which are called segments (Figure 1). These segments give the worm's body the appearance of being comprised of a series of ring-like units, which are called "annuli." Worms with a body made in this manner are called Annelid worms to differentiate them from other worms such as flat worms, ribbon worms, and peanut worms, none of which have a body constructed of segments. There are three major types of annelid worms: earthworms, leeches, and polychaetes. While all three groups are represented in marine environments by numerous species, the polychaetes are far and away the most common annelids in marine ecosystems.

Figure 1. A simplified diagram of the internal structures of a polychaete worm.

In the "basic" polychaete, each of the 100 or more segments is very similar to the one in front of it, and the one behind it. Now, if you can visualize this kind of basic worm, you probably visualize something that looks like an earthworm, and you would be right. However, the bristle worms are not earthworms; they have some additional structures that make them very different animals, indeed. These additional features are found on the sides of each segment. Earthworms are smooth sided; they have no appendages, whereas polychaete segments have small appendages that stick out from each side of the worm. These appendages, called parapodia, are not really constructed like legs. They are more like tissue flaps sticking straight out from each side of the worm. In the "basic worm" they are quite large and evident. Even though they don't bend or flex, they nonetheless may look quite "leggy." Some aquarists, on viewing some of the bristle worms for the first time, have said that they are reminded of centipedes because of the segments and the "legs" off of each side. The bristles that give the worms their common aquarium name are found attached to the parapodia. The same bristles, called chaetae by scientists, also give the group its scientific name of Polychaetes, which means "many bristles."

Figure 2. A diagram of a cross section through a “basic” polychaete worm, showing internal structures and parapodia.

Another feature distinguishes these worms from the standard earthworm, and that is the presence of additional specialized appendages, generally in the frontal region of the animal, but occasionally elsewhere. Often the bristle worms have a quite well-developed head, with a series of sensory appendages. In others, such as the feather-duster worms, the head appendages may also be modified for feeding.

Polychaete Diversity; It Is A Family Affair

These worms come in almost all shapes and sizes. The smallest are microscopic, so that they may live their entire lives on the surface of a sand grain. The largest may exceed fifty feet in length. Most are long cylinders, but some are quite "grub-like" while others give a pretty good impression of a soggy mouse. Many burrow through sediments, but others build reefs, and still others are pelagic animals and spend their entire lives swimming. While there may be many similarities of internal structures and perhaps even external anatomy, the polychaetes may be subdivided into about 100 smaller and quite unique groups. The all-inclusive larger group has been given the name of the "Class Polychaeta." The smaller groups are called "Families." Each family contains worm species that have similar characteristics. As these distinguishing characteristics are related to how the animals live, the families generally contain species that have the same basic biological attributes, even though they may live in such widely different areas such as Arctic mud and coral reef lagoonal silt. The family names all end in the same four letters, "-idae." As examples, all of the fireworm species are placed in the Family Amphinomidae, and are commonly referred to as "amphinomids," while marine blood worms are placed in the Family Glyceridae and are referred to as "glycerids."

Structural Characteristics Used In Identification

A lot of different characters are used in polychaete annelid identification. While I would truly like to say that these characters, and character sets, are consistently and logically used, I can't. Consider some of the variables that one has to work with, these include not only the worm and its anatomy, but also the type of life style it has. For example, it is useful to know whether is free living (basically just crawling around), or if it lives in a tube. If it does live in a tube, knowing the composition of the tube is useful in identification. Tubes may be composed of mucus, or mucus embedded with various materials such as sand grains or algal fragments. Tubes may also be composed of hardened proteinaceous material which are roughly the composition of parchment. Additionally, tubes may also be hard, and composed of sand grains cemented together or of calcium carbonate. The position of tubes is also sometimes important. The tubes may be on the surface of rocks, buried between rocks, or placed in the sediments. All of these tube characteristics are useful in identifications, and some of them, alone, may allow you to determine which major group or family of worms the specimen belongs to. For example, if the worm builds a calcareous tube, it is almost certainly a serpulid or spirorbid worm.

After determining whether or not it lives in a tube or some other structure, you have to examine the basic appearance of the worm, and pay close attention to specific details. Segmented worms by their very definition are comprised of similar appearing body units or segments, and while it is always useful to be sure of the basic segment shape and structure, it is often most useful to examine those segments that differ from the "average" segment. The differing segments are generally found at either end of the worm. The several most anterior segments may be formed into a recognizable head. This is not necessarily the case, however; consider the situation in earthworms. There are several marine polychaete groups that have species wherein the individuals have the appearance of earthworms, and which lack a defined head altogether.

Head Characteristics

If there is a head, it is composed of at least two regions. Although annelid worms are made with the classic "tube within a tube" construction of long wormy animals anywhere, the front end of the inner tube, or gut, does not occur at the absolute front end of the animal, but rather a short distance back. The front end of the worm is typically a small bulbous projection called the "prostomium" which is joined with a normal segmental boundary to the subsequent body region, called the peristomium. The mouth opens ventrally (in fact, it is the position of the mouth that defines the bottom of the worm) at the boundary between the prostomium and the peristomium. Generally, the peristomium and prostomium, together, are referred to as the most anterior, or the first segment, of the worm. Many more segments may be fused to them to form the head.

The head shape and the number, type and appearance of the appendages projecting from it, are probably the most important characters in determining the identity of the worm. Head shape may be very vastly modified from something simple and basic, and the number of appendages upon it may range from none at all to several dozen. To identify the worm, these head appendages need to be examined and characterized. In some very highly modified worms, there are a large number of feeding tentacles radiating from the head. These specialized tentacles are called "radioles," and have a feather-like appearance, and these worms are referred to a "feather-duster worms." They belong to a couple of different but, presumably closely-related groups. Often feather duster worms have a number of other types of head appendages as well, but these are often overlooked by the average aquarist as they are small and insignificant compared to the radioles. Although primarily used for feeding, the radioles are also loaded with sensory equipment, often including eyes and current sensors.

The other head appendages fall into four major types: antennae, palps, tentacles, and cirri (sometimes confusingly called tentacular cirri). Antennae are short and, if present, a pair of them is typically found right on the most forward part of the prostomium. They tend to point forward as little stubby sensory structures. Palps are generally found behind and to each side and, if present, there will only be two of them, one on each side. Generally in drawings of the worm's heads, they are shown extended forward, but in the living animals, they often point down and are found as sensory or manipulative appendages on either side of the mouth. Tentacles, are… well, tentacular. They are long, thin and tapering sensory appendages arising from the peristomium, and there are generally two to five of them. Generally, there a couple on either side of the head, and if a fifth is found, it arises from the top center or "forehead" of the worm. Tentacular cirri or, simply, cirri are found on the segments behind the head. There may be as many as four or five on each side of the worm on the second, third or fourth segments.

Figure 3. This diagram represents the head of a generalized polychaete as viewed from above. I know of no real animal with all of these structures in this pattern. However, there are worms having some of all of the structures illustrated here.

Additionally, many of these worms will have eyes. Typically the eyes are pigment spots on the top of the head, and typically the animals have either one or two pairs of them. These eyespots are incapable of resolving an image, but they are certainly capable of determining the direction of light or a sudden shadow. In some pelagic worms, however, the eyes are quite well developed and well capable of image formation. These worms are not found in aquaria.

In several groups of worms, the upper surface of the head is extended backward in the "neck" region to form what is called the "nuchal" or neck shield. This is presumed to be sensory, but rigorous data about its function are lacking. All of these head structures are useful in determining the identity of any worm that might show up in an aquarium, but perhaps it is most critical to note the number and position of the various tentacles, eyes, as well as the general shape of the head.

Body Characteristics

The body characteristics tell alot about the worms, but they are not generally as useful in determining the identity of the "usual suspects." This is a pity, as the head is generally small compared to the rest of the worm, and often aquarists can't see the head, but they can see some other part of the body. The general shape and color of the body should be noted. Typically, these worms are long relatively thin cylinders. However, a few groups have characteristically different shapes. These shape differences are apparent in cross section; that is what the worm would look like if it were cut across the body with a sharp knife that was perpendicular to the edges. The cross-section appearance of most worms is circular. However, several polychaete worm families vary significantly from the circular ideal. Some are definitely rectangular, and several have a flattened bottom region and a curving upper surface. In addition to body shape, one needs to examine the position and shapes of the various body appendages.

Body Appendages

Each body segment may have several different types of appendages arising from it, but generally only a couple of kinds of appendages concern us. In a few groups feeding tentacles, called palps (but not like nor related to the palps found on the head), may arise from various segments in the front section of the worm. These are often paired and there generally are only one or a few pairs, if they are present at all. They will be recognizable by their shape. They are long and thin, but they generally do not taper, and often have a "gutter" running along their length. Particulate food may be seen moving in them. Additionally, one may find gills arising from segmental surfaces. These are typically thread-like, often they coil, and often they are brightly colored red (most worm blood has hemoglobin as the respiratory pigment just like your blood).

The most characteristic body appendages, however, are the flap-like appendage found on either side of each segment. When viewed from above they make some of the worms look vaguely like centipedes but, unlike a centipede's long and essentially tubular legs, these appendages are tissue flaps without joints, so they can't bend or articulate. These appendages are the parapodia, and while they are not found on either the prostomium, the peristomium or the pygidium (the last body segment which contains the anus), they are found on all other segments of polychaete worms. The first thing to notice is their prominence. In some worms, particularly the scavenging fireworms common in aquaria, the parapodia are large and evident. In some worms, however, the parapodia may be very small, reduced in size to almost nothing. The only visible remnant of them may be a surface ridge or a small group of bristles.

The basic parapodium is a large and complex structure with several lobes which is supported internally by stiff chitinous rods. It bears on its outer edges the chaetae, or bristles, that give bristle worms their name. Additionally, it commonly bears gills, at least somewhere along the worm's length. The gills will often be visible as small reddish tufts at the top of the parapodia. The absence or presence of the gills and the position of the gills along the body are important clues as to what the animal's identity might be. One thing to establish is whether the parapodium has one, two, or no large lobes. Additionally, are any parts of the parapodia "extra" large? Some families of worms are characterized by expanded dorsal lobes that extend up and cover the upper surface of the worm. These scale-like coverings are characteristic of several groups of worms.

One of the factors that makes it difficult for hobbyists to identify these worms is that some of the diagnostic or distinguishing characteristics are not readily apparent to the untrained eye. Or, in some cases, to the trained eye, for that matter. Probably the most important of these are the shapes and structures of the bristles. These have to be examined microscopically, and under quite high power, 1000x or greater, but if they can be examined at that power, the amount of microstructure visible in the chaetae is really enormous and often quite helpful in determining worm types.

Finally, the other things that are difficult for the hobbyist to examine are the mouth parts, such as the jaws, accessory teeth and proboscis structure. Some of these are often large and evident structures, relative to the size of the worm. However, they are only seen when the worm is feeding or by examining the worm microscopically. The rest of the time they are withdrawn into the animals. As with the chaetae, they can provide very important clues about the identity of the worms.


These are the common character sets that are used to identify and differentiate between the major groups of polychaete worms. If you look over them, and consider them, you will see that they all relate in some manner to the actual natural history of the worms. Or in other words, they relate to the worm's life style: how it moves, eats, protects itself from predators or breathes. Next month, I will discuss how to differentiate some representatives of the many worm families found in aquaria and their role in structuring and maintaining our aquaria and natural coral reefs.

If you have any questions about this article, please visit my author forum on Reef Central.

Useful References About Polychaetes In General:

Fauchald, K. 1977. The polychaete worms; definitions and keys to the orders, families, and genera. Natural History Museum of Los Angeles County. Los Angeles, CA. 188pp.

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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Polychaete Annelid Identification, or “You Can Always Tell A Bristle Worm… by Ronald L. Shimek, Ph.D. - Reefkeeping.com