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…You just can't tell him very much…"
Worms
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.
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Figure
1. A simplified diagram of the internal structures
of a polychaete worm.
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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."
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Figure
2. A diagram of a cross section through a “basic”
polychaete worm, showing internal structures and parapodia.
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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.
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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.
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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.
Conclusion
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.
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