Over the past couple of years, I have devoted
several columns to the various kinds of segmented worms found
in reef aquaria. The bodies of these worms are composed of
repeated body units which are generally manifested on the
outside of the worm as a series of rings or annuli. The name
that scientists give this great group of worms is the Phylum
Annelida, recognizing that the "annuli" or segments
are the worm's major visible characteristic. Most annelids
found in reef tanks are polychaetes, known to most aquarists
as "bristle worms." These worms have a couple of
other notable visible characteristics such as the presence
of appendages off each side of the segments and tentacles
from the head or other regions of the body. These annelids
are often large and evident, and numerous species of them
thrive in aquaria. After looking into their tanks when the
"clean up crew" of bristle worms is active and visible,
reef aquarists might be forgiven for thinking all worms likely
to be found in their systems are annelids. However, upon reflection
most aquarists would also count flatworms among the wormy
denizens of their systems. These two types of worms are commonly
the basis for much discussion amongst aquarists, as they are
generally foreign to the average hobbyist. Nonetheless, they
are common inhabitants of marine aquaria, as eventually most
hobbyists come to realize.
Recently, however, a third variety of worm
has occasionally been found
in reef aquaria. These worms are moderately large, up
to several inches in length. Unlike the annelids, they don't
have segments, and they have no visible appendages such as
tentacles or the small paddle-like parapodia off the sides.
While these worms appear in some cases to look a lot like
flatworms, they aren't flat. They are decidedly "meaty"
and have some real depth to their length and width. And, unlike
most free-living flatworms, they may be quite large; in nature
some of them reach lengths well over 100 feet long. These
are the ribbon worms. Unlike a lot of the other worms as well,
many of them are strikingly beautifully colored, and well
worth keeping if we get the chance.
What's in a Name?
Ribbon worms are placed by taxonomists
in a group that goes by several names. The group is most commonly
referred to as the Phylum Nemertea, but it may also be called
the Phylum Nermertinea, the Phylum Nemertini or, alternatively,
the Phylum Rhynchocoela. All of these names refer to exactly
the same group of animals, and simply represent the names
given to that group by different researchers at different
times or in different countries. It is probably worth using
this example to illustrate the arbitrary nature of taxonomy
and the study of animals. Within any large scientific discipline,
there are differences of opinion, and methodology. For example,
the scientific discipline that most American reef aquarists
would recognize as "marine ecology" really doesn't
exist within the same framework in Russia. While the science
of marine ecology is present in Russia, the way in which it
is studied and concepts emphasized as important differ significantly
from what is studied, or even conceived of, in the United
States. Likewise, taxonomy differs from place to place. In
the middle part of the twentieth century, the American invertebrate
zoologists and taxonomists who studied ribbon worms considered
the worms' characteristic extensible prey-capturing proboscis,
called a rhyncocoele, as the most important defining structural
feature of the group. In use, the ribbon worm's proboscis
projects in front of the worm rather like an elephant's trunk
or proboscis. To emphasize that this notable feature had a
contained space or cavity within it, some American invertebrate
zoologists coined the term, "Rhynchocoela;" rhynchocoela
literally means "cavity in the nose, or beak." These
biologists then began to refer to the ribbon worms by that
name. However, most other invertebrate zoologists kept referring
to the group by its earlier name of Nemertea; a name derived
from the name of the ancient Greek Sea Nymph or Nereid, known
as Nemertes, "the unerring one." This was also an
appropriate name for these worms in that the proboscis is
unerring and always spears or grabs the prey. The name "Rhynchocoela"
was used extensively in the United States and Canada until
the 1970s when it began to fall out of fashion, nonetheless,
it is still occasionally used. Presently, the older name of
Nemertea is far more commonly used. In any event, should the
reader become interested in this group, information about
them will be found under all of the names listed above.
Shapes, Sizes, Structures and Functions
Ribbon worms don't have any manifestation
of segmentation. Unlike the annelids, the body is smooth.
They also lack appendages or any other permanent projections
from the body. The body is often brightly colored, and there
are often small dark eyespots found near the front end. On
careful examination, a pair of grooves is sometimes found
just behind the head, and the animal will be seen to have
either one or two openings at the front end, and one opening
at the rear end. They are typically long and thin, about the
length and width of some ribbons, hence giving the group their
common name. However, they are not flat like either ribbons
or flat worms. The bodies are typically circular or elliptical
in cross section. They really have only one claim to zoological
fame. Some species of nemerteans may well be the longest of
all animals. Nemerteans longer than 30 feet long are surprisingly
common, and some have been measured at well over 100 feet
in length. There is a nemertean story, probably apocryphal,
about the eminent late nineteenth and early twentieth century
British zoologist, E.Ray
Lankester. Lankester was pretty much the dean of British
scientists at the end of the Victorian period, and as one
of his many perks, enjoyed golfing privileges at St. Andrews
golf course in Scotland. During one round of golf, the eminent
doctor whacked a ball into the rough, which as the golf course
runs along the sea shore, was the intertidal zone. He is reputed
to have gone in search of the ball, only to be distracted
by finding a nemertean coming out of one hole and going into
a second hole over 100 feet away. Neither end of the worm
was seen, so the total length remains unknown, except that
it was in excess of 100 feet long. Lankester also collected
a sipunculan, or peanut, worm on one his forays into this
golfing intertidal zone. It was new to science, and he immortalized
his passion by naming the worm genus, Golfingia, but
that is another story. Other nemertean worms have subsequently
been collected with lengths similar to what Lankester found,
validating his observations. Alas, the largest nemerteans
that I have personally collected were only about 60 feet long.
Figure 1. The head of a large nemertean worm I photographed
about six feet of water near Seward, Alaska. The body of the
was well over 40 feet (12 m) long.
The 550, or so, species of ribbon worms,
are mostly marine, and most are found in the polar seas where
they are often both diverse and abundant. However, they are
by no means rare or uncommon in temperate or tropical habitats.
While most marine forms are bottom dwellers, there are few
species that are wholly pelagic, swimming
in mid-water. There are a few species found living on
the bottom in freshwater habitats. One species, called either
Prostoma rubrum or Prostoma graecense, is widely
reported from streams throughout North and Central America.
There are also numerous terrestrial species, found mostly
in the tropics, but at least one species of Geonemertes
has been accidentally introduced to the west coast of North
America and is now found from Vancouver, British Columbia,
to San Francisco. It lives in and under leaf litter and is
thought to prey upon small slugs.
Nemerteans have a misleading simplicity
of structure. Superficially, they look rather like "unflat"
flatworms. It is easy to look at small ones, particularly,
and see a relationship to flatworms. After all, both flatworms
and nemerteans are flattened (more-or-less) unsegmented worms,
with no clear definition of a head or other body regions.
They both move with the gliding type of movement characteristic
of ciliary locomotion when small and by a combination of gliding
and muscular movement when larger. Neither of these types
of worms has a hard shell or cuticle, and both have similar
simple eyespots. Internally, neither of these worm types has
a body cavity surrounding the gut. All of these things have
indicated to several generations of zoologists that nemerteans
and flatworms are closely related.
Figure 2. A small individual of Tubulanus polymorphus,
about six inches (15 cm) in length.
This species may eat small crustaceans, and reaches lengths
of about three feet.
Such a close relationship made for a pretty
story and allowed a lot interesting theory and discussion
about how nemerteans were somehow "advanced" animals
at the flatworm structural grade. Unfortunately, such stories
and discussions were well within the realm of science fiction.
Close examination of internal structures, as well as studies
of the genetic material of both groups, indicates no relationship
at all. Nemerteans are descended from a totally different
group of animals than those that gave rise to flatworms. Although
ribbon worms may look like flatworms, the most recent ancestor
they likely shared was alive over a billion years ago.
Ribbon worms are typically elongate worms
with a circular or elliptical cross section. They have a ciliated
epidermis which is the major means of locomotion for most
of them. Cilia on the bottom surface move the animals in a
gliding locomotion, in many ways similar to the gliding locomotion
seen in many snails. These snails, such as individuals of
the common aquarium snail, Nassarius vibex, also move
by ciliary means. These worms are, however, also highly muscular,
and the larger forms also use muscular means to move themselves
along. They may pass muscular waves of contraction down their
bodies using these to provide propulsive power. Some forms
are very active swimmers as well. Most, however, use a combination
of ciliary gliding and peristaltic muscular means for locomotion.
Figure 3. A composite of several images showing
one individual of Tubulanus albocinctus. Common
in the Northeastern Pacific, this species gets relatively
large. The animal illustrated was about 20 feet (6m)
long. Virtually all aspects of this species' natural
history are unknown, including why they aren't preyed
upon while foraging. The bright colors of many nemerteans
may be "aposematic" or "warning"
coloration indicating bad taste or some other deterrent
The internal anatomy of these animals is
complex, but not as complicated as the average annelid worm.
The gut is a complete gut with sequential digestion occurring
from front to back in the gut. Unlike the gut found in flatworms,
which has only one opening, the gut in ribbon worms has an
anterior mouth and a posterior anus. The mouth may be found
either at the front of the body, or on the bottom of the body
a short distance behind the front end. The anus is generally
terminal at the back end, although a few forms have a small
"pigtail-like" cirrus projecting behind it. The
gut has quite a few internal divisions, but they don't correspond
closely to what is seen in many other animals. Behind the
mouth is a short esophagus, often with a large pouch or caecum
extending from it. Located just after the esophagus is the
relatively small stomach, and then a long intestine extends
almost all the way to the end of the worm. The intestine has
many side pouches, referred to, collectively, as intestinal
caecae. It appears that much of the digestion occurs within
the cells lining the various pouches rather than in the gut
cavity. A short hindgut terminates in the anus. The gut is
embedded in the body musculature, and no body cavity is found
surrounding the gut as it is in annelid worms, fish, or crustaceans.
The ribbon worms possess a closed circulatory
system similar in concept to that found in annelids, fishes
or, for that matter, humans. The blood always is found with
vessels, unlike the situation in mollusks or crustaceans,
where the blood flows freely over and through the tissues.
There is a blood vessel loop around the brain region and lateral
blood vessels which extend down the length of the body on
either side, meeting at the posterior end of the body. Often,
but not always, a dorsal vessel is found running from the
anterior blood vessel ring to the juncture of the two lateral
vessels. There is no heart, nor are there valves within the
system. Blood simply sloshes around within the vessels and
is moved by muscle contractions throughout the body. The blood
may or may not contain hemoglobin, but if it does, the hemoglobin
is not found in cells.
As befitting long and narrow animals with
a permeable body surface, there are no specialized organs
for respiration. Dissolved gases simply diffuse in and out
over the body surface. Instead of using a high a high pressure
filter system as a kidney, such as found in octopuses, these
worms rely on a fine system of tubules with fluid motion dependent
upon a few small clusters of beating cilia. Such systems are
simple kidneys that are more adapted to control water balance
than to excrete nitrogenous wastes. Nonetheless, they do secrete
ammonium ion as a waste product. These kidneys empty through
pores near the front end of the animal.
Although they seem like simple worms with
not much going for them, they have a surprisingly complicated
nervous system. They have a large bilobed brain with a major
nerve running around the front of the gut, in the region of
the throat. Instead of a single spinal cord, they have paired
major nerves running the length of the body on either side.
Additionally, large nerves run from the brain to the proboscis.
They typically have eyespots, sometimes as many as several
dozen. These eyespots cannot form images, but in the most
sophisticated receptors can detect changes in light intensity
and the angle of light impingement. Additionally, they have
several well-enervated organs in the head region that are
thought to be mostly chemosensory.
Their reproductive systems are simple to
the point of being rudimentary. The sexes are separate, and
the gonads are found as masses of egg or sperm producing tissue
between the intestinal pouches. When the animals are gravid,
and "the time is right," temporary openings form
to the outside and spawning occurs. In a few nemerteans, such
as Cerebratulus species, a specialized hat-shaped larvae,
called a "pilidium" occurs. This larva develops
and then undergoes a metamorphosis into a juvenile worm. In
most nemerteans, the embryo develops directly into a small
worm, and has no larval form. They also develop asexually
Figure 4. A pilidium larva collected from the
plankton. The juvenile worm starts
development at different discrete points within the larvae.
These all grow together
as the larva ages to eventually form a worm which will emerge
from the larvae at
metamorphosis. This larva was about 0.004 inches (100 µm)
The single structure that sets these worms
apart from all other worms is their proboscis, the rhyncocoele.
Basically, the term "proboscis" simply means a large,
evident and sometimes mobile nose, such as an elephant's trunk.
However, that really isn't the case with these worms. Here
the proboscis is a rapid-strike prey capture organ. The nemertean
proboscis is normally retracted into an internal sheath or
pouch found above the worm's gut. The proboscis proper is
a thin walled closed tube, shaped like a rather long, flexible,
soda straw with a closed end. The open end is continuous with
the body surface at a pore on the front of the animal. The
closed end is held inside the sheath, pulled into the sheath
by the contraction of a long muscle called the proboscis retractor
muscle. The inside of the straw is continuous with the outside
of the animal, and is collapsed on itself when the proboscis
is retracted. The proboscis sheath is a closed impermeable
tube and is filled with fluid. When the animal senses a prey
organism nearby, a circular muscle layer around the proboscis
sheath rapidly and vigorously contracts. This contraction
forces the fluid from the proboscis sheath into the proboscis
and, in the process, literally turns it inside out, blowing
it out of the proboscis sheath. The proboscis will rapidly
(within a second or so) wrap itself around the prey, which
is then drawn to the mouth and eaten.
There are two basic types of nemerteans,
and the difference between them is based on the structure
of the proboscis. In the ones considered to be primitive,
in what taxonomists call the class Anopla, the mouth is located
ventrally, and the proboscis exits at the front of the worm.
These animals have a poisonous or sticky proboscis secretion
that acts to immobilize the prey, which is subsequently eaten.
Some common genera of Anoplans are Cerebratulus, and
Lineus. Ray Lankester's long nemertean found by the
golf course was probably a species of Lineus. Some
worms in the Anoplan genus Cerebratulus are notable
for their swimming ability. They are flattened top to bottom
and very muscular. They swim by undulating their bodies much
in the manner of a sea snake or leech. I have had several
of these worms, about two or three feet long swim by me, always
on night dives in murky dirty water. It is enough to make
one swear off alcoholic beverages
Figure 5. A diagram of the basic structure of the front
end of an Anoplan (stingless) nemertean showing
the retracted proboscis.
The other basic type of nemertean is placed
in the taxonomic class Enopla. Here the gut and the mouth
open through a common pore at the front of the animal. Additionally,
they possess a piercing calcareous stylet which punctures
the prey, introducing venom into it. The stylets bear a resemblance
to a sharpened golf tee. They are glued to a pad or thickened
area inside the proboscis. They typically are released or
break off upon striking the prey. When the proboscis is withdrawn,
it is reloaded with another stylet. The animal secretes extra
stylets and may have as many as a half dozen in reserve at
any one time. Some common genera of Enoplans are Paranemertes,
Amphiporus, Emplectonema, and Micrura.
Figure 6. A diagram of the basic structure of the front
end of an Enoplan (stinging) nemertean
illustrating the retracted proboscis. Note that there is only
one open to the outside, and that the
proboscis, when everted, will exit through the mouth.
Figure 7. A diagram of the basic structure of the front
end of an Enoplan (stinging) nemertean illustrating
the everted proboscis. Note that the proboscis can be much
larger than shown here, it may in fact be as
long as the animal's body.
Figure 8. An unidentified enoplan nemertean, about
0.04 inch (1mm) long.
This animal has four eyespots. The proboscis is clearly visible
above the gut.
The intestinal pouches, or caecae, are the site of much of
the animal's digestion.
Figure 9. This is the same nemertean as shown in the
The piercing stylet is visible in the introverted proboscis,
as is the gut
underlying the proboscis.
Most nemerteans are predatory, but a few
are parasites. The diets of relatively few of them have been
documented in detail. The prey appear to be primarily polychaete
annelids, crustaceans such as barnacles and amphipods, or
fish. Cerebratulus lacteus from Nova Scotia is odd
in that it eats Mya arenaria, a rather large clam.
Several species of some specialized nemerteans, such as species
of Malacobdella, are commonly found inside the mantle
cavities of large clams. Although it is widely thought that
the clam-dwelling nemerteans are parasites, in fact, they
eat small crustaceans and worms found in the mantle cavities.
They are probably best considered to be beneficial symbionts
that clean the mantle cavities of pests.
Some of the truly parasitic forms live
on crabs and are egg parasites. On the west coast of North
America, Carcinonemertes species are found infesting
several species of crabs. These are tiny nemerteans that live
on the outside of the crab's carapace. They eat the eggs of
the crabs; each worm can eat an egg or so a day. An infestation
of several dozen worms can effectively eat the entire yearly
spawn of the crab. These particular nemerteans can be of immense
economic importance, and have been credited with single handedly
destroying the economic viability of several Dungeness crab
fisheries along the coast.
Aquarium Occurrences and Care
Several different nemerteans have been
reported over the last couple of years in my forum
on Reef Central. They are all moderately sized animals, a
few inches long, and they probably hitchhiked into aquaria
on live rock. Nemerteans are mostly predatory, and although
a few, notably some large Antarctic
forms, have been seen to be scavengers. It is a pretty
safe bet that any turning up in aquaria will be predatory.
Some of them seem to be fairly specialized in their diets,
peregrina in the Northeastern Pacific has been well
studied and appears to eat mostly one species of polychaete
annelid, Platynereis bicanaliculata. Others may be
less picky in their foods.
I don't think I would consider any of these
animals, a priori, as a decided risk factor for any
aquarium inhabitants. Neither, however, would I consider them
as benign. I like nemerteans and consider them very neat creatures,
but I would not risk them in my reef aquarium. I would put
them in a small dedicated tank. In such a tank, they will
do very well if fed properly. I maintained a large (45 feet
long) worm in a chilled demonstration aquarium in one of my
teaching labs for over six months and it did very well, indeed.
At the end of the class year, I released it back into nature
where it presumably may still reside.
Nemerteans need good water quality! They
produce a lot of mucus and good filtration is a must. The
benthic species also need an appropriate substrate. Those
animals that enter aquaria as hitchhikers on rocks will benefit
from some rockwork in their home. They will generally hide
in the rocks when not hunting. Standard reef temperatures
and salinity are also necessary. Should you find yourself
caring for a nemertean, you are probably on pioneering ground.
While a few species have been studied, the biological interactions
and natural history of most of them are unknown. The small
species may not live lives of very long duration, but the
larger ones might well live several years to decades. Many
of these are exceptionally colorful and attractive animals
so if you find one in your aquarium, don't be in a hurry to
dispose of it. It might be well worth the time and expenditure
of setting up a small species tank.