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Some of the oddest types of animals found
on coral reefs belong to what are called, by biologists, the
Lophoporate phyla. There are several discrete and different
types of animals found in this group, but they all share a
specialized feeding and respiratory structure consisting of
a circular or horseshoe-shaped band of thin tentacles called
a lophophore. Most taxonomic authorities consider that there
are three to five discrete groups of animals with such a structure,
and I will be describing two of those groups, the Phoronida
and the Brachiopoda.
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Figure
1. Phoronopsis harmeri from the N.E. Pacific,
showing the animal extending from its tube and feeding.
The lophophore tentacles are clearly visible as is the
red line of the major internal blood vessel.
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A lophophore is a band of ciliated tentacles
found along a ridge slightly elevated from the surface of
the animal. This ridge is centered on the mouth so that the
tentacles are found in a symmetrical pattern on either side
of the mouth. These tentacles are covered with tracts of microscopic
cilia and mucous producing cells creating "food grooves"
to convey tiny particulate material to the mouth. The tentacles
are extended into the water and collect small particles, bacterial
particulates and phytoplankton cells. The particles stick
to the mucus and the mucus is moved by the cilia. The food
grooves run down the inner center surface of each tentacle
and meet a larger "collection" groove at the tentacle
base. The two collection grooves run along the bases of all
the tentacles on each side of the lophophore and one enters
each side of the mouth. This arrangement of mucus, cilia,
food grooves, and tentacles is called, for obvious reasons,
a ciliary-mucous suspension feeding apparatus. The tentacles
of the feather-duster worms have a similar architecture and
function, but unlike the feather dusters, the tentacles of
the lophophorate animals are generally smaller, unbranched,
more numerous for the size of the animal, and they don't have
a feather-like appearance. The tentacles of lophophorates
may appear quite like those of hydroids or most other small
cnidarian polyps; however, there are distinct differences.
The polypoid tentacles lack food grooves, so they can't be
ciliary-mucous suspension feeders, and the lophophore tentacles
lack nematocysts, so they cannot sting.
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Figure
2. The lophophore of a phoronid living in
sediments. The mouth of the animal is located in the
triangular-shaped structure on the inside of the middle
of the "back" of the "C" shaped
ridge of tentacles.
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The animals with lophophores are small,
but complex, beasts. All lophophorates possess organs arranged
into digestive, reproductive, excretory, circulatory and nervous
systems. There are three major groups of lophophorate animals,
each put into one of the major taxonomic groupings called
a phylum. These three phyla are the phoronids, the lamp-shells
(or brachiopods), and the moss animals or bryozoans. The most
successful and widespread of these groups is the Phylum Bryozoa,
and it will be discussed it in a later column. The other two
phyla are the subject of this column.
The Phylum Phoronida
Nobody sets out to purchase phoronids:
they are really a curiosity that may incidentally enter an
aquarium. The Phylum Phoronida is probably the least successful
major animal group in terms of species number and diversity.
There are only two genera, Phoronis and Phoronopsis,
and there are probably less than 30 species known. However,
total species diversity is only one measure of success, for
while there are less than fifty species of phoronids, there
are phoronids in darn near every marine habitat. Occasionally
very abundant, such as along the sand and mud flats of the
California coast, most often they are patchy and rare, but
they are almost always present.
These creatures are all tube-dwelling,
sessile worms. They form their tubes by secretions from the
glandular epidermis of the body surface using a combination
of some organic matrix and inorganic materials such as sand
grains. The tubes are straight and are typically longer than
the animals. Differing species may exist either as solitary
animals or as aggregations, and in some cases they form "colonies"
from asexual reproduction and budding. If they are aggregated,
the tubes are often intertwined into an inseparable mass.
Here is an image of a phoronid worm, with
its tube removed:
http://www.sms.si.edu/IRLSpec/images/Phyl_Phoron.jpg
Here is some information about phoronids,
including some interesting images of the worms and their tubes:
http://www.ucmp.berkeley.edu/brachiopoda/phoronida.html
The body wall is highly muscular, and the
animals are capable of rapid retraction into the tube, which
is often buried in sediments. Typically, the buried bulbous
bottom portion of the worm is anchored deeply in the sediments.
Strong longitudinal muscles run from the bulb to the upper
portion of the animal. When some threat is sensed, the muscles
contract and rapidly pull the animal down into the sediments.
The nervous system has some giant fibers allowing for very
rapid conduction of nervous impulses, and this can result
in the worm being able to retract completely into the sediments
within a few thousandths of a second. The tentacles are not
retractile, and when the animal withdraws into its tube, the
tentacles are simply pulled along with the rest of the body.
Here is a diagram of a phoronid showing
some of the internal structures:
http://www.biology.ucsc.edu/classes/bio136/phoronida/phoronida.gif
The gut is "U-shaped" with the
mouth and the anus on the upper exposed surface of the animal.
The digestive system is regionated with specialized digestive
regions. Phoronids collect particulate organic material with
the lophophore, and their food is rich in diatoms and bacteria.
Unlike mollusks and some other invertebrates, the digestion
of food occurs in the gut cavity and digested food is absorbed
across the gut lining, much as in vertebrates or annelid worms.
They have a well-developed circulatory system, which allows
the distribution of dissolved nutrients to all parts of the
body. They also possess hemoglobin, in corpuscles, as a respiratory
pigment, and the lophophore functions as a respiratory organ
as well as a food gathering one.
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Figure
3. A colonial mass of phoronids living in
a clam shell. The bright white objects are embryos being
protected in the bases of the lophophores. Each lophophore
is about one tenth of an inch across.
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Aquarium Occurrences
Nobody sets out to purchase phoronids:
they are really a curiosity that may incidentally enter an
aquarium. The most likely way for them to enter our aquaria
is either in burrows in live rock or associated with some
other animals. Many species of Phoronis are capable
of burrowing into calcareous rock or shells, and colonies
might well be imported with the rocks. Occasionally, they
are found living with other animals; Phoronis australis
is found living in the tubes of tropical cerianthids, or tube
anemones.
Phoronids are long worms which, like feather
dusters, have a crown of tentacles at the head end. The tentacle
crowns of phoronids are distinctive, but close examination
is required to distinguish them from feather-dusters. Generally,
these are small animals, with a tentacle crown less than a
centimeter in diameter. The tentacles are absolutely straight;
they cannot curl or bend. The most significant diagnostic
feature is their lack of side branches. So, unlike the feather-like
tentacles of the feather dusters, phoronid tentacles are perfectly
straight without side branches. No other worms with tentaculate
crowns have unbranched tentacles. The tentacle crowns may
be white, translucent gray, green, brown or black depending
on the species. When viewed from above, the crowns often look
horseshoe-shaped, usually with the ends of the horseshoe coiled
in on themselves a few times.
These are suspension-feeding animals,
living on fine plankton and will need supplemental small plankton,
such as phytoplankton, to survive in captivity. Phoronids
should be kept with their associated animals or rock and should
not be manipulated as they tend to break easily. If the species
is Phoronis australis, living on cerianthid tubes,
a deep sand bed will provide both the appropriate habitat
and supplemental food in the form of suspended bacterial particulates.
For species acquired on live rock, supplemental microplankton
feeding will be necessary. Given an appropriate habitat and
sufficient food, these animals are capable of living indefinitely
in aquaria.
The Phylum Brachiopoda
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Figure
4. Some individuals of the temperate articulate
brachiopod, Terebratalia transversa, on a rock.
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This phylum has a large fossil record (30,000
species) but relatively few living species (250 species).
Some ancient brachiopod fossils from the species Lingulella
have been used to define the beginning of the Cambrian period
about 525 million years ago and are effectively identical
to those of the living Lingula, which makes Lingula
a true living fossil. It is rather mind-boggling to consider
that this species may not have changed much in morphology
for over 500,000,000 years.
All brachiopods are marine animals, and
most are found in relatively shallow water. They have bivalved
shells, similar to clams. Unlike the mollusks, however, where
the shells are found laterally on each side of the animal,
brachiopod shells are dorsal and ventral to the body between
them.. In some of these animals, one of the shells has a hole
in it for some tissue to pass through. These particular shells
may look a bit like an ancient oil lamp, and have given the
whole group the common name of "lampshells." Brachiopod
individuals may be large with the shells of some species that
attach to rocks reaching up to four inches (10 cm) across.
While the free-living forms typically have smaller shells,
they have an equally long stalk or peduncle extending into
the sediments.
Brachiopods are probably closely related
to the phoronids discussed above, and may be considered to
be basically phoronid-like animals enclosed in a pair of shells.
They have an exceptionally complex lophophore enclosed within
the valves. Where the phoronid crown of tentacles is generally
in the shape of a simple horseshoeor slightly coiled horseshoe,
that of the brachiopods is much larger, and looks like a horse-shoe
with the ends rotated and spiraled, so that the tentacles
on each lateral branch look something like the tentacle arrangement
in a Christmas tree worm. Such a structure is structurally
very complex, and it is quite difficult to understand just
exactly how it works.
Brachiopods have two distinct functional
modes of living; they are either attached to rocks or burrowed
into sediments. There are also two taxonomic types of brachiopods
which are distinguished by their internal anatomy. To make
things a bit difficult, however, these two internal divisions
don't correspond to the two functional modes of life. Both
taxonomic groups may be found attached to hard substrata,
but only one type can burrow into sediments.
All brachiopods are suspension-feeding
organisms. The gut has a large digestive gland extending from
the stomach, and unlike the situation in the phoronids, digestion
appears to occur largely within the cells of some digestive
glands which sit adjacent to the gut. In one group of brachiopods,
those with shells connected with a hinge, the gut is blind-ending
and lacks an anus; in the other group, the shells are not
connected yb a hinge and there is an intestine and anus. As
in the phoronids, brachiopods are large enough to require
a circulatory system, with a pulsating vessel which moves
the blood, to move nutrients, wastes, and dissolved gases
around the inside of the body. Some of them have a blood pigment
present in cells or corpuscles. They have no specific respiratory
system, but the blood circulates in the lophophore and, as
in the phoronids, gas exchange undoubtedly occurs across the
tentacle surfaces. Sexes are separate and they generally have
planktonic larvae, although some may brood their young in
the shells. The nervous system in both brachiopod groups is
in the typical invertebrate pattern with the major nerves
connected to a nerve ring found surrounding the esophagus.
This ring has a small ventral ganglion which is dignified
with the name of brain. Sensory reception is poorly known,
but they have chemosensory and photosensory capabilities,
even though no discrete sensory organs are known.
Here is a link to a diagram of an articulate
brachiopod showing some of the internal structures: http://www.biology.ucsc.edu/classes/bio136/brachiopoda/brachiopoda.gif
Here is a link to an articulate brachiopod
opened to show the internal structures, as well as to provide
a good discussion of articulate brachiopod structure and anatomy:
http://www.ucmp.berkeley.edu/brachiopoda/brachiopodamm.html
The shells may be made of either calcium
phosphate or calcium carbonate and are opened and closed by
muscle action, from different muscle groups. Contrast this
with the bivalved mollusks where the shells are closed by
muscles, but opened by the hinge ligament.
For a wealth of brachiopod information,
images, and data, see this website:
http://emig.free.fr/BrachNet/
Articulate Brachiopods
or Brachiopods Whose Shells Are Connected By A Hinge
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Figure
5 . An articulate brachiopod, Hemithiris psittacea,
from the North Pacific. The animal is about one inch
wide and is fastened to the rock by a fleshy stalk.
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Articulate brachiopods look superficially
like clams, as they have two shells. The doral/ventral orientation
of the shells results in one characteristic that makes these
particular brachiopods easy to recognize, and which may be
used to distinguish them from clams. In clams the two shells
are generally very similar in shape, while in brachiopods,
they are always dissimilar. Additionally, while clams may
be fastened to rock by proteinaceous byssal threads or calcium
carbonate cement, articulate brachiopods are ALWAYS fastened
to the substrate with a fleshy stalk, and they are unable
to move from the position to which they are cemented.
Articulated brachiopods must remain attached
to their rock for survival. If removed from the rock, they
will die shortly thereafter. They are typically found in areas
of relatively high current, often on rock rubble. If such
rubble is collected and used as live rock in an aquarium,
the brachiopod comes along for the ride.
One common tropical species is Frenulina
sanguinolena. This particular species may be recognized
by its basically orange, tan or golden shell coloration, often
with a contrasting zigzag pattern of darker coloration, and
reaches a width of 2 to 3 cm. However, there are number of
other brachiopods with similar shell shapes and colors.
Here is an image of Frenulina sanguinolenta:
http://member.nifty.ne.jp/angursa/gallery/wa/11122/L.jpg
Articulate brachiopods feed by using the
cavity within the shells to assist the lophophore in collecting
their food, and need to be oriented correctly in relation
to current flow to filter efficiently. Frenulina sanguinolena
needs rather strong, consistent, currents blowing over it.
It will pivot on its stalk or peduncle to orient for maximum
feeding efficiency. Articulate brachiopods will need dietary
supplements of phytoplankton in most aquaria, and will benefit
from being in an aquarium with a well-established deep sand
bed community where planktonic bacterial aggregates are produced
by the sand bed fauna.
Inarticulate Brachiopods
or Brachiopods With Their Shells Connected Together Only By
Muscles
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Figure
6. An adult and several juvenile inarticulate brachiopods
fastened to a rock. These are Neocrania californica,
and the adult is about an inch across.
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Probably the most abundant of the tropical
inarticulate brachiopods, Lingula reeve is very abundant
in many tropical sand and mud flats where it lives buried
in the sand within a burrow, with just the tip of the shells
exposed. Lingula is an inarticulate brachiopod, and
these animals differ from articulate brachiopods in having
symmetrical shells. It may be recognized by its paired glossy
greenish or greenish-yellow to tan shells, which have a long
stalk or peduncle projecting from their rear. The stalk is
the burrowing and anchoring organ, and it allows the animal
to move up and down in its burrow.
For an image of Lingula, see this
link: http://paleo.cortland.edu/tutorial/Brachiopods/Brachiopod%20Images/lingula.GIF
For an image of fossil Lingula or Lingulella
follow this link:
http://www.library.csi.cuny.edu/dept/as/fossil/Lingula.jpg
Here is an image of Glottidia, a
Gulf of Mexico animal related to Lingula:
http://www.gulfspecimen.org/photographs/lo-580.GIF
As with other totally infaunal animals,
Lingula is a very decidedly null pet. On the other
hand, as a curiosity for the scientifically inclined, it is
a VERY neat animal, indeed. Shells effectively identical to
modern Lingula are found in some of the earliest fossil
beds dating from around a half a billion years old. This makes
this species a living fossil, and as such makes an interesting
addition to an aquarium.
As with many suspension-feeding animals,
individuals of Lingula will need supplemental small
plankton such as phytoplankton, and they should be housed
in a tank with a deep sand bed. The deep bed will provide
both the appropriate habitat and supplemental food in the
form of suspended bacterial particulates. Given a deep sand
bed, and sufficient food, these animals are capable of living
indefinitely in aquaria.
Conclusion
The larger Lophophorates, such as brachiopods
and phoronids, could make interesting additions to either
"species" tanks or to more diverse reef community
tanks. Phoronids are occasionally food for some fishes such
as bat rays, but probably are not eaten by most reef aquarium
animals, and should persist in reef tanks if given sufficient
food. Sand- or mud-dwelling brachiopods in the genera Lingula
or Glottidia are common in shallow areas in the tropics,
where they live in burrows. They can be kept in a tank with
a deep sand bed, and certainly could easily be collected.
However, I don't know of anybody that has actually been sold
one. If they were not purchased specifically, it is possible,
but unlikely, that individuals could enter the tank with some
uncleaned tropical sand. They do survive well in captivity,
however, as numerous individuals have been kept in research
aquaria for varying lengths of time. In contrast, the attached
brachiopods are mostly found on rocks, and have been brought
into aquaria with live rock, where they survive well. I have
maintained temperate species in tanks for several years. In
nature, brachiopods appear to have a long life expectancy,
20 years or more. Provided they get the appropriate food,
there is no reason to suggest that they won't live a long
life in a reef aquarium.
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