ISSN 10630740, Russian Journal of Marine Biology, 2010, Vol. 36, No. 2, pp. 109–116. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.N. Temereva, V.V. Malakhov, 2010, published in Biologiya Morya.
INVERTEBRATE ZOOLOGY
Filter Feeding Mechanism in the Phoronid Phoronopsis harmeri
(Phoronida, Lophophorata)1
E. N. Temereva and V. V. Malakhov
Moscow State University, Moscow, 119991 Russia
email: [email protected]
Received September 15, 2009
Abstract—Phoronids, like other Lophophorata (Bryozoa and Brachiopoda) are filter feeders. The lopho
phore performs various functions, the most important of which is the collection and sorting of food particles.
The mechanism of sorting has been well studied for many other groups of invertebrate, but until now it has
remained obscure for phoronids. With the help of functional morphology data we are proposing a possible
scheme of sorting in phoronids on the example of Phoronopsis harmeri. The lower limit of the particle size is
defined by the distance between laterofrontal cilia of tentacles and equals 1.2 µm. Larger particles are trans
ferred by frontal cilia to the basis of the tentacles, where they pass into the lophophoral groove. The distance
between the epistome and the external row of tentacles regulates the upper limit of the particle size that are
suitable for food. Only particles whose size does not exceed 12 µm get into the lophophoral groove and further
into the mouth. Larger particles collect in the space above the epistome and are removed from the lopho
phore. The size of the food particles that phoronids consume by filtration lies in a range 1.2–12 µm. These
are bacteria and small phytoplankton organisms. At the same time the significant individual mobility of the
phoronid tentacles plays an important role in the expansion of the pabular spectrum to large inactive zoop
lankton and phytoplankton organisms reaching a size of 50–100 µm.
Key words: Phoronids, sorting, lophophore, filter feeder, Phoronopsis harmeri.
DOI: 10.1134/S1063074010020057
1
Phoronids constitute a small (in terms of species
number) phylum of invertebrate animals, the Phoron
ida. Adult phoronids have a sessile mode of life; they
are tube dwellers and live within the thickness of solid
substrates or in soft grounds. Only the anterior part of
the body protrudes from the tubes; it bears the lopho
phore, a food capturing apparatus consisting of cili
ated tentacles. The lophophore of phoronids performs
different functions: gas exchange occurs through the
thin tentacular integument, tentacles bear mechan
oreceptor cells, in some phoronids the eggs are incu
bated in the lophophore, etc. However, the major
function of the lophophore is a trophic one; it carries
out water filtration and extraction of food particles [7,
8, 10]. In phoronids this process is still poorly studied
and many important aspects still remain uncertain. To
date, it has been revealed that phoronids belong to
organisms with an socalled upstream filtration pat
tern; i.e., water flow enters lophophore from above,
reaches the space between the distal extremities of the
outer and inner rows of tentacles and is discharged
through gaps between the lateral margins of the tenta
cles [11, 16–20, 22]. However, the available literature
provides no information about the dietary range of
1 The article was translated by the author.
phoronids, it is not known what they eat and how the
selection between edible and inedible particles occurs.
The target of this project was a morpho–functional
analysis of lophophore structure in phoronids, using
Phoronopsis harmeri Pixel, 1912 and the understand
ing of filtration mechanisms.
MATERIALS AND METHODS
As the materials for this project we used adult spec
imens of Phoronopsis harmeri collected in Vostok Bay
of the Sea of Japan, around the Vostok Marine Biolog
ical Station (Institute of Marine Biology FEB RAS) in
August–September of 1996–1998. The animals were
removed from tubes, fixed in a 4% formalin solution in
filtered seawater, rinsed from the fixative in distilled
water and stored in 70% ethanol. The heads with the
lophophores were then treated in an ascending ethanol
series, butanol, xylene, and paraplast. Finally, the
samples were embedded into paraplast and cut into
5 µm sections with a Leica RM 2125 rotational micro
tome. The sections were stained with Caracci hema
toxylin and embedded into Canada balsam. Alto
gether two sagittal series, two frontal series and seven
transverse series were prepared. The sections were
examined under a Zeiss AxioPLAN2 light microscope
109
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TEMEREVA, MALAKHOV
ep
lg
lg
ep
in
n
n
(c)
(b)
ep
lg
iк
lg
lс
er
(a)
Fig. 1. Histological sections through the lophophore of Phoronopsis harmeri. a, A transverse section through the base of the lopho
phore (arrow shows the medial line of the epistome; the circle of tentacles is broken in this area); b, A frontal section through the
side of ascending branch of intestine (arrowheads show the epidermis of the bases of the tentacles of the inner row); c, A sagittal
section through the epistome, mouth and a part of the descending branch of the intestine (muscle fibers traversing the cavity
inside the epistome are marked). Scale bar: a, c, 100 µm; b, 20 µm.
and photographed with an AxioCam HRm digital
photo camera.
Designations for figures afz) abfrontal zone;
ep) epistome; epa) the space above epistome; er) exter
nal row of tentacles; fz) frontal zone; in) ascending
branch of intestine; ir) internal row of tentacles;
lc) lophophoral concavity; lfz) laterofrontal zone;
m) mouth; n) nephridium canal; and tz) transitional
zone.
RESULTS
The structure of the lophophore in phoronids has
already been described elsewhere [2, 7], therefore we
will confine ourselves to information that is necessary
to understand the following concepts.
The lophophore of Phoronopsis harmeri represents
the part of body that bears tentacles; the latter fringe
the mouth opening and are arranged into two rows, an
outer one and an inner one (Fig. 1a). The outer row is
located in front of the mouth and the inner one,
behind the mouth. On lateral sides the outer and inner
rows are connected to each other; the inner row is bro
ken at the midline of the dorsal side of body, i.e., in the
zone where new tentacles arise (Fig. 1a).
In a tangential section through the middle of a
lophophoral tentacle as many as five zones of the epi
dermis can be distinguished, which differ from each
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FILTER FEEDING MECHANISM IN THE PHORONID
111
ir
er
er
ir
ep
lg
epa
ep
lg
(b)
(c)
afz
tz
l
lfz
(a)
fz
Fig. 2. Histological sections through a tentacle (a) and epistome (b, c) of Phoronopsis harmeri. a, A transverse section through the
middle part of a tentacle; b, c, transverse sections through the lophophoral groove in latero–dorsal (b) and latero–ventral (c) parts
of the lophophore. Scale bar: a, 50 µm; b, c, 10 µm.
other in the peculiarities of their histological organiza
tion and the density of flagella (Fig. 2a) (also see [2]).
The latter depends in phoronids on the shape of
monociliary cells (spindleshaped, cubical, or flat
tened) and, as a consequence, of their density. Facing
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the mouth is a frontal zone; flagella in this area usually
are more densely arranged than in other zones. In
transverse sections, each of the laterofrontal zones is
composed of one or two cells bearing long flagella. In
lateral zones numerous long flagella occur. The transi
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TEMEREVA, MALAKHOV
tional zones face neighboring tentacles; flagella are
very rare there. In the abfrontal zone, which faces out
of the mouth opening, flagella are scarce (Fig. 2a).
In Ph. harmeri the frontal zone in tentacles of the
outer row is densely covered with setae, from the tips
to the bases of the tentacles, whereas in tentacles of the
inner row the ciliary cover does not reach the tentacle
bases; the gap between the proximal margin of the cil
iary cover and tentacle basis is 40–50 µm (Figs. 1a,
1b). The epidermis of the frontal zone at the base of
tentacles is composed of cubical or flattened cells and
differs significantly from the frontal epidermis of the
distal extremities of the tentacles (Figs. 2b, 2c). This
peculiarity of ciliary cover organization in the tenta
cles is of great interest in order to understand filtration
mechanism function in phoronids (see below).
The mouth opening is crescent shaped; its angles
are somewhat curved onto the dorsal side (Fig. 3a).
Symmetrical lophophoral grooves lined with flagella
pass between the rows of tentacles and go toward both
the mouth angles (Figs. 1a, 3a). In sections through
latero–dorsal parts of the lophophore (the most
remote part from the mouth) the lophophoral groves
form a triangle, whose apex is directed downward (Fig.
2b); in the latero–ventral parts (the closest to mouth)
it takes the form of a flask with an expanded basis and
a relatively narrow neck (Fig. 2c). At the side of the
inner row of tentacles the mouth and lophophoral
groove are covered by the epistome, which represents a
fold of the epidermis. The height of the epistome is the
greatest above the mouth (Fig. 1c) and gradually
decreases in both directions from the mouth opening
(Figs. 1b, 2b, 2c). In latero–dorsal parts of the lopho
phore the epistome takes the shape of a small epider
mal tubercle (Fig. 1b).
The epistome covers the lophophoral tubercle,
leaving a small gap between the tentacles of the outer
row and the ventral wall of the epistome (Figs. 2b, 2c).
Inside the part of the epistome located above the
mouth there is a pronounced cavity traversed by sparse
muscle fibers (Fig. 1c). In the lateral and latero–dorsal
parts of the epistome the inner cavity is absent and no
muscle fibers are revealed. In sections of fixed speci
mens the diameter of the lumen in the lophophoral
groove, through the bulk of its length, equals 10–
12 µm (Figs. 2b, 2c). The high density of muscle fibers
in the epistome part located above the mouth opening
allows us to suppose that in this area the epistome is
movable and can regulate the diameter of the entrance
to the mouth (Fig. 1c). In sagittal sections this diame
ter equals 45–50 µm.
Water movement in the lophophore is ensured by
the beating of lateral cilia, so that the water flow leaves
the tentacular apparatus in two directions, through the
gaps between tentacles of the outer and inner rows
(Fig. 3b). Obviously, in phoronids, as in other lopho
phoral animals (bryozoans and brachiopods), these
are latero–frontal flagella that perform the function of
a filter arresting particles, that are large than the dis
tance between these flagella, within the lophophore
(Fig. 4a). Phoronids have monociliary cells, so the dis
tance between neighboring laterofrontal cilia can eas
ily be measured; it equals the apical diameter of the
ciliary cells and, in Ph. harmeri, averages 1.2 µm.
Thus, phoronids can seize particles that are larger than
1.2 µm. Particles arrested by the laterofrontal ciliary
mesh are then bounced onto the frontal surface of the
tentacles and transported to their bases (Fig. 4a).
The holding of the particles on the frontal surfaces
of the tentacles is obviously favored by mucous secre
tion provided by the glandular cells of the frontal zone.
The particles that are filtered out are transported by
frontal cilia toward the bases of the tentacles (Fig. 4b).
The particles transported along the frontal surface of
the outer tentacles pass into the lophophoral groove
through a gap between the tentacles of the outer row
and ventral wall of the epistome (Fig. 4b). The outer
and inner walls of the lophophoral groove are limited
by the basis of the tentacular lamella and the epistome
fold respectively. From inside, the lophophoral groove
is lined by a flagellar epithelium containing numerous
glandular cells, so the mucociliary mechanism that is
used to transport particles along the frontal side of ten
tacles also functions in the lophophoral groove, where
the particles are transported to the mouth (Fig. 3a).
The epistome fold obviously plays an important
role in the determination of the maximum size of the
particles that are suitable as diet items for phoronids.
In most of the lophophore length the diameter of the
gap between the epistome and the tentacles of
the outer row equals, as has been mentioned above,
10–12 µm; in living specimens the gap is evidently
more or less the same size, as the epistome in these
areas is devoid of muscle fibers. Larger particles can
not enter the lophophoral groove and are accumulated
in the supraepistomal space, inbetween the dorsal
wall of the epistome and the tentacles of the inner row
(Fig. 4b).
The destiny of the particles transported along the
frontal surfaces of the tentacles of the inner row is
somewhat more difficult (Fig. 4b). Obviously, these
particles go up to the end of the ciliary band stretched
along the frontal surface of tentacles of the inner row
(i.e., to a distance of 40–50 µm from the bases of the
tentacles), where they fall from the ciliary epithelium
into the food groove located between the tentacles of
outer and inner rows and, finally, are seized by the cilia
of the frontal zone of the outer row of tentacles
(Fig. 4b). The following speculations can be made
with respect to the particles that do not enter the
lophophoral groove and appear in the supraepistomal
space. The dorsal wall of the epistome is lined with a
monociliary cubical epithelium, whose cilia are beat
ing toward the midline. Due to the beating of the fla
gella large particles are gradually transported to the
midline of the epistome, where the row of tentacles is
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FILTER FEEDING MECHANISM IN THE PHORONID
113
lg
lg
epa
epa
lc
ep
m
fz
(A)
(B)
Fig. 3. Schemes of movement of food particles and water currents in the lophophore. a, a view from above (the scheme of the
lophophore is simplified, only the bases of the tentacles are shown; thin interrupted arrows show the movement of edible particles
along the lophophoral grooves into the mouth; thick interrupted areas show the movement of large inedible particles along the
dorsal side of the epistome, in supraepistomal space); b, view from the oral side (arrows show the direction of water currents).
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TEMEREVA, MALAKHOV
(A)
er
ir
epa
ep
(B)
lg
Fig. 4. Scheme of the movement of water currents and food particles along the tentacles of Phoronopsis harmeri. a, Scheme of a
transverse section through a part of outer row of tentacles (solid arrows show direction of water currents; interrupted arrows indi
cate the movement of arrested food particles); b, Scheme of a longitudinal section through the inner and outer rows of tentacles
(solid arrows show direction of water currents; interrupted arrows indicate movement of small food particles along the frontal sur
faces toward the lophophoral groove; thick interrupted arrows show the movement of large inedible particles).
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FILTER FEEDING MECHANISM IN THE PHORONID
broken (Fig. 1a) and discharged into the lophophoral
concavity, from which they are removed due to the
beating of cilia on the abfrontal surfaces of tentacles
(Fig. 3a).
The filtration apparatus of phoronids is very labile,
as it consists of isolated tentacles and each of these can
move. The solid row of tentacles can easily open, while
the tentacles can bend both outward and inward. It is
not improbable that the cleaning of the supraepistomal
space could also be performed due to the outward
bending of the tentacles of the inner row.
The individual mobility of the tentacles obviously
plays an important role, allowing phoronids to expand
their dietary range, as they can feed not only by filtra
tion. Observations on living specimens show that the
circumoral tentacles sometimes can capture large food
particles and push them toward the mouth opening.
For example, we observed the seizure and ingestion of
polychaete metatrochophores, whose diameter
exceeded 100 µm.
DISCUSSION
Filtration mechanisms of phoronids were studied
in three species, Phoronis vancouverensis Pixell, 1912;
P. muelleri SelysLangchamps, 1903; and Phoronopsis
harmeri (see [11, 19]). For this purpose, experiments
were performed using colloidal solutions and spherical
particles. The movement of the particles within the
lophophore was recorded in the course of direct obser
vations on the animals with following examinations of
the total preparations of lophophore [11] and using the
methods of vital videomicroscopy [19].
It has been shown that in phoronids the water flow
propelled by the lateral cilia of tentacles enters the
lophophore from above and leaves it through gaps
between tentacles. Thus, the phoronids have been
referred to as animals with an upstream filtration pat
tern. Besides the phoronids, the upstreamfiltrators
include bryozoans, brachiopods, and pterobranchs
[16–18, 20]. Water flow brings different particles of
small or large diameter into the lophophore. What is
the destiny of these particles? According Gilmour
[11], in P. vancouverensis and Ph. harmeri both large
and small particles, together with afferent water cur
rents, enter the space between the outer and inner rows
of tentacles. Large particles come to the frontal sur
faces of the tentacles; along these they are transported
toward the apexes of the tentacles and are removed
from the lophophore. Large particles are often agglu
tinated together by mucous secretions of the latero–
frontal glandular cells of the tentacles to form cords
that are also removed from the lophophore. Small par
ticles enter the lophophore at a certain distance from
the frontal surfaces of the tentacles and are accumu
lated on the lower surface of the epistome; the numer
ous cilia of the epistome push the particles into the
mouth opening [11].
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According to Risgård [19], in P. muelleri the parti
cles brought by water currents are arrested by a mesh
consisting of immovable latero–frontal cilia. Then
small particles come to frontal surface of the tentacles,
where cilia beat towards the mouth. When large parti
cles touch a tentacle it initiates their bending and they
are pushed into the mouth due to the individual activ
ities of the tentacles. When large particles fall onto the
tentacles the beating rhythm of the lateral cilia is often
disturbed.
The literature data and results of this paper allow us
to propose the following filtration mechanism in
phoronids. Lateral cilia of tentacles propel water cur
rents that enter the lophophore from above and leave it
through gaps between tentacles. Particles brought by
the water are arrested by a mesh consisting of the lat
ero–frontal cilia; the distance between the latter
determines the minimum size of the captured parti
cles. In Ph. harmeri this distance equals 1.2 µm, thus,
phoronids can seize even bacteria (except the smallest
ones) and the smallest particles of detritus and organic
colloids. The arrested particles are transferred to the
frontal surfaces of the tentacles and are transported
toward their bases.
The mechanisms of particle sorting into small (edi
ble) and large (inedible) are pronounced, to a greater
or lesser degree, in all filtrators. In bivalve mollusks a
triangular in the crosssection groove runs along the
margin of each hemibranch [3, 4]. Small particles
reach the inside of the groove and are transported
toward the mouth opening, whereas large particles fall
from the external margins of the groove and later are
removed from the mantle cavity as pseudofaeces. In
some bivalve mollusks, along the margins of the food
groove there are cilia that arrest particles that are too
large [3, 4]. In the polychaetes Sabellidae a jagged
groove that is triangular in cross section runs along the
axis of each gill filament. The smallest particles that
penetrate the groove are used as food; the particles of
medium size are used as the building material for the
tube; while the largest particles are rejected [15]. In
some gymnolaemate bryozoans ciliary grooves run at
the bases of the tentacles. Small food particles fall onto
the bottom of the groove and are transported into the
mouth, while larger ones remain at the margin of the
groove and are discharged from the lophophore [12].
In brachiopods a socalled brachial fold runs at the
bases of tentacles; it borders the brachial groove. The
greatest size of the edible particles that are transported
along the brachial groove is determined by the diame
ter of a narrow slit between the tentacles and adjacent
brachial fold [1].
A similar picture is observed in phoronids. The
upper size limit of particles that come into the mouth
of phoronids is determined by the distance from the
epistome fold and the tentacles of the outer row. In Ph.
harmeri this distance equals 10–12 µm. Thus, the
dietary range of phoronids is confined to organisms
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TEMEREVA, MALAKHOV
that are 1.2–12 µm in size. These are bacteria and dif
ferent organisms of phytoplankton. Semidigested
remnants of diatoms and shells of dinoflagellates are
often encountered in the lumen of the intestine and in
the stomach cells of phoronids [6, 9, 13, 21].
The feeding mechanisms of phoronids are not
restricted to filtration. The great individual mobility of
the tentacles allows phoronids to seize and push large
food particles that exceed the size of the slit between
the epistome and the tentacles of outer row into the
mouth opening. It is known that the individual activi
ties of the tentacles play and important role in the sei
zure of food particles in bryozoans. Active hunting for
relatively large prey has been described for cheilosto
mate [5, 22, 23] and phylactolaemate [14, 24] bryozo
ans.
ACKNOWLEDGEMENTS
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The project was supported by Russian Foundation
for Basic Research (grant no. 080400991) and Rus
sian Federal Agency of Science and Innovations (grant
no. 02.740.11.0280).
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Tube, Physiology of Digesting in Sabella povonia,
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