Flagellar movement of the sessile flagellates Actinomonas ...

Flagellar movement of the sessile flagellates
Actinomonas, Codonosiga, Monas,
and Poteriodendron
4°5
By M. A. SLEIGH
(From the Department of Zoology, The University, Bristol 8)
Summary
Water currents set up by flagellar activity are used to bring food particles to the
body in each of the sessile flagellates Actinomonas, Codonosiga, Monas, and Poteriodendron.
The water currents produced by the 4 organisms are all somewhat different,
and, while that set up by Codonosiga is in the expected direction with water flow
from the flagellar base towards the tip, the currents set up by the other 3 forms flow
from the tip towards the base. In all 4 types the flagellar movements take the form
of plane sinusoidal undulations propagated from the base of the flagellum towards
its tip, but the different types show adaptive modifications according to the pattern
of water currents required. The rates of beat of the flagellum (range 30 to 50 cycles/
sec) and the speeds of propagation of the contraction wave (range 100 to 600 /x/sec)
did not differ sufficiently to explain different current patterns. It is suggested that
the 'unexpected' direction of current flow in 3 of the types may be the result of the
presence of flagellar mastigonemes; these are known to be present in the chrysomonad
phytoflagellates, to which group Monas and probably also Actinomonas and Poteriodendron
belong. Attention is also drawn to the peculiar mode of coiling and unrolling
of the flagellum of the bicoecid Poteriodendron.
Introduction
THE movement of flagella has been studied in few organisms, and this lack
of knowledge is reflected in the confused accounts given in textbooks. Flagella
can evidently produce movement of water in a variety of ways: sometimes they
act as pushing organelles (pulsella), trailing behind the organism as it moves,
and in other cases they may act as pulling organelles (tractella), when they are
held in front of the organism and draw it along. The pushing action of flagella
is well known, in spermatozoa for example, and has recently been described
for the longitudinal flagellum of the dinoflagellate Ceratium (Jahn, Harmon,
and Landman, 1963), but few convincing descriptions of flagella acting as
tractella have been published until recently.
Both Lowndes (1941,1943) and Brown (1945) have discussed the movement
of the flagellum of those flagellates in which the flagellar activity causes a
gyration of the anterior end of the body, which 'screws' the organism through
the water. Observations on Trypanosoma by Jahn and Fonseca (1963) and on
trypanosomes and Strigomonas by Holwill (1963) have shown that in these
cases the propagated wave of contraction in the flagellum travels from the tip
of the flagellum towards the basal body when the organism moves with the
flagellum in front, so that the way in which these flagella act as tractella is
fairly obvious. The action of the flagellum of Chromulina described in a
preliminary report by Jahn, Landman, and Fonseca (1963) is much less
IQuart. J. micr. Sci., Vol. 105, pt. 4, pp. 405-14, 1964.]

406 Sleigh—Flagellar movement of sessile flagellates
obvious; high-speed cinematograph studies have shown that, although the
flagellum is held in front of the body as it moves, and acts as a tractellum,
the waves of contraction pass from the flagellar base towards the tip. In fact, the
flagellum is working in a similar manner to that described by Lowndes (1944)
for the longer flagellum of Monas, which was found to move particles towards
the flagellar base while the waves of contraction passed along the flagellum
from base to tip; Lowndes believed that the flagellum moved in a spiral, but
the movement of the flagellum of Chromulina takes place in a single plane.
Jahn, Landman, and Fonseca suggest that flagellar mastigonemes may play
a part in producing movement in a direction opposite to that expected.
Studies by high-speed cinematograph methods have shown that the movement
of the flagellum usually takes place in a single plane, but in the transverse
flagellum of Ceratium a helical movement of the flagellum has been observed
(Jahn, Harmon, and Landman, 1963). The motion of sperm tails of the sea
urchin and the bull is also believed to take the form of plane sine waves (Gray,
}
This study concerns observations on 4 sessile flagellates in which the
flagellum is held extended from the body and causes water currents from which
food particles are collected. The water currents caused have previously been
described in some cases, and occasional attempts have been made to describe
the flagellar motion. Descriptions relevant to the forms studied will be mentioned
later.
Materials and methods
The flagellates studied here were found in moss from a fresh-water stream
known as Alphin Brook, near Exeter, Devon, during June 1963, with an
abundance of other Protozoa, both ciliates and amoebae as well as flagellates.
Confirmatory observations have been made on material from the River Avon
near Bristol. These organisms were studied by phase-contrast microscopy
and also under dark-ground illumination in order to investigate their structure
and the motion of the water-borne particles that indicated the effects of
flagellar activity. The movement of the flagella themselves was studied under
stroboscopic illumination; this permitted an examination of the form of
flagellar beating, measurement of the frequency of beat, and, in some cases,
photography of the flagellum in action. These photographs have too little
contrast for reproduction, but form the basis for the drawings in this paper
and for measurements of the length of the waves of contraction.
Observations
The organisms are not generally very well known, and the structure of each
will therefore be described as far as is relevant here; after the description in
each case an account will be given of the motion of the flagellum and the water
movements that the flagellum produces. The different patterns of flagellar
movement will be compared later.

Actinomonas
Sleigh—Flagellar movement of sessile flagellates 407
This is an organism of uncertain affinities, having both heliozoan and
flagellate characters. The genus was originally described by Saville Kent
(1880) on the basis of two marine species, each with a single flagellum and
many fine straight pseudopodia. One fresh-water species, A. vernalis, with
few branched capitate pseudopodia, was described by Stokes (1888) and
another fresh-water type, A. radiosa, with 4 to 6 pseudopodia was described
by Roskin (1931). The description of
A. mirabilis by Greissmann (1914) from
marine collections at Roscoff is the most
complete, but refers to an organism that
is rather smaller than Kent's species of
the same name and has fewer pseudopodia;
it corresponds more closely with
Kent's A. pusilla. The fresh-water
specimens used in this investigation
agree closely with Greissmann's description
of A. mirabilis, though the freshwater
forms have fewer pseudopodia.
The genus Actinomonas is often included
in the Order Proteomyxida (see,
for example, Tregouboff, 1953), but
Pascher (1918) has suggested a relationship
with the chrysomonad flagellates,
since leucosin is a storage product.
The form of the body and the distribution
of the appendages are shown
in fig. 1. The almost spherical body
(diameter 8 to 10 p) carries a single flagellum
(length 20 to 30 /JL) at one pole, and
at the other pole there is a very slender
flagellum
filopodia
FIG. 1. Actinomonas, showing body form
and the water currents caused by flagellar
beating.
stalk (up to at least 60 (j, long) which attaches the body to a piece of debris or
plant material. This stalk is usually rigid, but is also contractile in the sense
that it suddenly bends, usually near the end that is attached to the substrate,
and carries the body to a new position. Contractions of the stalk were seen to
be especially violent when the organism was subjected to the bright illumination
required for dark-ground observations, and they caused rapid jerky movements
of the whole body when the stalk was detached from the substrate. The
very fine filopodia (length 10 to 15 fi) occur in two groups, a ring of 6 to 8
around the flagellar base, and another ring of about 4 above the stalk; they
are apparently straight and permanent even when food is being ingested.
Both Kent and Greissmann observed that the flagellum was held out stiffly
from the body and caused water movements that can be seen in the movement
of particles. The water circulation caused is shown in fig. 1; currents travel

408 Sleigh—Flagellar movement of sessile flagellates
towards the body from the region of the flagellar tip, and food particles
become entangled in the filopodia. Small amoebae were seen to be caught in
this way and carried down the filopodia to be ingested in the body.
The flagellar beat is rapid and regular, and under stroboscopic illumination
the form of beat is seen to be a sine wave with almost constant wavelength and
amplitude along its length (there is a slight tendency to an increase in both
towards the distal end). Sometimes the flagellum appears as shown in fig. i
when the sine wave is seen from the side, and sometimes the flagellum appears
as a straight line that is seen as a dotted line at a particular optical level, when
the sine wave is seen from the edge; evidently the beat takes place in a single
plane. In observations on a number of specimens the waves of bending were
always seen to pass up the flagellum from base to tip; it is certain that this
observation is not a stroboscopic artifact. The frequency of beat at i8° C was
on average about 50 cycles/sec, and the length of the waves of contraction was
about 7 to 10 JX, so that the contraction waves were propagated at about 350
to 500 /x/sec.
Codonosiga
This organism is well known from the studies of Kent (1880) and Lapage
(1925), and the species used here, C. botrytis, is a common fresh-water choanoflagellate.
Lapage has thoroughly described the organism and the water
currents that are produced by flagellar activity, but he did not venture to
describe the movement of the flagellum in any detail because it was normally
too rapid to be seen. Hollande (1952) has discussed evidence relating the
choanoflagellates to the chrysomonad phytoflagellates, but found it inconclusive.
The individuals observed were attached to debris or moss by slender rigid
stalks up to 100 fj, or more long; frequently there were several animals on a
single stalk. Each body (about 10 /u, long) carried a single flagellum (length 25
to 30 p), whose base was at the centre of a collar (about 8 to 10 JJL tall). It was
long believed that the collar was a conical membrane, but electron microscope
observations of Fjerdingstad (1961) have confirmed an alternative belief—that
the collar is made up of a palisade of microvilli, a structure that is shared with
the choanocyte cells of sponges.
Particles borne by water currents can be seen to flow in a three-dimensional
vortex in which the water moves in below the flagellar base and is swept away
along the axis of the flagellum towards its distal end (fig. 2, A). It is believed
that water passes through the meshwork of the collar, where particles are
strained from the water by the microvilli and carried down to the body by
protoplasmic flow, to be engulfed in food vacuoles near the base of the
collar.
The flagellum beats about 30 times/sec at 18° C, with a wavelength of 15
to 2011, giving a speed of 450 to 600 /it/sec for the propagation of the contraction
waves. The waves take place in a single plane, as is evident when organisms
turn so that the waves may be seen from the edge. The waves of contraction

Sleigh—Flagellar movement of sessile flagellates 409
pass from the base of the flagellum to the tip, and are sinusoidal, though with
perhaps some increase in amplitude of the broad waves as they near the tip.
Monas
Many organisms of the ochromonad type were found in the stream water
and among these were sessile forms that lacked plastids and had the body-form
shown in fig. 2, B ; they are believed to belong to the genus Monas. The movement
of Monas has been discussed by Krijgsman (1925) and Lowndes (1944),
FIG. 2. A, Codonosiga; B, Monas,
and figures by both authors have been published in textbooks to illustrate
flagellar movement. Krijgsman considered that the flagellum performed an
oar-like rowing stroke, but Lowndes pointed out that this movement was
unlikely to be the normal mode of progression. By the use of a stroboscope
Lowndes found that the flagellar waves moved from base to tip with a frequency
of about 19 cycles/sec, that the waves increased in amplitude towards
the tip, and that the water current set up was in the direction from the tip of
the flagellum towards the base. He pointed out that the organisms on which
these observations were made were swimming at much less than maximum
speed. The waves passing up the flagellum were believed to be spiral, and
this helical component provided a basis for Lowndes's explanation of the
mechanism of movement. He believed that the organisms always swam in a

41 o Sleigh—Flagellar movement of sessile flagellates
spiral and that the rotation and gyration provided the forward component of
movement. The sessile monads studied here could not move, nor turn quickly,
so that the motion of the flagellum is easily seen.
The body of the organism (some 10 to 12 \x in diameter) is attached to debris
or moss by a very fine strand (some 20 /J. long), which is flexible and noncontractile
(fig. 2, B). The two flagella emerge close together, the longer one
(25 to 30 fx in length) being extended almost straight out from the body, and
the shorter one (about 5 [x long) being held curved close to the body.
Strong water currents reminiscent of those caused by Actinomonas flow
around the active flagellum (fig. 2, B), differing slightly in that they impinge
on the surface of the body at the flagellar base. Food particles are seen to be
engulfed by activity of the body surface near the flagellar base and carried
down the side of the body and into vacuoles. Food objects as large as diatoms
30 to 40 ix long were seen enclosed in the body; intense flagellar activity must
be used to collect such large objects.
When seen under the stroboscope, the longer flagellum shows a roughly
sinusoidal beat, in which both the amplitude and wavelength increased
markedly towards the tip. The waves travel from base to tip and are planar.
The frequency of beat is about 50 beats/sec at 20 0 C, and the wavelength
increases from about 6 fx at the base to 10 to 12 fx distally, so that the waves
are propagated at 300 jtt/sec basally and 500 to 600 //,/sec distally.
The small flagellum appeared to flicker, but its motion was not found to be
regular enough for stroboscopic observation. It may be concerned with the
trapping and ingestion of food particles.
Poteriodendron
The movement of the flagellum of this colonial bicoecid has been briefly
discussed by Geitler (1942), but without stroboscopic or cinematographic
observations he was unable to see the true nature of the beating activity. Kent
(1880) noted that free-swimming bicoecids swam with their flagella held in
front, presumably acting as tractella. While Kent classed bicoecids with the
chrysomonad phytoflagellates, Geitler believed them to be choanoflagellates,
and Grasse and Deflandre (1952) found that their systematic position was
uncertain because of lack of knowledge of their structure.
Individuals of Poteriodendron occupy a stalked campanulate lorica (fig. 3, A),
which is attached to plant material or debris. The stalk is secreted first, and
then the lorica, which has a spiral structure (Robinow, 1956), is produced
from a continuous thread of secreted material laid down by rotation of the
animal; in one case the animal was observed to rotate continuously clockwise
during secretion of the lorica, performing one complete rotation every 7 or 8
min. The fully-formed lorica is about 15 /x long at the end of a stalk 5 to 30 fx
long.
The animal is attached to the base of the inside of the lorica by a contractile
'foot' which is said to be a modified flagellum. The body of the organism
(about 8 to 12 ju, long) is held near the mouth of the lorica at the end of the foot,

Sleigh—Flagellar movement of sessile flagellates 411
and from it projects the stiffly held, but slightly curved, flagellum, 25 to 40 /x
long. The flagellum is attached near the end of the body, at the base of an
apical bulge.
Flagellar activity sets up water currents which impinge on the body surface
near the base of the flagellum, in which region food particles are ingested. The
currents are different from those in either Actinomonas or Monas, since
the water is funnelled in from a considerable distance, but only close to the
flagellum (fig. 3, B). Stroboscopic observations show the reason for this: the
amplitude of the bending waves of the flagellum is small, and the sinusoidal
'/ water currents •
FIG. 3. Poteriodendron. A, part of a colony; B, water currents around the flagellum; c, unrolling
of the flagellum on relaxation after retraction of the animal into the lorica.
waves have a small wavelength which tends to decrease distally. Examination
of the animal while it is rotating during the secretion of the lorica shows
alternately the edge and flat face of a plane sine wave envelope of movement,
and this is especially obvious under the stroboscope. The frequency of beat
is 40 to 50 cycles/sec, and the wavelength 3 to 5 fj,, so that the rate of propagation
of the contraction waves from the base to the tip of the flagellum is 120
to 250 /x/sec.
On stimulation of the animal it was retracted to the base of the lorica by the
contraction of the foot, and at the same time the flagellum coiled up very
rapidly to assume the shape of a watchspring. On relaxation, the animal
emerged as the foot extended and the flagellum slowly uncoiled over a period
of a second or two (fig. 3, c). The contraction of a flagellum into a plane spiral
is most unusual, and the mechanism involved must be considerably different
from that which produces normal contractile waves in the flagellum.

412 Sleigh—Flagellar movement of sessile flagellates
Discussion
In all these 4 flagellates the flagellum is of a similar size, and plane waves
of approximately sinusoidal form are propagated from the base to the tip (fig.
4), yet the results of flagellar activity are all somewhat different. From studies
on the tails of spermatozoa one would expect that water currents would be
set up which travel in the same direction as the propagated bending waves
in the flagellum, and this is indeed what has been observed in Codonosiga. In
the other 3 examples the water currents were observed to travel in the direction
opposite to that in which the flagellar waves are propagated; this is not easily
explained.
Actinomonas Codonosiga. Monas Potehodendron
FIG. 4. Comparison of the form of flagella during beating.
At this point it is interesting to draw an analogy with the movement of
organisms of larger size. Comparison of the swimming of an eel with that
of a polychaete worm like Nepktys is instructive. The eel propagates waves of
bending from the head to the tail as it moves through the water, while in
Nephtys the waves of bending are propagated from the tail to the head as the
animal swims head-first. The difference here is the presence of the paddle-like
parapodia situated in the plane of the oscillations in the polychaete, while the
eel is smooth-bodied. Taylor (1952) approached this problem from a mathematical
point of view, and found from theoretical calculations that, while the
propagation of sinusoidal waves along a smooth thin cylinder will cause
movement of water in the same direction as the propagation of the waves, the
propagation of similar waves along a cylinder whose surface is sufficiently
rough, and whose roughness has a directional character, may cause movement
in a direction opposite to the direction of propagation of the waves.
It is possible that the flagellum of these flagellates uses a similar mechanism.
The presence of mastigonemes or flimmer filaments on many flagella is well
established (see Pitelka, 1963), but their arrangement on the flagellar shaft is
as yet unknown. Flimmer filaments appear to be of two distinct types: the
larger mastigoneme structures several micra long and 10 to 20 m/x in diameter,
and finer filaments only 2 m/x or so in diameter. Light microscopists found that
many flagella carry flimmer filaments, and Deflandre (1934) listed choanoflagellates
and chrysomonad flagellates among those forms whose flagella
carry lateral appendages; however, only by electron microscopy is it possible

Sleigh—Flagellar movement of sessile flagellates 413
to determine the type and arrangement of the flimmer filaments adequately.
The long flagellum of ochromonad flagellates, which no doubt has the same
structure as that of Manas, has been found to carry both mastigonemes and
fine filaments (Pitelka and Schooley, 1955), but no clear evidence is available
about the other types studied here. Electron micrographs of Codonosiga by
Petersen and Hansen (1954) show a fibrous zone around the flagellum which
may represent poorly-preserved flimmer filaments, and Afzelius (1961) found
that thin flimmer filaments were present in the plane of the central fibres of
the choanocyte flagellum of the sponge Microciona; the flimmer filaments of
choanoflagellates may well be of the same type. It is worth noticing that these
filaments are not situated in the presumed plane of oscillation of the sponge
flagellum, but in the plane at right angles to this. It is possible that in Actinomonas,
Monas, and Poteriodendron there are mastigonemes situated in the
plane of the oscillations (either in two rows, or all round like the hairs of a
bottle brush), which may act to move water in the direction opposite to that
expected, by providing the roughness mentioned in Taylor's theoretical consideration
of the movement of undulating thin cylinders. These mastigonemes
may perhaps be visualized as oars projecting from the flagellar shaft which row
water towards the flagellar base as the crests of the waves of bending move up
the flagellum.
There is doubt as to the systematic position of both Actinomonas and the
bicoecids, but the similarity of their flagellar activity to that of Monas and
other chrysomonads that I have studied suggest that they may belong to this
group; indeed Pascher has included Actinomonas with the chrysomonads for
another reason, and several probable bicoecids like Codonodendron are often
placed in the same group of phytoflagellates.
It is interesting to note the adaptive modification of the form of flagellar
beat in Actinomonas, Monas, and Poteriodendron. The food-catching pseudopodia
of Actinomonas only require a broad stream of water-carrying particles,
but in the other two forms the particles must hit the body near the flagellar
base. In Monas this is achieved by funnelling the water current towards the
body by increasing the amplitude of the bending waves of the flagellum distally,
and in Poteriodendron the amplitude of the waves is kept small so that the
main current is close to the flagellar shaft and impinges on the body surface.
Evidently there is a considerable variety of patterns of flagellar movement,
and a full range of these patterns must be studied before the mode of functioning
of the contractile mechanism of the flagellum can be properly understood.
The first observations were made at the University of Exeter, in the Department
of Professor L. A. Harvey, whom I should like to thank for his interest.
I am also grateful to Professor J. E. Harris, F.R.S., for reading the manuscript.

414 Sleigh—Flagellar movement of sessile flagellates
References
Afzelius, B. A., 1961. Nature, Lond., 191, 1318.
Brown, H. P., 1945. Ohio J. Sci., 45, 247.
Deflandre, G., 1934. Ann. Protistol., 4, 31.
Fjerdingstad, E. J., 1961. Z. Zellforsch, 54, 499.
Geitler, L., 1942. Arch. Protistenk., 96, 119.
Grasse, P.-P., and Deflandre, G., 1952. In Traite de Zoologie, ed. by P.-P. Grasses Tome I,
fasc. I, p. 599. Paris (Masson).
Gray, J., 1955. J. exp. Biol., 32, 775.
1958. Ibid., 35, 96.
Greissmann, K., 1914. Arch. Protistenk., 32, 1.
Hollande, A., 1952. In Traite de Zoologie, ed. by P.-P. Grass£, Tome I, fasc. I, p. 471.
Paris (Masson).
Hoi will, M. E. J., 1963. Personal communication.
Jahn, T. L., and Fonseca, J. R., 1963. J. Protozool., 10, suppl., p. 11.
Harmon, W. M., and Landman, M., 1963. Ibid., 10, 358.
Landman, M., and Fonseca, J. R., 1963. Proc. XVI int. Congr. Zool., 2, 292.
Kent, W. S., 1880. A manual of the infusioria. London (Bogue).
Krijgsman, B. J., 1925. Arch. Protistenk., 52, 478.
Lapage, G., 1925. Quart. J. micr. Sci., 69, 471.
Lowndes, A. G., 1941. Proc. zool. Soc. Lond. Ser. A., m, m.
1943- Ibid., 113, 99.
1944- Ibid., 114, 325.
Pascher, A., 1918. Arch. Protistenk., 38, 1.
Petersen, J. B., and Hansen, J. B., 1954. Bot. Tidsskr., 51, 281.
Pitelka, D. R., 1963. Electron-microscopic structure of protozoa. Oxford (Pergamon).
and Schooley, C. N., 1955. Univ. Calif. Publ. Zool., 61, 79.
Robinow, C. F., 1956. J. biophys. biochem. Cytol., 2, suppl., p. 233.
Roskin, G., 1931. Arch. Protistenk, 73, 203.
Stokes, A. C, 1888. J. Trenton Nat. Hist. Soc, 1, 71.
Taylor, G. I., 1952. Proc. Roy. Soc. A, 214, 158.
Tregouboff, G., 1953. In Traite de Zoologie, ed. by P.-P. Grasse^ Tome I, fasc. II, p. 437.
Paris (Masson).