AN ULTRASTRUCTURAL APPROACH TO FEEDING BEHAVIOUR IN PHILODINA ROSEOLA AND BRACHIONUS CALYCIFLORUS (ROTIFERS)

I. THE BUCCAL VELUM

P. CLEMENT, J. AMSELLEM, A.-M. CORNILLAC, A. LUCIANI’ & C. RICCI’

’ Laboratoire d’Histologie et Biologie Tissulaire (LA 244 CNRS) et CMEABG, Universite Claude Bernard (Lyon 1); Villeurbanne, France, 2 Instituto di Zoologia, Universita degli studi, via Celosia, IO, Milano, Italy

Keywords: Rotifers, ultrastructures, feeding behaviour, digestive tract, myelin-like structure

Abstract

The structure situated in the lumen of the buccal tube, called here ‘buccal velum’, separates the buccal space from the pharyngeal space. The velum is a supple membrane-pile. Its structure resem- bles that of a fish-trap. The food, once ingested, cannot go back into the buccal lumen.

Introduction

De Beauchamp (I 909) and Remane (1929-1932) noticed, in some rotifers, the presence of a buccal tube connecting the buccal field of the rotatory organ to the mastax.

The structure and function of this buccal tube are not well known, in spite of some ultrastructure studies in Monogonont rotifers (Notommata copeus: Clement & Pavans de Ceccatty, 1969; Trichocerca rattus: Clement, r977a) and in a Bdelloid Habrotrocha rosa: Schramm, 1978). Our preliminary ultrastructural observations in Brachionus caZycz$‘orus and more detailed ones in Phi- lodina roseola indicate that the buccal tube is a mixed cell-structure: buccal and pharyngeal. Between the buccal and pharyngeal lumens there is a very original structure: a pharyngeal membrane-pile which we have called ‘the buccal velum’. Schramm (1978) saw this membrane-pile but she did not reconstitute the structure as a whole, so she could not imagine its function like a fish-trap.

Material and methods

The animals tested are a clone of B. calyczji’orus, from a strain given by R. Pourriot, kept at about 2o°C and cultured on Phacus pyrum, and a clone of Philodina roseola provided by C. Ricci, kept at 24°C but fed on commercial fish food.

The animals, relaxed by 1% novocaine are fixed for two hours either by 2% 0~04 or by 2% glutaraldehyde fol- lowed by 2% 0~04. The fixative is buffered by sodium cacodylate giving 200 mosM osmotic pressure.

Fixed animals were embedded in epon, cut with a Reichert ultramicrotome and observed with an electron microscope Philips EM 300 or Hitachi HU 12 A at the ‘Centre de Microscopic Electronique Appliqute a la Biolo- gie et a la Geologic’ (Lyon I, Universitt Claude Bernard).

Results

Philodina roseola The buccal tube is formed by a simple epithelium. As in the buccal tube of N. copeus and T. rattus (Clement & Pavans de Ceccatty, 1969; Clement, r 977a, b) all the cells are mushroom-shaped: their caps are imbricated to make the tube-wall, and their stalks represent the cellbodies which contain the nuclei and an important part of the cytoplasm. The cell-bodies are interconnected by many gap-junctions.

Two easily recognized cell categories exist in the buccal tube. The first cell type bears buccal cilia characterized by the presence of an electron-dense substance at its apical extremity, while the other type has pharyngeal cilia lim- ited by a double cytoplasmic membrane (Fig. 9 and IO).

The apical side of pharyngeal cells also shows this double cytomembrane. The buccal velum consists of pile cyto- membranes (Fig. IO) which are continuous with the apical membranes of the pharyngeal cells (Fig. I I). Transverse serial sections of the tube (Fig. I to 6, Fig. 8) show the distribution of these two kinds of cells. The pharyngeal cells go up into the buccal tube as far as the buccal velum.

The velum is a symmetrical structure. It begins at about

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Hydrobiologia 73, 127-131(1980). 0018-8158/80/0732-0127$or.o0. 0 Dr. W. Junk b.v. Publishers, The Hague. Printed in the Netherlands.

Fig. I to 6. Transverse sections of the buccal tube (see the localization of these sections in Fig. 7). The epithelial wall is surrounded by muscles (M). A part of the tube wall is made of pharyngeal cells (arrows): they are caracterized by their myelin-like cuticle which also forms the buccal velum. In the buccal lumen, the buccal cilia(C) are

characterized by an electrondense substance at their apical extremity. Fig. I. (x 52oo). There is no pharyngeat cell; Fig. 2 (x 5300), Fig. 3. (x 6ooo), Fig. 4. (x 5700). There are pharyngeal and

buccal cells; Fig. 5. (x 5200), Fig. 6. (x 8500). There are only pharyngeal cells. BL: buccal lumen; C: buccal cilia; PL: pharyngeal lumen; V: buccal velum; arrows: pharyngeal cells.

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the anterior third of the tube and ends near the trophi of the mastax. The buccal velum separates the buccal from the pharyngeal lumen. The latter gives two cone shaped extensions up into the buccal tube.

space between the two symmetrical parts of the velum (Fig. 5,8i).

The buccal lumen narrows from its first third down to Brachionus calyc$‘orus its base. At this point this lumen is just formed by the We have noted that in this species, there is a velum which

Transversal sections fig.8

f

fig.7

Longitudinal dorso-ventral section

h

c- b fig1

-C

- d fig2

c- e fig3

i

BUCCAL TUBE DIAGRAMS

Fig. 7. Diagram of a longitudinal section of a buccal tube. The dorsal part is at left, the ventral one at right. The arrows show the transvere sections of Fig. 8. (a to j) and the transvere sections of the Fig. I to 6.

bl: buccal lumen; pl; pharyngeal lumen; m: mastax; v: buccal velum. The large arrows indicate the entrance of food particles into the pharyngeal lumen.

Fig. 8. Diagram of transverse sections of the buccal tube. The level of each section is indicated in the Figure 7. The legends are the same as in Figure 7.

In black/: epithelial buccal cells; In.....: epithelial pharyngeal cells. The interrupted line is the cuticle of the pharyngeal cells; this cuticle forms the buccal velum.

Figures 9 to 13. The structure of the buccal velum. Fig. 9, (x 78000) and Fig. IO. (x 150000). Philodina roseola. Transverse sections of the buccal tube at the level of the Figure 5. Note the myelin-like structure of the velum (V) which is formed by a membrane pile. In the pharyngeal lumen (PL) the cilia (C) have a double cytoplasmic membrane. (BL) buccal lumen. Fig. I I. (x 90ooo): Philodina roseola- Formation of the velum. The apical membranes of several pharyngeal cells (PC) meet up to form a membrane pile: the

130 velum (V). Fig. IZ. (x 4500): Bmchionus calyciflorus. Transverse section of the buccal tube. The velum (V) is more infolded than in P. roseola. Fig. 13. (x 134000): Brachionus calycifZorus. The buccal velum is a.membrane pile (a

myelin-like structure).

separates the buccal and the pharyngeal space. It shows the same structure (Fig. 13) and the same insertion on the pharyngeal cells as in P. roseolu (Fig. 12). Nevertheless our observations are not sufficient to reconstitute the three-dimensionnal morphology of the velum. Our pre- liminary results show many velum folds (Fig. 12) indicat- ing a greater complexity than in P. roseola.

Schramm, U. 1978. Studies on the ultrastructure of the rotifer Habrotrocha rosa Dormer (Aschelminthes) -The alimentary system. Cell. Tiss. Res., 189: 525-535.

Discussion

In P. roseola and probably in B. calyciflorus as well, the velum acts as a sort of fish-trap, preventing the alimentary particles from going back to the buccal lumen, once they have reached the mastax. Entrance into the pharyngeal lumen is thus made irreversible.

On the other hand, the buccal velum is a limp structure which cannot alone control ingestion of alimentary par- ticles. If it prevents the return of the food particles from the pharyngeal lumen back to the buccal lumen, the admission of the food particles into the buccal tube and then into the pharyngeal lumen is already controlled by different receptors and effecters which we describe else- where (cf. Clement et al., 1980, b and c).

References

De Beauchamp, P. 1909. Recherches sur les Rotiferes: les forma- tions ttgumentaires et l’appareil digestif. Arch. Zool. exp. g&n., 10: 1-410.

Clement, P. & Pavans de Ceccatty, M. 1969. Ultrastructures dune surface secretante salivaire: le tube buccal d’un rotifere Notommata copeus. Morphologic, desmosomes septts, secre- tions. Bull. Sot. 2001. Fr., 94: 599-612.

Clement, P. r977a. Introduction a la photobiologie des rotiferes dont le cycle reproducteur est control& par la photoperiode. Approches ultrastructurale et experimentale. These n”77r6. Universite Claude Bernard (Lyon I), Fr.

Clement, P. r977b. Ultrastructural research on rotifers. Arch. Hydrobiol. Bei.., 8: 270-297.

Clement, P., Amsellem, J., Cornillac, AM., Luciani, A. & Ricci, C. r98ob. An ultrastructural approach to feeding behaviour of P. roseola B. calyciflorus. (Rotifers). II- The oesophagus, In this volume, pp. I 33- I 36.

Clement, P., Amsellem, J., Cornillac, AM., Luciani, A. & Ricci, C. 1980~. An utrastructural approach to feeding behaviour in Philodina roseola and Brachionus calyciflorus (Rotifers). III. Cilia and muscles. Conclusions. In this volume, pp. 137-141.

Remane, A. 1929-1932. Rotatoria. In: Dr. HG. Bronn’s Klassen und Ordnungen des Tier-Reichs’, IV (Vermes), 2 (Asschel- minthes), I (Rotatorien, Gastrotrichen und Kinorhynchen), 3: 1-448.

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