34
THE PHYSIOLOGY OF CONTRACTILE VACUOLES
IV. A NOTE ON THE SOURCES OF THE WATER EVACUATED,
AND ON THE FUNCTION OF CONTRACTILE VACUOLES
IN MARINE PROTOZOA
BY J. A. KITCHING
Department of Zoology, University of Bristol
(Received 19 June 1938)
INTRODUCTION
IT has been shown for Paramecium caudatum that the rate of entry of water in food
vacuoles is considerably less than the rate of output of water by the contractile
vacuole (Eisenberg, 1925). Kitching (1938ft) has concluded that in most freshwater Protozoa the bulk of the water evacuated by the contractile vacuole enters by
the body surface; but that in certain forms in which a large quantity of fluid is
ingested at one "mouthful" or in a very short time, this may provide the greater
part of the output of the contractile vacuole. It seemed desirable to obtain an
estimate of the rate of uptake of water in food vacuoles for comparison with the rate
of output by the contractile vacuole in Protozoa other than Paramecium. Accordingly, this matter has been investigated in fresh-water and in marine peritrich
ciliates. It seemed possible that in marine Protozoa, in which the rate of entry of
water through the body surface was likely to be very low, the bulk of the water
eliminated by the contractile vacuole might have entered in food vacuoles. This
suggestion seemed to offer a reasonable functional explanation of the existence of
contractile vacuoles in so many marine ciliates.
MATERIAL AND METHODS
Fresh-water Peritricha were got from the legs of the isopod Asellus from a pond
in the University grounds at Bristol. Brackish water Peritricha were obtained on
the green alga Cladophora which was growing on a ship permanently moored in a
dock in the river Avon at Bristol. Marine Peritricha were received by post from
Plymouth, where they infest Cladophora in the aquarium. The actual species used in
each experiment is stated in Table I.
During observations, the organisms were irrigated in the way already described
(Kitching, 1934), except that the temperature was not controlled. Room temperature
lay between 14 and 20° C , but in any one experiment did not alter by more than
hah0 a degree.
Measurements of the diameter of vacuoles were made with a Zeiss screw micrometer eyepiece. In the case of food vacuoles there was probably considerable
The Physiology of Contractile Vacuoles
35
experimental error, as the vacuoles were not spherical when first ingested. They
were measured as soon as they became spherical (within £-1 min.), but they may
have undergone some shrinkage before this time in the case of fresh-water Peritricha, so that the calculated values of the rate of uptake of fluid in food vacuoles
are possibly a little too low.
THE WATER EXCHANGE BETWEEN FOOD VACUOLES AND THE CYTOPLASM
In Pentricha irrigated with sea water or filtered pond water (whichever was
appropriate), food vacuoles were taken in at regular intervals, usually of about
1-3 min. They were nearly all devoid of visible solid contents, although bacteria
may have been present in them. When surrounded by the cytoplasm of the body
these vacuoles shrank, either slowly or quickly, until they were invisible. The
shrinkage took place while they were some distance from the gullet or body surface,
so that it must be presumed that the water contained in them passed into the cytoplasm. (The actual rate of passage of fluid across the vacuolar membrane will be
dealt with in a later paper.)
"Empty" food vacuoles in shrinking disappeared from view, and no re-ejection
of the contents of food vacuoles into the pharynx was observed. It has been shown
by Greenwood (1894) that indigestible solids, together with some water, are got rid
of in this way, but no corresponding ejection of water could be detected by me in
the case of "empty " food vacuoles. Many empty food vacuoles are actually formed,
and from these all the water passes into the cytoplasm.
THE RATES OF UPTAKE AND OUTPUT OF FLUID
Various fresh-water, brackish water, and marine Peritricha were irrigated with
filtered pond water, filtered brackish water—of salinity equivalent to 5 % sea water—
from the river Avon at Bristol, and Plymouth "outside" sea water respectively.
Observations were made of the frequency and diameter of the contractile vacuole
and of the food vacuoles. The results are summarized in Table I.
In fresh-water Peritricha the rate of uptake of fluid in food vacuoles was found
in most cases to be between 8 and 20 % of the rate of output from the contractile
vacuole. On the other hand, in marine Peritricha the rates of uptake and of output
were approximately alike, and it is doubtful if the differences observed are significant
in view of the fairly large experimental error (see above).
DISCUSSION
In fresh-water Peritricha the rate of uptake of fluid from food vacuoles is only
a small fraction of the rate of output of the contractile vacuole. In the fresh-water
Zoothamnium sp.? used in previous work (Kitching, 1938 a) it is less than one-tenth.
In such forms it is to be presumed that the greater part of the water evacuated has
come in through the general body surface by osmosis, and that the contractile
vacuole is functioning as an osmoregulatory mechanism. But in marine Peritricha
3-3
The Physiology of Contractile Vacuoles
37
the rates of uptake by food vacuoles and output by the contractile vacuole approximately balance (within the rather wide limits of experimental error), and it may be
inferred that the main function of the contractile vacuole is the elimination of the
excess of water introduced by food vacuoles. The rate of removal, of water together
with indigestible material from old food vacuoles is negligibly small.
The question then immediately arises—why do not marine Rhizopoda require
contractile vacuoles? A possible clue to this problem lies in the fact that Rhizopoda
usually only form food vacuoles when food is available; they do not normally take
in "empty" or nearly "empty" ones. According to Mast & Hahnert (1935),
Amoeba sometimes evacuates from old food vacuoles the indigestible remains
together with a certain amount of water. It is possible that the balance between
uptake and output is maintained in this way in marine Rhizopoda, without the need
of a contractile vacuole.
In the case of marine Protozoa it is clear that a considerable quantity of salts
must be taken up in the food vacuoles, and it is not known whether these are
evacuated by the contractile vacuole or how they are removed. The possible evacuation of endogenous water by the contractile vacuole (suggested by Kamada, 1935),
has already been discussed (Kitching, 1936). The amount of such water must be
exceedingly small and could not provide more than a minute fraction of the output.
There remains the possibility that a small quantity of water might enter through
the body surface owing to the osmotic pressure of the cell proteins. However, it
seems that in marine Peritricha the main function of the contractile vacuole is the
removal of the water brought in by the food vacuoles.
SUMMARY
1. In peritrich ciliates many food vacuoles without visible solid contents may
be formed. The water in these vacuoles passes into the general cytoplasm.
2. In fresh-water Peritricha the rate of uptake of fluid in food vacuoles generally
amounts to between 8 and 20 % of the rate of output of fluid by the contractile
vacuole. The greater part of the water evacuated is presumed to enter the animal by
osmosis through the general body surface.
3. In marine Peritricha the rate of uptake of fluid by food vacuoles approximately balances the rate of output by the contractile vacuole. The elimination of
the water taken in by food vacuoles is believed to be the main function of the contractile vacuole in marine ciliates.
REFERENCES
EISKNBERG, E. (1925). Arch. Biol., Paris, 36, 441.
GREENWOOD, M. (1894). Pkiiot. Tram. B, 185, 355.
KAMADA, T. (1935). J. Fac. Set. Tokyo Univ. 4, 49.
KITCHING, J. A. (1934). J. exp. Biol. 11, 364.
(1936). Nature, Lond., 138, 287.
(1938a). J. exp. Biol. 15, 143.
(1938*). Biol. Rev. 13*(in the Press).
MAST, S. O. & HAHNERT, W. F. (1935). Phytiol. Z06I. 8, 355.