STUDIES ON THE GRAZING RATE OF NOTHOLCA SQUAMULA MiiLLER ON ASTERIONELLA FORMOSA HASS. AT DIFFERENT TEMPERATURES’

Linda MAY

Institute of Terrestrial Ecology, 78 Craighall Road, Edinburgh

IPart of a dissertation for the Degree of Doctor of Philosophy to the Council for National Academic Awards at Paisley College, Scotland, in

conjunction with the Institute of Terrestrial Ecology, Edinburgh, Scotland. Supported by Research Training Grant from the Natural Environment Research Council.

Keywords: Rotifers, diatoms, temperature, grazing

Abstract

The grazing rate of Notholca squamula on Asterionella formosa has been estimated to be 3.2 cells per female per hour at 6°C and I I .5 cells per female per hour at IO”C.

Introduction

Notholca squamula Miiller feeds on Asterionella formosa Hass. by breaking open the cell frustules and removing the contents (May, I 980). The broken, empty, frustules, which remain attached to the diatom colony, have been used as an index of the grazing rate of the rotifer since they rarely appear in ungrazed cultures of A. formosa.

Material and methods

Approximately 20 adult, female, Notholca squamula, from a laboratory culture fed on Asteriqnella formosa, were washed and added to each of thirty 3 cm x I cm glass tubes containing 2 ml aliquots of A. formosa suspension (I .6 x 10~ cells.ml-‘). Thirty similar tubes contained con- trol aliquots of A. formosa, without rotifers. Both sets of tubes were incubated at 6’C in a shaking waterbath under constant illumination. The experiment was later repeated at Io°C.

At intervals of o, 3, 9, 21 and 2g hours, 6 tubes were selected at random from both the experimental and con- trol batches and fixed with iodine. The number of broken and undamaged cells in subsamples of the alga were counted in a Lund chamber (Lund, 1959, 1962; Young-

man, 1971). and the grazing rate expressed as damaged cells per individual. The dry weight of A. formosa was determined by drying to constant weight at 8o°C.

Results

The grazing rate of Notholca squamula remained constant throughout the experiment at both temperatures (Fig. I). At 10% an individual consumed on average I I .5 cells per hour, whereas at 6°C the grazing rate was only 3.2 cells per animal per hour. The dry weight of Asterionella formosa was calculated to be 3.8 x IO-~ pg per cell, of which one half can be attributed to the undigestible frustule (Hughes & Lund, 1962). The feeding rate has therefore been estimated as 2.2 x IO-~ I.cg dry weight per animal per hour at IOT,

and 6.1 x IO-~ I.cg dry weight per animal per hour at 6’C.

Discussion

The method described, which has not previously been used for rotifers, is very sensitive even when grazing inten- sities are low. The grazing rate of Notholca squamula, expressed as cells per animal per hour, is much lower than those reported for other rotifers (Galkovskaja, 1963; Hirayama & Ogawa, 1972; Halbach & Halbach-Keup, I 974; Pilarska, I 977). This is probably due to the relatively large size of the A. formosa cell compared to the smaller food species of the filter feeders, and the difference in the temperatures at which the rates were determined.

79

Hydrobiologia 73, 79-81(1980). 0018-8158/80/0731-0079$00.6o. o Dr. W. Junk b.v. Publishers, The Hague. Printed in the Neiherlands.

Temp. Regression Equation Corr.Coeff.

6OC y = 3.2 x + 3.6 0.99

10°C y 111.5 x - 7.0 0.99

1 -

o*” 0 10 20 30

Time (hr.%) Fig. I. The grazing rate of Notholca squamula on Asterionella

formosa at 6°C and 10%

80

References

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