A COMPARISON OF FIXATIVES FOR THE ESTIMATION OF ABUNDANCE AND BIOVOLUME OF MARINE PLANKTONIC CILIATE POPULATIONS

The effects of five treatments with four commonly used fixatives on the abundance and cell volume of marine planktonic ciliates were investigated on a natural community from Plymouth Sound. The fixative treatments were 5% and 9% Bouin's solution, 0.4% acid Lugol's iodine, 1% buffered formaldehyde and 1% glutaraldehyde. The abundance of the aloricate ciliate community varied according to fixative treatment, with Lugol's maintaining the greatest cell numbers, followed by Bouin's, glutaraldehyde and formaldehyde. The effects of the fixatives on the abundance of the two most common aloricate taxa, Balanion sp. and Strombidium epidemum were similar to, and components of, those for the aloricate population as a whole. However, there was little difference between the effects of different fixatives on the abundance of the tintinnid community, or on the abundance of the most common tintinnid, Tintinnopsis nana. Mean cell volumes of Balanion sp., S.epidemum and T.nana were greatest in samples fixed in formaldehyde, followed by glutaraldehyde, Lugol's and Bouin's. The mean population volume of the aloricate and tintinnid populations generally reflected this trend with greatest values recorded in formaldehyde- and glutaraldehyde-fixed samples, followed by Lugol's and Bouin's; however, the differential effects of the fixatives were not as great. Lugol's therefore appears to be the most effective fixative tested for the estimation of ciliate abundance and population volume, the latter being a function of abundance and cell volume.