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The feeding biology of tintinnid protozoa and some other inshore microzooplankton Blackbourn, David John

Abstract

Tintinnids are among the largest and most abundant of the marine cillate microzooplankton but there is very little published information on their feeding rates and abilities. The feeding of Tintinnopsis subacuta (and to a lesser extent, that of 12 other species) was investigated with three methods 1) direct observation 2) counts of accumulated food cells and 3) Coulter Counts of the particles in the experimental medium. There was reasonable qualitative agreement between the results obtained by the three methods but quantitative agreement was poor. Many of the results showed no significant differences due to very great variability in the results for a single tintinnid species within and between experiments. Much of this variability may be due to the methods used but it also reflects the variability of tintinnids in natural populations. A wide variety of items was eaten by tintinnids, including smaller tintinnids; and the maximum food size can be related to tintinnid cell volume over a wide range but is dissimilar in tintinnid species of similar cell size. . Several tintinnid species showed differential predation on various types of laboratory phytoplankton. This differential predation was based upon the ability of the predator to handle prey, or on prey size or prey type depending upon the particular tintinnid species. 'Negative' selection of some types of laboratory phytoplankton in mixed-prey samples was also shown for some tintinnid species, particularly Tintinnopsis subacuta on members of the Cryptophyceae. Feeding rates measured with the accumulation method were equivalent to 0.65% ml/hr/tintinnid for T. subacuta and usually much less. Feeding rates for this species measured with the Coulter Counter technique ranged from 0.33 to 3.8% ml/hr/tintinnid. Very little feeding was observed directly but feeding rates estimated with this method were somewhat higher than those estimated for the same species from accumulation experiments. Tintinnids apparently both consumed, and caused the production of particles during experiments. Correlations between feeding rate and 9 other experimental variables were such that it would be impossible to predict the feeding rate of a tintinnid species using only the size distribution of avaifeble particulate biomass of less than 20 um diameter. There were large differences between the apparent feeding rate asymptotes of T. subacuta and S. ventricosa as measured with the Coulter Counter and the accumulation method. The latter method gave lower asymptotes than did the former. Ivlev electivity indices for T. subacuta were most consistently positive in those middle Coulter size classes which also showed the greatest growth in controls. Increased temperature had little effect on the rate of food accumulation by four tintinnid species, but there was some evidence of a faster rate of disappearance of ingested food at very high temperatures. The relationship between the gain of new food and the loss of old food in individual T. subacuta and Stenosomella ventricosa was highly variable and may strongly reflect the physiological history of the cell. The rate of gain of new food may be largely independent of the amount of old food in a tintinnid, but the average rate of loss of old food is faster in cells given new food than in starved cells. It was shown that natural concentrations of T. sub acuta can apparently control the growth of natural populations of phytoplankton of less than 20 μm dia. in under 24 hours. From a comparison with some other types of microzooplankton it was concluded that the larger species of tintinnid could probably have a potentially predominant effect upon the highly productive phytoplankton of less than 10 um diameter in English Bay and other coastal localities.

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