The feeding biology of tintinnid protozoa and some other inshore microzooplankton

Tin t i n n i d s are among the largest and most abundant of the marine c i l l a t e microzooplankton but there i s very l i t t l e published information on t h e i r feeding rates and a b i l i t i e s . The feeding of Tintinnopsis subacuta (and to a l e s s e r extent, that of 12 other species) was investigated with three methods 1) d i r e c t observation 2) counts of accumulated food c e l l s and 3) Coulter Counts of the p a r t i c l e s i n the experimental medium. There was reasonable q u a l i t a t i v e agreement between the r e s u l t s obtained by the three methods but quantitative agreement was poor. Many of the r e s u l t s showed no s i g n i f i c a n t differences due to very great v a r i a b i l i t y i n the r e s u l t s for a singl e t i n t i n n i d species within and between experiments. Much of t h i s v a r i a b i l i t y may be due to the methods used but i t also r e f l e c t s the v a r i a b i l i t y of t i n t i n n i d s i n natural populations. A wide v a r i e t y of items was eaten by t i n t i n n i d s , including smaller t i n t i n n i d s ; and the maximum food s i z e can be rela t e d to t i n t i n n i d c e l l volume over a wide range but i s d i s s i m i l a r i n t i n t i n n i d species of s i m i l a r c e l l s i z e . Several t i n t i n n i d species showed d i f f e r e n t i a l predation on various types of laboratory phytoplankton. This d i f f e r e n t i a l predation was based upon the a b i l i t y of the predator to handle prey, or on prey s i z e or prey type depending upon the p a r t i c u l a r t i n t i n n i d species. 'Negative' s e l e c t i o n of some types of laboratory phytoplankton i n mixedprey samples was also shown f o r some t i n t i n n i d species, p a r t i c u l a r l y Tintinnopsis subacuta on members of the Cryptophyceae. Feeding rates measured with the accumulation method were equivalent i i to 0.65% m l / h r / t i n t i n n i d f o r T_. subacuta and usually much l e s s . Feeding rates f o r t h i s species measured with the Coulter Counter technique ranged from 0.33 to 3.8% m l / h r / t i n t i n n i d . Very l i t t l e feeding was observed d i r e c t l y but feeding rates estimated with t h i s method were somewhat higher than those estimated f o r the same species from accumulation experiments. Ti n t i n n i d s apparently both consumed, and caused the production of p a r t i c l e s during experiments. Correlations between feeding rate and 9 other experimental v a r i a b l e s were such that i t would be impossible to predict the feeding rate of a t i n t i n n i d species using only the s i z e d i s t r i b u t i o n of avaifeble p a r t i c u l a t e biomass of l e s s 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 l a t t e r method gave lower asymptotes than d i d the former. Ivlev e l e c t i v i t y indices for T_. subacuta were most consistently p o s i t i v e i n those middle Coulter s i z e classes which also showed the greatest growth i n controls. Increased temperature had l i t t l e e f f e c t on the rate of food accumul a t i o n by four t i n t i n n i d species, but there was some evidence of a f a s t e r rate of disappearance of ingested food at very high temperatures. The r e l a t i o n s h i p between the gain of new food and the loss of old food i n i n d i v i d u a l T. subacuta and Stenosomella ventricosa was highly v a r i a b l e and may strongly r e f l e c t the p h y s i o l o g i c a l h i s t o r y of the c e l l . The rate of gain of new food may be l a r g e l y independent of the amount of old food i n a t i n t i n n i d , but the average rate of loss of o l d food i s f a s t e r i n c e l l s given new food than i n starved c e l l s .