Beat frequency of cilia in the branchial basket of the ascidian Ciona intestinalis in relation to temperature and algal cell concentration

To elucidate the effects of temperature and algal cell concentration on pum ping of water in the ascidian Ciona intestinalis a number of different expe riments were performed. Beat frequency of the lateral cilia in the openings of the branchial sac was measured in intact specimens using a microproject ion objective and a monochrome CCD video camera. At constant low algal cell concentration, beat frequencies increased linearly with temperature from 4 .0 Hz (+/-0.5) at 7.4 degrees C to 13.6 Hz (+/-1.6) at 20.1 degrees C. At a constant temperature of 15 degrees C, beat frequency decreased with increa sing algal cell concentration from approximately 3000 to > 10 000 Rhodomona s sp. cells ml(-1). The decrease was observed both in experiments where the ascidians had been acclimated to a fixed algal cell concentration and in e xperiments with changing concentrations. Effect of algal cell concentration on squirting/siphon closure and flow velocity in the exhalent siphon was m easured using a thermistor. At low algal cell concentrations, flow velocity in the exhalent siphon was stable apart from a few short squirts. At very high algal cell concentrations, the flow velocity was reduced and much less stable, with prolonged squirting. The effect of gut content on filtration was studied in experiments with specimens acclimated to high algal cell con centrations. Results showed a close relation between gut clearance and filt ration rate. From the experimental results and a qualitative analysis of th e Ciona-pump it was concluded that the ciliary beat frequency is proportion al to the water flow through the sea squirt and that changes in pumping cau sed by temperature or algal cell concentration are under nervous control or governed by enzyme kinetics, rather than being a result of physico-mechani cal properties, i.e. pump efficiency versus flow resistance, of the ascidia n pump.