Respiration, pumping activity and heart rate in Ciona intestinalis exposed to fluctuating salinities

Specimens of C. intestinalis were exposed to both gradual (sinusoidal) and abrupt (square-wave) salinity fluctuations and the changes in pumping activity and oxygen consumption monitored. Heart rate was monitored under steady state conditions using a new in vivo method, and pumping activity was monitored as spontaneous squirting by use of a pressure transducer. Oxygen consumption was also monitored under steady state conditions and under conditions of declining oxygen tension. It was found that during periods of regular beating the mean heart rate for 5 ascidians was 33.2 beats/min for the branchial pacemaker and 32.6 beats/min for the visceral pacemaker. Regular periods of beating lasted for similar to 1 min, followed by periods of irregular activity prior to heart beat reversal. The heart beat became irregular and reversal occurred sporadically when the ascidians were exposed to dilute seawater. The ascidians showed a pumping rate of 14 "squirts" /h in full-strength seawater. This rate declined during decreasing salinity and ceased entirely when the external seawater concentration reached similar to 60% seawater. During periods of decreased salinity, the siphons were tightly closed and oxygen consumption was zero. The rate of oxygen consumption decreased during decreasing salinities and there was no evidence of an oxygen debt. Oxygen consumption under steady state conditions varied with body weight according to the following: oxygen comsumption = 0.515 sub(w)0.831 (w body weight). The rate of oxygen consumption was found to be dependent on the external oxygen tension.