Lung collapse in the diving sea lion: hold the nitrogen and save the oxygen

Abstract

Lung collapse is considered the primary mechanism that limits nitrogen absorption and decreases the risk of decompression sickness in deep-diving marine mammals. Continuous arterial partial pressure of oxygen profiles in a free-diving female California sea lion (Zalophus californianus) revealed that (i) depth of lung collapse was near 225 m as evidenced by abrupt changes in during descent and ascent, (ii) depth of lung collapse was positively related to maximum dive depth, suggesting that the sea lion increased inhaled air volume in deeper dives and (iii) lung collapse at depth preserved a pulmonary oxygen reservoir that supplemented blood oxygen during ascent so that mean end-of-dive arterial was 74 ± 17 mmHg (greater than 85% haemoglobin saturation). Such information is critical to the understanding and the modelling of both nitrogen and oxygen transport in diving marine mammals.

Figure 1.

Arterial (black line, 5 s sampling interval) and depth profiles (blue line, 1 s sampling interval) from serial deep dives of a California sea lion. (a) Characteristic profiles of serial deep dives. (b) Typical abrupt changes in occur in this dive at approximately 200 m depth during descent and ascent (indicated with red arrows), consistent with significant cessation of gas exchange at depth. Profiles were characterized by a, an initial compression hyperoxia; b, an abrupt decline at approximately 200 m depth; c, a gradual decline in in the bottom segment of the dive; d, a rapid increase in during early ascent; e, a decline in during ascent.

Figure 2.

Relationship between depth of lung collapse (m) and maximum depth (m) of the dive. The positive relationship indicates that lung collapse occurs at greater depths in deeper dives, suggesting sea lions increase inhaled air volume in deeper dives (F_1,46 = 146.97, p < 0.001).