Ventilation Frequencies and Stroke Volumes in Acute Hypoxia in Octopus

Octopus can regulate its oxygen uptake down to a of 70 mmHg, and in some instances well beyond this (Borer & Lane, 1971; Maginnis & Wells, 1969; Wells & Wells, 1983). In part, this capacity depends upon a rise in the pH of the blood, which increases the oxygen affinity of the animal’s haemocyanin as the external falls (Houlihan, Innes, Wells & Wells, 1982). The fact that the blood can remain saturated down to quite low Po₂ values means that the animal does not need to circulate any more blood to keep the tissues supplied in an environment that is becoming depleted of oxygen. But it does have to pass more water through the gills. The percentage of the available oxygen that is extracted from the ventilatory stream sometimes increases a little as the inhalant falls; but the volume removed (% x content) plainly does not (Wells & Wells, 1982). To maintain the same total uptake the animal must increase the ventilation frequency and/or the ventilation stroke volume. The purpose of the present work was to establish what happens.

Ten Octopus vulgaris (eight males and two females), from 666 to 1687 g, were used. The animals were kept at Banyuls during September 1984, at temperatures between 21 and 24°C. Temperature change during any one experiment was not greater than 1 °C. The animals were undamaged and fed regularly on crabs. Each octopus was anaesthetized in 2·5 % ethanol and had a 3-mm o.d. ‘Portex’ cannula sewn to the mantle adductor, just below the funnel in the midline of the animal. The cannula was led out through the muscle at the back of the mantle, as described in Wells & Wells (1982). The operation was very quick (<5min) and could be carried out as soon as ventilatory movements ceased. Recovery, aided by passing a stream of sea water over the gills, was rapid. Tests were carried out not less than 1 h after breathing had begun again. Previous studies (Wells, O’Dor, Mangold & Wells, 1983; Wells & Wells, 1984) and work to be reported (D. F. Houlihan, G. Duthie, P. J. S. Smith, M. J. Wells & J. Wells, in preparation) have indicated that the maximum O₂ debt that an octopus can sustain (about 20 ml kg^-1, following exhausting exercise or prolonged anaesthesia) is paid off within 2 h. The cannula sampling exhalant led to a Perspex chamber surrounding the electrode of an EIL 7130 dissolved oxygen meter. The water in the chamber was kept stirred by a paddle activated by a magnetic stirrer. The outflow was led to a beaker with a floating lid to minimize the free surface, and this water, collected at a rate of about 30 ml min^-1, was periodically returned to the respirometer. The respirometer itself has been described in Wells, O’Dor, Mangold & Wells (1983). The oxygen tension in the respirometer was measured by circulating the water past an EIL 15A oxygen probe. The circulation pump, with an output of several 1 min ^-1 also served to keep the tank water stirred.

The two oxygen electrodes were checked and matched to give the same output in saturated sea water before each experiment. Experimental runs normally lasted about 75 min, during which the animal would reduce the from about 140 to about 40 mmHg.

Ventilation frequency was observed directly, mantle movements being counted through the transparent ceiling of the respirometer while remaining, as far as possible, out of sight of the animal.

The results obtained are summarized in Table 1. Oxygen uptake and percent extraction from the ventilatory stream both declined a little as the fell (the latter by about 20 % for a three-fold decline in ⁠. Ventilation frequency and ventilation minute volume both increased. Ventilation stroke volume changed very greatly compared with ventilation frequency. The former increased by up to 400 %, the latter by a maximum of 50 % (average figures were 161 % and 32 %).

If the animals are ranked according to their capacity to maintain their O₂ uptake as the fell (columns 3-5 in Table 1) and these rankings are compared with their capacity to increase ventilation frequency (columns 9, 10) and stroke volume (13, 14), it is immediately obvious that it is the stroke volume, and not the frequency, that matters so far as O₂ uptake is concerned (Spearman Rank Correlation, rs = 0·042 for frequency, not significant and 0·0756, P= <0·01, for stroke volume).

The lack of correlation between O₂ uptake and ventilation frequency is in keeping with earlier results (Boyle, 1983; Wells & Wells, 1982) which have shown that frequency tends to increase with behaviour such as locomotion, defaecation, defence and arousal, activities that are not always correlated with immediate changes in O₂ uptake; percent extraction normally falls on these occasions (Wells & Wells, 1982). Frequency generally increases as falls. It rose by an average of 32% in the present series. Borer & Lane (1971), working with O. briareus, found that frequency rose by 4 cycles min^-1 for each 12-mmHg drop in Pio₂ below 85 mmHg. But the variability introduced by other factors swamps the relationship between frequency and the capacity to regulate.

There has been one previous attempt to measure ventilation volumes in O. vulgaris. Winterstein (1925) attached the exhalant funnel to the finger of a rubber glove and thence, via a condom, to a collecting vessel outside the respirometer. He found ventilation volumes ranging from 100 to 1500 ml min^-1 as the Pio₂ fell in animals from the same size range as those considered in Table 1. Johansen (1965) used a similar technique to examine ventilation in the much larger O. dofleini. The difficulty about direct collection is that it is liable to impede flow, now known to be driven by very small pressure gradients, in the region of 0·5 cmH₂O(∼0·05 kPa) (Wells & Smith, 1985). The lack of obstruction achieved by using an indirect method presumably accounts for the considerably larger volumes (260 to 4000 ml min^-1) that we have now recorded.