Oxygen consumption and responses to hypoxia of ammocoetes of the southern hemisphere lampreyGeotria australis.

The standard rate of oxygen consumption of ammocoetes (larvae) ofGeotria australis with a mean weight of c. 0.5 g was 9.6, 31.4 and 59.4μl g(-1) h(-1) at 4.5, 15.5 and 25.0°C respectively, which gives an overall Q10 of 2.4. The regression coefficient for the logarithmic relationship between oxygen consumption and body weight at 15.5°C was 0.704. The ammocoetes ofG. australis have a much lower rate of oxygen consumption at 15.5 and 25.0°C than those of holarctic lampreys. This presumably reflects the lower oxygen delivery pressure to their tissues and helps account for their slow growth rate. At 15.5°C, ammocoetes ofG. australis emerged from the substrate at 21-25 mm Hg and, unlike those of the Northern HemisphereIchthyomyzon greeleyi, died at 14-17 mm Hg. Thus, despite having a thinner water/blood barrier in the gills and blood with a higher oxygen affinity and capacity than holarctic ammocoetes, the larvae ofG. australis cannot survive very low dissolved oxygen tensions. This is apparently related to an inability of larvalG. australis to meet the high oxygen requirements of the respiratory pump at these oxygen tensions. During metamorphosis, oxygen consumption at 15.5°C rose from approximately 27μl g(-1) h(-1) at the beginning of transformation to 33.2μl g(-1) h(-1) by Stage 3 and then rapidly to 66μl g(-1) h(-1) at Stage 6. It remained near this level in Stage 7 and the downstream migrant.