The brine shrimp Artemia has been a favourite subject of study for molecular biologists and is an extremely important economic resource. Despite this we know little of the physiology of brine shrimp as it relates to their natural environment. While some data are available on physiological responses to temperature and salinity, little is known of respiratory responses to hypoxia, particularly long-term and periodic hypoxia. Consequently the main aim of this thesis was to examine the development of respiratory function in the brine shrimp Artemia franciscana Kellogg 1908, cultured under different oxygenation regimes. Not only were developmental patterns documented but particular attention was given to possible underlying physiological mecruunsms. Culture of A. franciscana under chronic hypoxia (50 % normoxic saturation) resulted in accelerated development at least in the initial stages (5-9). The weight of individuals of any particular developmental stage was independent of experimental treatment, i.e. hypoxic cultured individuals did not increase developmental rate by producing smaller individuals. By the time they reached sexual maturity there was no difference in growth or development between experimental and control tre3:tments. Individuals cultured under chronic hypoxia contain~d considerably more hemoglobin than normoxic controls, with those cultured under periodic hypoxia being intermediate. This hemoglobin was present not only in the hemolymph but was also localised in many of the highly aerobic tissues ( appendage muscles). There was very little difference in the pattern of culture mortality between chronic hypoxia and normoxia, with a peak in mortality during the thoracic stage of development. Artemia franciscana cultured under periodic hypoxia (8 h, hypoxia, 16 h normoxia) showed a response intermediate to that of chronic hypoxia and normoxia. Culture under chronic hypoxia resulted in a shift from a mixed oviparity/ovoviviparity to oviparity alone. Although the total number of offspring produced was lower for individuals cultured under chronic hypoxia, the number of cysts· produced was the same as for the control treatments. The relationship between oxygen uptake and dry body weight in A. franciscana could be predicted on an allometric basis. The recorded decrease in oxygen uptake with development, between Stage 0 and Stage 3 could be explained by the fact that at this point development was accompanied by a decrease in body weight. the nauplii displayed a marked ability to regulate oxygen uptake when exposed acutely to declining oxygen tensions (critical oxygen tension or Pc= 6.5 ± 0.3 kPa). This ability improved from Stage 0 -6, with a decrease in Pc to 4.6 ± 0.3. This coincided with beginning of the thoracic stage of development, which was characterised by the formation of gills and a functional heart. Thereafter there was little change in regulatory ability with continuing development. Culture under conditions of periodic or chronic hypoxia had no significant effect on rates of oxygen uptake but did result in the improvement in respiratory performance which normally accompanies development being brought forward. Improved regulation under conditions of declining oxygen tension was achieved by Stage 3 and regulatory ability was more highly developed than normoxic controls. Individuals cultured under periodic hypoxia showed a response intermediate to hypoxic and normoxic individuals indicating that it was probably the duration of exposure to hypoxia that was important not the pattern. Regulation of oxygen uptake during conditions of declining oxygen tensions was compromised if the respiratory pigment hemoglobin was inactivated (using CO) in individuals cultured under hypoxic conditions but not in normoxic controls. Whether or not cysts were produced by hemoglobin-rich or hemoglobin-poor parents, made little· difference to the respiratory performance of the offspring. Furthermore this respiratory performance was not compromised by CO exposure. The aerobic capacity of the tissues of A. franciscana was improved by culture under chronic hypoxia. This was seen both in the dramatic increase in the total activity of cytochrome c oxidase in individual animals, determined enzymatically and by staining for the enzyme in thin sections, and also by the dramatic changes in mitochondria number and morphology observed, particularly for the most aerobic tissues. It was concluded that respiratory regulation of A. franciscana 'improved' early in development and that this 'improvement' could be 'brought forward' by culturing individuals under hypoxic conditions. The development of respiratory regulation in normoxia cultured animals could not be attributed to the presence of hemoglobin, as could its development in hypoxia cultured animals. Furthermore, as well as improving the uptake and transport of oxygen to the tissues hypoxic cultured A. franciscana also 'improved' the aerobic capacity of the tissues. The fact that A. franciscana cultured under periodic hypoxia showed an intermediate response for most of the features studied suggests that it is the duration not the pattern of hypoxic exposure that is important in modifying respiratory regulation. So A. franciscana in response to hypoxic exposure, hurries through early hypoxia-sensitive life stages, but also, and at the same time, develops a respiratory physiology which is better able to cope with hypoxia as an environmental stress. So this study has demonstrated that some animals, early in development may respond to environmental stress by increasing developmental rate to bring the adult pattern of physiological regulation nearer in time and also, if possible, bring that adult pattern of regulation forward in development itself.