The plasticity of the cardiovascular system during development in response to environmental perturbations is certainly important, because embryos of lower vertebrates usually are free-living in very early stages and do not develop under the protection of the maternal organism. This means that they usually are exposed to the same or a similar environment as the adults. Alterations of the environment, such as natural fluctuations in temperature, salinity, or oxygen availability, as well as pollution, thus also affect the developing animal. Embryos and larvae may be more sensitive to environmental changes than adults because their organs and organ systems are still in the process of growth and development. In addition, metabolic pathways, pathways of ion regulation, and even pathways for hormonal and neuronal control of organ function may not be fully developed to allow for a coordinated response to changing environmental conditions.
Many studies have looked at the effect of environmental variables like oxygen availability, temperature, and osmolarity on oxygen consumption (ṀO2) or mortality of aquatic larvae. In terms of circulatory physiology, heart rate was the initial variable that was accessible. Recent technical developments like micropressure systems, pulsed Doppler systems, and video imaging techniques have opened up the possibility of gaining insight into variables like blood pressure and blood flow in miniature animals (Keller 1995; Burggren & Fritsche, 1995; Colmorgen & Paul, 1995).
The aim of this chapter is not to present a complete review of the literature available in this field but to excerpt and describe general physiological responses that typically are observed in embryos when facing hypoxia or changes in temperature, osmolarity, or pH.