Adaptations to low tissue pO2 and the molecular stress response in bivalves

DORIS ABELE; GEORGINA RIVERA-INGRAHAM; CYRIL DEGLETAGNE; IARA ROCCHETTA; JULIA STRAHL; SILVIA HARDENBERG; EVA PHILIPP

Congreso; PHYSIOMAR 14 International Meeting; 2014

Maximum performance, such as highest respiration and heart rates, or pumping activities are often interpreted as marking the range of ?optimal physiological performance? in a species.  In bivalves this can be misleading, and many species actively avoid maximum hemolymph and shell water oxygenation. Maximum heart beat and pumping rates are seldom observed and either mark stress conditions or episodic high scope for activity. Contrary, bivalves are highly tolerant of hypoxic conditions, and even self-induce low oxygen partial pressure in shell water and a low metabolism. This behavior reduces cellular oxygen turnover and possibly contributes to the long life expectancy of some clams species. We have recently studied patterns of shell water oxygenation in different mobile and sessile bivalve ecotypes and found all species to reduce internal shell water pO2 below normoxic levels in the surrounding seawater, often with frequent oscillation between high and low pO2 at species specific patterns. Alternations of tissue pO2 between hypoxia and re-oxygenation to normoxia produce oxidative stress in mammals. For hypoxia tolerant species the hypothesis of ?preparative up-regulation of antioxidant systems? has been put forward, which would help these animals to avoid oxidative damage as a consequence of fluctuation tissue oxygen levels. We investigated the physiology of the bivalve respiratory organ, the gills, at different pO2 levels and at different levels of energy demand. Firstly we monitored respiratory coupling in permeabilized excised gills and found that addition of ADP reduces the oxygen uptake, whereas current trials show the expectable increase of respiration in saponized mantle tissue. The response to hypoxia and reoxygenation was studied in gills of blue mussels and of mud clams exposed to hypoxia and reoxygenation. We analyzed oxygen radical formation using life imaging techniques, and in parallel the response in antioxidant enzyme activity and found no clear evidence of a preparatory response to hypoxia, but up-regulation of antioxidants during re-oxygenation. Finally, we tested the response in stress and antioxidant gene expression to hypoxia and anoxia in 2 populations of the ocean quahog from environments with differing natural stress levels and found that only animals from the high stress environment induced stress genes under severe (anoxic), while not under exposure to mild (hypoxic ) stress. In the talk we will discuss the specific strategies of stress tolerance in bivalves as hypoxia adapted and   group of animals, partly with extraordinary long life expectancy, and highlight differences and similarities to hypoxia sensitive vertebrate models.