Slow heating rates increase thermal tolerance and alter mRNA HSP expression in juvenile white sturgeon (Acipenser transmontanus)

Freshwater fish such as white sturgeon (Acipenser transmontanus) are
particularly vulnerable to the effects of anthropogenically induced global
warming. Critical thermal maximum tests (CTmax) are often conducted to
provide insight of the impacts of changing temperatures; however, little
is known about how the rate of temperature increase in these assays
affects thermal tolerance. To assess the effect of heating rate
(0.3°C/min, 0.03°C/min, 0.003°C/min) we measured thermal tolerance,
somatic indices, and Hsp mRNA expression. Contrary to what has been
observed in most other fish species, white sturgeon thermal tolerance was
highest at the slowest heating rate of 0.003°C/min, suggesting an ability
to rapidly acclimate to slowly increasing temperatures. Hepatosomatic
index decreased in all heating rates relative to control fish, indicative
of the metabolic costs of thermal stress. At the transcriptional level,
slower heating rates resulted in higher mRNA expression of Hsp90a, Hsp90b,
and Hsp70. Hsp70 mRNA expression was increased in all heating rates
relative to controls, whereas expression of Hsp90a and Hsp90b mRNA only
increased in the two slower trials. Together these data indicate that
white sturgeon have a very plastic thermal response, which is likely
energetically costly to induce. Acute temperature changes may be more
detrimental to sturgeon as they struggle to acclimate to rapid changes in
their environment, however under slower warming rates they demonstrate
strong thermal plasticity to warming.