Cardiac performance and heat shock response variation related to shell colour morphs in the mudflat snail Batillaria attramentaria

Physiological and transcriptomic response to thermal stress were investigated in the mudflat snail Batillaria attramentaria of different shell colour morphs. Despite no difference in upper lethal temperatures among shell colour morphs, their Arrhenius breakpoint temperatures (ABT) for cardiac thermal performance differed significantly, and the ABT of snails with D type morph was higher than snails with UL type morph. The transcriptomic analysis showed that D type snails exhibit higher levels of four HSPs than UL type snails at control temperature. Unfolded protein response was activated in UL type snails but not in D type snails under moderate thermal stress. And there were 11 HSPs increased in UL type snails in contrast to 1 HSP in D type snails, suggesting a "preparative defense" strategy of heat shock response in D type snails under moderate thermal stress. When exposed to sublethal temperature, eight molecular chaperones were uniquely upregulated in D type snails, suggesting these genes may allow for D type snails improved their cardiac thermal tolerance. The "preparative defense" strategies and higher ABT of cardiac thermal performance may adapt the dark shell snails to the rapid and stronger thermal stress in the field. Our results suggested that physiological selection imposed by moderate and sublethal thermal stress, instead of upper lethal limits, may be a driving force shaping shell colour frequency in the mudflat snail B. attramentaria.