The role of thermal stress phenomena in setting stratified distribution limits was investigated in the bearded horse mussel Modiolus barbatus in response to the seasonal temperature regime in the field. Mussels were transplanted from their natural depth range at ca. 20 m to 3 m depth, where they then experienced enhanced variability of ambient conditions. In specimens from both depths, thermal stress was assessed from the inducible heat shock response (HSR), the accumulation of irreversibly damaged proteins, and from metabolic characters including the putative shift from aerobic to anaerobic metabolism. During both winter and summer, the HSR became involved more at shallow depths than at 20 m depth. The accumulation of succinate during summer indicates transition to anaerobiosis. The results suggest that the development of anaerobic conditions and the exploitation of the HSR are closely intertwined. The field data corroborate that glycolytic capacity, the level of energy turnover, and also protection from protein damage play a role in setting passive tolerance to extremes in environmental temperature. We suggest that limits to vertical distribution of M. barbatus are set by the time and degree of exploitation of the mechanisms sustaining passive thermal tolerance and the avoidance of protein damage in the warmth. © 2012 Inter-Research.