Physiological responses of two acorn barnacles, Tetraclita japonica and Megabalanus volcano, to summer heat stress on a tropical shore

Species living on rocky shores are subject to periodic tidal emersions and can experience intense thermal stress during daytime low tides. Tetraclita japonica and Megabalanus volcano are common intertidal barnacles in Hong Kong, with T. japonica occupying the high to mid-intertidal zone where its lower limits overlap with the upper limits of M. volcano, which is found in the lower intertidal to subtidal zone. In this study, daily profiles of body temperature and hemolymph osmolality on the shore, and responses to thermal challenge under laboratory conditions were compared between the two species to investigate their behavioral and physiological responses to heat and desiccation stress. On-shore measurements of body temperature revealed contrasting thermal profiles, with T. japonica from the upper shore experiencing greater thermal stress than T. japonica and M. volcano from the lower shore. Aerial exposure of the barnacles was associated with an increase in hemolymph osmolality. M. volcano, however, showed a greater increase in hemolymph osmolality than T. japonica at similar shore levels. When the two species were exposed to rock temperatures of 40 °C for 5 h, more individuals (94%) of T. japonica entered a coma state, and at an earlier time than M. volcano (36%). All T. japonica individuals resumed activity when re-immersed in water, whereas in contrast, all M. volcano that went into a coma state did not recover and died within 24 h. Quiescence and reversible coma during the early stages of acute heat stress, as observed in T. japonica, could therefore be adaptive responses to cope with thermal stress in the upper rocky shore.