Summer mortality: effects on the distribution and abundance of the acorn barnacle Tetraclita japonica on tropical shores

In the Hong Kong rocky intertidal, the demography of the acorn barnacle Tetraclita japonica is largely determined by regular, intense mortality events in the summer and subsequent pulses of recruitment in late summer. During 2 consecutive summers from 2000 to 2001, 34 to 52 and 98 to 99% of T japonica were killed on the mid- and high shore, respectively, with younger cohorts suffering higher mortality in the mid-shore. Recruitment of T japonica occurs in late summer; as a result, the population in the high shore consists of a single cohort of new recruits, while the mid-shore supports 2 cohorts (new recruits and adult survivors). To investigate how thermal stress affects acorn barnacle populations, air and rock temperatures and sub-lethal physiological measures (body temperature, sinus pulsation rate and haemolymph osmolality) of T japonica were taken on the mid- and high shore during daytime, low spring tides in June-July 2001. Mean body temperature increased gradually after emersion, reaching a maximum of similar to 48 degrees C at noon, 6 to 8 degrees C higher than the adjacent rock surface. T japonica entered a heat-induced coma when body temperatures reached 45 degrees C. Osmolality of the haemolymph and sinus pulsation rate increased with body temperature as a result of water loss and possibly altered haemolymph pressure; both were greater in high than in mid-shore individuals, with barnacles on horizontal surfaces being hotter than those on vertical surfaces. Body temperature and sinus pulsation rate differed between shore levels, but were similar at spatial scales of 1 to 20 m at the same tidal height. Haemolymph osmolality, however, varied over this distance, probably due to individual variation in the amount of mantle water trapped before emersion and possible micro-habitat differences. Heat and desiccation stress, therefore, play an important role in determining the life history of T japonica by limiting their distribution and abundance and influencing the demography of the population at different spatial scales.