The ecophysiology and dynamic energy budget of Septifer virgatus

Abstract

The black-ribbed mussel, Septifer virgatus, is a northern species which forms dominant bands in the mid-low levels of exposed rocky shores along the Northwest Pacific coast. S. virgatus is thought to reach its southern distributional limits just south of Hong Kong. In Hong Kong, mass mortalities of S. virgatus were recorded with the onset of the hot and wet season, with mussel cover decreasing at the upper limit of the mussel bed at Shek O from 82 % (~1746 ind m-2) in May to 9 % (~ 38 ind m-2) in August 2012. Elevated temperatures played an important role in driving physiological responses of the mussel, including heart rate, oxygen consumption and mantle water and haemolymph osmotic concentrations. The upper thermal limit in water (36 °C, as indicated by the Arrhenius breakpoint temperature, ABT), was unlikely to be reached by local seawater, but the ABT in air (~ 41 °C) was often lower than rock temperatures on Hong Kong shores, suggesting S. virgatus is living at its thermal limit. When immersed, both heart rates and oxygen consumption increased from 25 – 35 °C and dropped beyond 35 °C; whilst in air, haemolymph and mantle water osmolalities increased with duration (0 vs 6 hrs) and temperature (30 vs 40 °C) and 60 % of individuals died at 40 °C after 6 hours. Mortality of S. virgatus is, therefore, linked to the high temperatures and desiccation stress experienced on the shore, which are likely to impose high energetic costs. To better understand the performance of S. virgatus under natural conditions, and the energetic implications of thermal stress on its survival and fitness and hence potential distribution, a Dynamic Energy Budget (DEB) model was constructed. The model had a high goodness of fit. Mussels were predicted to live 5 years, and their main growth and energy reserves were derived from high assimilation during the hot and wet season when food was most abundant, while another slower growth period and reproduction occurred during the cool and dry season. Energy stored in the reserve during growth periods was likely to be allocated to combat thermal stress during the hot and wet season and reproduction during the cool and dry season. The DEB model was used to investigate the energy allocation strategies of S. virgatus under the most conservative and extreme scenarios associated with climate change as projected by the IPCC. The model predicted that reproductive events throughout the mussels’ life-span would be reduced by 30 – 70 %, despite an increase (18 – 470 %) in number of eggs per event. This reduction in the number of reproductive events might not be able to sustain local populations due to the high annual mortality. Such a decrease in performance and fitness, associated with not being able to reach the energy threshold necessary for somatic maintenance, suggests that S. virgatus may be limited to living lower on the shore, where conditions and duration of exposure are more benign, and ultimately experience a northwards range shift as environmental temperatures increase.