Resilience of the amphipod Hyale niger and its algal host Sargassum linearifolium to heatwave conditions

Abstract

Ocean warming and increasing incidence of marine heatwaves (MHW) challenge the survival of marine organisms. While the impacts of climate scenario-based ocean warming are well investigated, the response of organisms to extreme events such as MHW is less understood. In this study, the thermal tolerance of the amphipod Hyale niger, an ecologically important amphipod in southeast Australia, and its algal host Sargassum linearifolium, were determined. For H. niger, a broad temperature range (2–33 °C) was tested in 6-day exposures in three seasons (summer, autumn and winter) and extended to 11 days in summer. Despite the 7 °C difference in mean habitat temperature across seasons, H. niger had a similar broad thermal optimum (Topt) range with ≥ 90% survival across a 17–20 degree temperature range. The upper lethal temperature with 50% mortality in the 6-day tests was also similar across seasons (LT50 29–30 °C). In 11-day tests in summer, the LT50 was reduced to 26 °C. Cold tolerance of H. niger indicated potential for poleward migration. Sargassum linearifolium had a similar broad thermal tolerance with a 50% reduction in the photosynthetic capacity at 33 °C. Temperature significantly affected survival of H. niger depending on the size and sex. The decrease in the Topt range over time pointed to the deleterious influence of prolonged heatwaves. The broad thermal tolerance of H. niger and S. linearifolium suggests that this important amphipod–host system may be resilient to habitat warming.