Abstract
In the rocky intertidal environment, the frequency and duration of heatwaves have increased over the last decade, possibly due to global climate change. Heatwaves often result in lethal or sub-lethal disturbances in benthic animals by changing their metabolic activities. In this study, we investigated the impacts of extreme heatwave stress on the hemocyte functions of Mytilisepta virgata and subsequent mortality to gain a better understanding of the potential causes and consequences of mass mortality events in this mussel during summer. We discriminated three types of hemocytes in the hemolymph, granulocytes, hyalinocytes, and blast-like cells, using flow cytometry and revealed that granulocytes were the major hemocyte involved in cellular defensive activities, such as phagocytosis and reactive oxygen species (ROS) production. For the experiment, mussels were exposed to a 40°C air temperature for 12 h per day over 5 days under laboratory conditions as a simulated semi-diurnal tidal cycle. Mortality began to occur within 3 days after beginning the experiment, and all mussels had died by the end of the experiment. Flow cytometry indicated that the mussels exposed to high air temperatures produced significantly more ROS than did the control mussels within 2 days after the onset of the experiment, which may have caused oxidative stress. Such high levels of ROS in the hemolymph increased DNA damage in hemocytes after 3 days of exposure and decreased the phagocytosis of hemocytes 4 days after the experiment began. The observed mortality and decline in immune capacity suggested that an extreme heat event occurring in the rocky intertidal ecosystem during summer could exert sublethal to lethal impacts on macrobenthic animals.
Highlights
IntroductionThe earth has experienced a steady increase in temperature due to global climate change (IPCC, 2018)
Over the last century, the earth has experienced a steady increase in temperature due to global climate change (IPCC, 2018)
The light microscopic analysis revealed that M. virgata had three distinct types of hemocytes in the hemolymph, as in other marine bivalves [Mytilus coruscus (Yang et al, 2015), Crassostrea gigas (Donaghy et al, 2010), C. ariakensis (Donaghy et al, 2009b), C. nippona (Hong et al, 2014), Saccostrea kegaki (Hong et al, 2013), S. glomerata (Aladaileh et al, 2007), Ostrea circumpicta (Hong et al, 2013), Hyotissa hyotis (Hong et al, 2013), Anodonta cygnea (Jamili et al, 2009)], including granulocytes, hyalinocytes, and blast-like cells (Figure 3)
Summary
The earth has experienced a steady increase in temperature due to global climate change (IPCC, 2018). Concurrent with global warming, an increase in the intensity and frequency of extreme high-temperature events, referred to as heatwaves, have been observed worldwide The earth’s average temperature in 2018 was the fourth highest in 140 years (2018 heat wave, 2021). The summer of 2018 in the Northern Hemisphere, such as Europe, North America, and Northeast Asia, experienced extremely high temperatures (2018 heat wave, 2021). In Northeast Asia, including South Korea, Japan, and China, the highest air temperature during the summer of 2018 was >40◦C (2018 Northeast Asia heat wave, 2021). The frequency of extreme heatwaves is expected to increase due to global warming (Perkins-Kirkpatrick and Lewis, 2020)
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