Ocean warming reduces gastropod survival despite maintenance of feeding and oxygen consumption rates

Short-term, sub-lethal response variables are increasingly used to provide rapid indications of whole organism responses to future climate conditions. Accumulating evidence suggests, however, that these response variables may not consistently reflect whole organism responses which manifest over longer time scales. Here, we consider the effect of moderate warming on longer-term whole organism fitness, as reflected by survival, as well as two shorter-term response variables, feeding rate and oxygen consumption, for two tropical gastropod species. We found a significant reduction in survival under warming, despite no significant effect of warming on feeding or oxygen consumption rates. This result demonstrates that the maintenance of physiological rates alone is not sufficient to sustain organism survival under elevated temperatures; rather an increase in physiological process rates is likely required. Consequently, isolated short-term physiological processes may not adequately reflect longer-term whole organism responses to altered climate. For improved understanding, both short- and long-term responses need to be considered.R Script files walks user through transforming the raw data into usable data sets. Analyses are completed in Primer instead of R. There are simplified data sets that are prepared for that purpose, however the cleaning of the data and transforming raw data into rate data can be found in R.Experimental design and set-up To determine the effects of warming on tropical gastropods, we quantified the response of two species (Chlorostoma argyrostoma, Lunella coronata) to experimentally manipulate temperatures in a laboratory-based tank experiment. We considered adults of these species (maximum shell diameter C. argyrostoma 22.2 mm ± 0.4 mm, L. coronata 24.3 ± 0.5 mm; mean ± SE), which typically live to be between 5 and 10 years of age, although they can live for 30 years (Walsby 1977, Cooper & Shanks 2011). We exposed experimental tanks to current or future temperatures for six weeks from July to August 2019. The 'current' temperature (25.5 °C) was based on long-term measurements of the collection site in the summer experimental period (25.4 ± 0.2 °C was the average temperature over the experimental months, i.e. July – August, for the last 10 years of data published, i.e. 2009 – 2018, at the site nearest our collection location, i.e. Eastern Buffer station EM2; Hong Kong Environmental Protection Department 2019), while the 'future' temperature (+ 2.5 °C) simulates ocean temperature anticipated between the years 2081-2100 (Stocker et al. 2013). We used six replicate tanks for each treatment, with replicate specimens of each gastropod species in each tank (n = 3 of each species per tank). Collection of gastropods and holding conditions We collected adult gastropods (C. argyrostoma, L. coronata) and algae (Gelidium spp.) in early July at Joss House Bay in Tai Miu Wan, Hong Kong (22.27°S, 114.29°E). We then brought experimental organisms back to the Simon F.S. Li Marine Science Laboratory, Hong Kong, and cleaned all gastropod shells of epibiotic organisms. To allow for acclimation, prior to the experimental period we kept all gastropods and algae in separate holding tanks (19 L seawater; n = 10 individuals per tank) filled with natural seawater under current conditions (temperature = 26.1 °C, salinity = 35, pH = 8.03, 12:12h light:dark regime) for seven days. During this period, we provided gastropods algae collected from the field to graze upon. Experimental treatments Following acclimation, we transferred individual gastropods into experimental tanks (n = 3 gastropods of each species, 6 total individuals per tank) where each were held isolated from the others in a cage, with the same conditions as in the holding tanks described above. We provided each gastropod a piece of algae for food which was replenished weekly throughout the experimental period. Half of the tanks we maintained at current temperatures (n = 6), while gradually increasing temperature of the other half to the future treatment (n = 6). The water temperature in the 'future' treatment tanks was warmed at a rate of 0.5 °C per hour to be 2.5 °C hotter than the 'current' treatment tanks on the day before the experimental period. This rate was used to minimise the chance of mortality to result from initial heat shock (the first gastropod mortality was recorded 9 days after establishment of treatment conditions). Bar heaters were used inside of the tanks to establish and maintain experimental temperatures. iButtons monitored temperature in each tank, where they took recordings every 10 minutes (Thermochron iButton logger, Embedded Data Systems). Mean water temperature was significantly different between the current and future tanks (mean ± SE, current 25.7 ± < 0.1, future 28.2 ± <0.1 °C, F1, 10 = 98.22, p =