Intertidal organisms experience a dramatic suite of fluctuating abiotic stressors. Among others, daily shifts in tide height and seasonal changes in water temperature are key governing factors in species zonation and viability. Mytilus californianus (M.c), the California Mussel, can be found in both inter‐ and subtidal populations and experience unique cycles of emersion as a result. Intertidal (I) organisms, which are subject to daily emersion periods during low tide, experience rapid changes in body temperature and oxygen availability. This fluctuation contrasts sharply with subtidal (S) M.c. which remain submerged for most or all of tidal each cycle. We conduct reciprocal treatments with four experimental populations (I‐>I, I‐>S, S‐>S, S‐>I) using respirometry and qPCR to quantify adaptive capacity, acclimation, and physiological differentiation within these two populations.Mussels were collected from intertidal and subtidal populations in Ventura, California and subjected to simulated reciprocal transplant within lab aquaria (origin/treatment: S/S, S/I, I/I, I/S). We tested constant water temperatures of 14°C and 19°C. The intertidal treatments underwent daily six‐hour emersion periods at 20°C over a period of four weeks. At the conclusion, a final air emersion treatment was performed at one of three different temperatures (7°C, 20°C, 35°C), followed by two‐hour respirometry trials. Gills and mantle were dissected for respirometric rate calculations and molecular analysis. We synthesized cDNA from extracted RNA was used to perform gene expression analysis of genes implicated in desiccative stress and the glyoxylate shunt (MLS, TPS, AQP4), redox homeostasis (CAT), oxidative stress (FLBN1), and heat shock response (HSP70, HSP104) using a CFX96 RT‐PCR System. We quantify gene expression across all treatments and pair these results with corresponding respirometric rates for full‐factorial analysis of stress response and adaptive capacity.Multiple Iinear regression analysis indicated tidal origin and tidal treatment were significantly correlated with respiration rate (p=.001, p=.025, respectively). The S/I mussels had significantly higher respiration rates than I/S (p<.0001), suggesting that transfer of subtidal mussels to an intertidal treatment is more challenging than transfer of intertidal mussels to a subtidal treatment. I/S mussels also had significantly lower respiration rates that I/I and S/S (p =.0011, p =.0410 respectively). Respiration was profoundly influenced by emersion temperature in mussels collected from the subtidal zone, but not from the intertidal zone (p=.0027, p=.0934, respectively). The 7°C emersion temperature spiked respiration rate relative to 20°C emersion temperature (p =.0018). Intertidal mussels acclimated to periodic emersion are likely better suited to deal with fluctuations in air temperatures. Coupled with our quantitative PCR findings, we demonstrate differential adaptive capacities for these populations and the divergent fates these populations of Mytilus californianus will incur with increasing sea and air temperatures.Support or Funding InformationWe would like to thank CSUCI and RSCA for funding this research.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Rocky intertidal organisms, such as the sessile West Coast mussel Mytilus californianus (M.c.), experience a dramatic set of acute and chronic environmental stressors. Mussels transition daily from submerged to emerged conditions, experiencing seasonal temperature changes in water, daily shifts in air temperature, and fluctuating durations of exposure based on tidal periods. M.c. must therefore tolerate both gradual and rapid shifts in environmental conditions. To better quantify differential individual outcomes of M.c. to simultaneous acute and chronic stressors, we employ both phenotypic and genotypic methods. We couple respirometric rates to differential gene expression on a suite of genes implicated in stress response: Heat Shock Protein 70 (HSP70; chaperone), Hypoxia‐Induced Factor 1α (HIF‐1α; hypoxia), Glutathione Peroxidase (GPX; oxidative stress), Thioredoxin Reductase (TRR; oxidative stress), and Caspase‐8 (CASP8; apoptosis), referenced to Elongation Factor 1α (EF‐1α; reference).Mussels were collected from Ventura, CA and acclimated for three weeks prior to study. Our full‐factorial experiment subjected M.c. to a range of conditions: water temperatures (14°C, 19°C, 24°C), air temperatures (7°C, 18°C, 35°C), and emersion times (4 hours, 8 hours) using 12 mussels per treatment. Two‐hour in vivo respirometry trials were completed using lab‐designed Chambers for Organismal Response to Environmental Stressors (CORES) and a NeoFox spectrophotometer (Ocean Optics, Dunedin, FL). Gills were excised and divided evenly for dry‐weight respirometry measurements and RNA extraction. Extracted RNA was converted to cDNA for quantitative PCR in technical triplicate using a CFX96 RT‐PCR System (BioRad, Hercules, CA). We quantify gene expression of HSP70, HIF‐1α, GPX, TRR, and CASP8 across all 18 experimental conditions and pair these results with corresponding respirometric rates.Mytilus californianus show a strong acute response when exposed to high air temperature (35°C) across all water conditions, with HSP70 expression elevated by 750x −2460x over 18°C air treatments. However, HSP70 is slightly downregulated (2.5–6.3x) as water temperature increases with highest expression found at 14°C. This may be due to the differential response to acute (air) versus chronic (water) stressors. When considering genes implicated in longer term organismal response, such as CASP8 apoptotic repair mechanisms, the response rate is different, however. We see slight rate changes (1–4x) between air temperature conditions, but a significant downregulation (11–59x) of CASP8 at 19°C water temperature when compared to 14°C and 24°C treatments. We also see a characteristic increase in respiration rates (2–3x) with increasing water temperature (14 to 24°C) which is broadly in line with the Q10 hypothesis. Notably, M.c. exhibit a wide range of individual respirometric responses to the most stressful condition set (24°C water, 35°C air, 8‐hour emersion). Variability also increases in larger deviations of HSP70 expression rates at higher temperatures, suggesting that individual variation may play a large role in determining outcomes under extreme multi‐stress conditions.Support or Funding InformationThis work is supported by CSU Channel Islands and the Santa Rosa Island Research Station.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.