Quantifying Adaptive Capacity and Differentiation in California Mussels (Mytilus californianus) through Respirometry and Gene Expression
Authors: Dilly Geoff , Senger Christiana , Brazier Garrick , Williamson Clay 
Published: June 2021

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.