Emersion Profiles and Recovery: Quantifying Physiological Responses in Mussels

Rocky intertidal zones are in a constant state of flux. Sessile intertidal organisms must tolerate a suite of dynamic abiotic conditions: temperature, wave action, pH, hypoxia, and most dramatically, emersion. The duration of emersion also changes daily depending on the tides; additionally, air temperatures shift with diurnal cycles, seasonal differences, and weather conditions. Thus, the West Coast mussel, Mytilus californianus ( M.c .), a key indicator species of intertidal community health, must tolerate rapid shifts between a wide variety of submerged and emerged conditional regimes. Previous literature on M.c. stress responses to conditional shifts have focused primarily on respirometric rates during immersion experiments and/or protein turnover with relation to emersion. Our full‐factorial study combines these approaches to explore the respirometric responses of M.c. to emersion at a range of environmentally relevant thermal regimes (air: 7, 18, and 35°C; water: 14 and 24°C), emersion durations (4 and 8 hour), and recovery times (0 and 30 minute) using a custom‐designed respirometry system. To conduct in vivo respirometry experiments, we built a new customizable isolation system named the Chambers for Organismal Response to Environmental Stressors (CORES) to interface with a NeoFox spectrophotometer (Ocean Optics, Dunedin, Fl). This low‐cost, novel system allows for continual non‐invasive measurements of dissolved oxygen on a variety of marine invertebrates. Mytilus californianus were collected from Ventura, California (34.272° N, −119.287° W) and acclimated at temperature in lab aquaria. After 2‐hour respirometric trials, gills were excised and divided evenly for dry‐weight respirometric calculations and RNA extractions. We quantify key gene expression stress responses using RT‐qPCR with HSP70, HSC71 (thermal), catalase (oxidative), phosphofructokinase (metabolic), and caspase (apoptosis) on a CFX96 RT‐PCR System (Bio Rad, Hercules, CA). Calculated respiration rates were positively correlated with water temperature and emersion duration, indicating that Mytilus californianus needed to regenerate their depleted aerobic capacity after exposure to these stresses. Additionally, our data suggest M.c. maintained at 24°C experience a stress‐induced state of torpor following 8‐hour periods of emersion at boundary air temperatures (7 and 35°C). A differential response in respiration rate was observed at the median air temperature of 18°C, indicating that air temperatures at the boundaries of environmental conditions increase the time of recovery to homeostasis after mussels are re‐submerged. Support or Funding Information This work is supported by CSU Channel Islands and an NSF I‐Corps CSUPERB sub‐award.