Cardiac and valve-gaping activity of mussels in response to common stressors in San Diego estuaries

Committee Members

• Dr. Luke Miller (Chair), Biology
• Dr. Kevin Hovel, SDSU
• Dr. Trent Biggs, SDSU

Abstract

Cardiac and Valve-Gaping Activity of Mussels in Response to Common Stressors in San Diego Estuaries

Urban estuaries face mounting pressures from anthropogenic activity and climate change. However, effective tools for monitoring biological responses alongside abiotic stressors remain limited. The need for robust monitoring is especially prevalent in San Diego, CA, where heavily urbanized, low-inflow estuaries frequently experience stressful conditions, such as hypoxia and low salinity. One promising approach is the use of bivalve biosentinels. Monitoring the behavior and physiology of bivalve biosentinels within these estuaries can provide a detailed understanding of ecosystem stress in real time.

In this thesis, I aimed to understand how the Mediterranean mussel (M. galloprovincialis) responds to common stressors in three San Diego estuaries (Los Peñasquitos Lagoon, San Diego Bay, and the Tijuana River Estuary) by measuring their heart rate and gaping behavior in the laboratory and comparing those results to what is observed in the field. Mussels were equipped with custom infrared heart rate sensors and Hall-effect gape sensors to continuously monitor cardiac activity and valve behavior.

To evaluate mussel responses to common estuarine stressors, I conducted laboratory experiments exposing individuals to either hypoxia (≤ 3 mg L1) or low salinity (<3 psu). I also conducted field deployments to track cardiac and gaping responses under naturally fluctuating conditions at two depths within the three estuaries. During the lab trials, hypoxia increased heart rate variability and short bursts of gaping activity. Exposure to low salinity induced prolonged valve closure and decreased average heart rate.

Both stressors elicited pronounced spikes in cardiac activity during recovery, likely in response to oxygen debt accrued during a transition to anaerobic metabolism during stressful periods. Mussels from the different estuaries responded similarly during laboratory trials, suggesting that gene flow homogenizes physiological traits along the San Diego coastline.

Field deployments, however, revealed strong site-specific differences: mussels in San Diego Bay and Los Peñasquitos Lagoon experienced relatively benign conditions, while mussels in the Tijuana River Estuary experienced chronic hypoxia, salinity crashes, and elevated turbidity that reduced overall survival and growth. Collectively, these findings highlight the value of bivalve biosentinels for capturing biologically meaningful responses to estuarine stress and demonstrate their potential as a complementary tool to abiotic monitoring for managing and conserving urban estuaries.