Date of Award

Spring 2023

Project Type

Thesis

Program or Major

Biological Sciences

Degree Name

Master of Science

First Advisor

Stephen H Jones

Second Advisor

Brittany Jellison

Third Advisor

Christopher Hunt

Abstract

Bivalve aquaculture garners global ecological and economic benefits, which makes the continued health of bivalve populations paramount. Ocean acidification presents a novel pressure on bivalves. Decreased pH due to acidification limits the carbonate available for bivalves to take-up, thus inhibiting the formation and growth of their calcium carbonate-based shells. The aragonite saturation (Ω) of seawater serves a useful biologic contextualization for pH in studies of acidification and bivalves, as an Ω of 1 indicates that calcium and carbonate ions can bond to form calcium carbonate. Shell health is a central aspect of bivalve fitness as it is the main defense of these organisms against predation and exposure to disease or other unfavorable environmental conditions. Resistance of the shell against crushing force is highly relevant as aquaculture bivalves are reportedly becoming increasingly predated upon by crustaceans in the Gulf of Maine as more invasive crab species become established in the region. Juvenile bivalves seem exceptionally susceptible based on reports from the Gulf of Maine shellfish industry that has recently been losing large portions of their juvenile stock to crab predation.The blue mussel Mytilus edulis was selected for this study as it is a model species in bivalve research and has commercial relevance in the Gulf of Maine. Three cohorts of M. edulis were housed for this study. One cohort was housed in aquaria to simulate ambient pH Gulf of Maine seawater, whereas two were exposed to moderately and highly acidified conditions (0.25 pH and 0.5 pH below ambient) in aquaria. Exposure was conducted for three months and enabled examination of the significance of pH, Ω, and the length of exposure time to the physical conditions of resistance to crushing force, lengthwise growth, and mortality. Mussels with greater shell length had more resistance to crushing force than smaller mussels, and the force required to crush mussels of all length classes increased at a consistent rate of 1.25 lbf per millimeter of shell length within each length class. The average force required to crush mussels between 40 and 45 mm long was significantly greater than for mussels between 35 and 39.99 mm long. The average force required to crush mussels greater than 45 mm long was also significantly greater than for mussels between 40 and 45 mm long. Exposure time did not have a significant effect on the force required to crush the shells of M. edulis housed for the acidification treatment, but resistance to crushing force increased linearly with increased Ω. Crushing force resistance, standardized against length, was expected to increase by 0.609 lbf/mm per one unit increase of Ω. Lengthwise growth rates were unaffected by the amount of time that mussels were exposed to treatment, but lower Ω decreased lengthwise growth rates. For every one unit increase of Ω, lengthwise growth was expected to increase by 176%. Mussels housed at ambient pH levels had the highest average lengthwise growth rate, whereas the average lengthwise growth was lowest in the -0.5 pH treatment. One additional mortality out of one-thousand mussels per day was estimated by the end of the study period, but Ω did not significantly affect mussel mortality. The number of mussel mortalities in the aquaria increased over the course of the study period, but percent mortality never exceeded 5% of the total cohort in any of the treatment aquaria, and biweekly percent mortality ranged between 0.18% to 4.92%. Acidification may lower blue mussel shell resistance to crushing force and may increase the timeframe when mussels are more actively susceptible to predation by crabs and other damage through the paired effects of acidification on force resistance and lengthwise growth.

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