UBC Theses and Dissertations
Mechanisms and consequences of surviving freezing in the bay mussel, Mytilus trossulus Kennedy, Jessica
Many intertidal invertebrates are freeze tolerant, meaning that they can survive ice formation within their bodies when exposed to freezing air temperatures during low tides. In my thesis I addressed two key questions regarding intertidal invertebrate freeze tolerance using the intertidal mussel Mytilus trossulus. First: What biochemical mechanisms enable freeze tolerance in intertidal invertebrates? Second: How do sublethal single and repeated freeze exposures negatively impact intertidal invertebrates? To address the first question, I investigated the role of osmolytes in mussel freeze tolerance, which may be cryoprotective by mitigating osmotic stress caused by freezing. I sought to determine if different osmolytes are interchangeable cryoprotectants (acting as colligative cryoprotectants), or if each osmolyte has unique a cryoprotective role, beyond just contributing to increased intracellular osmolarity (and thus act as non-colligative cryoprotectants). I did this by manipulating the composition of mussels’ intracellular osmolyte pools, and then testing how mussel freeze tolerance changed. I found that mussel freeze tolerance did not change after taurine and betaine increased in concentration, significantly decreased after alanine and glycine increased in concentration, and increased with increasing TMAO concentrations, indicating that TMAO may be cryoprotective. Overall, my findings indicate that osmolytes are non-colligative cryoprotectants. Next, I explored how mussels are impacted by sublethal freezing. I found that mussels do not filter feed for the first four hours post-freeze, but resume filter feeding 24 hours after freezing, which corresponds to my microscopic examinations of mussel gill tissues after freezing which reveal freeze-related damage. I also found that freezing decreased mussel posterior adductor strength, although this effect did not lead to an increase in mussel susceptibility to sea star predation. Finally, I found that mussels survived shorter, repeated freezes (where mussels received 1 day for recovery between freezes) better than prolonged freezes, when total time frozen is held constant. Thus, mussels are well-adapted to survive the short freezing events which they regularly encounter in their habitat, and one mechanism behind this survival could be TMAO accumulation. Further, the effects of sublethal freezing on mussel performance are limited, although how these effects scale up to entire mussel beds remains unknown.
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