The above being the words coming out of the UBC biology teaching lab everyday two weeks ago. Not a phrase you’d expect to hear in a place of esteemed learning, or anywhere for that matter. Well I was the one saying those words, and I was saying them to the 76 of 80 blue mussels in my seawater tanks that just would not die! (Four did die, but that wasn’t enough!).
Now, before you go animal rights on me, let me explain. I would normally never wish death on anything, I mean, I am the kind of person that cries my eyes out when an animal dies or gets injured in a movie. If a human character dies . . . meh. But this was a special case, I had spent countless hours freezing 80 mussels to -7°C over the past week, and freezing 30 mussels the week before that for my preliminary trials for my final Biology 326 project. The goal? To find the LT50 for my sample of blue mussels. The LT50 is the lower lethal temperature threshold, or the temperature causing fatality in approximately 50% of my sample individuals. Why did I want this? I was interested in the effect of ocean acidification (OA) on the lower lethal temperature of the blue mussel, a hardy and abundant species on our Pacific west coast.
And yes, some mussels are exposed to subfreezing temperatures, and can even survive temperatures less than -15°C! That’s pretty cold for a little animal (especially considering smaller mussels have a lower tolerance to cold temperatures, as I found out in my study). Intertidal mussels, including the blue mussel, are exposed to extreme fluctuations in salinity, dehydration risk, and temperature, mostly due to twice daily tidal falls. During low tides they are exposed to air, which, depending on its temperature, can cause mussel temperatures to drop well below 0°C, especially in the winter months.
I wanted to see if the predicted ocean acidification waters of the future would decrease the lower lethal temperature of blue mussel. I had mussels that were grown in either elevated CO2, an acidic environment representing future ocean conditions due to OA, or ambient CO2, representing current ocean acidity levels. Considering elevated CO2 has multiple different negative effects on many marine organisms, especially those that build calcium carbonate shells, such as the mussel, I thought for sure it would affect the lower lethal temperature of the blue mussel, likely causing the mussels to die at a higher temperature, due to a decreased tolerance of subfreezing conditions.
Despite running preliminary trials to establish the appropriate LT50 temperature to use, it turns out that the chosen -7°C was not cold enough. And because I did not have significant deaths in my mussel populations I couldn’t investigate the effect of acidified waters on LT50. Which meant I had to find a different measure to represent mussel health post freezing. The solution was counting the number of byssal threads produced by each of the 76 surviving mussels post freezing, as a proxy to their health. Counting byssal threads is not an easy task when they are in mesh containers, and produce up to 44 intertwined threads. But it was done, and kind of therapeutic in a monotonous way, maybe. And it turns out that elevated CO2 did not have a strong effect on byssal thread production, and so was not detrimental to mussel health post-freezing.
I am of course happy the mussels didn’t die, as it meant they were not strongly affected by the freezing temperature of -7°C, nor were their freezing tolerances impaired by the acidified conditions. That’s GOOD NEWS EVERYONE! However, this does not mean elevated CO2 does not have detrimental effects on mussel lower temperature tolerance, it just doesn’t at this temperature.
Overall, it worked out. I learned a new way to assess mussel health post-freezing event, and that you gotta go with the flow . . . of science.