Bioenergetics and Morphology of Mussels (Mytilus trossulus) in Kenai Fjords National Park

We sampled Pacific blue mussels (Mytilus trossulus) at three rocky intertidal sites in the northern Gulf of Alaska. Two sites were located in Resurrection Bay and the third site was in Aialik Bay. Mussels were collected at the same tidal height across all three sites on a nearly monthly basis from 2014-2015. Due to logistical difficulties, we were unable to sample mussels from October to December. We collected only mussels > 20 mm in length because they are the size class typically consumed by vertebrate predators. After collection, mussels were frozen until analysis.

Mussel morphology and energetics were quantified in the lab once all samples had been collected. After thawing samples, we measured the length of mussels to the nearest millimeter, then dissected the gonads from the somatic tissue. We measured shell mass, wet gonadic mass, and wet somatic mass, and then freeze-dried the samples at -40 degrees C for 48 hours using a Labconco FreeZone freeze drier. Next, we reweighed the samples for dry gonadic and somatic mass. Using these values, we calculated the gonado-somatic index (dry gonadic mass/total dry soft tissue mass x 100) for each mussel. We then homogenized the gonadic and somatic tissues using a mortar and pestle and combined them together. Using a pellet press, we pelletized the sample. After weighing the pellet, we measured the energy density (calorie per gram) by combusting the sample using a Parr semi-micro oxygen bomb calorimeter. Then we calculated the mussel’s energy content by multiplying its energy density by the sum of the dry gonadic and somatic tissue masses.