Video A. Inner and outer demibranchs of Dreissena polymorpha as observed through the endoscope (looking dorso-anteriorly). The visceral mass is on the left, inner demibranch (and its marginal groove) in the center of the frame, and outer demibranch on the right. The outer demibranch is just visible in the lower right-hand background before the inner demibranch moves to the right. The demibranchs are held curved towards the visceral mass. These gill postures are maintained despite variation in the overall orientation of the mussels.