ABSTRACT

Two morphological features (relative length of the inhalant siphon to shell length and relative shell height to shell length) of the deepwater morphotype of the quagga mussel (Dreissena rostriformis bugensis) were examined among individuals collected at three depths (25, 45, and 93 m) in Lake Michigan. Based on laboratory observations, mean siphon length to shell length ratios were 0.19, 0.44, and 0.20 at the three depths, respectively, and the mean ratio was signicantly greater at 45 m than at the other two depths. Mean ratio of in situ deepwater morphs estimated from still-capture frames of underwater video taken at 53-57 m in Lake Michigan was 0.74 but ranged from 0.57 to 0.78 depending on the location. In comparison, the ratio of quagga mussel shallow morphs from western Lake Erie,

as observed in the laboratory, was 0.19. We speculate that longer siphons relative to shell lengths at 45 m are an adaptation to greater mussel densities at this depth compared to 25 and 93 m. Siphon lengths of in situ mussels also appeared to be longer relative to shell lengths at locations where mussels were more dense. A longer siphon provides more exibility in placement and would thus offer a competitive advantage in obtaining food when population densities are high. The mean shell width to shell length ratio of the deepwater morph in Lake Michigan was 0.52, and there was no difference in ratios between the three depths. Water temperatures at the three depths in Lake Michigan were broadly different during seasonal stratication (June to October) and, hence, our results do not agree with previous studies that showed water temperature plays an important role in width to length ratios in shells of quagga

Thomas F. Nalepa, Vera Pavlova, Wai H. Wong, John Janssen, Jeffrey S. Houghton, and Kerrin Mabrey

CONTENTS

Abstract .................................................................................................................................................................................315 Introduction ...........................................................................................................................................................................316 Study Sites .............................................................................................................................................................................317 Methods ................................................................................................................................................................................317