Species and epilimnion/hypolimnion‐related differences in size at larval settlement and metamorphosis in Dreissena (Bivalvia)

Recently settled postlarval quagga mussels ( Dreissena bugensis ) and zebra mussels ( D. polymorpha ) were examined using optic microscopy to determine planktonic shell growth and size at settlement and metamorphosis from two habitats in Eastern Lake Erie: nearshore epilimnion and offshore hypolimnion. Postlarvae (shell length ~400‐2000 µm) were collected from various substrates between 1992 and 1995. Planktonic shell growth and size at settlement and metamorphosis were determined by measuring height of the prodissoconch I (PI) and prodissoconch II (PII) on right valves. Mean PI height was 79.07 (SD = 4.64) and 79.62 µm (SD = 4.28) for the quagga and zebra mussel, respectively, and did not differ between species or across habitats. There was, however, a distinct between‐species difference in size at settlement and metamorphosis (PII size), with larvae of the quagga mussel settling at significantly larger sizes than those of the zebra mussel (nearshore/epilimnion data: quagga PII means, 256‐284 µm; zebra PII means, 236‐249 µm). In addition, quagga mussel larvae settled at a greater size in the offshore hypolimnion habitat (PII mean = 313.64 µm, SD = 24.69, n = 320) compared to nearshore epilimnion habitat (mean = 261.89 µm, SD = 19.41, n = 207). The additional 28% of larval shell (PII) secreted by offshore hypolimnion quagga mussels may be linked to several factors, including a prolonged planktonic period. This study is the first to document the relationship between offshore distance and size at settlement in a bivalve.