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 twit 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.