In laboratory experiments, quagga mussels (Dreissena bugensis) survived as well as zebra mussels (Dreissena polymorpha) and equaled or exceeded their growth rate (–3 to 242% change in wet mass) when reared at 6 or 23°C and fed natural seston or Chlamydomonas at food levels ranging from 0.05–7.4 µg·L–1 chlorophyll a (chl a). Superior growth of quagga mussels was most pronounced at low food levels. We found no significant differences in per capita clearance rates (CR), functional responses, or feeding behavior between zebra and quagga mussels fed Chlamydomonas, Nannochloris, or mixed suspensions of Nannochloris and clay. Per capita CR ranged from 0.018 to 0.402 L·mussel–1·h–1 for zebra mussels and from 0.010 to 0.407 L·mussel–1·h–1 for quagga mussels. Because quagga mussels had more biomass per unit shell length, we found lower biomass-specific CR for quagga mussels. When fed natural seston, zebra and quagga mussels could selectively reject inorganic material and at the lowest seston level the assimilation efficiency of quagga mussels (81%) was significantly higher than that of zebra mussels (63%). Our experiments suggest that quagga mussels can survive, grow, and feed as well or better than zebra mussels in epilimnetic waters with either low or high productivity.

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Round gobies and dreissenid mussels, exotic species in the North American Great Lakes basin, are euryhaline organisms whose geographic spread and ecological impacts in freshwaters may be limited by low levels of dissolved ions such as calcium (Ca). We measured source populations of these exotics in the St. Lawrence River and found population densities of dreissenids (range of *1,000-6,400 individuals m -2 ) and round gobies (6-32 individuals m -2 ) similar to those in other Great Lake locations from which they have spread inland. However, we found little evidence for their secondary invasion of inland tributary rivers and lakes of northern New York State. Using natural waters collected from inland ecosystems, we ran laboratory bioassays of reproduction, growth, and survival of several life stages of zebra and quagga mussels as well as the round goby. We found little difference in the responses of zebra and quagga mussels, with each species showing moderate reproductive success, growth, and survival at Ca concentrations [ 13 mg L -1 and dramatic improvements at[18 mg L -1 . Round gobies showed moderate survival in waters with Ca concentrations [ 8 mg L -1 and high survival [ 18 mg L -1 . These bioassays are the first such experiments for quagga mussels and round gobies and show how all three species may be similarly restricted in their ability to invade and permanently colonize significant geographic regions of New York State and perhaps the US.

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1. Invasive dreissenid mussels are known to cause large ecosystem changes because of their high filter-feeding capacity, while native bioturbators may interfere with the mussels filter feeding. In this experiment, we investigated indirect environmental interactions between invasive filter-feeding dreissenid mussels (zebra and quagga mussels) and native recolonizing bioturbating hexagenid mayflies (Hexagenia) at two mussel densities and two Hexagenia densities in a 2-month long laboratory experiment. 2. Mean turbidity increased with increasing density of Hexagenia and decreased with increasing density of mussels. Turbidity showed the fastest decline at the highest mussel density, and no decline or a lower rate of decline at the low mussel density, dependent on Hexagenia density. 3. Mussel growth decreased with increasing Hexagenia density at low but not at high mussel density. Moreover, growth of mussels decreased as a function of increased mean turbidity at low mussel density but not at high mussel density. Filtering activity at the highest mussel density increased after introduction of food at the lower two densities of Hexagenia, but was constantly high at the highest Hexagenia density. 4. There was no difference in emergence of Hexagenia among the treatments, but mortality of Hexagenia was higher in the presence of mussels than in their absence. 5. Our results indicate that interactions between dreissenids and hexagenids are mediated through the sediment, and depend on density of both dreissenids and hexagenids. As the natural densities of these animals vary considerably within lakes, their growth and survival because of indirect environmental interactions is expected to vary spatially.

Abstract-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 ϳ400-2000 m) 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 ϭ 4.64) and 79.62 m (SD ϭ 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 m; zebra PII means, 236-249 m). In addition, quagga mussel larvae settled at a greater size in the offshore hypolimnion habitat (PII mean ϭ 313.64 m, SD ϭ 24.69, n ϭ 320) compared to nearshore epilimnion habitat (mean ϭ 261.89 m, SD ϭ 19.41, n ϭ 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.

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The identification of bivalve larvae and early postlarvae in plankton and benthic samples has long been a challenge, hampering both basic and applied research efforts in marine, estuarine, and freshwater environments. The usefulness of published optical micrographs of the early life-history stages of bivalves is limited because of the great morphological similarity of the imaged articulated shells, particularly at the early (straight-hinge) developmental stages. While a number of techniques have been refined in recent years and show promise for use in routine identifications of larval and post-larval bivalves (e.g., single-step nested multiplex polymerase chain reaction; in situ hybridization protocols through color coding with taxon-specific, dye-labeled DNA probes; coupled fluorescence in situ hybridization and cell sorting; and image analysis techniques using species-specific shell birefringence patterns under polarized light), no adequate comprehensive reference source exists that accurately depicts the morphology and morphometry of the shells of larval and post-larval stages of target bivalve species in a consistent format to assist in identification of such stages. To this end, scanning electron micrograph (SEM) sequences are presented of the disarticulated shell valves of laboratory-reared larval and post-larval stages of 56 species of bivalve molluscs from a wide spectrum of marine, estuarine, and freshwater habitats. Emphasis is placed on the usefulness of the morphology and morphometrics of consistentlyoriented, disarticulated shell valves and associated hinge structures in discriminating the early life-history stages of these various bivalve species. Although the scanning electron micrograph sequences presented accurately depict the gross morphologies/ morphometrics and hinge structures of the disarticulated shell valves of the larvae and/or postlarvae of the 56 species of bivalves, it is important to emphasize that a scanning electron microscope is not necessary to observe even fine hinge structures associated with the early ontogenetic stages of these species. Such structures are readily visible using a wide range of optical compound microscopes equipped with high-intensity reflected light sources, although the disarticulated shell valves must be viewed in several planes of focus to discern the often subtle details seen clearly in the scanning electron micrographs. These morphological characters provide researchers with invaluable aids for the routine identification of the early life-history stages of these species isolated from plankton and benthic samples.