Mortality of zebra mussel, Dreissena polymorpha, veligers during downstream transport

1. Streams flowing from lakes which contain zebra mussels, Dreissena polymorpha , provide apparently suitable habitats for mussel colonization and downstream range expansion, yet most such streams contain few adult mussels. We postulated that mussel veligers experience high mortality during dispersal via downstream transport. They tested this hypothesis in Christiana Creek, a lake-outlet stream in south-western Michigan, U.S.A., in which adult mussel density declined exponentially with distance downstream.
2. A staining technique using neutral red was developed and tested to distinguish quickly live and dead veligers. Live and dead veligers were distinguishable after an exposure of fresh samples to 13.3 mg L⁻⁻¹ of neutral red for 3 h.
3. Neutral red was used to determine the proportion of live veligers in samples taken longitudinally along Christiana Creek. The proportion of live veligers (mean ± SE) declined from 90 ± 3% at the lake outlet to 40 ± 8% 18 km downstream.
4. Veligers appear to be highly susceptible to damage by physical forces (e.g. shear), and therefore, mortality in turbulent streams could be an important mechanism limiting zebra mussel dispersal to downstream reaches. Predictions of zebra mussel spread and population growth should consider lake-stream linkages and high mortality in running waters.     

Specific amplification of the 18S rRNA gene as a method to detect zebra mussel (Dreissena polymorpha) larvae in plankton samples

An important issue in the management of zebra mussel (Dreissena polymorpha) populations is early, rapid, and accurate detection of the planktonic larvae (veliger) of the zebra mussel. The goal of this study was to explore the feasibility of developing a molecular approach for the detection of zebra mussel larvae in diverse environments. In this study a Dreissena polymorpha-specific 18S ribosomal RNA gene targeted oligonucleotide primer (ZEB-715a) and Polymerase Chain Reaction (PCR) assay was developed and compared with cross-polarized microscopy as a means to detect zebra mussel veligers in plankton samples. The design of the zebra mussel-specific primer was facilitated by sequencing nearly the complete 18S rRNA gene from the zebra mussel and three other closely related freshwater Veneroids including the quagga mussel (D. bugensis), the dark false mussel (Mytilopsis leucophaeata), and the Asian freshwater clam (Corbicula fluminea). The specificity of the primer for the zebra mussel was empirically tested by using the primer as a direct probe in a blot hybridization format. A single veliger in a plankton sample could be detected by PCR using this approach. Veliger detection sensitivity using the PCR approach was estimated to be over 300 times more sensitive than cross-polarized light microscopy based techniques. Cross-polarized light microscopy and the PCR technique were used to identify the presence of zebra mussel larvae in plankton samples that were collected from a variety of natural and industrial water sources. Detection results (presence or absence) were generally consistent between the two methods. Although additional studies will be required before routine application of molecular based veliger detection technology is available, a long-term goal of this work is the application of molecular technology to the development of a field device for the routine detection and quantification of zebra mussel veligers.     

Suppression of microzooplankton by zebra mussels: importance of mussel size

1. The zebra mussel (Dreissena polymorpha) is amongst the most recent species to invade the Great Lakes. We explored the suppressive capabilities of mussels 6–22-mm in size on Lake St Clair microzooplankton (< 240)μm) in laboratory experiments. 2. Absolute suppression of rotifers and Dreissena veliger larvae was proportional to mussel shell length for individuals larger than 10 mm; larger zooplankton, mainly copepod nauplii and Cladocera, were not affected. Mussel clearance rates on rotifers generally exceeded those on veligers, although rates for both increased with increasing mussel size. Rotifer-based clearance rates of large (22 mm) mussels approached published values for phytoplankton food. 3. Most zooplankton taxa, particularly rotifers, declined significantly in western Lake Erie during the late 1980s concomitant with the establishment and population growth of zebra mussels in the basin. Densities of some taxa subsequently increased, although rotifers and copepod nauplii densities remained suppressed through 1993. Available evidence indicates that direct suppression by Dreissena coupled with food limitation provides the most parsimonious explanation for these patterns.     

Comparative Biology of Zebra Mussels in Europe and North America: An Overview

SYNOPSIS. Since the discovery of the zebra mussel, Dreissenapolymorpha, in the Great Lakes in 1988 comparisons have beenmade with mussel populations in Europe and the former SovietUnion. These comparisons include: Population dynamics, growthand mortality rates, ecological tolerances and requirements,dispersal rates and patterns, and ecological impacts. NorthAmerican studies, mostly on the zebra mussel and a few on asecond introduced species, the quagga mussel, Dreissena bugensis,have revealed some similarities and some differences. To dateit appears that North American populations of zebra musselsare similar to European populations in their basic biologicalcharacteristics, population growth and mortality rates, anddispersal mechanisms and rates. Relative to European populationsdifferences have been demonstrated for: (1) individual growthrates; (2) life spans; (3) calcium and pH tolerances and requirements;(4) potential distribution limits; and (5) population densitiesof veligers and adults. In addition, studies on the occurrenceof the two dreissenid species in the Great Lakes are showingdifferences in their modes of life, depth distributions, andgrowth rates. As both species spread throughout North America,comparisons between species and waterbodies will enhance ourability to more effectively control these troublesome species.     

A developmental bottleneck in dispersing larvae: implications for spatial population dynamics

We found evidence for a critical population bottleneck at a developmental‐stage transition in larvae of the zebra mussel Dreissena polymorpha Pallas from field estimates of mortality. Identification of this critical period in the field was made possible by closely tracking cohorts of larvae over 5 days of development as they dispersed 128 km in a river system. The presence of a survival bottleneck during development was confirmed in laboratory studies of zebra mussel larvae. Development‐specific mortality has important implications for spatial population dynamics of the zebra mussel in particular, and all species with indirect development in general. Marine reserves that do not take development‐specific mortality into account may dramatically underestimate reserve size needed to protect rare and/or exploited marine populations. Conversely, for the zebra mussel, the lower contribution of dispersing individuals to population growth downstream of reserves can lead to more feasible control through the blocking of dispersal.     

Biology of the exotic zebra mussel,Dreissena polymorpha,in relation to native bivalves and its potential impact in Lake St. Clair

The zebra mussel, Dreissena polymorpha, is a new exotic species that was introduced into the Great Lakes as early as the fall of 1985. It differs markedly from native species of bivalves in its: (i) shell form; (ii) mode of life; (iii) reproductive potential; (iv) larval life cycle; (v) population dynamics; (vi) distribution, (vii) dispersal mechanisms; (viii) physiology; (ix) potential impact on the ecosystem; and (x) impact on society and the economy. In body form, it has an anterior umbone, a flat ventral surface with permanent aperature for the byssal apparatus and a shape that together make the animal well adapted for life on a hard surface. The shell has a zebra-stripe pattern, a heteromyarian muscle condition and lacks hinge teeth which make it easily identifiable from native bivalves. The zebra mussel is strongly byssate and has an epifaunal mode of life not seen in native bivalves. The species is dioecious and has external fertilization, the eggs developing into pelagic veligers which remains planktonic for approximately 4 weeks. Gametogenesis begins in late winter to early spring, veligers appear in the water column in late May to early June and disappear in mid to late October in Lake St. Clair. Adults live for about 2 years and have very rapid growth rates. Maximum shell lengths average 2.3 to 2.5 cm. Standing crops as high as 200 000 m^-2 are present in the 1-m depths of the Ontario shores. Infestations may be interfering with the normal metabolism of native unionid clams and there is potential of the unionid clam populations being reduced or even eliminated from Lake St. Clair.     

Retention and supply of zebra mussel larvae in a large river system: importance of an upstream lake

1. Persistence of zebra mussel populations in river systems probably depends upon the presence of upriver sites capable of hosting self‐recruiting adult populations that act as sources of larvae. In this paper we examine the importance of Lake Pepin, a natural riverine lake in the Upper Mississippi River, as an upriver source of larvae to the downstream populations of zebra mussels. 2. Field studies and modelling suggest that Lake Pepin plays a major role in maintaining zebra mussel populations in the Upper Mississippi River. Long water residence times in Lake Pepin allow for self‐recruitment under the right hydraulic conditions. Larval abundance was low to absent upstream of the lake but increased dramatically downriver in all 3 years of the study. Travel time estimates in the Upper Mississippi River show that newly fertilised larvae drifting out of Lake Pepin can contribute substantially to the major downstream peak in larval abundance. In contrast, backwater and other off‐channel sites are unlikely to drive main‐channel abundance patterns. Larval abundances in off‐channel sites were less than or equal to those in the main channel. 3. A key factor in assessing the importance of Lake Pepin as a source population was the abundance of early stage, unshelled larvae. Studies that consider only abundances of older shelled stages (visible by cross polarised lighting) may yield misleading results. Results of this study suggest that efforts to control zebra mussels in the Upper Mississippi River should focus on controlling adult populations within Lake Pepin and reducing or eliminating larvae exiting the lake.     

Scale‐dependent post‐establishment spread and genetic diversity in an invading mollusc in South America

Aims Our study aimed to characterize the dispersal dynamics and population genetic structure of the introduced golden mussel Limnoperna fortunei throughout its invaded range in South America and to determine how different dispersal methods, that is, human‐mediated dispersal and downstream natural dispersal, contribute to genetic variation among populations. Location Paraná–Uruguay–Río de la Plata watershed in Argentina, Brazil, Paraguay and Uruguay. Methods We performed genetic analyses based on a comprehensive sampling strategy encompassing 22 populations (N = 712) throughout the invaded range in South America, using the mitochondrial cytochrome c oxidase subunit I (COI) gene and eight polymorphic nuclear microsatellites. We employed both population genetics and phylogenetic analyses to clarify the dispersal dynamics and population genetic structure. Results We detected relatively high genetic differentiation between populations (F_ST = ?0.041 to 0.111 for COI, ?0.060 to 0.108 for microsatellites) at both fine and large geographical scales. Bayesian clustering and three‐dimensional factorial correspondence analyses consistently revealed two genetically distinct clusters, highlighting genetic discontinuities in the invaded range. Results of all genetic analyses suggest ship‐mediated ‘jump’ dispersal as the dominant mode of spread of golden mussels in South America, while downstream natural dispersal has had limited effects on contemporary genetic patterns. Main conclusions Our study provides new evidence that post‐establishment dispersal dynamics and genetic patterns vary across geographical scales. While ship‐mediated ‘jump’ dispersal dominates post‐establishment spread of golden mussels in South America, once colonies become established in upstream locations, larvae produced may be advected downstream to infill patchy distributions. Moreover, genetic structuring at fine geographical scales, especially within the same drainages, suggests a further detailed understanding of dynamics of larval dispersal and settlement in different water systems. Knowledge of the mechanisms by which post‐establishment spread occurs can, in some cases, be used to limit dispersal of golden mussels and other introduced species.     

Filtering impacts of larval and sessile zebra mussels (Dreissena polymorpha) in western Lake Erie

Summary We assessed the feeding biology of veliger larvae of the introduced zebra mussel (Dreissena polymorpha Pallas) in laboratory experiments using inert microspheres as food analogues. Mean clearance rate on 2.87-m beads ranged between 247 and 420 L veliger^–1 day^–1. Clearance rate was unrelated to bead concentration up to 100 beads L^–1, but was positively correlated with veliger shell length. Clearance rates of Dreissena veligers are within the range of those reported for marine bivalve veligers of similar size and for herbivorous Great Lakes microzooplankton, but are orders of magnitude lower than those of settled, conspecific adults. The impact of settled zebra mussel grazing activities on phytoplankton stocks may be up to 1162 times greater than that exerted by veliger populations in western Lake Erie. Based on 1990 size-frequency distributions and associated literature-derived clearance rates, reef-associated Dreissena populations in western Lake Erie (mean depth 7 m) possess a tremendous potential to filter the water column (up to 132 m³ m^–2 day^–1) and redirect energy from pelagic to benthic foodwebs. Preliminary analyses indicate that chlorophyll a concentration is strongly depleted (<1 g L^–1) above Dreissena beds in western Lake Erie.     

Long-term demography of a zebra mussel (Dreissena polymorpha) population

1. We used long‐term data and a simulation model to investigate temporal fluctuations in zebra mussel populations, which govern the ecological and economic impacts of this pest species. 2. The size of the zebra mussel (Dreissena polymorpha) population in the Hudson River estuary fluctuated approximately 11‐fold across a 13‐year period, following a cycle with a 2–4 year period. 3. This cycling was caused by low recruitment during years of high adult population size, rapid somatic growth of settled animals, and adult survivorship of 50% per year. 4. Adult growth and body condition were weakly correlated with phytoplankton biomass. 5. The habitat distribution of the Hudson's population changed over the 13‐year period, with an increasing proportion of the population spreading onto soft sediments over time. The character of soft‐sediment habitats in the Hudson changed because of large amounts (mean = 34 g DM m^?2) of empty zebra mussel shells now in the sediments. 6. Simulation models show that zebra mussel populations can show a range of long‐term trajectories, depending on the balance between adult space limitation, larval food limitation, and disturbance. 7. Effective understanding and management of the effects of zebra mussels and other alien species depend on understanding of their long‐term demography, which may vary across ecosystems.     

Wetlands as barriers: effects of vegetated waterways on downstream dispersal of zebra mussels

1. Stream flow is a major vector for zebra mussel spread among inland lakes. Veligers have been found tens to hundreds of km from upstream source lakes in unvegetated stream and river systems. It has been suggested, however, that the downstream transport of zebra mussels is restricted by wetland ecosystems. We hypothesized that vegetated waterways, (i.e. wetland streams) would hinder the downstream dispersal of zebra mussels in connected inland lake systems. 2. Veliger abundance, recruitment and adult mussels were surveyed in four lake‐wetland systems in southeastern Michigan, U.S.A. from May to August 2006. Sampling was conducted downstream of the lakes invaded by zebra mussels, beginning at the upstream edge of aquatic vegetation and continuing downstream through the wetland streams. 3. Veliger abundance decreased rapidly in vegetated waterways compared to previously reported rates of decrease in non‐vegetated streams. Veligers were rarely found more than 1 km downstream from where vegetation began. Newly recruited individuals and adults were extremely rare beyond open water in the wetland systems. 4. Densely vegetated aquatic ecosystems limit the dispersal of zebra mussels downstream from invaded sources. Natural, remediated and constructed wetlands may therefore serve as a protective barrier to help prevent the spread of zebra mussels and other aquatic invasive species to other lakes and ecosystems.     

Zebra mussel (<Emphasis Type="Italic">Dreissena polymorpha</Emphasis>) affects the feeding ecology of early stage striped bass (<Emphasis Type="Italic">Morone saxatilis</Emphasis>) in the Hudson River estuary

Variability in the feeding ecology of young fishes over short and long time scales in estuaries is likely to affect population dynamics. We studied 14 years of early stage Striped Bass feeding ecology in the Hudson River Estuary over a 25-year time span, including years in which invasive zebra mussels markedly altered energy flow within the estuary. We predicted that feeding success would be low and that diet composition would be altered during years of high zebra mussel impact, particularly in upriver locations where mussels occur. Feeding success in the short term was indicated by volume of gut contents and in the long term by dry mass at length, i.e. condition; these measures were positively intercorrelated and varied significantly year to year. We tested for associations between condition and multiple biotic and abiotic environmental variables. There was a strong negative effect of zebra mussel grazing rate on condition in upriver locations and a weak positive effect in downriver locations. In upriver locations, condition was 33% higher when local salinity was high and zebra mussel grazing rates were low, whereas in downriver locations, condition was 35% higher when zebra mussel grazing rates and copepod abundance were high and local dissolved oxygen was low. Copepods, amphipods, mysids, and Leptodora constituted the highest prey-specific index of relative importance throughout the estuary. There was no evident effect of the zebra mussel invasion on diet composition. This long-term study corroborates the inferences of earlier studies that zebra mussels reduced early-stage striped bass growth rate.     

Range expansion of quagga mussels Dreissena rostriformis bugensis in the Volga River and Caspian Sea basin

In 1992, we discovered populations of the nonindigenous quagga mussel Dreissena rostriformis bugensis in the middle reaches of the Volga River. The same species was found in samples collected between 1994 and 1997 in the Volga delta and in shallow areas of the Northern Caspian Sea. D. r. bugensis always co-occurred with its more widespread congener, the zebra mussel D. polymorpha (Pallas 1771). The quagga mussel's contribution to total Dreissena abundance increased over time in the middle Volga reservoirs and Volga River delta. D. r. bugensis was common in the Volga portion of Rybinsk Reservoir during 1997 and, by 2000, it was in Uglich, Rybinsk and Gorky Reservoirs on the Upper Volga River. D. r. bugensis was neither found in Ivankov Reservoir, nor in terminal sections of the Volga-Baltic corridor including the eastern Gulf of Finland. Presently, all but the northern-most regions of the Volga River have been colonized by D. r. bugensis. We hypothesize that its introduction into the Volga River and Caspian basin occurred no later than the late 1980s via commercial shipping that utilized the Volga-Don waterway to navigate between the source Black-Azov Sea region and recipient areas on the Volga River. Larval drift likely contributed to establishment of populations at downstream sites, while human-mediated vectors may be responsible for introductions to upstream locations on the Volga River. We anticipate continued northward dispersal in conjunction with shipping activities.     

Enhanced early detection and enumeration of zebra mussel (Dreissena spp.) veligers using cross-polarized light microscopy

Zooplankton with calcareous skeletons are birefringent under cross-polarized light, and thus this technique can be quite useful, indeed sometimes almost essential, for the detection and enumemeration of these types of organisms in plankton samples. A simple and inexpensive application of this technique is described and illustrated with quantitative examples from research on the veligers of the zebra mussel Dreissena polymorpha (Pallas). The time required to detect veligers in plankton samples was decreased by an order of magnitude; the accuracy of counts was substantially improved (15% more than controls), and the time required for counts was greatly reduced (41% of control times). This technique is especially useful in situations in which veligers are difficult to find or see (e.g., at low densities, in samples cluttered with extraneous organisms or material) or when the investigator is inexperienced with plankton sampling and planktonic organisms. The major limitations are its inability to discriminate among various bivalve species that have planktonic larvae and the similar appearance of ostracods which also have calcareous shells. Expanded use of this technique should (1) increase our ability to use plankton sampling for the early detection of veligers during range expansion and reproductive cycles and (2) permit more accurate estimates of veliger abundance.     

Invasion genetics of a freshwater mussel (Dreissena rostriformis bugensis) in eastern Europe: high gene flow and multiple introductions

In recent years, the quagga mussel, Dreissena rostriformis bugensis, native to the Dnieper and Bug Limans of the northern Black Sea, has been dispersed by human activities across the basin, throughout much of the Volga River system, and to the Laurentian Great Lakes. We used six published microsatellite markers to survey populations throughout its native and introduced range to identify relationships among potential source populations and introduced ones. Mussels from 12 sites in Eurasia, including the central Caspian Sea and one in North America (Lake Erie), were sampled. Field surveys in the Volga River basin suggested that the species first colonized the middle reach of the river near Kubyshev Reservoir, and thereafter spread both upstream and downstream. Evidence of considerable gene flow among populations was observed and genetic diversity was consistent with a larger, metapopulation that has not experienced bottlenecks or founder effects. We propose that high gene flow, possibly due to multiple invasions, has facilitated establishment of quagga mussel populations in the Volga River system. The Caspian Sea population (D. rostriformis rostriformis (=distincta)) was genetically more distinct than other populations, a finding that may be related to habitat differences. The geographical pattern of genetic divergence is not characteristic of isolation-by-distance but, rather, of long-distance dispersal, most likely mediated by commercial ships' ballast water transfer.     

Origin of Spanish invasion by the zebra mussel, Dreissena polymorpha (Pallas, 1771) revealed by amplified fragment length polymorphism (AFLP) fingerprinting

The zebra mussel, Dreissena polymorpha is an aquatic nuisance invasive species originally native to the Ponto-Caspian region where it is found in lakes and delta areas of large rivers draining into the Black and Caspian seas. The dispersal of D. polymorpha began at the end of the 18th century, at a time when shipping trade become increasingly important and many canals were built for linking different navigable river systems in Europe. Over the past 200 years, zebra mussels spread to most of the lakes, rivers and waterways in Europe by a combination of natural and anthropogenic dispersal mechanisms. D. polymorpha invaded Spain around 2001, being found for the first time in the Riba-roja reservoir at the lower part of the Ebro River, North-East Spain. The relatively late invasion of Spain was most likely caused by the presence of the Pyrenees, which isolated the Iberian Peninsula from the rest of the European continent, and acted as a barrier to the dispersal of D. polymorpha. In recent studies, molecular genetic methods have successfully been used to determine phylo-geographic relationships, which may reflect invasion corridors and can help retrace source populations. Zebra mussels from populations in Great Britain, The Netherlands, Belgium, France, Germany, Spain, Italy, Romania and North America were analyzed using PCR based amplified fragment length polymorphism (AFLP)-fingerprinting to determine the source population of D. polymorpha in Spain. The phylogenetic analyses and pair-wise genetic distances revealed that the recent invasion of zebra mussels in Spain is most likely from France.     

Range expansion of quagga mussels Dreissena rostriformis bugensis in the Volga River and Caspian Sea basin

In 1992, we discovered populations of the nonindigenous quagga mussel Dreissena rostriformis bugensis in the middle reaches of the Volga River. The same species was found in samples collected between 1994 and 1997 in the Volga delta and in shallow areas of the Northern Caspian Sea. D. r. bugensis always co-occurred with its more widespread congener, the zebra mussel D. polymorpha (Pallas 1771). The quagga mussel's contribution to total Dreissena abundance increased over time in the middle Volga reservoirs and Volga River delta. D. r. bugensis was common in the Volga portion of Rybinsk Reservoir during 1997 and, by 2000, it was in Uglich, Rybinsk and Gorky Reservoirs on the Upper Volga River. D. r. bugensis was neither found in Ivankov Reservoir, nor in terminal sections of the Volga-Baltic corridor including the eastern Gulf of Finland. Presently, all but the northern-most regions of the Volga River have been colonized by D. r. bugensis. We hypothesize that its introduction into the Volga River and Caspian basin occurred no later than the late 1980s via commercial shipping that utilized the Volga-Don waterway to navigate between the source Black-Azov Sea region and recipient areas on the Volga River. Larval drift likely contributed to establishment of populations at downstream sites, while human-mediated vectors may be responsible for introductions to upstream locations on the Volga River. We anticipate continued northward dispersal in conjunction with shipping activities.     

Rapid range expansion of the invasive quagga mussel in relation to zebra mussel presence in The Netherlands and Western Europe

Since its appearance in 2006 in a freshwater section of the Rhine–Meuse estuary (Hollandsch Diep, The Netherlands), the non-indigenous quagga mussel has displayed a rapid range expansion in Western Europe. However, an overview characterising the spread and impacts of the quagga mussel in this area is currently lacking. A literature study, supplemented with field data, was performed to gather all available data and information relating to quagga mussel dispersal. Dispersal characteristics were analysed for rate and direction and in relation to hydrological connectivity and dispersal vectors. To determine ranges of conditions suitable for quagga mussel colonisation, physico-chemical characteristics of their habitats were analysed. After its initial arrival in the freshwater section of the Rhine-Meuse estuary and River Danube, the quagga mussel demonstrated a rapid and continued range expansion in Western Europe. Quagga mussels have extended their non-native range to the network of major waterways in The Netherlands and in an upstream direction in the River Rhine (Germany), its tributaries (rivers Main and Moselle) and the River Meuse (Belgium and France). The calculated average quagga mussel dispersal rate in Europe was 120 km year^?1 (range 23–383 km year^?1). Hydrological connectivity is important in determining the speed with which colonisation occurs. Dispersal to water bodies disconnected from the freshwater network requires the presence of a suitable vector e.g. pleasure boats transferred over land. Upstream dispersal is primarily human mediated through the attachment of mussels to watercraft. The relative abundance of quagga mussel to zebra mussel has greatly increased in a number of areas sampled in the major Dutch rivers and lakes and the rivers Main and Rhine and the Rhine–Danube Canal leading to a dominance shift from zebra mussels to quagga mussels. However, evidence for displacement of the zebra mussel is limited due to the lack of temporal trends relating to the overall density of zebra and quagga mussel.     

In situ experiments on predatory regulation of a bivalve mollusc (Dreissena polymorpha) in the Mississippi and Ohio Rivers

1. In situ exclosure experiments in the Mississippi and Ohio Rivers determined the importance of fish predation in regulating zebra mussels (Dreissena polymorpha), an increasingly important constituent of the benthic invertebrate assemblages in both rivers. 2. We evaluated the effects of predatory fish on the density, biomass and size distribution of zebra mussels in a floodplain reach of the upper Mississippi River and in a naturally constrained reach of the Ohio River. Fifty, six-sided, predator-exclusion cages and fifty ‘partial’ cages (mesh at the upstream end only) were deployed, with half the cages containing willow snags and half clay tiles suspended 12–16 cm above the bottom. A single snag or tile sample unit was removed from each cage at approximately monthly intervals from July to October 1994. Types and relative abundances of molluscivorous fish were evaluated by electrofishing near the cages in both rivers. Actual and potential recruitment of young zebra mussels on to the substrata were measured using benthic samples in both rivers and estimated (Ohio River only) from counts of planktonic veligers. 3. Zebra mussels were consumed by at least three fish species in the upper Mississippi River (mostly carp, Cyprinus carpio, and redhorse suckers, Moxostoma sp.) and five species in the Ohio River (primarily smallmouth buffalo, Ictiobus bubalus, and channel catfish, Ictalurus punctatus), but potential recruitment seemed adequate to replace consumed mussels, at least in the Ohio River. The number of juvenile benthic mussels showed no apparent link with the density of veligers soon after initiation of reproduction. Recruitment of juveniles on snags and tiles was not affected by cage type (thus eliminating a potentially confounding ‘cage effect’). 4. Fish significantly influenced mussel populations, but the impact was often greatest among low density populations in the upper Mississippi. Density and biomass differed in both rivers for cage type (higher inside cages), substratum (greater on tiles), and date (increased over time). Presumed size-selective predation was present in the Mississippi (greater on larger size classes) but was not evident in the Ohio. We hypothesize that fish in the Mississippi can more easily select larger prey from the low density populations; whereas size-selective predation on tightly packed zebra mussels in the Ohio would be difficult. 5. Although fish can reduce numbers of Dreissena polymorpha in the two rivers, current levels of fish predation seem insufficient to regulate zebra mussel densities because of its great reproductive capacity. The recent invasion of zebra mussels, however, could lead to larger fish populations while promoting greater carbon retention and overall ecosystem secondary production.     

Veligers from the nudibranch <Emphasis Type="Italic">Dendronotus</Emphasis><Emphasis Type="Italic">frondosus</Emphasis> show shell growth and extended planktonic period in laboratory culture

The planktonic period of planktotrophic veliger larvae from the nudibranch Dendronotus frondosus was characterized by laboratory culture methods. Larvae in culture successfully metamorphosed at 73–86 days after hatching. These veligers have Type 2 (Thompson) larval shells that significantly increased in length over the first 7–14 days after hatching. Direct observations of the development of nudibranch larvae with Type 2 protoconchs are limited, and these data help clarify previous attempts to correlate shell type and growth with minimum planktonic periods. Although these are not absolute values for the planktonic period of D. frondosus larvae, these data show the potential for extended larval dispersal and may help explain reports of an extensive geographic range in north-temperate waters for this species.