Size Classes Of Mussels Research Articles

Factors determining selective predation of the common carp on quagga versus zebra mussels

Abstract Selective predation may affect interspecific competition between coexisting prey species. Ponto‐Caspian zebra (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) have become common components of benthic communities in invaded ecosystems in Europe and North America, where they are exposed to predation by molluscivorous fish. In a pairwise food selection experiment, we examined whether an efficient predator of dreissenids, the common carp (Cyprinus carpio), selects between the two mussel species and, if so, which mussel traits (attachment, shell strength, nutritional value) are responsible for the discrimination. The fish were offered simultaneously the two mussel species in three separate treatments: (1) attached individuals; (2) unattached individuals; and (3) freshly removed soft tissue, as well as in three mussel size classes. The fish consistently selected quagga over zebra mussels, irrespective of mussel size and attachment status, including even soft tissues of both species. The success rate of fish attacks did not differ between the prey species. Smaller mussels were easier to crush and swallow, but more difficult to detach from the substratum. Thus, although interspecific differences in attachment and shell strength existed (small and medium zebra mussels had stronger shells and were more strongly attached than corresponding quagga mussels), these traits did not affect fish preferences. However, the differences in chemical composition of the body—the glycogen content (higher in large quagga vs. zebra mussels) and the caloric content (higher in small and medium quagga vs. zebra mussels)—were responsible for fish selection. The higher predatory pressure on the quagga versus zebra mussel suggests that differential predation is not a likely reason for the displacement of zebra by quagga mussels, currently occurring in many European and North American ecosystems. Thus, this phenomenon must be driven by other traits making the quagga mussel more competitive.

Understanding the ontogenetic changes in particle processing of the greenshell™ mussel, Perna canaliculus, in order to improve hatchery feeding practices

Despite the commercial aquaculture importance of the Greenshell™ mussel, Perna canaliculus, the morphological development of gill structure (ctenidial filaments) in relation to potential particle feeding abilities have not been described. Scanning electron microscopy (SEM) and light microscopy were used to examine the ontogenetic changes across five size classes of mussels from post-settlement larvae to adults ranging from 300μm to >100mm in shell height. Key morphological characteristics were measured including filament width, the eulatero-frontal cirri (ELF) length, interfilamentary space (IFS) and the mean distance between adjacent ELF, dELF. Overall the development of the ctenidial filaments in P. canaliculus, shared some similarities to other homorhabdic filibranch bivalves with differences in the timing of development. Filament width and ELF length followed sigmoidal growth curves in relation to increasing shell height of mussels. The morphological changes of the ctenidia, reflected in changes in the ELF/IFS ratios, together with the development of cilia in the ELF may help explain the differences in the capture efficiency of small particles between small individuals (<6mm in shell height) and bigger mussels (around 60mm in shell height). Ctenidial filaments in small individuals are not fully developed and resemble the “leaking” model, letting small particles through and capturing particles between 15–25μm, while the filaments in bigger mussels resemble the “complete block” model being able to capture bacteria-sized particles around 2μm. This information is not only useful to further the understanding of the feeding biology of this species, but also for the feeding management in hatcheries producing large numbers of juveniles of this commercially important species, considering that under current hatchery practices small-sized mussels are being fed microalgal species mainly with a size of around 5μm. Statement of relevance–This article provides information regarding the gill ontogenetic development of the commercially important Greenshell™ mussel, Perna canaliculus.–The information given will help improve hatchery practices, especially regarding the selection of adequate particle size for feeding post-settlement P. canaliculus.–Considering that mussel aquaculture is a global phenomenon, these results could be used worldwide in order to help the industry find economically feasible alternatives for mussel juvenile supply.The New Zealand Greenshell™ industry has recently invested $20M to build a large scale hatchery/nursery to address an on-going shortage with seed supply which has been constraining the growth of the industry. Shortages of mussel seed, caused by poor feeding and starvation, have resulted in seed shortages in the industry for the last three years, shaving off almost 20% production of the entire industry, in the order of $30M lost production a year. Hence there is intense interest in better understanding the feeding biology of juvenile mussels and using this knowledge to resolve the major production issues faced by this aquaculture industry. From our international contacts, we understand that other countries with significant mussel farming industries face similar issues, such as Chile. Therefore, we believe that there is strong reader interest and value in the publication of this fundamental research.

Trematode infections reduce clearance rates and condition in blue mussels Mytilus edulis

Suspension feeders are important players in coastal food webs: by filtering particles suspended in the water column and depositing faeces and pseudofaeces in sediments, they mediate the coupling of pelagic primary and benthic secondary production. However, the potential interference of parasite infections on suspension feeding is not well investigated. We experimentally determined the effect of infections with metacercariae of the common trematode Renicola roscovita on clearance rates and condition of blue mussels Mytilus edulis. We also investigated whether there were differences in the effects of infections between different size classes of mussels. Using controlled infections under lab conditions, our experiment showed that infections significantly reduced the clearance rates of mussels. This effect was less pronounced in small (42% reduction) than in large mussels (71%). In addition, infections significantly reduced mussel body mass index in large (23% reduction) and small (17%) mussels compared to uninfected mussels. These reductions most likely resulted from a combined effect of an interference of the parasites with the mussel’s feeding apparatus (with the palps being the preferred infection site of the parasite) and of the energetic demands of the parasites (with metacercariae growing in size after initial infection). These negative effects on mussel filter feeding capacity indicate that parasites can alter a crucially important functional trait of marine suspension feeders.

About the Cover.

Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverThe sea star Pisaster ochraceus is a dominant rocky intertidal predator that lives along the west coast of North America. It plays a large role in determining local patterns of biodiversity via its interactions with mussels in the genus Mytilus. The cover image depicts juvenile P. ochraceus (the maroon, purple, and ochre sea stars) foraging among Mytilus trossulus (the bluish-black mussels) on a floating dock at the Hakai Beach Institute on Calvert Island, British Columbia. Several empty mussel shells are evidence of recent sea star predation events. The Pisaster-Mytilus interaction is famous for providing the inspiration for the “keystone predator” concept, and is now widely featured in textbooks.As a well-studied predator-prey pair, Pisaster and Mytilus make an ideal model system for addressing ecological and behavioral theory relating to the effects of body size. Body size of both predator and prey are key attributes of trophic interactions, as differences in body size can determine ease of prey capture, time required to handle and consume the prey, and number of prey required to meet energetic demands. On pages 192–200 of this issue, Gooding and Harley explore the ways in which variation in body size can affect the relationship between the predator P. ochraceus and its preferred prey in wave-protected waters, Mytilus trossulus.Gooding and Harley found that predation rates, expressed as mussels consumed per unit time, increased with increasing sea star size and decreased with increasing mussel size. Larger sea stars preferred to consume larger mussels. For juvenile sea stars, in which size relationships could be worked out in the greatest detail, prey-size preference corresponded with mussel size classes that allowed Pisaster to consume the most tissue in the least amount of time. Overall, the study found that even small changes in predator or prey body size could play a large role in predator energetics (tissue consumed per unit time) and prey population dynamics (individuals consumed per unit time). Because sea star and mussel growth rates and body size frequency distributions are affected by temperature and ocean acidification, changes in body size and the accompanying shifts in predation rates may be one important pathway through which global change alters community structure.Credits: Photo, Christopher D.G. Harley, University of British Columbia; cover design, Beth Liles, Marine Biological Laboratory. Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 228, Number 3June 2015 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv228n3cover © 2015 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.

The effects of transportation stress and barnacle fouling on predation rates of starfish (Asterias rubens) on mussels (Mytilus edulis)

Abstract The starfish, Asterias rubens , is widely distributed throughout the northern hemisphere and is an important predator on benthic mussel ( Mytilus edulis ) beds. Whilst several studies have examined how the size of individuals determines this predator–prey relationship, less is known about how the physiological condition of the prey (mussels) and the extent of their fouling may alter these relationships. Such issues are of particular interest to those working within the benthic mussel cultivation industry to inform best management practice and to help minimise losses during the aquaculture process. The potential role of starfish in the removal of epibiotic barnacles from mussels, the presence of which increases processing costs within the industry, is also of interest. We tested whether stressing mussels by aerial exposure for 48 h and whether the extent of barnacle fouling on mussels affected the feeding rates of three different size classes of starfish feeding on two different size classes of mussels. Feeding rates on stressed and unstressed mussels were similar for each starfish–mussel size combination. Barnacle fouling reduced the feeding rate of medium-sized starfish on larger-sized mussels. We also observed starfish, of all size classes, preying directly on the epibiotic barnacles on mussels, however, feeding rates were low and considered unlikely to reduce the extent of fouling on mussels. Our findings show that the predator–prey relationship between starfish and mussels does not differ between unstressed mussels and those experimentally stressed by aerial exposure for 48 h so that this level of stress is unlikely to affect predation rates by A. rubens following relaying in commercial operations. Whilst barnacle fouling suppressed predation rates in one of our experimental treatments, it does not appear that fouling by barnacles would provide a significant refuge from predation for the majority of mussels in benthic aquaculture stocks. Instead we found the size relationship between starfish and mussels was more important in determining predation rates. Starfish are also unlikely to help reduce barnacle fouling on cultured mussels by preying solely on fouling barnacles and the need to control starfish predation during culture remains.

Size‐structured vulnerability of the colonial cyanobacterium, Microcystis aeruginosa, to grazing by zebra mussels (Dreissena polymorpha)

Summary We quantified the vulnerability of colonies of the bloom‐forming cyanobacterium, Microcystis aeruginosa, to grazing by the invasive filter‐feeding zebra mussel (Dreissena polymorpha) as a function of size in both organisms with laboratory feeding experiments. In one experiment, size‐selectivity of 16‐ to 21‐mm mussels was assessed for a single M. aeruginosa clone across a wide size range (~5–88 μm median equivalent diameter, ED). Consumption of colonies ≥80 μm median ED (109 μm median maximum linear dimension) was undetectable, indicating a size threshold of grazing invulnerability. Smaller colonies and single cells were consumed at rates similar to a highly palatable alga (Ankistrodesmus). In a second experiment, the size‐selectivity of three size classes of mussels (8–11, 17–20 and 25–28 mm shell length) was assessed across three size classes of M. aeruginosa (~32–75 μm median ED). There were no systematic differences in the abilities of the different mussel size classes to consume the largest colonies within this size range. An 8‐year field survey of the M. aeruginosa population in Gull Lake, MI (U.S.A.), the source of the experimental organisms, revealed that median colony size consistently decreased during each summer, from above to below the size threshold of effective mussel feeding we identified, which suggests major within‐season shifts in the overall vulnerability of the M. aeruginosa population to mussel grazing. Variation in the size structure of M. aeruginosa may help explain highly variable effects of D. polymorpha on the dynamics of this harmful phytoplankter within and across systems.

Impacts of the invasive ascidian Didemnum vexillum on green-lipped mussel Perna canaliculus aquaculture in New Zealand

Biofouling can pose a significant threat to shellfish aquaculture, as fouling organisms are often strong spatial competitors that are able to reach high densities or biomass in relatively short time frames. This study investigated the potential impacts of the colonial ascidian Didem- num vexillum on cultured New Zealand green-lipped mussels Perna canaliculus at one farm in the Marlborough Sounds region. Three size classes of mussels were examined: small (20 to 40 mm shell length at deployment), medium (40 to 60 mm), and large (60 to 70 mm). Replicate 4 m mussel lines were assigned to 1 of 3 treatments: (1) ambient fouling, (2) fouling enhanced by D. vexillum fragment inoculation (in addition to ambient fouling), or (3) control lines that were kept free of D. vexillum. After 15 mo, subsections of lines (0.5 m length) were processed to determine the effects of fouling cover on mussel density within lines, as well as on individual mussel size and condition. A highly significant negative relationship was shown between D. vexillum biomass and mussel density for small mussels, and to a lesser extent for medium mussels. Values of mussel condition indices were similar across size classes and across fouling treatments within each size class. These results indicate that mussels may only be vulnerable to direct D. vexillum fouling impacts at early stages of production, and that impacts may be restricted to displacement of mussels as opposed to reduced size and condition. This information will assist in the implementation of management procedures through increased understanding of D. vexillum effects at different stages of mussel production.

Quantifying the Effects of Diet and Mussel Size on the Biophysical Properties of the Blue Mussel,Mytilusspp., Feces Egested Under Simulated Imta Conditions

Three size classes of mussels (Mytilus spp.) (small, 26–35 mm; medium, 45–54 mm; and large, 65–74 mm) were exposed to 4 experimental diets consisting of mixed algae, diatom pastes, salmon feed “fines,” or salmon feces. Salmon culture byproduct particles (feces and feed fines) were found to have minimal effect on the biophysical properties of mussel feces when compared with those from an algal-based diet. Differences in fecal morphology (feces widths) of mussel feces were found to be minimal in small mussel sizes, but became more significant as mussel shell length increased (45–74 mm). Furthermore, faeces from fish farm-based diets were found to be significantly narrower than algal based diets. Absorption efficiencies of the 4 different diets were 87%, 81%, 90%, and 86%, respectively. Regardless of diet, small mussels produced feces that dispersed as a function of settling velocity (small, 0.18 cm/sec; medium, 0.29 cm/sec; and large, 0.54 cm/sec (settling velocity of 50% of particles)) over much larger areas than those feces produced by larger mussels, suggesting that the influence of mussel culture on benthic loading of organic material around an aquaculture site will tend to increase over time as the mussel crop grows to maturity.

Predatory blue crabs induce byssal thread production in hooked mussels

Abstract. Blue crabs (Callinectes sapidus) prey on hooked mussels (Ischadium recurvum) growing epizoically on oyster clumps in estuaries along the Louisiana coast. In prey size-selection experiments, blue crabs preferred small mussels (<30-mm shell length) to larger mussels, possibly because handling time increased with mussel size. When crabs were given a choice of solitary mussels versus mussels in clumps on oysters in the laboratory, mortality was lower by 86% in clumped mussels. However, no size selection by crabs occurred with mussels in clumps, likely because smaller mussels escaped predation in crevices between larger mussels or oysters. When individuals of two size classes of mussels were exposed to water containing the scent of crabs and of mussels consumed by blue crabs, an increase in byssal thread production was induced in all mussels, but byssal thread production rate was higher for small mussels than for large mussels. We conclude that increased predation risk for small mussels has resulted in higher size-specific production of byssal threads, and that predator-induced production of byssal threads, which may increase clumping behavior, may reduce their risk of mortality to predatory blue crabs.

Absorption efficiency of blue mussels ( Mytilus edulis and M. trossulus) feeding on Atlantic salmon ( Salmo salar) feed and fecal particulates: Implications for integrated multi-trophic aquaculture

Blue mussels ( Mytilus edulis and M. trossulus) are grown adjacent to Atlantic salmon ( Salmo salar) cages in some open-water, Integrated Multi-Trophic Aquaculture (IMTA) systems. Absorption efficiency (AE) data of salmon feed ‘fines’ or salmon feces consumed by blue mussels is limited, but is crucial for determining organic load delivery to secondary niches (e.g. deposit feeders placed under mussel rafts) and the efficiency of IMTA systems. This study aimed to investigate AE of salmon particulates relative to some commercial shellfish diets and potential effects of mussel size. In the laboratory trials, there were significant differences of fecal OC ( p = 0.00) across diets but no significant differences ( p = 0.07) of fecal OC between mussel size classes (30–35 mm, 50–55 mm and 70–75 mm lengths) fed the same diet. The AE of a commercial spat formula ( Phaeodactylum tricornutum, Chaetoceros-B, and Nanochloropsis oculata), a commercial diatom diet ( Thalassiosira weissflogeii), salmon feed, salmon feces, and total particulate matter (TPM) at salmon cages; averaged 87, 81, 90, 86, and 54%, respectively. TPM at a salmon cage increased with total organic matter ( r 2 = 0.76), but decreased in OC ( r 2 = 0.87), suggesting periods of substantial inorganic particle (e.g. silt) flux. Consequently, mean values of OC and AE of TPM at salmon cages was less than what would be expected if mussels were ingesting significant portions of salmon particulates. The results of this study emphasize the temporal/spatial variation of ‘particle plumes’ exiting fish cages is a major consideration for the placement of suspension or filter feeders at open-water IMTA sites. However, the AE data presented suggests that organic material in particulate salmon culture waste can effectively be utilized by blue mussels.

Evaluating the Use of Exhalent Siphon Area in Estimating Feeding Activity of Blue Mussels,Mytilus edulis.

We evaluated the technique of measuring the exhalant siphon area (ESA) as an indicator of feeding activity in the blue mussel, Mytilus edulis. To accomplish this, we established the relationships between ESA measured using video and image analysis and clearance rate (CR) measured simultaneously in mussels exposed to various concentrations of microalgae as a food source in the laboratory. Two size classes of mussels (30 and 60 mm shell length) were fed 6 and 7 concentrations of the unicellular alga Isochrysis galbana., respectively (0, 0.6, 1.2, 1.8, 2.4, 3.0, 6.0 mg L-1). ESA and the variation in feeding activity associated with individual mussels significantly influenced CR in 60mm mussels, whereas effects of algal concentration were not significant within the range tested. Individual variation in feeding activity significantly influenced CR in 30 mm mussels. However, unlike the 60 mm mussels, ESA did not significantly influence CR, whereas algal concentration did have a significant effect in 30 mm mussels. We did observe significant relationships between ESA and CR in some groups of mussels suggesting it may be a useful indicator of feeding activity under certain circumstances. However, the high degree of variation observed in our laboratory-based experiments on the relationship between CR and ESA leads us to conclude that measurements of ESA may be better used as an estimate of general behavior trends in feeding rather than a quantitative measure of clearance rate.

Infection of Nematopsis oocysts in different size classes of the farmed mussel Perna viridis in Thailand

We report on the infection intensity of Nematopsis oocysts in different size classes of the green mussel Perna viridis farmed in Chonburi province, eastern upper Gulf of Thailand. Infection of the apicomplexan parasite was investigated from November 2003 to November 2004 in small (2.3 ± 0.2 cm shell length, n = 130), medium (5.6 ± 0.3 cm, n = 130) and large (8.6 ± 0.4 cm, n = 130) size classes of the green mussel. Heavy prevalence of infection was found in all size classes from November 2003 to June 2004, whereas low prevalence was observed from July to October 2004. From statistical analysis, prevalence of the parasite did not differ between the three mussel size classes. However, infection intensity calculated per unit weight of dried gill tissue was significantly lower in the smallest size class. From light and transmission electron microscopy, the Nematopsis oocysts were ellipsoidal 12.6 ± 0.3 μm wide and 17.4 ± 0.9 μm long and 12.0 ± 0.3 μm wide and 16.6 ± 0.5 μm long, respectively. Oocyte wall thickness was between 0.5 and 0.7 μm. Oocysts were located within a parasitophorous vacuole (PV) 22–30 μm in diameter. The PVs were in turn engulfed within phagocyte sacs produced by the mussel with diameters ranging from 30–95 μm depending on the number of oocysts within. Microscopic examination suggested that this infective stage of the parasite causes severe damage to P. viridis gill tissue in conditions of heavy infection causing inflammation and disruption of gill epithelia.

Population dynamics of the green-lipped mussel, Perna canaliculus, at various spatial and temporal scales in northern New Zealand

Ecological processes that differentiate and maintain intertidal populations of mussels, Perna canaliculus, were studied within three sites at Ninety Mile Beach, northern New Zealand. At these three sites (Scott Point, The Bluff and Tonatona Beach), the dynamics of larval availability, primary and secondary settlement, recruitment and mortality rates were investigated at various spatial and temporal scales. (1) Mussel concentrations in seawater were variable with respect to study site and time of year, with highest abundances at the northernmost population (Scott Point) and lowest concentrations at the middle population (The Bluff). In seawater at all three sites, small mussels (< 0.25 mm in shell length) were more abundant in August 2000, while larger mussels (> 0.5 mm in shell length) were more abundant in March 2001. (2) Primary and secondary settlement patterns were investigated during short-term (daily) and long-term (monthly) settlement experiments, within quadrats that were cleared of all mussels in both the mussel bed and in adjacent algal band habitats. At all sites, primary settlement (< 0.5 mm in shell length) was high within the algal band habitat in August 1999, 2000 and 2001. Conversely, secondary settlement (> 2.0 mm in shell length) was high within cleared areas in the mussel beds in November–March 1999–2000 and 2000–2001. Abundance of mussels settling on artificial substrates placed in the intertidal did not differ greatly from comparable areas of natural substrates (bare rock or algae within cleared quadrats). (3) Recruitment and mortality rates were recorded during monthly surveys of the adult populations. Within three mussel size classes (< 24, 25–74 and > 75 mm in shell length), peak recruitment coincided with high mortality in August of the 2 years studied. However, the most dramatic turnover of the population was observed at Scott Point in both years, following a spawning event. In adjacent waters at Scott Point, large accumulations of drift algae covered (up to 100% cover) with juvenile mussels may deplete food supplies usually delivered to intertidal adult mussels, causing their demise. Mats of adult mussels were observed “peeling-off” from the rocky shore at this time of the year, making space available to the new recruits. Where nearshore algal accumulations were moderate to low, only moderate to low mussel turnovers were observed (e.g. Tonatona Beach and The Bluff).

Seasonal and spatial variability in growth of Mytilus edulis L. in a brackish sound: comparisons of individual mussel growth and growth of size classes

Growth of Mytilus edulis L. in a brackish Danish sound was estimated from individually measured mussels and measurements of discrete size classes. In winter, individual mussels grew faster than size classes of mussels, and in summer, the inverse situation occurred. The effect of size-specific mortality of larger mussels in winter, and smaller mussels in summer on the population dynamics is discussed. Spatial variability in growth was analysed by comparing von Bertalanffy growth rates estimated for discrete size classes of mussels at different locations in the sound. Faster growth rates were measured in the western part of the sound characterised by a higher salinity and current speed. The observations were supported by results of cage experiments with individually marked mussels.

Size-Selective Predation on Zebra Mussels (Dreissena polymorpha) by Crayfish (Orconectes propinquus)

Zebra mussels (Dreissena polymorpha) are now a common component of benthic communities in the lower Great Lakes, but information regarding North American predators remains scant. I measured predation by crayfish (Orconectes propinquus) on different size classes of zebra mussels in aquarium experiments. Crayfish showed a strong preference for small (3-5 mm) mussels, although larger individuals (8-10 mm, 12-14 mm) were also eaten when presented simultaneously. Feeding rates on different mussel size classes were affected by crayfish size and sex, although results varied between experiments. Survival of zebra mussels was enhanced between 3 and 30% when crayfish were offered alternative macrophyte foods (Potamogeton, Vallisneria); however, size selective predation patterns on mussels remained virtually unchanged in the presence of alternative foods. Time-lapse videographic studies revealed that both rejection rate and handling time increased with mussel size. Despite high predation rates on small mussels, cray...

The Role of the Zebra Mussel, Dreissena polymorpha, in Contaminant Cycling: I. The Effect of Body Size and Lipid Content on the Bioconcentration of PCBs and PAHs

The zebra mussel, Dreissena polymorpha, a recent invader to the Great Lakes, may influence contaminant cycling by bioconcentrating high levels of hydrophobic contaminants in its tissue. To better understand zebra mussel bioconcentration and ultimately, contaminant cycling, we measured bioconcentration factors (BCFs) and kinetic parameters for accumulation of polychlorinated biphenyl and poly cyclic aromatic hydrocarbon congeners for two size classes of mussels and for a pre-spawning (high lipid) and post-spawning (low lipid) mussel population. High lipid, pre-spawning mussels had greater BCFs and faster uptake kinetics for the highly hydrophobic compounds, i.e, hexachlorobiphenyl and benzo(a)pyrene, than the low lipid, post-spawning mussels. BCFs and uptake kinetics determined for the less hydrophobic compounds, i.e., tetrachlorobiphenyl and pyrene, were not measurably influenced by differences in lipid content. Small mussels (15 mm shell length) had higher BCFs and faster uptake kinetics for all compounds compared to larger (21 mm shell length) mussels. Contaminant elimination was not affected by size or differences in lipid levels. For both lipid levels and size classes of mussels, the BCFs were positively correlated with compound log octanohwater partition coefficient. Hence, the lipid affinity of a compound can be a good indicator of mussel contaminant accumulation. Potentially large contaminant concentration in zebra mussels may alter contaminant cycling in the Great Lakes by increasing contaminant transfer to mussel predators. Selective predation on small, pre-spawning (high lipid) mussels may present a greater hazard to predators than predation on larger, post-spawning (low lipid) mussels.

Comparative Growth and Survival of Dreissena polymorpha and Dreissena bugensis, Exotic Molluscs Introduced to the Great Lakes

Zebra ( Dreissena polymorpha) and quagga mussels ( D. bugensis) are among the most recent species to invade the Great Lakes. This study explored growth rates of two size classes (5 and 15 mm) of each species reciprocally transplanted into western and eastern Lake Erie, and in laboratory experiments in which environmental temperature and food concentration were varied. Both species and size classes experienced high survival rates in western (>95%) and eastern (> 85%) Lake Erie. Length-and mass-specific growth rates varied significantly between locations and mussel size classes, but not by species. Mussels incubated in the warm, western basin of Lake Erie exibited growth {length and mass), whereas those in the cooler eastern basin did not. In a laboratory experiment, temperature and size were the most important determinants of growth, though species differences were evident. Growth of both species was greater at 15°C than at 6°C. Specific growth rates of zebra mussels were greater than those of quagga mussels, except with respect to mass growth of smaller mussels at the higher temperature. Large mussels of both species experienced no mortality, whereas small quagga mussels experienced a significantly higher rate of mortality (50%) at the higher temperature than did small zebra mussels (10%). Results of the present study indicate that mussel growth of both species corresponds with initial size and environmental temperature, and that the distribution of quagga mussels in the Great Lakes is unlikely to be determined by high water temperature.

The effects of size-selective feeding by starfish ( Asterias vulgaris) on the production of mussels ( Mytilus edulis) cultured on nets

Laboratory feeding trials were conducted in which excess rations of four different size classes of mussels were provided to three different size classes of starfish. Although differences in size-selective feeding behaviour occurred between different size classes of starfish feeding separately and those feeding aggregately, the total food intake remained the same. The consumption of mussel meat by starfish based on the relative ash-free dry weights of both prey and predator ranged from 6 to 8.6% day −1. Patterns of size-selective feeding behaviour and feeding rates of starfish observed in the laboratory appear consistent with the changes in relative abundance among year class cohorts of mussels and starfish residing on culture nets at a mussel farm. It is concluded that the impact of starfish predation at the mussel farm is not likely to affect adversely the marketable yield of older mussels, and may provide a beneficial influence through the culling of undersized mussels.