The density and spatial arrangement of the invasive oyster Crassostrea gigas determines its impact on settlement of native oyster larvae

Understanding how the density and spatial arrangement of invaders is critical to developing management strategies of pest species. The Pacific oyster, Crassostrea gigas, has been translocated around the world for aquaculture and in many instances has established wild populations. Relative to other species of bivalve, it displays rapid suspension feeding, which may cause mortality of pelagic invertebrate larvae. We compared the effect on settlement of Sydney rock oyster, Saccostrea glomerata, larvae of manipulating the spatial arrangement and density of native S. glomerata, and non‐native C. gigas. We hypothesized that while manipulations of dead oysters would reveal the same positive relationship between attachment surface area and S. glomerata settlement between the two species, manipulations of live oysters would reveal differing density‐dependent effects between the native and non‐native oyster. In the field, whether oysters were live or dead, more larvae settled on C. gigas than S. glomerata when substrate was arranged in monospecific clumps. When, however, the two species were interspersed, there were no differences in larval settlement between them. By contrast, in aquaria simulating a higher effective oyster density, more larvae settled on live S. glomerata than C. gigas. When C. gigas was prevented from suspension feeding, settlement of larvae on C. gigas was enhanced. By contrast, settlement was similar between the two species when dead. While the presently low densities of the invasive oyster C. gigas may enhance S. glomerata larval settlement in east Australian estuaries, future increases in densities could produce negative impacts on native oyster settlement. Synthesis and applications: Our study has shown that both the spatial arrangement and density of invaders can influence their impact. Hence, management strategies aimed at preventing invasive populations reaching damaging sizes should not only consider the threshold density at which impacts exceed some acceptable limit, but also how patch formation modifies this.     

A cDNA Microarray for Crassostrea virginica and C. gigas

The eastern oyster, Crassostrea virginica, and the Pacific oyster, C. gigas, are species of global economic significance as well as important components of estuarine ecosystems and models for genetic and environmental studies. To enhance the molecular tools available for oyster research, an international group of collaborators has constructed a 27,496-feature cDNA microarray containing 4460 sequences derived from C. virginica, 2320 from C. gigas, and 16 non-oyster DNAs serving as positive and negative controls. The performance of the array was assessed by gene expression profiling using gill and digestive gland RNA derived from both C. gigas and C. virginica, and digestive gland RNA from C. ariakensis. The utility of the microarray for detection of homologous genes by cross-hybridization between species was also assessed and the correlation between hybridization intensity and sequence homology for selected genes determined. The oyster cDNA microarray is publicly available to the research community on a cost-recovery basis.     

Predation by the endemic whelk Tenguella marginalba (Blainville, 1832) on the invasive Pacific oyster Crassostrea gigas (Thunberg, 1793)

The endemic mulberry whelk (Tenguella marginalba) is a common predator on Australian intertidal rocky shores. The introduced Pacific oyster (Crassostrea gigas), found within the natural range of T. marginalba, is potential prey for the whelk. In experiments designed to increase our understanding of predatory behaviour by the whelk on oysters, we found that adult T. marginalba detected C. gigas and increased movement in the presence of oyster prey. Tenguella marginalba showed a preference for smaller C. gigas, but consumed oysters up to 60?mm in shell height. To access oyster flesh, whelks used their radula to drill holes in the oyster’s shell. These holes were on average 0.68?±?0.09?mm in diameter, most frequently located central to the pericardial cavity on the right (upper) valve. Predation was greatest when predator and prey were both submerged, but was unaffected by a diurnal light cycle. When offered a choice among the native Sydney rock oysters (Saccostrea glomerata), mussels (Trichomya hirsuta) or the invasive C. gigas, whelks displayed no preference among prey. We conclude that the invasive oyster C. gigas represents a viable food source for T. marginalba, which may help to slow the spread of this invasive oyster throughout eastern Australia.     

Mycobiota of the giant oyster <Emphasis Type="Italic">Crassostrea gigas</Emphasis> (Thunberg, 1787) (Bivalvia) from the Peter the Great Bay of the Sea of Japan

Mycological investigation of the Pacific (giant) oyster Crassostrea gigas (Thunberg, 1793) (Bivalvia) from the Peter the Great Bay of the Sea of Japan was carried out. The taxonomic composition of filamentous fungi associated with C. gigas was studied. The taxonomic composition of the fungi associated with the giant oyster included 22 species of filamentous fungi of which 17 species were identified. The latter belonged to six genera: Alternaria, Aspergillus, Botrytis, Fusarium, Penicillium, and Trichoderma. The distribution of filamentous fungi in the internal organs of the bivalve mollusk was studied.     

Comparison of quantitative gonad maturation scales in a temperate oyster (Crassostrea gigas) and a sub-tropical oyster (Crassostrea corteziensis)

Quantitative scales to evaluate maturity of female gonads were compared for a temperate oyster, Crassostrea gigas and a tropical oyster, Crassostrea corteziensis grown in the same locality. C. gigas had well-defined maturation and spawning periods and a resting phase in winter; in C. corteziensis mature individuals occurred most of the year and there were several spawning peaks. Of the quantitative scales used here, average oocyte diameter and gonad coverage area, much used for C. gigas, and ovary maturity index, less used, were inadequate to distinguish the reproductive pattern of C. corteziensis, since they both skewed the degree of maturation in vitellogenic stages in favor of C. gigas. Maximum oocyte diameter and maximum cytoplasm area were different among species at vitellogenic stages and also in previtellogenic stages. Nucleus:cytoplasm ratio was significantly different in previtellogenic and spawned stages between C. gigas and C. corteziensis. The Index of gonadal development was skewed in favor of C. gigas in postvitellogenic stage. The only scale that was comparable between species was reproductive potential, but it also was one of the most laborious. Other ordinal scales commonly used, such as a visual external evaluation of the gonad, only classified correctly a quarter of the stages.     

Introduced Pacific oysters (Crassostrea gigas) in the northern Wadden Sea: invasion accelerated by warm summers?

Among the increasing number of species introduced to coastal regions by man, only a few are able to establish themselves and spread in their new environments. We will show that the Pacific oyster (Crassostrea gigas) took 17 years before a large population of several million oysters became established on natural mussel beds in the vicinity of an oyster farm near the island of Sylt (northern Wadden Sea, eastern North Sea). The first oyster, which had dispersed as a larva and settled on a mussel bed, was discovered 5 years after oyster farming had commenced. Data on abundance and size-frequency distribution of oysters on intertidal mussel beds around the island indicate that recruitment was patchy and occurred only in 6 out of 18 years. Significant proportions of these cohorts survived for at least 5 years. The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north- and southwards along the coastline. Abundances of more than 300 oysters m^–2 on mussel beds were observed in 2003, only after two consecutive spatfalls in 2001 and 2002. Analyses of mean monthly water temperatures indicate that recruitment coincided with above-average temperatures in July and August when spawning and planktonic dispersal occurs. We conclude that the further invasion of C. gigas in the northern Wadden Sea will depend on high late-summer water temperatures.Communicated by H.D. Franke     

Specific Detection of Pacific Oyster (Crassostrea gigas) Larvae in Plankton Samples Using Nested Polymerase Chain Reaction

Management of sustainable Pacific oyster fisheries would be assisted by an early, rapid, and accurate means of detecting their planktonic larvae. Reported here is an approach, based on polymerase chain reaction (PCR), for the detection of Pacific oyster larvae in plankton samples. Species-specific primers were designed by comparing partial mitochondrial cytochrome oxidase subunit I (COI) sequences from Crassostrea gigas, with other members of the family Ostreidae including those of Crassostrea angulata. Assay specificity was empirically validated through screening DNA samples obtained from several species of oysters. The assay was specific as only C. gigas samples returned PCR-positive results. A nested PCR approach could consistently detect 5 or more D-hinge-stage larvae spiked into a background of about 146 mg of plankton. The assay does not require prior sorting of larvae. We conclude that the assay could be used to screen environmental and ballast water samples, although further specificity testing against local bivalve species is recommended in new locations.     

New Resources for Marine Genomics: Bacterial Artificial Chromosome Libraries for the Eastern and Pacific Oysters (Crassostrea virginica and C. gigas)

Large-insert genomic bacterial artificial chromosome (BAC) libraries of two culturally and economically important oyster species, Crassostrea virginica and C. gigas, have been developed as part of an international effort to develop tools and reagents that will advance our ability to conduct genetic and genomic research. A total of 73,728 C. gigas clones with an average insert size of 152 kb were picked and arrayed representing an 11.8-fold genome coverage. A total of 55,296 clones with an average insert size of 150 kb were picked and arrayed for C. virginica, also representing an 11.8-fold genome coverage. The C. gigas and C. virginica libraries were screened with probes derived from selected oyster genes using high-density BAC colony filter arrays. The probes identified 4 to 25 clones per gene for C. virginica and 5 to 50 clones per gene for C. gigas. We conducted a preliminary analysis of genetic polymorphism represented in the C. gigas library. The results suggest that the degree of divergence among similar sequences is highly variable and concentrated in intronic regions. Evidence supporting allelic polymorphism is reported for two genes and allelic and/or locus specific polymorphism for several others. Classical inheritance studies are needed to confirm the nature of these polymorphisms. The oyster BAC libraries are publicly available to the research community on a cost-recovery basis at     

Recently established <Emphasis Type="Italic">Crassostrea</Emphasis>-reefs versus native <Emphasis Type="Italic">Mytilus</Emphasis>-beds: differences in ecosystem engineering affects the macrofaunal communities (Wadden Sea of Lower Saxony,southern German Bight)

Since 1998 the non-indigenous Pacific oyster Crassostrea gigas (Thunberg 1793) has been invading the Wadden Sea of Lower Saxony, southern German Bight. C. gigas settles predominantly on intertidal Mytilus-beds (M. edulis L.) and subsequently create rigid reef-like structures. Both bivalve species are ecosystem engineers in sedimentary tidal flats. They provide hard substrate for sessile species, mobile organisms find refuge within the habitat matrix of dense suspension feeders, and biodeposits enrich the sediments with organic matter. The transformation of Mytilus-beds into Crassostrea-reefs gives rise to the question whether the invader may affect the native community. We investigated two parts of a changing bivalve bed in the backbarrier area of the island of Juist in March 2005. One part was still dominated by M. edulis whereas the other part was already densely colonized by C. gigas. Crassostrea-reefs compensate for the conceivable loss of Mytilus-beds in the intertidal of the Wadden Sea by replacing the ecological function of M. edulis. There was no indication of a suppression of indigenous species. This even applied to M. edulis, which persisted at the site invaded by C. gigas. The associated macrofaunal community showed increased species richness, abundance, biomass, and diversity in the Crassostrea-reef. The latter particularly favored sessile species like anthozoans, hydrozoans, and barnacles. Higher abundance and biomass for vagile epizoic species like the shore crab Carcinus maenas and the periwinkle Littorina littorea also occurred among oysters. Abundance of deposit feeding oligochaetes was enhanced by oysters as well. More opportunistic, facultative filter-feeding polychaetes occurred in the Crassostrea-reef.     

Warm temperature acclimation impacts metabolism of paralytic shellfish toxins from Alexandrium minutum in commercial oysters

Species of Alexandrium produce potent neurotoxins termed paralytic shellfish toxins and are expanding their ranges worldwide, concurrent with increases in sea surface temperature. The metabolism of molluscs is temperature dependent, and increases in ocean temperature may influence both the abundance and distribution of Alexandrium and the dynamics of toxin uptake and depuration in shellfish. Here, we conducted a large‐scale study of the effect of temperature on the uptake and depuration of paralytic shellfish toxins in three commercial oysters (Saccostrea glomerata and diploid and triploid Crassostrea gigas, n = 252 per species/ploidy level). Oysters were acclimated to two constant temperatures, reflecting current and predicted climate scenarios (22 and 27 °C), and fed a diet including the paralytic shellfish toxin‐producing species Alexandrium minutum. While the oysters fed on A. minutum in similar quantities, concentrations of the toxin analogue GTX1,4 were significantly lower in warm‐acclimated S. glomerata and diploid C. gigas after 12 days. Following exposure to A. minutum, toxicity of triploid C. gigas was not affected by temperature. Generally, detoxification rates were reduced in warm‐acclimated oysters. The routine metabolism of the oysters was not affected by the toxins, but a significant effect was found at a cellular level in diploid C. gigas. The increasing incidences of Alexandrium blooms worldwide are a challenge for shellfish food safety regulation. Our findings indicate that rising ocean temperatures may reduce paralytic shellfish toxin accumulation in two of the three oyster types; however, they may persist for longer periods in oyster tissue.     

Reproduction of the introduced oyster Crassostrea gigas (Bivalvia: Ostreidae) cultured on rafts in Spain

The oyster Crassostrea gigas was introduced in Spain for aquaculture purposes; however, until now, it is not known whether populations are established in the wild, being necessary to define whether this species is spawning and which environmental variables trigger this process. The influence of environmental parameters on the reproduction of C. gigas was evaluated from January 2008 to October 2009 with oysters grown on a raft in the Ría de Arousa (Galicia, NW Spain). Temperature and chlorophyll a are directly correlated to sexual maturation. Oysters can mature at temperatures below 14°C. The temperature necessary for spawning differs between the sexes, requiring a temperature above 15°C for males and 18°C for females. Females had a single massive spawn between June and September, while males had partial spawning from May to December with two peaks, one in May–September and another in October–December, with the second peak more pronounced. The first spawning peak is related to high temperatures and concentrations of chlorophyll a, and the second spawning peak is mainly related to the food availability in the water. The spawning asynchrony may be impeding establishment of wild C. gigas populations in Spain.     

The Distribution of an Introduced Mollusc and its Role in the Long-term Demise of a Native Confamilial Species

The estuarine mud snail, Batillaria attramentaria, was transported to the Pacific coast of North America with the Pacific oyster, Crassostrea gigas (C. gigas), imported from Japan in the early part of this century, and has proliferated in several bays where the oyster was introduced. Since the arrival of Batillaria there have been declines in populations of its native ecological equivalent, Cerithidea californica. This study documents the distribution of the exotic Batillaria throughout its entire introduced range, concentrating on the few bays in northern California where both snails exist sympatrically. Using dates of initial importation of C. gigas and dates of first documentation of Batillaria within a bay, I established the earliest possible date for the introduction of Batillaria in a particular area. In cases where Cerithidea also was, or had been, present within a bay, I calculated a range of time for either the continued coexistence of the two species or the time until local exclusion of the native. Density measurements of Cerithidea within these bays where the species co-occurred allowed comparison of present Cerithidea numbers to historical accounts. Results indicated that Batillaria is replacing Cerithidea in the northern marshes of California. This replacement of the northernmost Cerithidea populations is not only reducing Cerithidea's overall range, but also eliminating a race recently shown to be a genetically distinct from southern Cerithidea populations. Other studies that have demonstrated superior exploitative competitive ability by Batillaria provide a potential mechanism for this displacement pattern. Regardless, the results presented here indicate that the displacement process is slow, taking on average >50 years to complete. This study illustrates a gradual, but predictable process of exotic replacement of native species, and argues strongly against complacency toward invaders that may currently seem innocuous. This revised version was published online in July 2006 with corrections to the Cover Date.     

High survival and growth rates of introduced Pacific oysters may cause restrictions on habitat use by native mussels in the Wadden Sea

Pacific oysters Crassostrea gigas (Thunberg, 1793) were introduced to the northern Wadden Sea (North Sea, Germany) by aquaculture in 1986 and finally became established. Even though at first recruitment success was rare, three consecutive warm summers led to a massive increase in oyster abundances and to the overgrowth of native mussel beds (Mytilus edulis L.). These mussels constitute biogenic reefs on the sand and mud flats in this area. Survival and growth of the invading C. gigas were investigated and compared with the native mussels in order to predict the further development of the oyster population and the scope for coexistence of both species. Field experiments revealed high survival of juvenile C. gigas (approximately 70%) during the first three months after settlement. Survival during the first winter varied between > 90% during a mild and 25% during a cold winter and was independent of substrate (i.e., mussels or oysters) and tide level. Within their first year C. gigas reached a mean length of 35-53 mm, and within two years they grew to 68-82 mm, which is about twice the size native mussels would attain during that time. Growth of juvenile oysters was not affected by substrate (i.e., sand, mussels, and other oysters), barnacle epibionts and tide level, but was facilitated by fucoid algae. By contrast, growth of juvenile mussels was significantly higher on sand flats than on mussel or oyster beds and higher in the subtidal compared to intertidal locations. Cover with fucoid algae increased mussel growth but decreased their condition expressed as dry flesh weight versus shell weight. High survival and growth rates may compensate for years with low recruitment, and may therefore allow a fast population increase. This may lead to restrictions on habitat use by native mussels in the Wadden Sea.     

Mitochondrial and nuclear DNA phylogeography of Crassostrea angulata, the Portuguese oyster endangered in Europe

The respective status of the Portuguese oyster, Crassostrea angulata, and the Pacific oyster, Crassostrea gigas, has long been a matter of controversy. Morphological and physiological similarities, homogeneity of allozyme allelic frequencies between populations of the two taxa and the demonstration of hybridization lead most authors to suggest that they should be regrouped within the same species. The risk of introgression and the present expansion of C. gigas aquaculture in Europe raises the question of the need for preservation of C. angulata in Europe, as only a few populations remain. We studied European and Asian populations of C. gigas and C. angulata using microsatellite and mitochondrial DNA markers to estimate their genetic diversity and differentiation. The analysis of genetic distances and the distribution of allelic and haplotype frequencies revealed significant genetic differences between taxa, showing two clusters: (1) C. gigas French and Japanese populations and (2) C. angulata Portuguese and Taiwanese populations. The Asian origin of the Crassostrea angulata taxa is therefore confirmed. Unlike previous studies based on allozymes, significant nuclear genome differences were noted between C. angulata and C. gigas. Despite the presumed history of the introduction of C. angulata into Southern Europe, these populations did not show any significant reduction of variability compared to Taiwanese populations. Any conservation plans for European C. angulata populations should take its non-native origin into account. They represent a valuable genetic resources for European breeding program.     

Characterization of 79 microsatellite DNA markers in the Pacific oyster Crassostrea gigas

We characterized 79 microsatellite DNA markers, which were obtained from genomic libraries enriched for CA, GA, ATG and TAGA motif repeats, in the Pacific oyster Crassostrea gigas. For eight F₁ grandparents or great‐grandparents of mapping families, the average heterozygosity, 0.705, and average number of alleles per locus, 5.7, did not vary among motif‐repeat or motif‐complexity categories. Non‐amplifying polymerase chain reaction null alleles, which were confirmed by segregation in the mapping families, were detected at 41 (51.9%) of the 79 loci. Cross‐species amplifications from C. angulata, C. sikamea, C. ariakensis and C. virginica showed a precipitous decline with distance from the focal species C. gigas.     

Development of SNP‐genotyping arrays in two shellfish species

Use of SNPs has been favoured due to their abundance in plant and animal genomes, accompanied by the falling cost and rising throughput capacity for detection and genotyping. Here, we present in vitro (obtained from targeted sequencing) and in silico discovery of SNPs, and the design of medium‐throughput genotyping arrays for two oyster species, the Pacific oyster, Crassostrea gigas, and European flat oyster, Ostrea edulis. Two sets of 384 SNP markers were designed for two Illumina GoldenGate arrays and genotyped on more than 1000 samples for each species. In each case, oyster samples were obtained from wild and selected populations and from three‐generation families segregating for traits of interest in aquaculture. The rate of successfully genotyped polymorphic SNPs was about 60% for each species. Effects of SNP origin and quality on genotyping success (Illumina functionality Score) were analysed and compared with other model and nonmodel species. Furthermore, a simulation was made based on a subset of the C. gigas SNP array with a minor allele frequency of 0.3 and typical crosses used in shellfish hatcheries. This simulation indicated that at least 150 markers were needed to perform an accurate parental assignment. Such panels might provide valuable tools to improve our understanding of the connectivity between wild (and selected) populations and could contribute to future selective breeding programmes.     

Genetic identification of source and likely vector of a widespread marine invader

The identification of native sources and vectors of introduced species informs their ecological and evolutionary history and may guide policies that seek to prevent future introductions. Population genetics provides a powerful set of tools to identify origins and vectors. However, these tools can mislead when the native range is poorly sampled or few molecular markers are used. Here, we traced the introduction of the Asian seaweed Gracilaria vermiculophylla (Rhodophyta) into estuaries in coastal western North America, the eastern United States, Europe, and northwestern Africa by genotyping more than 2,500 thalli from 37 native and 53 non‐native sites at mitochondrial cox1 and 10 nuclear microsatellite loci. Overall, greater than 90% of introduced thalli had a genetic signature similar to thalli sampled from the coastline of northeastern Japan, strongly indicating this region served as the principal source of the invasion. Notably, northeastern Japan exported the vast majority of the oyster Crassostrea gigas during the 20th century. The preponderance of evidence suggests G. vermiculophylla may have been inadvertently introduced with C. gigas shipments and that northeastern Japan is a common source region for estuarine invaders. Each invaded coastline reflected a complex mix of direct introductions from Japan and secondary introductions from other invaded coastlines. The spread of G. vermiculophylla along each coastline was likely facilitated by aquaculture, fishing, and boating activities. Our ability to document a source region was enabled by a robust sampling of locations and loci that previous studies lacked and strong phylogeographic structure along native coastlines.