Donald W. Meritt

Growth Rates and Prevalence of Perkinsus marinus in Restored Oyster Populations in Maryland

ABSTRACT Since 1995, hatchery-produced juvenile oysters have been planted on numerous natural oyster bars in Maryland in an effort to restore degraded reefs. As part of the monitoring effort, 27 discrete hatchery plantings spanning 10 y of restoration were sampled during late summer and fall 2009. Oyster shell height, dry meat weight, shell weight, and clump height all increased significantly with age. Perkinsus marinus infections were low in all sampled populations, but increased with ago. These data enable estimates of growth and shell production rates, and highlight the low prevalence of disease in restored Maryland oyster populations. The longevity of these dense patches suggests that local metapopulation restoration may provide substantial ecological services. The trends presented in this study may provide valuable insights for refining management tools, adapting ongoing restoration, and improving population modeling efforts.

Effects of age and composition of field-produced biofilms on oyster larval setting

Lack of success in restoring the native Eastern oyster, Crassostrea virginica, to Chesapeake Bay has been linked to the low occurrence of oyster larval setting in tributaries to the Bay. Among the many potential factors that could affect efforts to produce oysters through aquaculture or supplementation of shell beds is substratum condition. The present study examined larval setting on field-produced biofilms from Little Wicomico River (Virginia, USA) to assess whether bacterial community structure (examined by terminal restriction fragment length polymorphism, T-RFLP) or other characteristics of contemporary biofilms in this tributary (biofilm age and mass, algal abundance, and percentage organic matter) inhibited setting of larval oysters. The structure of the natural and heterogenous bacterial community in the biofilms and the success of oyster set were correlated, suggesting that specific microbial species may play a role in oyster setting. Larval set increased with biofilm age and mass, suggesting that established field-produced biofilms have no inhibitory effect. In contrast, the percentage of organic matter was negatively correlated with oyster set, whereas chlorophyll a concentration had no observed effect. The study expands prior knowledge by providing a more realistic timeframe for biofilm development (weeks as opposed to days), recounting effects of biofilms that are more representative of the natural dynamic and disturbance processes that would be expected to occur on submerged structures, and by incorporating seasonal and spatial variation. An important negative effect observed during the study period was heavy predation by Stylochus ellipticus on newly set oysters. Overall, the results of this study, which is the first assessment of the effects of biofilms produced naturally within a Chesapeake Bay tributary, suggest that the absence of large numbers of oysters in Little Wicomico River is not related to microbes or other specific characteristics of biofilms that develop on suitable setting substrata, but rather to heavy predation of newly set larvae.

HARMFUL ALGAE POSE ADDITIONAL CHALLENGES FOR OYSTER RESTORATION: IMPACTS OF THE HARMFUL ALGAE KARLODINIUM VENEFICUM AND PROROCENTRUM MINIMUM ON EARLY LIFE STAGES OF THE OYSTERS CRASSOSTREA VIRGINICA AND CRASSOSTREA ARIAKENSIS

Abstract The eastern oyster, Crassostrea virginica (Gmelin, 1791) has been in decline along the eastern seaboard, and especially in Chesapeake Bay, for decades because of over-harvesting, disease and declines in water quality and suitable habitat. Eutrophication has also been increasing over the past half century, leading to increases in hypoxia and harmful algal blooms (HABs). The effects of two common Chesapeake Bay HAB dinoflagellates, Karlodinium veneficum, and Prorocetnrum minimum were tested on larvae of C. virginica and the Asian oyster being considered for introduction to Chesapeake Bay, C. ariakensis. When embryos from freshly spawned C. virginica and C. ariakensis were exposed immediately to K. veneficum at 104 cells mL−1, virtually all of the developed larvae were deformed within 48 h in one experimental trial, but not in a second trial in which algae were at a different growth stage. No deformities, and mortalities of <45%, were observed in controls to which a standard diet of the haptophyte Isochrysis was added. When 2-wk-old larvae of both species were exposed to the same HAB species, the effect was a severe reduction in motility with K. veneficum, but with P. minimum only C. ariakensis was affected and not C. virginica. Comparisons were made of the frequency of these HABs in Chesapeake Bay from long-term data analysis and the temporal period of spawning. Whereas both blooms are more common during the summer months, the frequency of blooms of K. veneficum and the period of oyster spawning, June to September, coincide more strongly. To compare spatial similarity, results of a larval transport model were compared with observational data for K. veneficum. This comparison demonstrated a significant overlap in July, particularly in the northern reaches of the Bay. These eutrophication-related HABs thus have the potential to reduce survival of early life history stages of oysters and hence to reduce oyster recruitment. Any reduction in recruitment either spatially or temporally, combined with an overall reduction in sheer numbers of larvae that survive, will make restoration or establishment of significant, self-sustaining populations of natural or introduced oyster species much more difficult.

Survival and Growth of Triploid Crassostrea virginica (Gmelin, 1791) and C. ariakensis (Fujita, 1913) in Bottom Environments of Chesapeake Bay: Implications for an Introduction

ABSTRACT Survival and growth of triploid Crassostrea virginica and triploid C. ariakensis were investigated at four sites surrounding Chesapeake Bay, United States, that varied in salinity, tidal regime, water depth, predation intensity and disease pressure. Four experimental treatments were established at each site: C. virginica; C. ariakensis; 50:50 of C. virginica: C. ariakensis; and shell only. Oysters were deployed at mean shell heights of 12.80 mm and 13.85 mm (C. virginica and C. ariakensis, respectively), at an overall density of 347.5 oysters m-2. Oyster survival and growth varied significantly with site and species. Survival was significantly higher in C. virginica than C. ariakensis at the intertidal site, and significantly higher in C. ariakensis than C. virginica at the highest salinity, subtidal site. Survival did not differ significantly between species at the mid and low salinity, subtidal sites. For both species, survival differed significantly between sites, with lowest survival in both species occurring at the intertidal site. Among the subtidal sites, C. virginica survival varied inversely with salinity, whereas C. ariakensis had the lowest survival at the mid salinity site. Eight months after deployment C. ariakensis were significantly larger than C. virginica at all sites. This difference generally persisted throughout the experiment, though the size differences between oyster species at the lowest salinity site were small (< 10%). Shell heights within single-species treatments differed significantly between sites; highest growth rates were observed at the high salinity, subtidal site, whereas lowest growth rates were observed at the high salinity, intertidal site. At low and mid salinity subtidal sites, C. ariakensis shell heights were significantly greater in the single-species treatment compared with the mixed-species treatment. Perkinsus marinus infections occurred in both species at all sites, with prevalences varying between sites. In C. virginica, moderate and high intensity infections were only common at the two higher salinity sites, whereas infections in C. ariakensis were generally low to rare. Haplosporidium nelsoni infections in C. virginica were only observed at the two higher salinity sites and prevalences were generally low. Two out of 53 C. ariakensis tested at the high salinity, subtidal site had rare H. nelsoni infections. Bonamia spp. infections were never observed. Our study supports previous laboratory findings and observations from its native range that C. ariakensis survives poorly in intertidal habitats. In subtidal habitats, however, C. ariakensis displayed broad environmental tolerances, often exceeding native oyster survival and growth rates. Post-introduction C. ariakensis populations would be shaped by the survival and growth patterns described here, but also by their reproductive success, larval survival, predator-prey interactions and prevailing disease dynamics.

Crassostrea ariakensis in Chesapeake Bay: Growth, Disease and Mortality in Shallow Subtidal Environments

Abstract In April 2004, triploid native (Crassostrea virginica) and nonnative (Crassostrea ariakensis) oysters were deployed in cages at four sites along a salinity gradient in Chesapeake Bay. In Maryland, the lowest salinity site was located in the Severn River and two low to mid-salinity sites were located in the Choptank and Patuxent Rivers. The highest salinity site was located in the York River in Virginia. Growth, disease acquisition, and mortality were measured in the deployed oysters through August 2006. Although ANOVA revealed that the nonnative oysters were significantly larger at the end of the experiment than the native oysters at all sites, the differences were much greater at the Virginia site (59 mm) than in Maryland waters (9–23 mm). With the exception of C. ariakensis in the Severn River, Perkinsus marinus infected both species at all sites. Prevalences and weighted prevalences in both species remained relatively low throughout the experiment, but native oysters consistently acquired higher prevalences and weighted prevalences than C. ariakensis by August 2006. With the exception of several mortality-inducing events including winter freezing and hypoxic exposure, mortality was generally low in both species. No disease-related mortality was suspected in either species given the low weighted prevalences observed. In the York River, where a substantial natural spatfall occurred in 2004, more native spat were found on C. ariakensis than on C. virginica. To our knowledge, this is the first comparison of triploid C. ariakensis to triploid C. virginica conducted in the field. Because we did not observe substantial disease-related mortality, it is too soon to draw conclusions regarding the disease tolerance of C. ariakensis in the field or its viability as a replacement for the native species.

A Comparison of Crassostrea Virginica and C . Ariakensis in Chesapeake Bay : Does Oyster Species Affect Habitat Function ?

ABSTRACT We examined the possibility that a nonnative oyster species would provide an ecologically functional equivalent of the native oyster species if introduced into the Chesapeake Bay. Habitat complexity and associated benthic communities of experimental triploid Crassostrea virginica and Crassostrea ariakensis reefs were investigated at 4 sites of varying salinity, tidal regime, water depth, predation intensity, and disease pressure in the Chesapeake Bay region (Maryland and Virginia). Four experimental treatments were established at each site: C. virginica, C. ariakensis, 50:50 of C. virginica and C. ariakensis, and shell only. Abundance, biomass, species richness, evenness, dominance, and diversity of reef-associated fauna were evaluated in relation to habitat location and oyster species. Although habitat complexity varied with location, no differences among complexity were associated with oyster species. Similarly, differences in faunal assemblages were more pronounced between sites than within sites. Our results show functional equivalency between oyster species with respect to habitat at the intertidal site and the low-salinity subtidal location. At subtidal sites of higher salinity, however, the numbers of organisms associated with C. virginica reefs per unit of oyster biomass were significantly greater than the numbers of organisms associated with C. ariakensis reefs. Multivariate analyses of data from subtidal high-salinity sites revealed unique communities associated with C. virginica treatments, whereas mixed-oyster species assemblages were functionally equivalent to monospecific C. ariakensis experimental treatments. Our study represents the first effort to quantify the potential habitat function of C. ariakensis, which has been proposed for an intentional introduction into Chesapeake Bay, and provides evidence of species-specific similarities and differences in reef-associated communities.

Differential Production of Feces and Pseudofeces by the Oyster Crassostrea ariakensis When Exposed to Diets Containing Harmful Dinoflagellate and Raphidophyte Species

Abstract The Asian oyster, Crassostrea ariakensis, is being evaluated for its potential success in the restoration of oyster populations in Chesapeake Bay. Critical to an understanding of its potential success is knowledge of the impacts of common harmful algae in its diet; blooms of such algae are common in Chesapeake Bay. In these experiments, C. ariakensis were exposed to a standard algal diet, Isochrysis sp., alone, and in combination with the harmful dinoflagallates Prorocentrum minimum and Karlodinium veneficum and the raphidophytes Heterosigma akashiwo and Chattonella subsalsa. Two experiments were run, with varying proportions of Isochrysis to the test algal species. Feces and pseudofeces were examined microscopically and by high-performance liquid chromatography for changes in diagnostic pigments relative to the initial diet and for production of degradation pigments of chlorophyll. All species were cleared from suspension to varying degrees by the oysters. In the Isochrysis control and in the Isochrysis + P. minimum treatment, intact, solid feces were produced, and the highest proportion of chlorophyll degradation pigments were found in the Isochrysis control diet suggesting algal digestion. Thin, “ropey” feces and pseudo-feces were observed in the K. veneficum + Isochrysis treatment. Virtually no degradation pigments were found for oysters fed a diet containing K. veneficum, suggesting lack of digestion and assimilation of algal food. With the raphidophytes, reduced production of feces and pseudofeces was evident, and these contained a lower proportion of recognizable harmful algal cells, and higher proportion of degradation pigments than found with the other test species. Amorphous and cellular material that appears to be partly derived from the oyster digestive system was evident in the feces of oysters exposed to K. veneficum, H. akashiwo, and C. subsalsa; this was particularly pronounced in oysters exposed to H. akashiwo and suggests damage to the oyster gut. The impact of the presence of harmful algae in the diet of the oysters thus varied by algal species, but in all cases oyster digestion was altered relative to the control diet.

Impact of Environment and Ontogeny on Relative Fecundity and Egg Quality of Female Oysters (Crassostrea virginica) from Four Sites in Northern Chesapeake Bay

Resource allocation to reproduction is a primary physiological concern for individuals, and can vary with age, environment, or a combination of both factors. In this study we quantified the impact of environment and individual age on the reproductive output of female oysters Crassostrea virginica. We determined the relative fecundity, egg total lipid content, and overall and omega-3/omega-6 (ω3/ω6) fatty acid signatures (FAS) of eggs spawned by female oysters over a 2-year period (n = 32 and n = 64). Variation was quantified spatially and ontogenetically by sampling young and old oyster populations from two rivers in Chesapeake Bay, totaling four collection sites. During Year 1, when oysters underwent oogenesis in different locations, overall and ω3/ω6 egg FAS varied significantly by river, with no significant differences observed in the FAS of oysters by age in Year 1. In Year 2, when oysters from different sites underwent oogenesis in a single location, no significant differences in the overall egg FAS or ω3/ω6 egg FAS by river or age were observed. These findings suggest that oysters integrate environment into their reproductive output, but that time spent growing at a specific location (in this case, represented by oyster age) plays a relatively minor role in the biochemical composition of oyster eggs. These results have consequences for our understanding of how resources are allocated from the female oyster to eggs and, more generally, the impact of environment and ontogeny on reproductive physiology.