Kathryn A. Ashton-Alcox

Haemocyte parameters associated with resistance to brown ring disease in Ruditapes spp. clams

Brown ring disease (BRD) is a shell disease caused by Vibrio tapetis. This pathogen disturbs the periostracal lamina causing the appearance of a brown conchiolin deposit on the inner face of the shell, within the extrapallial space. Although differences in resistance to BRD have been documented, their relationship to possible defense functions has never been investigated. In this study, flow cytometry was used to analyze cellular parameters in asymptomatic and experimentally infected Ruditapes philippinarum from France and the west coast of the USA. Parallel analyses were made on Ruditapes decussatus, the native European clam, which is highly resistant to BRD. In the haemolymph and extrapallial fluid of animals without BRD, total haemocyte counts, the percentage of granulocytes, and the phagocytic activity against latex beads or V. tapetis by the haemocytes were significantly higher in American R. philippinarum than in French R. philippinarum. In most cases, levels in R. decussatus were the highest of all three groups. Four weeks following challenge with V. tapetis, BRD prevalence reached 52 in American clams and 100% in French specimens, but only 37% in R. decussatus. In symptomatic animals, phagocytosis of V. tapetis increased significantly in the resistant species of clam, R. decussatus, was unchanged in US clams, and decreased significantly in FR specimens when compared to asymptomatic individuals from each population. Ingestion of V. tapetis by haemocytes in the extrapallial fluid, which is in contact with the periostracal lamina, could be the main defense mechanism used to counter the pathogen. Our results suggest that resistance to BRD may well be related to the concentration of granular haemocytes and the phagocytic activity of haemocytes.

VARIABILITY IN MOLLUSCAN HEMOCYTES : A FLOW CYTOMETRIC STUDY

Reported variability in numbers and relative proportions of hemocytes in marine bivalves may be related to environmental conditions and laboratory method differences. An automated identification assay, flow cytometry, removes much laboratory bias, but its usefulness is limited because the putative cell types in delineated subpopulations have never been confirmed. The present study was designed to: (1) confirm the identity of oyster hemocyte subpopulations described by flow cytometry, and (2) use flow cytometry in an experimental analysis of potential causes of variation. Light-scatter flow cytometry consistently differentiated three subpopulations in oysters from two mid-Atlantic (USA) sites. Cell sorting and microscopy identified them as granular, small granular, and agranular (hyalinocytes and apparently degranulated) hemocytes. Subpopulation proportions estimated by microscopy and by flow cytometry were significantly correlated (r(2) = 0.27 to 0.50). In a 4-week laboratory experiment, neither temperature (12 vs. 22 degrees C) nor food (fed vs. not fed) had a statistically significant effect on total or differential counts, or on hemocyte viability. Most of the variability was attributable to individual differences and was very similar to that reported for vertebrates. We hypothesize that variability in molluscan hemocytes may be more immediately linked to individual metabolic condition than to ambient changes.

Long-term Trends in Oyster Population Dynamics in Delaware Bay: Regime Shifts and Response to Disease

Abstract We evaluate a 54-y survey time series for the Delaware Bay oyster beds in New Jersey waters to identify the characteristics of regime shifts in oyster populations and the influence of MSX and Dermo diseases on population stability. Oyster abundance was high during the 1970s through 1985. Oyster abundance was low at the inception of the time series in 1953, remained low through 1969, and has been low since 1985 and very low since 2000. Natural mortality was low in most years prior to the appearance of MSX in 1957. From 1957 through 1966, natural mortality generally remained above 10% annually and twice exceeded 20%. Natural mortality remained well below 15% during the 1970s and into the early 1980s when oyster abundance was continuously high. The largest mortality event in the time series, an MSX epizootic that resulted in the death of 47% of the stock, occurred in 1985. Mortality rose again with the incursion of Dermo in 1990 and has remained above 15% for most years since that time and frequently has exceeded 20%. The primary impact of MSX and Dermo diseases has been to raise natural mortality and ultimately to cause a dispersed stock to retreat into its habitat of refuge in the moderately low salinity reach of the bay. The time series of oyster abundance on the New Jersey oyster beds of Delaware Bay is dominated by two regime shifts, the 1970 abundance increase that was maintained for about 15 y thereafter, and the 1985 abundance decrease that continues through today. These two regime shifts ushered in long-term periods of apparent constancy in population dynamics. The 1985 regime shift was induced by the largest MSX epizootic on record that produced high mortalities throughout a population distributed broadly throughout its habitat range after 15 y of high abundance. A putative new regime commenced circa 2000 as a consequence of a series of Dermo epizootics. Mortalities routinely exceeded 20% of the population annually during this period, with the consequence of a greater degree of stock consolidation than any previous time in the 54-y record. Extreme consolidation of the stock would appear to be a characteristic of the populations response to Dermo disease. The 1970–1984 and post-1985 regimes each were ushered in by a confluence of events unique in the 54-y time series. Each was characterized by a period of relative stability in population abundance. However, the stability in total population abundance belies a more dynamic process of stock redistribution during both time intervals, demonstrating that the appearance of constancy in stock abundance is not necessarily a result of invariant stock dynamics. Rather, the Delaware Bay oyster time series suggests that regime shifts delimit periods during which differential, often offsetting, local trends impart similar abundance levels, and thus constancy at the level of the stock masks substantive changes in local population dynamics potentially fostering future catastrophic changes in population-level attributes. Understanding such regime shifts will likely determine the success of decadal management goals more so than measures designed to influence population abundance.

The Rise and Fall of Crassostrea Virginica Oyster Reefs: The Role of Disease and Fishing in Their Demise and a Vignette on Their Management

We describe a model designed to simulate the shell carbonate budget of an oyster reef. We identify five parameters descriptive of basic characteristics of the shell carbonate budget of a reef that limit simulation accuracy. Two describe the TAZ (taphonomically-active zone) and the distribution of shell carbonate within it. One is the taphonomic rate in the TAZ. Two determine the volume contribution of shell carbonate and the taphonomic loss rate within the reef framework. For Mid-Atlantic estuaries, model simulations suggest that reef accretion only occurs if oyster abundance is near carrying capacity. Simulations further suggest that reef accretion is infeasible for any estuarine reach where dermo is a controlling influence on population dynamics. We forecast that the oyster disease dermo is a principal antagonist of reef persistence through its ability to limit shell addition. Model simulations suggest that reefs with inadequate shell addition ‘protect themselves’ by limiting the volumetric content of shell carbonate in the TAZ. Thus, a dominant process is the transient expansion and contraction of the shell resource, otherwise termed cultch, within the TAZ, rarely expanding enough to generate reef accretion, yet rarely contracting enough to foster erosion of the reef framework. The loss of framework carbonate thusly is curtailed during periods when the surficial shell layer deteriorates. Stasis, a reef neither accreting nor eroding, is a preferred state. Reef recession requires an inordinately unbalanced shell carbonate budget. Results strongly argue for expanded focus on the dynamics of the shell resource within the TAZ, as this likely fosters a feedback loop with abundance through recruitment, serves as the protective layer for the reef during periods of reef stasis, and establishes the threshold conditions for reef accretion and recession. Model simulations suggest that attaining maximum sustainable yield and maintaining a biomass capable of supporting sufficient shell production for reef accretion are irreconcilable goals over a large component of the oyster’s range. Reef stasis would appear to be the only achievable restoration goal in Mid-Atlantic estuarine reaches where dermo holds sway. Exploitation rates much above 5% of the fishable stock per year restrict availability of surficial shell and foster reef erosion. In contrast, in the Gulf of Mexico at the high-productivity end of the oyster’s range, an enhanced fishery and reef accretion may be compatible goals. 1. Rutgers University, Institute of Marine and Coastal Sciences and The New Jersey Agricultural Experiment Station, Haskin Shellfish Research Laboratory, Port Norris, New Jersey, 08349, U.S.A. 2. Corresponding author. email: eric@hsrl.rutgers.edu 3. Center for Coastal Physical Oceanography, Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, Virginia, 23508, U.S.A.

In vitro Interactions between Bivalve Hemocytes and the Oyster Pathogen Haplosporidium nelsoni (MSX)

Resistance to disease caused by the oyster pathogen Haplosporidium nelsoni (MSX) has developed in nature and through selective breeding; however, examination of tissue slides shows no evidence that phagocytic cells are involved. To investigate this phenomenon further, we quantified in vitro phagocytosis of plasmodial stages of H. nelsoni by hemocytes from 2 oyster and 1 mussel species. Our results show that hemocytes from oysters Crassostrea virginica, whether selected or unselected for resistance to MSX disease, did not ingest plasmodia unless the parasites were damaged or killed. Time-lapse photography suggested that granular hemocytes retreated rapidly after initial contact with living plasmodia and thatnearby agranular hemocytes showed little movement

MOLLUSCAN SHELL CONDITION AFTER EIGHT YEARS ON THE SEA FLOOR—TAPHONOMY IN THE GULF OF MEXICO AND BAHAMAS

Abstract In 1993 and 1994, the shelf and slope experimental taphonomy initiative (SSETI) deployed shells of a suite of molluscan species in a range of environments of deposition (EODs) representing a range of depths, sediment types, and environmental conditions with the goal of measuring taphonomic rates over an extended period of time. In 1999 and 2001, SSETI retrieved skeletal remains from 41 locations in the Bahamas and on the Gulf of Mexico continental shelf and upper slope that had been on the seafloor for eight years. Here, we compare taphonomic processes in two different ocean basins, across 24 environments of preservation (EOP) to evaluate the influence of species, sedimentary environment, degree of burial, and water depth on the preservational process. Taphonomic signature after eight years was almost exclusively a function of location of deployment and, frequently, taphonomically-distinctive locations of deployment were subsumed within distinctive EODs. EOD-level characteristics were insufficiently discriminative to delineate environments of preservation. EOPs and EODs are not synonymous concepts. Across all sites and species, the dominant taphonomic process was discoloration. Dissolution was of penultimate importance; nevertheless the cumulative impact over eight years was insufficient to produce a significant loss in shell weight in any EOP. Maximum dissolution intensity was normally observed on the outer shell surface; the inner and outer shell surfaces are inherently different in their time course of shell deterioration. Principal components analysis (PCA) demonstrated limited co-occurrence of discrete taphonomic processes among the 24 EOPs. Breakage and edge rounding fell on the same PCA axis, but these two processes were independent of all others. PCA divided dissolution into three independent components that discriminated the inner and outer shell surface of bivalves (and spire and body whorl of gastropods) and pitting from the development of a chalky surface. Discoloration was dissembled into five distinctive discoloring processes: fading without subsequent discoloration, the development of a brown-to-red coloration, orange/orange mottled discoloration, development of a green/green mottled color, and gray-to-black discoloration. The only concordance of ostensibly distinctive taphonomic processes was the association of small pits on the shell surface with orange discoloration on the shell. Depth did not exert a single significant effect on any of the eight primary taphonomic factors resolved by PCA, likely because of burial processes. The trends in taphonomic signature cannot be explained by any simple combination of sediment type and degree of exposure. A comparison between two-year and eight-year deployments suggests that important revelations can be gleaned from short-term experimental deployments, yet the same comparison discloses the spuriousness of other inferences. Thus, long-term experiments are essential to understand the time course of preservation. The taphonomic process is, in general, slow, and nonlinearity in rates over time constrains the subset of inferences that can be deduced accurately from short deployment periods.

THE TAPHONOMIC SIGNATURE OF A BRINE SEEP AND THE POTENTIAL FOR BURGESS SHALE STYLE PRESERVATION

Abstract Unusually fine preservation of soft anatomy in the fossil record, often referred to as Lagerstätte deposits, has led to great advances in understanding the evolution of life. An understanding of the potential environments of deposition that might lead to exquisite preservation may help to reconstruct the effects of the taphonomic filter and thereby better interpret the completeness of fossil Lagerstätten. Seafloor brines are potential environments leading to exceptional preservation. The Shelf and Slope Experimental Taphonomy Initiative (SSETI) placed mollusc shells, decapod crustaceans, sea urchins, and wood into a Gulf of Mexico seafloor brine pool environment to study the rates and modes of skeletal and soft tissue decay. We found that skeletons, soft tissue, and wood placed directly in the sulfidic anoxic brine were essentially not degraded or discolored over nearly a decade. Where the brine mixed with overlying seawater in a brine stream, the taphonomic signature was quite different. Calcium-carbonate shell and urchin tests underwent severe dissolution, whereas terrestrial plant remains were unaltered. Farther from the brine, shell and urchin carbonate was only slightly dissolved, wood was completely consumed by xylophagus animals, and decapods were reduced to claw parts only. From these experiments, we conclude that the taphonomic signature of a brine seep can be recognized by a unique juxtaposition of preservation styles that varies across phyla. The central area of the anoxic brine would promote exquisite preservation of carbonate, soft-animal tissue, and cellulose. The central area would be ringed by a zone of near total loss of shell carbonate, but paradoxically would promote the preservation of organic tissue such as shell periostracum and ligament, wood, nuts, and cones. Where seawater salinity is normal, the taphonomic signature would return to a seafloor assemblage appropriate to the depth and depositional environment. Brine seep systems may provide a mechanism for maintaining unaltered organism remains at the sediment-water interface long enough to become buried with soft anatomy intact and undisturbed. The very important fossil deposit known as the Burgess Shale exhibits preservation styles and patterns that might be explained by presence of brine. Our experimental work in a modern brine system may shed some light on the taphonomic conditions that led to preservation known as the “Burgess Shale type.”

Reevaluation of Eastern Oyster Dredge Efficiency in Survey Mode: Application in Stock Assessment

Abstract Dredge efficiency measurements were conducted in 2003 by comparing paired dredge and diver samples taken on eastern oyster Crassostrea virginica beds in the New Jersey waters of Delaware Bay. We evaluated whether ancillary data collected during the tow could be used to estimate dredge efficiency rather than periodically conducting costly field experiments. With the exception of market-size eastern oysters, dredge efficiency did not vary between the time when it was last measured (2000) and 2003. Dredge efficiencies were lower upbay in both studies. Capture efficiency was significantly greater for live eastern oysters than for boxes (dead, articulated valves) and significantly greater for boxes than for cultch (shell clumps, shells, and shell fragments without attached live eastern oysters or boxes) in both studies. In the single significant exception, dredge efficiency was significantly lower for market-size eastern oysters in 2003. Survey quantification of eastern oyster abundance by calibrating...

FISHERMAN CHOICE AND INCIDENTAL CATCH: SIZE FREQUENCY OF OYSTER LANDINGS IN THE NEW JERSEY OYSTER FISHERY

Abstract We randomly sampled the landings of oyster boats fishing on the New Jersey oyster beds of Delaware Bay during the 2004 fishing season (April to mid November) to determine (a) the viability of a simple conversion between the numbers-based stock assessment and the bushel-based quota-setting process and (b) the degree to which undersized oysters are taken during the fishing process because of imperfect culling. Rarely were more than 3% of the chosen (unattached) animals less than 2.5″ in length: fisherman choice is a knife-edge process. Most smaller oysters were attached to chosen oysters. Incidental catch caused by imperfect culling contributed 9.9% to fishing mortality. Season influenced landings size frequency more than did bed of harvest. Oysters chosen for market were larger in the Fall; consequently the number per bushel was lower in the Fall. Attached oysters were more numerous in the Fall, so that the total number of oysters per bushel did not differ significantly over the year. Trends in the uniformity of sizes harvested were dominantly related to within-bed variations in size frequency. Trends in the average size of oysters harvested were partly determined by fisherman choice. For the purposes of management, a single average conversion is adequate: 272 oysters per bushel. Regulations furthering an increase in culling efficiency are unnecessary.

Evaluation of Dredging Effort by the Delaware Bay Oyster Fishery in New Jersey Waters

Abstract As part of a study to assess the effect of commercial dredging for eastern oysters Crassostrea virginica on their beds in Delaware Bay, we evaluated the total dredging effort for the Delaware Bay oyster industry in New Jersey waters for 1999-2000 and examined some of the behavioral and gear-related factors that determined this total effort. In a standard 8-h fishing day, a one-dredge boat traverses about 3.8 ha (38,000 m2) of oyster bed. A two-dredge boat traverses nearly twice that area, about 6.4 ha. Oyster boats typically fish in a single area for most of the day, yet catch per unit effort (CPUE) does not decline during the course of the day. Catch per unit effort is stable because of low dredge efficiency. Although an oyster dredge is capable of routinely achieving efficiencies of 10-60%, dredge efficiency during fishing is usually only 4-7%. Low dredge efficiency means that swept-area coverage (area of the bottom traversed by a dredge) is high for the number of oysters taken. The most heavil...