Discriminatory predation by three invertebrates on eastern oysters (Crassostrea virginica) compared with non-native Suminoe oysters (C. ariakensis)

Abstract. Diminished populations of eastern oysters Crassostrea virginica in Chesapeake Bay have stimulated proposals to introduce Crassostrea ariakensis from Asia to restore oyster stocks. As part of a program evaluating possible ramifications of such an introduction, we studied how invertebrate predators responded to this non-native oyster. We compared predation activity under laboratory conditions by oyster drills ( Urosalpinx cinerea; Eupleura caudata ) that bore through an oyster's shell and by the seastar Asterias forbesi that pulls shell valves apart. These three predators preyed significantly (p<0.05) more on the familiar C. virginica than on the novel C. ariakensis . We previously reported that five crab species preyed significantly more on C. ariakensis than on C. virginica , with predation by polyclad flatworms similar between oyster species. Thus, the drills and the seastar differed from the crabs and the flatworms in their response to novel prey. When Urosalpinx cinerea was placed in a Y-maze after being held for 40 d with oysters of one species or the other, the drills moved toward C. virginica effluent more than toward C. ariakensis effluent. This response did not depend on the species of oyster the drills had been held with, suggesting that the drills were responding to more familiar infochemicals from eastern oysters than from the non-native oysters.     

Inducible defenses in Olympia oysters in response to an invasive predator

The prey naiveté hypothesis suggests that native prey may be vulnerable to introduced predators because they have not evolved appropriate defenses. However, recent evidence suggests that native prey sometimes exhibit induced defenses to introduced predators, as a result of rapid evolution or other processes. We examined whether Olympia oysters (Ostrea lurida) display inducible defenses in the presence of an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea), and whether these responses vary among oyster populations from estuaries with and without this predator. We spawned oysters from six populations distributed among three estuaries in northern California, USA, and raised their offspring through two generations under common conditions to minimize effects of environmental history. We exposed second-generation oysters to cue treatments: drills eating oysters, drills eating barnacles, or control seawater. Oysters from all populations grew smaller shells when exposed to drill cues, and grew thicker and harder shells when those drills were eating oysters. Oysters exposed to drills eating other oysters were subsequently preyed upon at a slower rate. Although all oyster populations exhibited inducible defenses, oysters from the estuary with the greatest exposure to drills grew the smallest shells suggesting that oyster populations have evolved adaptive differences in the strength of their responses to predators. Our findings add to a growing body of literature that suggests that marine prey may be less likely to exhibit naiveté in the face of invasive predators than prey in communities that are more isolated from native predators, such as many freshwater and terrestrial island ecosystems.     

Preference Alters Consumptive Effects of Predators: Top-Down Effects of a Native Crab on a System of Native and Introduced Prey

Top-down effects of predators in systems depend on the rate at which predators consume prey, and on predator preferences among available prey. In invaded communities, these parameters might be difficult to predict because ecological relationships are typically evolutionarily novel. We examined feeding rates and preferences of a crab native to the Pacific Northwest, Cancer productus, among four prey items: two invasive species of oyster drill (the marine whelks Urosalpinx cinerea and Ocenebra inornata) and two species of oyster (Crassostrea gigas and Ostrea lurida) that are also consumed by U. cinerea and O. inornata. This system is also characterized by intraguild predation because crabs are predators of drills and compete with them for prey (oysters). When only the oysters were offered, crabs did not express a preference and consumed approximately 9 juvenile oysters crab⁻¹ day⁻¹. We then tested whether crabs preferred adult drills of either U. cinerea or O. inornata, or juvenile oysters (C. gigas). While crabs consumed drills and oysters at approximately the same rate when only one type of prey was offered, they expressed a strong preference for juvenile oysters over drills when they were allowed to choose among the three prey items. This preference for oysters might negate the positive indirect effects that crabs have on oysters by crabs consuming drills (trophic cascade) because crabs have a large negative direct effect on oysters when crabs, oysters, and drills co-occur.     

The Effects of Hydraulic Dredging to Control Oyster Drills on Benthic Macrofauna of Oyster Grounds in Delaware Bay,New Jersey

This study describes the extent and nature of the effects of hydraulic dredging to control oyster drills (Urosalpinx cinerea and Eupleura caudata, Family Muricidae, Order Neogastropoda) on benthic macrofauna and sediments of the oyster grounds in Delaware Bay, New Jersey. The immediate effects of hydraulic dredging were reductions in numbers of species as well as in total numbers of animals on the three oyster grounds selected. However, oyster drills were most affected. Benthic populations have recovered three to ten months after dredging. The sediments of the dredged grounds can be described as muddy sands. Immediately after dredging, additional mud was brought up from subsurface layers which reduced the median grain size on Ground 154 test plot. On Ground 515 test plot, however, there was a slight loss in the mud which increased the median grain size.     

Physical constraints on the foraging ecology of a predatory snail

We studied the effects of aerial exposure and high summer temperatures on the southern oyster drill ( Stramonita haemastoma ), feeding on the American oyster, Crassostrea virginica . In the laboratory, oyster drill feeding rates and growth were highest at 25 and 30°C, some mortality occurred at 35°C, all snails died at 40 and 45°C, and the 28-day LC 50 was 35.7°C. In a second experiment where both water temperature (25 vs . 33°C) and aerial exposure were varied, only simulated tidal exposure lowered oyster drill feeding and growth rates. In field cage experiments, oyster drills had reduced feeding rates and growth at intertidal sites, but snail growth rates increased in late summer with warmer water temperature. We therefore conclude that aerial exposure, not high temperature, is the major factor limiting oyster drill feeding and growth in intertidal oyster reefs. Field experiments with partial cages also suggested that ambient predation rates were much higher at a subtidal than at a nearby intertidal site. Because southern oyster drills have depressed feeding, growth, and possibly lower fitness in intertidal oyster reefs during the summer, this reduced predation risk may provide a refuge for intertidal oysters.     

Predator-induced changes in fluted giant clam (Tridacna squamosa) shell morphology

Predator-induced defences have been demonstrated in numerous marine molluscs, but never in giant clams (Bivalvia: Tridacnidae). Water-borne cues from the predatory crab Myomenippe hardwickii were tested for their ability to induce changes in the shell morphology, strength and growth of juvenile fluted giant clams, Tridacna squamosa. Specimens were maintained in effluent from tanks holding ‘fed crabs’, ‘starved crabs’ and ‘no crabs’. To ensure changes were due to predator cues only, food for the crabs was provided ex situ and measures were taken to minimise the prey signal. After 182 days, MANOVA identified differences in various shell parameters relating to shape and strength. CDA showed a clear change in overall morphology, with strong separation among the three treatments. Effluent from ‘fed crabs’ had a greater influence than effluent from ‘starved crabs’, possibly because the starved crabs were perceived as weaker, and thus less risk. Significantly more clams died in ‘fed crabs’ treatments (25%) and ‘starved crabs’ treatments (30%) compared to the ‘no crabs’ control (0%). This degree of mortality has not been observed in similar experiments elsewhere and represents a new challenge to interpret. We suggest that the cause is unlikely to be related to the plastic responses, but rather a result of some form of crab-associated contaminant.     

An invasive crab alters interaction webs in a marine community

Over the last decade, the non-native, filter-feeding crab Petrolisthes armatus invaded oyster reefs of the South Atlantic Bight at densities of thousands m⁻². Mesocosm and field experiments demonstrated that P. armatus at ∼10–75% of mean summer densities: (1) suppressed growth of small oysters, biomass of benthic microalgae, and recruitment of native mud crabs, (2) enhanced oyster, mussel, and total bivalve recruitment, macroalgal cover, and survivorship of predatory oyster drills, but (3) did not affect native taxonomic richness. Laboratory feeding assays, field tethering experiments, and population changes in field and mesocosm experiments suggest that P. armatus is a preferred prey for native mud crabs and other consumers, thus relieving predation on native species and enhancing recruitment or survival of bivalves and oyster drills. In contrast, the invasive crab can consume crustacean larvae and via this feeding may suppress recruitment of native mud crabs. Our findings should be conservative given the low densities of P. armatus seeded into experimental plots and our inability to run longer-term experiments due to controls rapidly being colonized by non-native crabs recruiting from the plankton. Invasive crabs commonly impact native communities via predation, but community impacts of this invasive crab may be as much due to its role as a preferred prey of native consumers as to its predation on native prey. Given that oysters are foundation species for shallow reefs in the South Atlantic Bight, the long-term effects of this invasion could be considerable.     

Inheritance of allozymes of a marine snail (Urosalpinx cinerea).

Crosses using virgin oyster drills (Urosalpinx cinerea) show that both the ODH and LAP loci of foot muscle tissue have two co-dominant alleles segregating in Mendelian fashion. This constitutes the first demonstration of allozyme inheritance in a marine gastropod.     

Chemically induced predator avoidance behaviour in the burrowing bivalve Macoma balthica

The responses of the burrowing bivalves Macoma balthica and Cerastoderma edule to chemical cues emitted by feeding shore crabs Carcinus maenas were investigated. M. balthica held in the laboratory and exposed to chemical signals in effluent water discharging from tanks containing C. maenas fed 20 M. balthica day^− 1 reacted by increasing their burial depths from approximately 30 mm to depths of > 60 mm, over a period of several days. When the signal was removed the bivalves gradually returned to their original depth over 5 days. C. edule similarly exposed to effluent from crabs feeding on conspecifics showed no response. In an attempt to identify the signal inducing this burrowing response, M. balthica were exposed to a variety of chemical signals. Crabs fed M. balthica elicited the strongest response, followed by crabs fed C. edule. There were also small responses to effluent from crabs fed on fish, crabs previously fed on M. balthica and to crab faeces, but no responses to starved crabs, crushed M. balthica, or controls. We conclude that increased burrowing depth of M. balthica is induced by some as yet unidentified chemical cue produced by feeding crabs and is strongest when the crabs were fed on M. balthica. Unexpectedly, neither the presence of crabs themselves, nor of damaged conspecifics, was effective in eliciting a burrowing response. The mortality rates of M. balthica and C. edule selected by crabs when burrowed at normal depths and after exposure to effluent from feeding crabs were different. Crabs selected 1.5 times more C. edule than M. balthica when both species were burrowed at their normal depths, but 15 times more after the tanks had been exposed to effluent from feeding crabs for 5 days. The burrowing response of M. balthica thus appears to reduce mortality significantly by displacing predation pressure on to the more accessible C. edule.     

Insight into Invertebrate Defensin Mechanism of Action: OYSTER DEFENSINS INHIBIT PEPTIDOGLYCAN BIOSYNTHESIS BY BINDING TO LIPID II*

Three oyster defensin variants (Cg-Defh1, Cg-Defh2, and Cg-Defm) were produced as recombinant peptides and characterized in terms of activities and mechanism of action. In agreement with their spectrum of activity almost specifically directed against Gram-positive bacteria, oyster defensins were shown here to be specific inhibitors of a bacterial biosynthesis pathway rather than mere membrane-active agents. Indeed, at lethal concentrations, the three defensins did not compromise Staphylococcus aureus membrane integrity but inhibited the cell wall biosynthesis as indicated by the accumulation of the UDP-N-acetylmuramyl-pentapeptide cell wall precursor. In addition, a combination of antagonization assays, thin layer chromatography, and surface plasmon resonance measurements showed that oyster defensins bind almost irreversibly to the lipid II peptidoglycan precursor, thereby inhibiting the cell wall biosynthesis. To our knowledge, this is the first detailed analysis of the mechanism of action of antibacterial defensins produced by invertebrates. Interestingly, the three defensins, which were chosen as representative of the oyster defensin molecular diversity, bound differentially to lipid II. This correlated with their differential antibacterial activities. From our experimental data and the analysis of oyster defensin sequence diversity, we propose that oyster defensin activity results from selective forces that have conserved residues involved in lipid II binding and diversified residues at the surface of oyster defensins that could improve electrostatic interactions with the bacterial membranes.     

Glycosis in the eggs of the echiuroid, Urechis unicinctus and the oyster, Crassostrea gigas. Rate-limiting steps and activation at fertilization.

The content of glycolytic intermediates and of adenine nucleotides was measured in eggs of the echiuroid, Urechis unicinctus and the oyster, Crassostrea gigas, before and after fertilization. On the whole, the profile of the change in each glycolytic intermediate in Urechis eggs upon fertilization was found to be essentially similar to that in oyster eggs. Calculation of the mass action ratio for each glycolytic step from the amounts of glycolytic intermediates determined suggests that there are at least three limiting enzymes in the glycolysis system in unfertilized and fertilized eggs of each species examined. Phosphorylase (EC 2.4.1.1), phosphofructokinase (EC 2.7.1.11), and pyruvate kinase (EC 2.7.1.40) may be rate-limiting enzymes for the glycolysis system in Urechis eggs as well as in oyster eggs. These enzymes are thought to be activated upon fertilization, though even the reactions of the enzymes in fertilized eggs do not reach a state of equilibrium. In eggs of Urechis and oyster, phosphorylase is the first enzyme to be activated following fertilization. In Urechis eggs, pyruvate kinase is activated after the instant increase in the phosphorylase activity upon fertilization, followed by phosphofructokinase activation. In oyster eggs, however, pyruvate kinase and phosphofructokinase seem to be stimulated simultaneously, subsequent to phosphorylase activation upon fertilization. The mechanism controlling phosphorylase and pyruvate kinase activity is unknown, but the phosphofructokinase activity in both species may be regulated by the intracellular concentration of adenine nucleotides, since the enzyme activity is enhanced along with a decline in the phosphate potential in the eggs of both Urechis and of oyster.     

Chemotactic attraction between hemocytes of the oyster,Crassostrea virginica,and bacteria

Experiments were conducted to ascertain whether there is chemotactic attraction by Bacillus megaterium and Micrococcus varians, both Gram-positive species, and Escherichia coli and Vibrio parahaemolyticus, both Gram-negative species, for hemocytes of the American oyster, Crassostrea virginica. It was ascertained quantitatively that oyster hemocytes are attracted to live E. coli, B. megaterium, and M. varians but not to heat-killed bacteria. Furthermore, oyster cells are not attracted to either live or heat-killed V. parahaemolyticus. It is concluded that the chemoattractant is some molecule emitted by living vegetative cells of certain Gram-positive as well as Gramnegative bacteria.     

Perkinsus marinus Extracellular Protease Modulates Survival of Vibrio vulnificus in Eastern Oyster (Crassostrea virginica) Hemocytes

The in vitro effects of the Perkinsus marinus serine protease on the intracellular survival of Vibrio vulnificus in oyster hemocytes were examined by using a time-course gentamicin internalization assay. Results showed that protease-treated hemocytes were initially slower to internalize V. vulnificus than untreated hemocytes. After 1 h, the elimination of V. vulnificus by treated hemocytes was significantly suppressed compared with hemocytes infected with invasive and noninvasive controls. Our data suggest that the serine protease produced by P. marinus suppresses the vibriocidal activity of oyster hemocytes to effectively eliminate V. vulnificus, potentially leading to conditions favoring higher numbers of vibrios in oyster tissues.     

Use of extracts from oyster shell and soil for cultivation of Spirulina maxima

Calcium ion and trace metals play important roles in various metabolisms of photosynthetic organisms. In this study, simple methods were developed to extract calcium ion and micronutrients from oyster shell and common soil, and the prepared extracts were tested as a replacement of the corresponding chemicals that are essential for growth of microalgae. The oyster shell and soil were treated with 0.1 M sodium hydroxide or with 10 % hydrogen peroxide, respectively. The potential application of these natural sources to cultivation was investigated with Spirulina maxima. When compared to standard Zarrouk medium, the Spirulina maxima cultivated in a modified Zarrouk media with elements from oyster shell and soil extract exhibited increases in biomass, chlorophyll, and phycocyanin by 17, 16, and 64 %, respectively. These results indicate that the extracts of oyster shell and soil provide sufficient amounts of calcium and trace metals for successful cultivation of Spirulina maxima.     

Effects of hemolymph of the American oyster, Crassostrea virginica, on marine cercariae

The interactions of cellular and humoral factors of hemolymph of the American oyster, Crassostrea virginica, and several species of marine cercariae were studied. Attraction of hemocytes to dead but not to living cercariae was observed. Dead cercariae were encapsulated in vitro by oyster hemocytes. The plasma of C. virginica was apparently not toxic to the species of cercariae tested.     

Hemocytes and Plasma of the Eastern Oyster (Crassostrea virginica) Display a Diverse Repertoire of Sulfated and Blood Group A-modified N-Glycans

The eastern oyster (Crassostrea virginica) has become a useful model system for glycan-dependent host-parasite interactions due to the hijacking of the oyster galectin CvGal1 for host entry by the protozoan parasite Perkinsus marinus, the causative agent of Dermo disease. In this study, we examined the N-glycans of both the hemocytes, which via CvGal1 are the target of the parasite, and the plasma of the oyster. In combination with HPLC fractionation, exoglycosidase digestion, and fragmentation of the glycans, mass spectrometry revealed that the major N-glycans of plasma are simple hybrid structures, sometimes methylated and core α1,6-fucosylated, with terminal β1,3-linked galactose; a remarkable high degree of sulfation of such glycans was observed. Hemocytes express a larger range of glycans, including core-difucosylated paucimannosidic forms, whereas bi- and triantennary glycans were found in both sources, including structures carrying sulfated and methylated variants of the histo-blood group A epitope. The primary features of the oyster whole hemocyte N-glycome were also found in dominin, the major plasma glycoprotein, which had also been identified as a CvGal1 glycoprotein ligand associated with hemocytes. The occurrence of terminal blood group moieties on oyster dominin and on hemocyte surfaces can account in part for their affinity for the endogenous CvGal1.     

Behavioral plasticity in an invaded system: non-native whelks recognize risk from native crabs

Inducible defenses have the potential to affect both invasion success and the structure of invaded communities. However, little is known about the cues used for risk-recognition that influence the expression of inducible defenses in invasive prey, because they involve a novel threat. In laboratory experiments, we investigated behavioral defenses induced by a native crab on two invasive oyster drills (marine whelks Urosalpinx cinerea and Ocinebrina inornata). Both drills hid more often and reduced their feeding rates when they detected predators consuming conspecific prey. Examination of the responses of U. cinerea to specific cue sources (predator kairomones, conspecific alarm cues) indicated that this species had the strongest responses to cues from injured conspecifics, but that it did recognize the novel crab predator. Our observation of native predator (per se) recognition by an invasive marine prey is novel. In addition, we observed that neither species of drill reduced their defensive behavior to reflect predation risk shared by a group of prey. The lack of density dependence in risk-assessment could cause populations of invasive prey to transmit both quantitatively and qualitatively different community effects over the course of an invasion as abundance changes. Together, these findings demonstrate several ways that the risk-assessment strategies could be important in establishment and post-establishment dynamics of invasive prey.     

Effects of Light and CO on the Survival of a Marine Ammonium-Oxidizing Bacterium during Energy Source Deprivation

The chemolithotrophic ammonium-oxidizing bacterium Nitrosomonas cryotolerans responds uniquely to nutrient deprivation by lowering its endogenous respiration and anabolic processes to undetectable levels during starvation, thus appearing to enter a dormant state. To ascertain whether this state protects the cells from further stresses (as seen with endospore-forming bacteria), the starved cells were subjected to two known inhibitors, CO and light. It was found that long-term-starved cells were less resistant than freshly starved cells to light inhibition. Both long-term-starved cells and freshly starved cells were unaffected by CO.     

Supplementation of Perkinsus marinus Cultures with Host Plasma or Tissue Homogenate Enhances Their Infectivity

The protozoan oyster parasite Perkinsus marinus can be cultured in vitro in a variety of media; however, this has been associated with a rapid attenuation of infectivity. Supplementation of defined media with products of P. marinus-susceptible (Crassostrea virginica) and -tolerant (Crassostrea gigas, Crassostrea ariakensis) oysters alters proliferation and protease expression profiles and induces differentiation into morphological forms typically seen in vivo. It was not known if attenuation could be reversed by host extract supplementation. To investigate correlations among these changes as well as their association with infectivity, the effects of medium supplementation with tissue homogenates from both susceptible and tolerant oyster species were examined. The supplements markedly altered both cell size and proliferation, regardless of species; however, upregulation of low-molecular-weight protease expression was most prominent with susceptible oysters extracts. Increased infectivity occurred with the use of oyster product-supplemented media, but it was not consistently associated with changes in cell size, cell morphology, or protease secretion and was not related to the susceptibility of the oyster species used as the supplement source.