Immunological detection of winter flounder (Pseudopleuronectes americanus) eggs and juveniles in the stomach contents of crustacean predators

Predation on the early life history of fish is an important factor regulating year-class strength. Verifying predation events, however, is difficult when analyses rely on visually identifying the remnants of partially digested fish in the stomachs of suspected predators. The objective of this study was to assess the utility of using immunological assays to detect the presence of winter flounder eggs and juveniles (Pseudopleuronectes americanus) in the gut contents of sand shrimp (Crangon septemspinosa) and green crab (Carcinus maenas). After defining assay capabilities, the stomach contents of field-collected shrimp and crabs were examined to determine if these predator-prey relationships occur under natural conditions. Winter flounder-specific antisera developed and used in this study successfully identified homologous antigens (egg or juvenile flounder extracts) without appreciably cross-reacting with antigenic material from predators or nontarget prey. Moreover, antisera detected flounder eggs 10.8-16.4 h after initial feeding by various sized shrimp, and identified juvenile flounder 9.4 and 7.8 h after initial ingestion by shrimp and crabs, respectively. Immuonological dietary analysis of decapod crustaceans collected from Niantic River, Connecticut, revealed that C. septemspinosa and C. maenas are potentially important predators on the early life stages of winter flounder. The temporal trends and magnitude of flounder predator-induced mortality was affected primarily by the spatial and temporal overlap between predator and prey (egg mortality), and the size-dependent relationships underlying crustacean and flatfish predator-prey interactions (juvenile mortality).     

Stage dependent habitat use under conflicting predation pressure: An experimental test with larval and juvenile two-spotted gobies, Gobiusculus flavescens Fabricius

The effect of size, predator types and presence of multiple predators on the microhabitat use of larvae and juveniles of a sublittoral, semipelagically schooling fish, the two-spotted goby (Gobiusculus flavescens), was tested in two experiments. Larvae (15 and 25 days old, Experiment I) and juveniles (mean ± 1 S.E.: small, 15.9 mm ± 1.28; medium, 19.2 mm ± 1.43; and large, 23.4 mm ± 1.67, Experiment II) were allowed to choose between two sections of the tanks; an upper, representing a water column habitat, and a lower, artificially vegetated, representing the hyperbenthic habitat. Position of larvae or juveniles and the activity level of juveniles were recorded. Predator treatments were: (I) no predators (control), (II) a pelagic predator, the jellyfish Aurelia aurita L., (III) a hyperbenthic predator, the mysid Praunus flexuosus O.F. Müller or (VI) both predator types simultaneously. In Experiment I predators were restricted to the habitat which they were chosen to represent, while goby larvae could move freely. In Experiment II both predators and juvenile gobies were allowed to move freely between compartments.Increasing age caused larval gobies, but not juveniles to shift downwards. Only 25-day-old larvae and small juveniles avoided the mysid by shifting upwards. Larval response to A. aurita was also size dependant: 25-day-old larvae avoided medusae by shifting downwards, while 15 day olds did not. Emergent multiple predator effects were found for the vertical distribution of 15-day-old larvae and small juveniles. Larger juveniles were more active than smaller, both in the upper and the lower sections of tanks. P. flexuosus caused juvenile gobies in their vicinity (i.e. in the lower section) to increase their activity level, while small juveniles (but not medium-sized or large) increased their activity level even when further away (i.e. in the upper section). The presence of A. aurita led to a reduction in activity of small juveniles in its vicinity (i.e. in the upper section), while no response was observed among older juveniles or juveniles further away from the predator (i.e. in the lower section). Emergent multiple predator effects on the activity level of juveniles were not observed.     

Predatory impact of the green crab (Carcinus maenas Linnaeus) on post-settlement winter flounder (Pseudopleuronectes americanus Walbaum) as revealed by immunological dietary analysis

Predation on flatfish during the early juvenile stage is an important factor regulating year-class strength and recruitment. In this study, immunological dietary analysis was performed on green crabs (Carcinus maenas) collected from the Niantic River, Connecticut, in an effort to evaluate the predatory impact of this species on post-settlement winter flounder (Pseudopleuronectes americanus). Through the use of species-specific antiserum, winter flounder proteins were identified in 4.8% of the green crab stomachs analyzed (n = 313, size range = 14-74 mm carapace width, CW), revealing that crabs ≥ 29 mm CW are predators of post-settlement winter flounder in natural populations. The most significant factor underlying the predator-prey interaction was the relative size relationship between species, such that the incidence of winter flounder remains in the stomach contents of green crabs was positively correlated with predator-to-prey size ratio. Results from dietary analysis were incorporated into a deterministic model to estimate the average daily instantaneous mortality and cumulative mortality of winter flounder owing to green crab predation. Accordingly, green crabs may account for 0.4% to 7.7% (mean = 2.2%) of the daily mortality of winter flounder and consume 1.1% to 32.3% (mean = 10.2%) of the flounder year-class. Model simulations further indicate that variations in green crab abundance and size-structure account for the greatest variability in winter flounder mortality. Relative to other macro-crustacean predators, however, predation by green crabs has a minimal effect on winter flounder survival, due in large part to the low densities of these crabs in temperate estuaries.     

Field evaluation by RNA/DNA ratios on post-release nutritional status of released and wild Japanese flounder Paralichthys olivaceus juveniles

A large-scale juvenile Japanese flounder (Paralichthys olivaceus) release-recapture experiment was undertaken to find the optimal release season by evaluating the nutritional status of released fish at different seasons during which food abundance was significantly different. Forty thousand fish were released at depths of 1.5 m for early-release (May 29, 1997) and 2 m for late-release (July 2, 1997) (42.1±3.5 and 40.9±4.2 mm body length, respectively) in an experimental field, Wakasa Bay, the Sea of Japan. Samples were taken, after the releases, at Wada beach at intervals of 1, 2, 3, 6, 10, 16 and 30 days after release (DAR), including pre-surveys before each release. Released fish recaptured from the two different release groups totaled 764; 467 from the early-release group (ER) and 297 from the late-release group (LR). A total of 1956 wild flounder juveniles were simultaneously collected (1041 ER, 915 LR). ER fish were subject to higher food availability and were exposed to less pressure from predation by smaller wild juvenile flounder. RNA/DNA ratios in ER juveniles were significantly higher than those of LR fish during all samples. Especially, RNA/DNA ratios in ER juveniles were higher than in wild juveniles from 3 to 50 DAR. In the LR group, the nutritional status of juveniles was relatively low in shallower water. These findings corresponded well with feeding incidence examined by coworkers. Mass release of hatchery-reared juveniles apparently reduced RNA/DNA ratio of the wild juveniles right after releasing. The present study showed that earlier release of hatchery-reared juvenile Japanese flounder with higher RNA/DNA ratio could increase the possibilities of survival right after release in the nursery ground, and that RNA/DNA ratio appeared to be a good tool in evaluating nutritional status of released juveniles as well as wild juveniles in Japanese flounder.     

Effectiveness and immunomodulation of chemotherapeutants against scuticociliate Philasterides dicentrarchi in olive flounder

Philasterides dicentrarchi is a histophagous scuticociliate causes fatal scuticociliatosis in farmed olive flounder Paralichthys olivaceus. The average monthly prevalence of scuticociliatosis due to P. dicentrarchi infections was increased from May to July (40 ± 3.1% to 79.4 ± 1.7%) and it decreased from August to November (63 ± 2.3% to 30 ± 2.6%) in olive flounder farms at Jeju Island, South Korea during 2000-2006. The prevalence of mixed infection along with Vibrio spp. bacterial infection was 49 ± 7.2% than that of other mixed infection. At present no effective control measure for P. dicentrarchi infection has been described and large production losses continue. In the present study, formalin, hydrogen peroxide and Jenoclean chemotheraputants were used for bath treatment. Among Jenoclean at a low concentration of 50 ppm proved effective. The results were confirmed with in vitro motility assessments and morphological changes scoring system in P. dicentrarchi. On the other hand, similar trend was noted following hydrogen peroxide treatment at this concentration, but formalin was only moderately effective. Either hydrogen peroxide or Jenoclean are the promising compounds effective at low concentrations with short application time for P. dicentrarchi. Therefore, these substances were evaluated on day 10, 20 and 30 for their ability to enhance innate immune response and disease resistance against P. dicentrarchi in olive flounder after chemotheraputants bath treatment with 100 ppm for 30 min per day. All the tested immune parameters were enhanced by treatment with Jenoclean, but not formalin and hydrogen peroxide. These findings suggest that Jenoclean bath treatment can be used for ensuring the heath of cultured marine fish against internal parasites such as P. dicentrarchi.     

Nutritional value of the cryptophyte Rhodomonas lens for Artemia sp.

Juvenile or adult Artemia sp. are often used as live prey for the rearing of early life stages of some crustacean, fish and cephalopod species. The improvements of both Artemia growth and its biochemical composition are key issues for the suitable use of Artemia biomass in these rearing processes. In this study we evaluated the growth and survival rates of Artemia fed with the cryptophyte Rhodomonas lens in comparison with different microalgal species commonly used in aquaculture: the prasinophyte Tetraselmis suecica, the prymnesiophyte Isochrysis galbana Parke, and the eustigmatophyte Nannochloropsis gaditana. Microalgae were cultured semi-continuously in nutrient saturated conditions and with a daily renewal rate of 30% of the volume of cultures, to obtain biomass of controlled and optimized composition. Considerable differences in Artemia growth were observed, as well as in the survival rate. At day 8 of rearing, Artemia fed R. lens had the highest length (4.9 ±0.6 mm, P < 0.001), followed by individuals fed T. suecica (4.2 ± 0.7 mm), I. galbana (3.6 ± 0.7 mm) and finally those fed N. gaditana (1.5 ± 0.2 mm). The survival rate of Artemia fed N. gaditana (18 ± 3%) was much lower (P < 0.001) than values found for the remaining groups (69 to 88%). The growth rate of Artemia obtained with R. lens was in general much higher than with other microalgal diets previously reported in the literature. The higher protein content of R. lens could explain the higher growth obtained with this species, but differences of Artemia growth with the different diets could not be explained solely on the basis of the gross composition of microalgae. Factors such as cell size and digestibility all seem to contribute to the results observed. Another trial was carried out to investigate differences in Artemia growth and on its biochemical composition when fed the best two diets: R. lens or T. suecica. The fatty acid (FA) and total amino acid (AA) composition of both microalgal species and the composition of Artemia were assessed as well. As found in the first experiment individuals fed R. lens (group ARHO) grew faster than those fed T. suecica (group ATET), attaining 3.6 ± 0.3 mm and 3.2 ± 0.4 mm (P < 0.001), respectively, after 5 days of rearing. The much higher AA content obtained in R. lens may be on the basis of the higher growth obtained with this species. Protein and carbohydrate levels in Artemia juveniles were very similar in both groups (64-68% of dry weight, and 8-10%, respectively). Lipid was slightly lower in ARHO (12%) than in ATET (15%, P < 0.01). Regarding the FA composition, juveniles from group ARHO contained higher levels of eicosapentaenoic acid (EPA, 6.2%) than juveniles from ATET (4.1%, P < 0.01), whereas docosahexaenoic acid (DHA) was only found in juveniles from ARHO (1.1%). Taking into account that the daily productivity of R. lens culture was higher than, or at least equal, the remaining microalgal species this cryptophyte is confirmed as an excellent diet to optimize the growth of Artemia, as well as to improve its biochemical composition.     

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.     

Weak meiofaunal trophic linkages in Crangon crangon and Carcinus maenus

To document the relative importance of meiofauna as prey for juvenile Crangon crangon and Carcinus maenus, short interval (1.5-2 h) collections were made in the muddy Lynher Estuary (Plymouth, Great Britain) and in the sandy-bottom Ythan Estuary (Aberdeenshire, Scotland) in 1990. Gut passage times of Crangon fed flaked fish food and fluorescent tracer in the laboratory at 13 °C ranged from 4 to 20 h. Wild shrimp exhibited feeding periodicity, with guts fullest during high tide in both locations. Visual and immunological gut contents analyses revealed that meiofaunal nematodes and harpacticoid copepods were present only in recently settled shrimp from 8 to 12 mm total length on muddy bottoms. Larger shrimp collectively consumed up to 33 different macrobenthic prey types. Shrimp were fullest at night (mean gut contents weight = 8% wet body weight, Lynher) or at dawn (6%, Ythan). The Lynher Carcinus gut contents—from animals 8 to 30 mm carapace width, examined visually only—contained mostly fluids, green benthic algae, sediment particles, and masses of unidentifiable prey remains plus digestion-resistant hard parts visually identifiable as macrobenthic in origin. None of the 203 crabs examined from the 24-h collection contained meiofaunal prey. Crangon shrimp probably eat meiofaunal prey for only a brief period of time after their initial settlement to the bottom. Evidence for significant top-down impacts on meiofauna from these two abundant shallow-water predators was weak. More trophic studies are needed on newly settled epibenthic predators to test the hypothesis that biological control of shallow-water meiofauna is important.     

Predation by gulls on crabs in rocky intertidal and shallow subtidal zones of the Gulf of Maine

Most organisms in intertidal areas are marine in origin; many have distributions that extend into the subtidal zone. Terrestrial predators such as mammals and birds may exploit these animals during low tide and can have considerable effects on intertidal food webs. Several studies have shown that avian predators are capable of reducing densities of sessile and slow-moving intertidal invertebrates but very few studies have considered avian predation on mobile invertebrate predators such as crabs. In this study, we investigated predation by Great Black-backed Gulls (Larus marinus Linnaeus) on three species of crabs (Cancer borealis Stimpson, Cancer irroratus Say, and Carcinus maenas Linnaeus). The study was at Appledore Island, ME (a gull breeding island) and 8 other sites throughout the Gulf of Maine, including breeding islands and mainland sites. On Appledore Island, intertidal and subtidal zones provided over one-third of prey remains found at gull nests, and crabs were a substantial proportion (∼ 30% to 40%) of the total remains. Similarly, collections of prey remains from intertidal areas indicated that crabs were by far the most common marine prey. C. borealis was eaten far more often and C. irroratus and C. maenas less often than expected at each site. Comparing numbers of carapaces to densities of crabs in low intertidal and shallow subtidal zones at each site, we estimated that gulls remove between 15% and 64% of C. borealis during diurnal low tides. The proportion of C. borealis eaten by gulls was independent of proximity to a gull colony. Approximately 97% of the outer coast of Maine is within 20 km of a breeding island. Thus, a lot of gull predation on crabs may occur throughout the Gulf of Maine during summer months. Crabs are important predators of other invertebrates; if predation by gulls reduces the number of crabs in intertidal and shallow subtidal areas, gulls may have important indirect effects on intertidal food webs.     

Behavioral plasticity of the soft-shell clam, Mya arenaria (L.), in the presence of predators increases survival in the field

Soft-shell clams, Mya arenaria, are sessile, suspension-feeding bivalves that are preyed upon by the exotic green crab, Carcinus maenas. Clams evade crab consumers by burrowing deeper into the sediment after perceiving a threat from a nearby predator. The purpose of this study was to determine the types of signals that M. arenaria use to detect predators and the types of behaviors clams use to avoid being eaten. In a field study, clams increased their burial depth in the presence of green crab predators consuming conspecifics that were caged nearby, and also increased burial depth after artificial tactile stimulation in the laboratory assay. These results indicate that clams can use chemical and mechanical cues to detect potential predatory threats. We performed a field study to examine the difference in survivability of clams that had burrowed deeper into the sediment in response to predators vs. control clams that were burrowed less deeply. Significantly higher survival rates were observed in clams that had initially burrowed more deeply, suggesting that increasing burial depth is a valid predator avoidance strategy. Some bivalves also alter their pumping rates in the presence of predators, making them less apparent and providing more structural defense by covering soft tissue, and we measured pumping time of soft-shell clams in the presence and absence of predators, when burrowing was not an option for escape. Soft-shell clams did not alter their pumping time in the presence of green crab predators, possibly because they employ a burrowing method called “hydraulic” or “jet-propelled” burrowing, where it is necessary for the clam to pump in order to burrow. Chemical signals and tactile cues instigated behavioral changes in M. arenaria, and this change in behavior (increasing burial depth) increased clam survival in the field.     

Predation by blue crabs, Callinectes sapidus, on rapa whelks, Rapana venosa: possible natural controls for an invasive species?

Blue crabs Callinectes sapidus are voracious predators in Chesapeake Bay and other estuarine habitats. The rapa whelk Rapana venosa is native to Asian waters but was discovered in Chesapeake Bay in 1998. This predatory gastropod grows to large terminal sizes (in excess of 150 mm shell length (SL)) and has a thick shell that may contribute to an ontogenetic predation refuge. However, juvenile rapa whelks in Chesapeake Bay may be vulnerable to predation by the blue crab given probable habitat overlap, relative lack of whelk shell architectural defenses, and the relatively large size of potential crab predators. Feeding experiments using three size classes of blue crab predators in relation to a size range of rapa whelks of two different ages (Age 1 and Age 2) were conducted. Blue crabs of all sizes tested consumed Age 1 rapa whelks; 58% of all Age 1 whelks offered were eaten. Age 2 rapa whelks were consumed by medium (67% of whelks offered were eaten) and large (70% of whelks offered were eaten) blue crabs but not by small crabs. The attack methods of medium and large crabs changed with whelk age and related shell weight. Age 1 whelks were typically crushed by blue crabs while Age 2 whelk shells were chipped or left intact by predators removing prey. Rapa whelks less than approximately 35 mm SL are vulnerable to predation by all sizes of blue crabs tested. Rapa whelk critical size may be greater than 55 mm SL in the presence of large blue crabs indicating that a size refugia from crab predation may not be achieved by rapa whelks in Chesapeake Bay until at least Age 2 or Age 3. Predation by blue crabs on young rapa whelks may offer a natural control strategy for rapa whelks in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.     

Effects of phenanthrene on growth and basic physiological functions of the olive flounder, Paralichthys olivaceus

Flounder were exposed to waterborne phenanthrene (0.5, 1 and 2 μM) for 4 weeks to test effects of waterborne phenanthrene on growth and hematological properties of the olive flounder (Paralichthys olivaceus). The average weight gain (WG) of flounder was significantly decreased in fish exposed to phenanthrene at 2.0 μM for 2 weeks, whereas WGs of fish treated by 1.0 and 2.0 μM phenanthrene for 4 weeks were significantly decreased. However, hepatosomatic index (HSI) and condition factor (CF) of flounder were not significantly affected by phenanthrene exposure. Red blood cell (RBC) count, hemoglobin (Hb) concentration, hematocrit (Ht), the mean corpuscular hemoglobin (MCH) and the mean corpuscular hemoglobin concentration (MCHC) mean levels were decreased with an increase in exposure time of phenanthrene to the fish, but the level of the mean corpuscular volume (MCV) was increased. Plasma bilirubin concentrations were significantly increased following exposure to waterborne phenanthrene (2.0 μM) for 2 and 4 weeks; however, there were no significant changes in plasma total cholesterol in fish of all treated groups compared to control. The phenanthrene-exposed groups (≥1.0 μM) showed significantly higher mean plasma lysozyme activity. Kidney lysozyme activity of fish exposed to phenanthrene (≥1.0 μM) was also significantly higher than that of control fish. The central finding from these data is that olive flounder exposed to waterborne phenanthrene at more than 1.0 μM are likely to experience negative impacts on fish health and basic physiological functions.     

Associative learning in ants: Conditioning of the maxilla-labium extension response in Camponotus aethiops

Associative learning has been studied in many vertebrates and invertebrates. In social insects, the proboscis extension response conditioning of honey bees has been widely used for several decades. However, a similar paradigm has not been developed for ants, which are advanced social insects showing different morphological castes and a plethora of life histories. Here we present a novel conditioning protocol using Camponotus aethiops. When the antennae of a harnessed ant are stimulated with sucrose solution, the ant extends its maxilla-labium to absorb the sucrose. We term this the “maxilla-labium extension response” (MaLER). MaLER could be conditioned by forward pairing an odour (conditioned stimulus) with sucrose (unconditioned stimulus) in the course of six conditioning trials (absolute conditioning). In non-rewarded tests following conditioning, ants gave significantly higher specific responses to the conditioned stimulus than to a novel odour. When trained for differential conditioning, ants discriminated between the odour forward-paired with sucrose and an odour forward-paired with quinine (a putative aversive stimulus). In both absolute and differential conditioning, memory lasted for at least 1 h. MaLER conditioning allows full control of the stimulation sequence, inter-stimulus and inter-trial intervals and satiety, which is crucial for any further study on associative learning in ants.     

Recognizing biotic breakage of the hard clam, Mercenaria mercenaria caused by the stone crab, Menippe mercenaria: An experimental taphonomic approach

The ability to assign lethal traces left on prey to particular durophagous predators enhances our understanding of predation pressure in the fossil record. To determine whether stone crabs (Menippe mercenaria Say 1818) leave diagnostic traces in the act of feeding on hard clams (Mercenaria mercenaria Linnaeus 1758), live clams were offered to crabs in laboratory aquaria over several months and the fragments produced during predation were examined for diagnostic breakage patterns. These fragments were then compared both macroscopically and using scanning electron microscopy to the fracture patterns produced by tumbling clams in a rock tumbler which simulated breakage during transport in the surf zone, and crushing clams using an Instron which simulated breakage resulting from sediment compaction. Fossil specimens of Mercenaria mercenaria were also examined to determine whether the criteria for recognizing predation traces generated experimentally could be recognized. While not all acts of predation produce diagnostic traces, when larger fragments (greater than 50% shell remaining) are produced during feeding, predatory-diagnostic breakage ranges from 70 to 80%. Macroscopic breakage patterns generated during the predation experiments were also present in fossil specimens. Damage caused by abiotic mechanisms (tumbling and crushing) is highly unlikely to be confused with damage produced by this predator.     

Type and nature of cues used by Nucella lapillus to evaluate predation risk

The ability of prey to detect and adequately respond to predation risk influences immediate survival and overall fitness. Chemical cues are commonly used by prey to evaluate risk, and the purpose of this study was to elicit the nature of cues used by prey hunted by generalist predators. Nucella lapillus are common, predatory, intertidal snails that evaluate predatory risk using chemical cues. Using Nucella and a suite of its potential predators as a model system, we explored how (1) predator type, (2) predator diet, and (3) injured conspecifics and heterospecifics influence Nucella behavior. Using laboratory flumes, we determined that Nucella responded only to the invasive green crab (Carcinus maenas), the predator it most frequently encounters. Nucella did not respond to rock crabs (Cancer irroratus) or Jonah crabs (Cancer borealis), which are sympatric predators but do not frequently encounter Nucella because these crabs are primarily subtidal. Predator diet did not affect Nucella responses to risk, although starved predator response was not significantly different from controls. Since green crabs are generalist predators, diet cues do not reflect predation risk, and thus altering behavior as a function of predator diet would not likely benefit Nucella. Nucella did, however, react to injured conspecifics, a strategy that may allow them to recognize threats when predators are difficult to detect. Nucella did not react to injured heterospecifics including mussels (Mytilus edulis) and herbivorous snails Littorina littorea, suggesting that they are responding to chemical cues unique to their species. The nature of cues used by Nucella allows them to minimize costs associated with predator avoidance.     

Predator-prey dynamics between recently established stone crabs (Menippe spp.) and oyster prey (Crassostrea virginica)

Range expansion and population establishment of individual species can have significant impacts on previously established food webs and predator-prey dynamics. The stone crab (Menippe spp.) is found throughout southwestern North Atlantic waters, from North Carolina through the Gulf of Mexico and the Central American Caribbean, including the Greater Antilles. Recent observations suggest that stone crabs have become better established on certain oyster reefs in North Carolina than in the early 1900s when they we first observed in NC. To assess the predatory impact of stone crabs on oysters, we (1) quantified stone crab densities on subtidal oyster reefs in Pamlico Sound, NC using scuba surveys, and (2) conducted laboratory predation experiments to assess the functional response of stone crabs to varying densities of oysters. We then (3) analyzed previously unpublished functional response data on another important oyster predator, the mud crab Panopeus herbstii. Finally, we (4) compared and contrasted potential predatory impacts of stone, mud and blue crabs (Callinectes sapidus). The functional response data and analyses for both stone crabs and mud crabs were consistent with a type II functional response. Mud crabs, on a m² basis, inflicted the highest proportional mortality on oysters over a 24 hour period, followed by stone and then blue crabs. Proportional mortality did not vary significantly with oyster size; however, relatively small and large oysters were consumed disproportionately less than medium-sized oysters, likely due to the mechanical inability of stone crabs to handle small oysters, and the inability to crush large oysters. Although stone crabs appear to be established in Pamlico Sound at densities equivalent to densities in other systems such as the U.S. Florida Panhandle, their predatory activities on oysters are not expected to have as significant a negative impact on oyster populations compared to other resident predators such as mud crabs.     

Endogenous swimming rhythms of juvenile blue crabs, Callinectes sapidus, as related to horizontal transport

The blue crab Callinectes sapidus settles and metamorphoses in areas of aquatic vegetation in estuaries. Crabs in the first-fifth instar stages (J1-5) then emigrate from these areas by walking on the bottom or pelagic dispersal throughout estuaries. The present study was designed to characterize the timing of this migration pattern relative to the light-dark and tidal cycles. Field sampling in Pamlico Sound, NC, USA indicated that J4-5 juveniles were most abundant in the water column during the night. J4-5 juveniles were collected from Pamlico Sound in an area near Oregon Inlet that has semi-diurnal tides, a Mid-Sound area where tides are weak, and on the West side where regular tides do not occur. Crabs from all three sites had a circadian rhythm in which they swam up in the water column during the time of darkness in the field. Peak swimming consistently occurred at about 0300 h, but was not related to the timing of the tidal cycle. Similar results were obtained for juvenile crabs from an adjacent estuary having semi-diurnal tides. Dispersal at night reduces predation by visual predators, and allows early juvenile blue crabs to disperse planktonically from initial settlement sites.     

Predatory behavior and preference of a successful invader, the mud crab Dyspanopeus sayi (Panopeidae), on its bivalve prey

Predator-prey relationships between the panopeid crab, Dyspanopeus sayi, and the mytilid, Musculista senhousia, were investigated. Through laboratory experiments, prey-handling behavior, prey size selection, predator foraging behavior and preferences for two types of prey (M. senhousia and the Manila clam Ruditapes philippinarum) were assessed. Handling time differed significantly with respect to the three prey sizes offered (small: 15.0-20.0 mm shell length, SL; medium: 20.1-25.0 mm SL; and large: 25.1-30.0 mm SL); mud crabs were more efficient in predating medium-small than large prey. Although differences in prey profitability were not evident, D. sayi exhibited a marked reluctance to feed on larger-sized prey whilst smaller, more easily predated mussels were available. Size selection may be the result of a mechanical process in which encountered prey are attacked but rejected if they remain unbroken after a certain number of opening attempts. D. sayi exhibited inverse density-dependent foraging. A significant higher mortality of prey was evident at low prey density. Thus, at low predator density, the D. sayi-M. senhousia interaction was a destabilizing type II functional response. Interference responses affected the magnitude of predation intensity by D. sayi on M. senhousia, since as the density of foraging crabs increased, their foraging success fell. At high density (4 crabs tank⁻¹), crabs engaged in a high amount of agonistic activity when encountering a conspecific specimen, greatly diminished prey mortality. Finally, presenting two types of prey, Manila clam juveniles were poorly predated by mud crabs, which focused their predation mostly on M. senhousia. It is hypothesized that, when more accessible prey is available, mud crabs will have a minimal predatory impact on commercial R. philippinarum juvenile stocks.     

Hematodinium sp. infection of red Paralithodes camtschaticus and blue Paralithodes platypus king crabs from the Sea of Okhotsk, Russia

A disease caused by a parasitic dinoflagellate of the genus Hematodinium was identified in red, Paralithodes camtschaticus, and blue, Paralithodes platypus, king crabs from the north-east region of the Sea of Okhotsk, Russia, during annual stock surveys. No carapace color change was observed even in heavily infected crabs, but diseased crabs possessed creamy-yellow hemolymph, which was visible through the arthrodial membranes of the abdomen and appendages. Several stages of the parasite’s life history, including trophonts, plasmodia, sporonts and macrodinospores, were observed in tissues of infected king crabs. Numerous parasite cells were observed in the lumina of the myocardium, the gills, the connective tissue of antennal glands and the sinuses of nerve ganglia, eyestalks and gastrointestinal tract of king crabs with gross signs of infection. Based on sequencing of the 18S rDNA, it appears that the Hematodinium sp. found in red and blue king crabs is identical or closely related to Hematodinium sp. isolated from crabs of the genera Chionoecetes and Lithodes. Observed prevalences were 0.33% in sublegal male red king crabs, 0.18% in female red king crabs, 0.34% in sublegal male blue king crabs and 0.31% in female blue king crabs.     

Reproductive costs of the symbiotic hydroid Hydractinia symbiolongicarpus (Buss and Yund) to its host hermit crab Pagurus longicarpus (Say)

Studies on the interaction between the hermit crab Pagurus longicarpus and its symbiotic hydroid Hydractinia symbiolongicarpus have focused on positive effects of hydroids on their host hermit crabs (e.g., protection from predators). Yet, these benefits may be balanced with reproductive costs, which are rarely studied. Results from field observations, laboratory trials, and a mesocosm experiment indicate that female hermit crabs in hydroid-colonized shells exhibit depressed ovigery, smaller clutch sizes, and increased clutch failure relative to females in bare shells. Frequent switching between bare and hydroid-colonized shells may alleviate negative effects when the density of hydroids in the environment is low, but at high densities Hydractinia may significantly impact hermit crab reproduction.