Diet selectivity of juvenile blue crabs (Callinectes sapidus) in Chesapeake Bay

Shallow coves in Chesapeake Bay have abundant food and serve as nursery grounds for juvenile blue crabs. In this study, we examined the relationships between the diet of very small (4-40?mm CW) juvenile blue crabs and the benthic infauna in shallow, unvegetated nursery coves. We compared infauna in benthic samples with gut contents of juvenile blue crabs from six shallow coves in each of two sub-estuaries (Rappahannock and York Rivers) in Chesapeake Bay, Virginia, USA. Benthic communities differed depending on river and location, with abundant clams in upriver regions and abundant polychaetes in downriver regions. Juvenile crabs, like adults, appeared to be opportunistic feeders, with gut contents including clams, amphipods, polychaetes, small crustaceans, plant matter, and detritus. There was a positive relationship between polychaetes in the benthic samples and in crab guts, suggesting that juvenile crabs are opportunistic feeders on polychaetes in the benthos. Moreover, Ivlev's electivity index and foraging ratio showed that clams and polychaetes were selectively eaten at all locations. Alternatively, crabs selectively rejected amphipods. Crab densities corresponded positively with polychaete densities, which suggests that there may be bottom-up control of crab distributions and that food resources are important in nursery habitats.     

Trophic relationships of juvenile blue crabs (Callinectes sapidus) in estuarine habitats

Salt marshes and shallow-water macroalgal beds are known to provide nursery habitat for many species of fish and invertebrates. The role of these habitats as refuge from predation is well established, but the degree to which indigenous primary production within the nursery provides food for growth and development of estuarine species remains unresolved. In this study, we tested the hypothesis that juvenile blue crabs depend on indigenous primary production, directly or indirectly, during their entire stay within the nursery. To test this hypothesis, we conducted isotopic studies and stomach content analyses of juveniles from habitats near the mouth of Delaware Bay and from an adjacent lagoonal estuary (ca. 39.5° N, 75.1° W). Primary producers, marsh detritus, various life-history stages of blue crabs and potential prey species were sampled in the main estuary and in an adjacent marsh during the summer and early fall of two consecutive years. Newly settled juveniles (<15 mm carapace width) from the marsh were about 1.8‰ lighter in carbon (−17.2‰) relative to larger juveniles from the marsh (15–30 mm carapace width) and appeared to have retained a carbon isotopic signature indicative of the phytoplankton-based food web associated with larval stages. However, the signature of juveniles changed as a function of size. Large juveniles and crabs >60 mm were enriched in δ¹³C (−14.7 ± 0.1‰) compared to small crabs, suggesting a gradual shift in diet from a planktonic to a detritus-based food web with increasing size. As with crabs from Delaware Bay, the δ¹³C signature of juvenile crabs sampled from macroalgal beds in the lagoonal estuary (Rehoboth Bay) changed as a function of size. Also, δ¹³C ratios of crabs varied among the various species of macroalgae. The δ¹⁵N composition of primary producers in the marsh and main estuary also was reflected in the δ¹⁵N values of crabs and other benthic consumers in the respective habitats. Results of stomach-content analysis in this study were consistent with isotope data. Observed changes in prey preferences were related to changes in size of juvenile crabs and also differed among habitats. Gut content analyses of the three size classes of juveniles in macroalgal beds from Rehoboth Bay indicated that the crabs depend heavily on various amphipod species that occur on the seaweeds. These amphipods graze directly on the macroalgae and are among the most abundant invertebrates in the macroalgal beds. This implies a direct trophic relationship between the juvenile crabs and the macroalgae. In summary, our study provides strong evidence that the value of nursery areas such as salt marshes and macroalgal beds goes beyond that of providing refuge from predation, and that species using these nurseries (e.g. juvenile blue crabs) are ultimately dependent on primary production originating in benthic plants indigenous to the nursery.     

The impact of limb autotomy on mate competition in blue crabs Callinectes sapidus Rathbun

Summary This study is the first to demonstrate experimentally that autotomy (self-amputation of a body part) adversely affects competition for mates. Experiments were conducted using blue crabs Callinectes sapidus Rathbun to examine the consequences of limb loss and pairing precedence on mate acquisition by males. Two adult males of equivalent size were introduced sequentially into pools containing a sexually-receptive female and observed after 24 h and 48 h. One male in each pair was left intact, while the other experienced: (1) no autotomy, (2) autotomy of one cheliped, or (3) autotomy of both chelipeds, one walking leg, and one swimming leg. In the absence of a competitor (first 24 h), both intact and injured males established precopulatory embraces with females. Intact males were highly successful (84–95%) in defending females from intact or injured intruders in the second 24 h period. Both autotomy treatments, however, significantly reduced the ability of males to defend females from intact intruders. Females in experiments suffered greater frequency of limb loss than did males. In the field, paired blue crabs showed significantly higher incidence of limb loss than unpaired crabs. Limb loss frequency increases with body size, and field observations indicated that larger males may be more successful than smaller males in obtaining females. Both experimental manipulations and field studies provide strong evidence for mate competition in this ecologically and commercially important portunid species.     

A Minchinia-like haplosporidan parasitizing blue crabs,Callinectes sapidus

A haplosporidan parasite was found infecting two moribund blue crabs, Callinectes sapidus, from Virginia and North Carolina. Spore stages were not found in either crab. Because of the presence of haplosporosomes in the cytoplasm, the parasites are thought to be a species of Minchinia or Urosporidium.     

Concentration of rotavirus and enteroviruses from blue crabs (Callinectes sapidus)

A simple method for concentration and detection of rotavirus and enteroviruses in the blue crab is described. Virus was separated from tissue homogenates at pH 9.5, concentrated by adsorption to protein precipitates at pH 3.5, and recovered by elution of precipitates at pH 9.2. Test samples of 12 to 15 ml were produced from an initial 100 g of crab tissues. Cat-floc precipitation was used to remove sample toxicity for cell cultures. Recovery effectiveness averaged 52% with poliovirus 1, echovirus 7, and coxsackievirus B5 and 23% with rotavirus SA11.     

Concentration of rotavirus and enteroviruses from blue crabs (Callinectes sapidus).

A simple method for concentration and detection of rotavirus and enteroviruses in the blue crab is described. Virus was separated from tissue homogenates at pH 9.5, concentrated by adsorption to protein precipitates at pH 3.5, and recovered by elution of precipitates at pH 9.2. Test samples of 12 to 15 ml were produced from an initial 100 g of crab tissues. Cat-floc precipitation was used to remove sample toxicity for cell cultures. Recovery effectiveness averaged 52% with poliovirus 1, echovirus 7, and coxsackievirus B5 and 23% with rotavirus SA11.     

Size-selective foraging behaviour of blue crabs,Callinectes sapidus (Rathbun), when feeding on mobile prey: Active and passive components of predation

Size-selective predation by small (30-50 mm carapace width) and large (90-110 mm) Callinectes sapidus when feeding on three different size classes of Litopenaeus setiferus was investigated. Laboratory experiments using no-choice and multiple-choice designs were carried out. Active and passive components of predation were evaluated by comparing the probability of attacking (active) and capturing (passive) shrimp. Small C. sapidus consumed shrimp of all size classes in similar frequencies during multiple and no-choice experiments, and the probability of attacking and capturing shrimp of all size classes was similar. Large crabs consumed large shrimp with a significantly higher frequency during multiple than during no-choice experiments, and the probability of capturing large shrimp was significantly higher than medium and small ones. Results in the present study suggest that size-specific consumption rates of C. sapidus are mainly the result of a passive mechanism associated with the salience of shrimp to predation. However, an active component of foraging behaviour related to the decision to attack an alternative prey whilst consuming one that has already been captured, helps to explain the differences in shrimp consumption between multiple and no-choice experiments. The complex interaction between active, passive components and satiation is discussed.     

Heterogeneous humoral and hemocyte-associated lectins with N-acylaminosugar specificities from the blue crab, Callinectes sapidus Rathbun

Callinectes sapidus serum and hemocyte microsomal fraction agglutinated a panel of untreated and enzyme treated vertebrate erythrocytes and cultured lymphoid cell lines. Crossed absorption experiments suggested the presence of multiple specific lectins in the serum. The microsomal fraction showed a 35-fold increase in specific activity when compared to the hemocyte lysate suggesting that hemocyte lectins are membrane-associated. Agglutination by serum and hemocyte lectins was inhibited by low concentrations of N-acylamino compounds including sialic, N-acetylmuramic and N-acetylglutamic acids, GalNAc, GlcNAc, ManNAc, and glycoproteins and polysaccharides which contain these carbohydrates: bovine submaxillary mucin, human orosomucoid, porcine stomach mucin and colominic acid. Hemagglutination by lectins of both serum and hemocyte microsomal fraction required divalent cations as suggested by the reduction in hemagglutination titer in the presence of the chelators EDTA, EGTA, CDTA and citrate.     

Incidence of Vibrio species associated with blue crabs (Callinectes sapidus) collected from Galveston Bay, Texas.

Bacteria were readily isolated from the hemolymph of a majority (88%) of the blue crabs collected from Galveston Bay, Texas. The hemolymph of most crabs contained moderate (greater than 10(3) bacteria/ml) to heavy (greater than (10(5) bacteria/ml) infections. Large variances were observed in the bacterial number associated with individual crabs, but no significant difference was observed between the mean bacterial levels in the hemolymph of crabs collected during different seasons of the sampling year. Vibrio spp. were the predominant bacterial types in the hemolymph of infected crabs and increased in number significantly during the summer season. Warmer water temperatures were thought to be responsible for this increase. Bacterial numbers and the percentage of Vibrio spp. were highest in the interior of the crab bodies, especially in the digestive tract. The exterior of the crabs did not appear to be the source of the hemolymph's bacterial flora. Bacteria taxonomically identical to Vibrio cholerae. V. vulnificus, and V. parahaemolyticus were routinely isolated from the crab hemolymph and external carapace. V. parahaemolyticus was the most prevalent of the pathogenic Vibrio spp. and was isolated from 23% of the hemolymph samples. V. vulnificus (7%) and V. cholerae (2%) occurred less commonly in the hemolymph. The incidences of V. parachaemolyticus and V. vulnificus were related and increased in the summer months. Both organisms were frequently isolated from the same crab.     

Respiratory and digestive responses of postprandial Dungeness crabs, Cancer magister, and blue crabs, Callinectes sapidus, during hyposaline exposure

Respiratory responses and gastric processing were examined during hyposaline exposure in two crab species of differing osmoregulatory ability. The efficient osmoregulator, Callinectes sapidus, displayed an immediate increase in oxygen uptake when exposed to low salinity in isolation. In contrast, the weak osmoregulator, Cancer magister, showed no change in oxygen uptake upon acute exposure (<6 h), but slight increases in oxygen uptake tended to occur over longer time scales (12–24 h). These changes were likely attributable to an increase in avoidance activity after 6 h hyposaline exposure. Following feeding in 100% SW, oxygen uptake doubled for both species and remained elevated for 15 h. When postprandial crabs were exposed to low salinities, C. sapidus were able to sum the demands of osmoregulation and digestion. Thus, gastric processes continued unabated in low salinity. Conversely, postprandial C. magister prioritized responses to low salinity over those of digestion, resulting in a decrease in oxygen uptake when exposed to low salinity. This decrease in oxygen uptake corresponded to a reduction in the rate of contraction of the pyloric stomach and a subsequent doubling of gastric evacuation time. The current study is one of the few to illustrate how summation or prioritization of competing physiological systems is manifested in digestive processes.     

Endogenous swimming rhythms underlying secondary dispersal of early juvenile blue crabs, Callinectes sapidus

Blue crab, Callinectes sapidus Rathbun, megalopae settle in seagrass or other complex submerged aquatic habitats in estuaries, where they metamorphose to the first juvenile (J1) crab stage. Within tidal areas, early juveniles (J1-2) leave such nursery areas by undergoing secondary dispersal during nocturnal flood tides. The present study determined whether J1-2 blue crabs have a biological rhythm in vertical swimming activity that contributes to secondary dispersal. Endogenous rhythms in vertical swimming were determined for (1) J1-2 crabs collected from two estuaries with semi-diurnal tides, (2) J1 crabs that metamorphosed from the megalopal stage in the laboratory the day after collection, and (3) premolt megalopae that metamorphosed to J1 crabs under constant conditions during the experiment. In all cases, a circadian rhythm was present in which crabs swam vertically during the time of night in the field. The time of peak vertical swimming did not correspond to the time of flood tide at the collection sites, but did consistently occur at night, with a mean around midnight. While responses to environmental factors probably control the onset and end of vertical swimming by early juvenile blue crabs during flood tides in tidal areas, a circadian rhythm underlies secondary dispersal at night.     

Effects of hypercapnic hypoxia on the clearance of Vibrio campbellii in the Atlantic blue crab, Callinectes sapidus Rathbun

Callinectes sapidus, the Atlantic blue crab, encounters hypoxia, hypercapnia (elevated CO(2)), and bacterial pathogens in its natural environment. We tested the hypothesis that acute exposure to hypercapnic hypoxia (HH) alters the crab's ability to clear a pathogenic bacterium, Vibrio campbellii 90-69B3, from the hemolymph. Adult male crabs were held in normoxia (well-aerated seawater) or HH (seawater with PO(2) = 4 kPa; PCO(2) = 1.8 kPa; and pH = 6.7-7.1) and were injected with 2.5 x 10(4) Vibrio g(-1) body weight. The animals were held in normoxia or in HH for 45, 75, or 210-240 min before being injected with Vibrio, and were maintained in their respective treatment conditions for the 120-min duration of the experiment. Vibrio colony-forming units (CFU) ml(-1) hemolymph were quantified before injection, and at 10, 20, and 40 min afterward. Total hemocytes (THC) ml(-1) of hemolymph were counted 24 h before (-24 h), and at 10 and 120 min after injection. Sham injections of saline produced no change in the bacterial or hemocyte counts in any treatment group. Among the groups that received bacterial injections, Vibrio was almost completely cleared within 1 h, but at 10-min postinjection, Vibrio CFU ml(-1) hemolymph was significantly higher in animals held in HH for 75 and 210-240 min than in those held in normoxia. Within 10 min after crabs were injected with bacteria, THC ml(-1) significantly decreased in control and HH45 treatments, but not in the HH75 and HH210-240 treatments. By 120 min after injection of bacteria, hemocyte counts decreased in all but the HH45 group. These data demonstrate that HH significantly impairs the ability of blue crabs to clear Vibrio from the hemolymph. These results also suggest that HH alters the normal role of circulating hemocytes in the removal of an invading pathogen.     

The effect of salinity on experimental infections of a Hematodinium sp. in blue crabs, Callinectes sapidus

The parasitic dinoflagellate Hematodinium sp. parasitizes blue crabs along the Atlantic seaboard of the United States. Infections in blue crabs have only been reported from waters where salinity is >11 practical salinity units (psu). Blue crabs maintain a hyperosmotic internal concentration at low salinities (0-5 psu), roughly comparable to 24 psu, and should be capable of maintaining an infection in low-salinity waters even if Hematodinium spp. cells are intolerant of low salinities. We tested this notion by observing the effect of low salinity on the progression of disease in crabs experimentally infected with the parasite. Blue crabs were acclimated to 5 psu or 30 psu salinity treatments. They were inoculated with Hematodinium sp. and necropsied 3, 7, 10, and 15 days post-inoculation. The low-salinity treatment did not have an effect on the proliferation of Hematodinium sp. infections in blue crabs; moreover, a greater proportion of infections in crabs in the low-salinity treatment developed dinospore stages than did those in the high-salinity treatment, indicating that salinity may affect the development of the parasite. However, dinospores from in vitro cultures rapidly became inactive when held in salinities <15 psu. Our experiments indicate that Hematodinium spp. can develop in blue crabs at low salinities, but that the parasite is incapable of transmission in this environment, which explains the lack of natural infections in crabs at low salinities.     

Gas-bubble disease in the blue crab, Callinectes sapidus.

Exposure of crabs to water supersaturated with air resulted in formation of gas emboli in the hemal system, which in turn caused localized ischemia. More than one-third of the exposed crabs died during the 2 days following the episode. In surviving crabs, the most severely affected organs and tissues were the gills, heart, and antennal gland. Especially in gills, emboli were still present in apparently healthy crabs 35 days following exposure to water supersaturated with air. Evidence of ischemic injury was focal in character except in the antennal gland, where the epithelium of the labyrinth was sometimes extensively degenerate. Repair processes apparently did not involve hemocytes except for occasional fibroblastic infiltration in damaged gill lamellae.     

Risk of handling as a route of exposure to infectious waterborne Cryptosporidium parvum oocysts via Atlantic blue crabs (Callinectes sapidus)

Commercial Atlantic blue crabs (Callinectes sapidus) were exposed to 2.0x10(4) infectious waterborne oocysts of Cryptosporidium parvum. The study demonstrated that blue crabs can transfer C. parvum oocysts to persons involved in handling or preparing crabs and that they may contaminate other surfaces or products during storage.