Juvenile Blue Crabs Research Articles

Habitat selection of the pink shrimp Farfantepenaeus paulensis and the blue crab Callinectes sapidus in an estuary in southern Brazil: influence of salinity and submerged seagrass meadows

This study was conducted in two estuarine inlets (Saco da Mangueira and Saco do Arraial) at the Patos Lagoon estuary, southern Brazil. The changes in relative abundance and size of post-larvae and juvenile shrimp Farfantepenaeus paulensis and juvenile blue crab Callinectes sapidus were compared, considering the influence of salinity and the presence of submerged seagrass meadows. The analyses were performed using generalized linear models (GLM) for abundance variations and ANOVA for variations on the size of individuals. The pink shrimp was more abundant at Saco da Mangueira, in seagrass meadows and areas of higher salinity. The blue crab was more abundant at Saco do Arraial and in lower levels of salinity. The importance of submerged vegetation for the blue crab lies in a preference of smaller crabs of the species for the seagrass meadows. It has been shown that these species choose different habitats in the estuary, and both the salinity and the presence of submerged seagrass meadows influence the selection of habitat.

Toxicity of the dispersant Corexit 9500 to early life stages of blue crab, Callinectes sapidus.

The Deepwater Horizon well released 4.4 million barrels of light crude oil offshore of Louisiana into one of the world's largest and most productive blue crab (Callinectes sapidus) fisheries. The objectives of this paper were to determine the toxicity of the dispersant Corexit(®) 9500A used in the 2010 oil spill on juvenile and larval blue crabs, and the long-term effects of sublethal acute exposure. Only the highest treatment levels of dispersant significantly increased mortality in larval and juvenile blue crabs (100 mg/L and 1,000 mg/L, respectively). This correlated to concentrations well above levels found in the Gulf of Mexico following the spill. Smaller and younger crabs showed higher mortality than older and larger crabs. This research indicates direct application of dispersants on crab larvae could cause acute mortality, but dilution through diffusion and natural weathering processes would minimize long-term effects.

Modelagem de ocorrência e densidade de juvenis de Callinectes sapidus (Decapoda, Portunidae) em dois estuários do Rio Grande do Sul, com dados zero-inflacionados

Dados de contagem de juvenis de siri-azul (Callinectes sapidus Rathbun, 1896) coletados em dois estuários do Rio Grande do Sul são objeto do presente estudo. Por se encontrarem zero-inflacionados, esses dados motivaram a formulação de modelos hierárquicos, que quantificam o efeito das covariáveis categóricas mês e local sobre a probabilidade de ocorrência e densidade dessas populações, levando em conta a detecção imperfeita. Foram também desenvolvidos modelos não-hierárquicos para comparação. Uma abordagem Bayesiana foi adotada para a estimação dos parâmetros dos modelos por simulação Monte Carlo com Cadeias de Markov (MCMC). A comparação entre modelos foi feita com o Critério de Informação da Deviância (DIC). Os modelos hierárquicos apresentaram ajustes melhores que os modelos convencionais, mitigaram o problema do excesso de zeros e permitiram analisar simultaneamente as probabilidades de ocorrência e a densidade de juvenis de siri-azul. No estuário da Lagoa dos Patos, a probabilidade de ocorrência de juvenis na Classe 2 aumenta com a distância da desembocadura, enquanto em Tramandaí os pontos intermediários apresentam as maiores probabilidades. Em ambos os estuários a ocorrência é mais provável nos meses de verão e de inverno. A densidade de juvenis da Classe 2 apresenta marcada variação em relação aos meses do ano sendo, em geral, maior no estuário de Tramandaí.

Broad-scale association between seagrass cover and juvenile blue crab density in Chesapeake Bay

Although numerous small-scale laboratory, mesocosm, and field experiments have demonstrated that abundance, survival, and growth of juvenile fish and invertebrates are higher in vegetated than in unvegetated habitats, the effect of habitat quality (i.e. habitat complexity) within vegetated habitats has not been documented at a broad spatial scale. We examined the relationship between percent cover in seagrass beds (eelgrass Zostera marina, widgeon grass Ruppia maritima, and associated macroalgae) and juvenile blue crab Callinectes sapidus density at a broad spatial scale. We quantified the functional relationship between juvenile density and percent cover of vegetation by sampling in Chesapeake Bay (USA) seagrass beds utilized by juve- nile blue crabs in the fall of 2007 and 2008, following peak postlarval blue crab recruitment. Based on Akaike's information criterion model comparisons, the most plausible model included both percent cover of vegetation and region of Chesapeake Bay. Juvenile crab density was a positive exponential function of percent cover of vegetation, and was augmented by 14 to 30%, depending on year, for every 10% increase in cover. Density was approximately 2 times higher on the western shore of Chesapeake Bay than on the eastern shore. Seagrass bed area, presence or absence of algae, and distance to the mouth of the bay did not significantly influence density. An expected threshold (i.e. sigmoid) response of juvenile density to percent cover of vegetation was not evident, probably because this study was undertaken when recruitment was low, so habitats may not have been at carrying capacity. This study is the first to document the functional relation- ship between habitat quality and juvenile density at a broad spatial scale for a marine fish or invertebrate, and suggests that the quality of seagrass habitat influences population dynamics.

Mechanism by which coarse woody debris affects predation and community structure in Chesapeake Bay

Coarse woody debris (CWD) provides a structured habitat in aquatic systems. It is associated with a higher abundance, diversity, and productivity of aquatic animals and can provide a shelter from physical stressors and predation. We investigated the effects of CWD on predation risk and community structure in two field experiments in the Rhode River, Chesapeake Bay. In the first experiment we assessed the effects of the orientation (horizontal or vertical) and complexity (simple or crossed) of CWD, and in the second experiment we investigated the effects of interstitial space (ISS). We measured predation risk in a tethering experiment for three species in each experiment: juvenile blue crab, Callinectes sapidus; mummichog (a type of killifish), Fundulus heteroclitus; and grass shrimp, Palaemonetes pugio. These three species are common in nearshore Chesapeake Bay habitats but differ in their size and habitat use, allowing for a holistic examination of the function of CWD. At the end of the experiments, the benthic community inhabiting the CWD was sampled. By interpreting the community data in light of the predation experiments we were able to examine how the structural characteristics of the CWD affected predation to shape the benthic community. In the first experiment, lowest predation was experienced by blue crabs at the treatments with high complexity at the sediment surface, by mummichogs at all CWD treatments relative to control plots, and by grass shrimp in horizontally oriented treatments. In the second experiment, lowest predation was experienced by blue crabs at moderate to high ISSs, by mummichogs at intermediate ISSs, and by grass shrimp at low and high ISSs. Predation rates for each experimental species could be predicted by the size of the prey, and by both the size of their predators and the size of species that prey on the predators relative to the ISS of the structure. Community structure differed among treatments in both experiments and appeared to be driven primarily by predation risk; species whose predators were in high abundance in a treatment were less abundant themselves. This work demonstrates the importance of CWD and of wooded shorelines as important habitat for both economically and ecologically important species. It also highlights the importance of top-down forcing in the small scale community dynamics of these near-shore habitats.

Preference for feeding at habitat edges declines among juvenile blue crabs as oyster reef patchiness increases and predation risk grows

Both habitat patchiness and behaviorally-mediated indirect effects (BMIEs; predator- induced changes in prey behavior that affect the prey's resources) are important in many food webs, but the relationships between these 2 factors have yet to be investigated. To explore effects of habitat patchiness and variation in perceived risk of predation on food-web dynamics, we conducted a factorial experiment in a model aquatic food chain of predator-prey-resource using 2 contrasting predators (adult blue crab Callinectes sapidus and toad fish Opsanus tau), juvenile blue crab as prey, and mussel Geukensia demissa as resource. Both predator presence and habitat patchiness influenced the prey's preference for consuming resources at patch edges instead of interiors. The preference of prey for consuming resources at habitat edges was 4 times stronger in continuous oyster reef habitat than in smaller habitat patches. This suggests that interior resources in continuous habitat experience a refuge from consumption, but this refuge is largely lost in patchy habitat. The mere presence of predators reduced the prey's preference for consuming resources at habitat edges. This BMIE was significant for the ambush predator (toadfish) and the treatment containing both predators, but not for the actively hunting predator (adult blue crab). We conclude that habitat patchiness and predator presence can jointly affect resource distribution by inducing shifts in prey foraging behavior, revealing a need to incorporate BMIEs into habitat fragmentation studies. This conclusion has broad and growing relevance as anthropogenic factors increasingly modify predator abundances and fragment coastal habitats. © Inter-Research 2012.

Acute toxicity and sub-lethal effects of common pesticides in post-larval and juvenile blue crabs, Callinectes sapidus

The coastal plains of the southeastern United States support extensive agricultural operations that apply pesticides and herbicides. The shallow tidal creeks and ditches that directly drain agricultural fields are home to the young of the ecologically and economically important blue crab, Callinectes sapidus (Rathbun, 1896). Massive mortality observed by peeler crab fishers led us to investigate the acute toxicity of several commercial pesticide formulations and their active ingredients to blue crab megalopae and J1–J4 stage juveniles. Twenty-four hour acute toxicity assays were conducted with the organophosphate acephate (Orthene®), the carbamate aldicarb, the chloro-nicotinyl imidacloprid (Trimax™), the pyrethroid lambda-cyhalothrin (Karate® with Zeon Technology), and the glyphosate-based herbicide Roundup® Pro. LC50 values ranged from 0.22μg/L for megalopae exposed to lambda-cyhalothrin to 316,000μg/L for juveniles exposed to Roundup. The acute toxicities of active ingredient insecticides to blue crabs followed the order: lambda-cyhalothrin>imidacloprid≈aldicarb>acephate. Megalopae were almost always more sensitive than juveniles and there was little difference between the LC50s of each commercial formulation and its active ingredient. Treatment of intermolt megalopae with LC20 levels of Roundup resulted in significantly reduced time to metamorphosis (TTM) compared to estuarine water controls, while no differences resulted from treatment with the four active ingredient insecticides. Treatment with acephate, aldicarb, imidacloprid, and Roundup significantly increased the frequency of juveniles that died within 6h of molting. The sensitivity of molting blue crabs to these pesticides makes frequently molting juveniles particularly vulnerable to pesticides in estuaries.

Improving the survival, growth and hemolymph ion maintenance of early juvenile blue swimmer crabs, Portunus pelagicus, at hypo- and hyper-osmotic conditions through dietary long chain PUFA supplementation

Blue swimmer crab, Portunus pelagicus, juveniles are weak osmoregulators leading to reduced survival and growth outside salinities of 15–35‰. Therefore improving the performance of this commercially important crab at sub-optimal salinities through diet manipulation will likely benefit the aquaculture industry. The aim of the current experiment was to determine whether increased dietary LC-PUFA, in the form of fish oil, could improve the survival, growth and hemolymph ion maintenance of early P. pelagicus juveniles at salinities of 14, 30 and 42‰. Crabs were fed five diets containing 0, 3, 6, 9 and 12% fish oil inclusion levels (% dry diet weight) and their survival and growth were measured over 20days. After 20days, the hemolymph Na+, K+ and Ca2+ levels of the crabs were measured. Seven replicates, with three crabs in each replicate, were used in each salinity/dietary treatment (total of 315 crabs). Results showed diets containing 0% fish oil inclusion led to the lowest survival in all salinities, although this was only significant (p<0.05) for the crabs cultured at a high salinity of 42‰. Further, a salinity of 42‰ generally caused lower growth, compared to 14 and 30‰, however the highest fish oil inclusion level of 12% (or 3.73% LC-PUFA) caused no significant growth or development difference (p>0.05) compared with those fed the same diet but at a salinity of 30‰. Between salinities of 14 and 30‰, no significant growth difference was detected although the growth benefit to increased dietary LC-PUFA at 14‰ was more obvious than at 30‰. Meanwhile, at salinities of 14 and 42‰, hemolymph Na+ regulation was significantly improved through dietary LC-PUFA supplementation, indicating that LC-PUFA is an essential nutrient for undisrupted ion-regulation. These findings indicate that the productivity of P. pelagicus juveniles may be increased at sub-optimal salinities through dietary LC-PUFA supplementation, which may be linked with improved hemolymph ion maintenance.

Preference for feeding at habitat edges declines among juvenile blue crabs as oyster reef patchiness increases and predation risk grows

Preference for feeding at habitat edges declines among juvenile blue crabs as oyster reef patchiness increases and predation risk grows

Effects of anthropogenic shoreline hardening and invasion by Phragmites australis on habitat quality for juvenile blue crabs ( Callinectes sapidus)

Unvegetated, shallow water habitats adjacent to marshes are an important nursery for juvenile blue crabs, Callinectes sapidus, in Chesapeake Bay. Alteration of the shoreline, either through the replacement of marshes with anthropogenic structures, such as riprap and bulkheads, or through the replacement of the native marsh grass Spartina sp. ( Spartina) with the invasive Phragmites australis ( Phragmites), may affect the value of this habitat as a nursery. In this study, we compared the effects of four common shoreline types, bulkheads, riprap, Phragmites marshes, and Spartina marshes, on food availability, feeding, growth, and survival of juvenile blue crabs in adjacent subtidal areas, as well as on the abundance and size of predators in the South River, Maryland. Sites with each shoreline type were randomly selected. We used benthic cores to sample macrobenthic prey and performed gut content analysis on caged crabs to examine food availability and feeding. Growth was estimated using caged crabs. Survival was assayed with a tethering experiment and predators were sampled with a seine net. Riprap had a lower abundance of macrofaunal prey, and the macrofaunal community differed from both marsh types in that it had it had smaller an more opportunistic species such as nematodes and small polychaetes compared to more bivalves and larger polychaetes at the marsh sites; however, gut contents and growth did not vary among shoreline types. Predation pressure on juvenile blue crabs was highest at bulkhead sites and lowest at riprap. Predator abundance did not vary among the shoreline types, though piscine predators were smaller in size near Spartina marshes compared to the other shorelines. We conclude that shoreline hardening substantially reduced the value of shoreline habitats for juvenile blue crabs, but that Spartina and Phragmites are functionally equivalent.

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.

Living in a Contaminated Estuary: Behavioral Changes and Ecological Consequences for Five Species

reichmuth, and aLLiSon candeLmo Killifish, grass shrimp, fiddler crabs, blue crabs, and young bluefish in contaminated estuaries differ ecologically from reference populations in relatively uncontaminated environments. All five of these species show reduced activity and feeding, but only fishes show reduced growth. In these areas, killifish are poor predators, eat much detritus, have poor predator avoidance, and are smaller and less abundant. Bluefish have reduced rates of feeding and growth. Both killifish and bluefish have altered thyroid glands and neurotransmitters, which may underlie behavioral changes. Shrimp in contaminated environments show unchanged levels of predator avoidance; compensatory energetic partitioning favors growth and reproduction despite reduced feeding. With less predation pressure, shrimp are larger and more numerous. Fiddler crabs tend to spend more time in burrows and experience reduced predation. With ample food, metal depuration through molting, and reduced population size, they grow larger. In contaminated estuaries, we’ve found that juvenile blue crabs are less likely to be eaten by adults; adults are impaired in prey capture but are larger, despite eating much detritus and algae. Release from top-down effects from humans—as a result of a fishery advisory—may allow crabs to live longer. It appears that differences in physiology and trophic interactions modify the effects of reduced feeding on the different species.

Fight or flight: an investigation of aggressive behavior and predator avoidance in two populations of blue crabs (Callinectes sapidus Rathbun) in New Jersey

Recent literature has suggested aggression may be context dependent. The purpose of this investigation was to examine aggressive and predator avoidance behaviors in juvenile blue crabs of two populations. Furthermore, we wanted to determine whether aggression persisted into the adult stages. Juvenile blue crabs collected from an impacted estuary, the Hackensack Meadowlands (HM), were found to attack a threatening stimulus significantly more often (70%) than conspecifics from a less impacted estuary (Tuckerton—TK). TK juveniles responded significantly more often with a flight (~35%) or mixed response (~30%). Additionally, HM juveniles were significantly more successful than TK juveniles at avoiding an adult blue crab predator when sandy substrate was present in laboratory experiments. However, the video clarity made it impossible to determine which interactions were allowing survival. To determine if “aggression” exhibited by the HM juveniles was the reason for their enhanced survival, follow-up predator avoidance experiments were conducted without substrate and videotaped. The results of these experiments suggest that aggression per se is not the reason since aggressive juveniles were no more successful than non-aggressive individuals. The aggressive behavior exhibited by HM juveniles continues into the adult stages. This behavior may be important to recognize when estimating population size as well as local fishery efforts.

Population density, survival and movement of blue crabs in estuarine salt marsh nurseries

The importance of a broad suite of complex structured habitats as nurseries for estuarine fauna is well recognized. In contrast, recent evidence indicates the nursery value of salt marshes and contiguous unvegetated mud flats for blue crabs are underestimated. To assess the nursery value of salt marsh tidal creeks for the blue crab Callinectes sapidus in coastal North Carolina, USA, we quantified population density, survival and movement patterns of juvenile blue crabs in 2 tidal salt marsh creeks during summer and fall. Survival rates of blue crab juveniles were high (0.98 d–1) and similar in both creek systems. Juvenile crabs exhibited a high degree of site fidelity to a given marsh creek during summer–fall, suggesting that losses were predominantly due to mortality, not emigration. Our study provides critical information on the demographic processes underlying the importance of salt marshes as nurseries for estuarine-dependent species, and was novel in that it: (1) measured density, survival and emigration concurrently; and (2) enabled the identification of individuals, which allowed for the assessment of the relationship between blue crab size, survival and capture probability. We conclude that the observed patterns of abundance, survival and habitat utilization of blue crabs within tidal salt marsh creeks in North Carolina are consistent with the hypothesis that salt marsh creeks are important nurseries for blue crabs. Further, the relatively high use of the marsh surface by juvenile blue crabs, combined with a general lack of directed sampling within these complex habitats, suggests that crab densities may be even higher in salt marshes than previously thought.

Blue Crab (Callinectes sapidus Rathbun, 1896) Settlement at Three Georgia (USA) Estuarine Sites

The blue crab, Callinectes sapidus Rathbun, 1896, represents the second most important fishery for coastal Georgia; yet, little is known about environmental forces that affect planktonic postlarval settlement in the region. Here, we describe a study to examine the physical mechanisms responsible for blue crab settlement in the extensive salt marsh system of coastal Georgia. Bottom and surface samplers were placed at three sites along a salinity gradient from a low-salinity site in the Altamaha River to a high-salinity area of the Duplin River, Sapelo Island, GA, USA during 2005. Megalopae and juvenile monitoring occurred from July through December. The majority of both megalopae (86.8%) and juvenile (89.3%) blue crabs were recovered in bottom samplers at the low-salinity Altamaha River site during August and early September. Few megalopae were collected at the surface of the Altamaha River or at the two higher-salinity sites in the Duplin and North Rivers. Downwelling winds were unable to explain all settlement events; however, winds with an onshore component regularly preceded settlement events. The use of a multiple-regression model revealed a lagged relationship (r = 0.5461, \( lag = 0–2 days \)) between wind events, temperature, salinity, maximum tidal height, and settlement.

Ambient Malathion concentrations modify behavior and increase mortality in blue crabs

Organophosphate insecticides can compromise water quality and harm non-target species unintentionally. In this study, we examined the effects of a commonly used organophosphate insecticide, Malathion, on blue crabs Callinectes sapidus, which are an economically and ecologically important estuarine species. Adult and juvenile crabs were exposed to environmentally-occurring concentrations of Malathion at 2 salinity levels to determine how these factors would affect crab behavior and mortality. Initial Malathion concentrations of 1.0 ppb and 11.2 ppb caused a significant increase in mortality of juvenile blue crabs, and concentrations of 11.2 ppb caused a significant increase in mortality of adults, all within 36 h after exposure. Our results indicated that salinity did not affect blue crabs' susceptibility to Malathion at these concentrations. We measured the time needed for adult blue crabs to right themselves when placed on their backs before and after exposure to Malathion. After 1 h of exposure to Malathion concentrations of 11.2 ppb, adult crabs took signifi- cantly longer to right themselves, indicating that short-term exposure to Malathion at ecologically relevant concentrations can interfere with essential behaviors of blue crabs. Blue crab populations have been declining in recent years, and their decline has primarily been linked to overfishing. However, other mechanisms including pollution and disease may also contribute to this problem and require further investigation. Since blue crabs are important components of estuarine food webs, changes in blue crab mortality rates and behavior may have important consequences for entire estuarine systems.

Hemolymph ecdysteroids during the last three molt cycles of the blue crab, Callinectes sapidus: quantitative and qualitative analyses and regulation

The profiles of circulating ecdysteroids during the three molt cycles prior to adulthood were monitored from the juvenile blue crab, Callinectes sapidus. Ecdysteroid patterns are remarkably similar in terms of peak concentrations ranging between 210-330 ng/ml hemolymph. Analysis of hemolymph at late premolt stage revealed six different types of ecdysteroids with ponasterone A (PoA) and 20-OH ecdysone (20-OH E) as the major forms. This ecdysteroid profile was consistent in all three molt cycles. Bilateral eyestalk ablation (EA) is a procedure that removes inhibitory neurohormones including crustacean hyperglycemic hormone (CHH) and molt-inhibiting hormone (MIH) and often results in precocious molting in crustaceans. However, the inhibitory roles of these neuropeptides in vivo have not yet been tested in C. sapidus. We determined the regulatory roles of CHH and MIH in the circulating ecdysteroid from ablated animals through daily injection. A daily administration of purified native CHH and MIH at physiological concentration maintained intermolt levels of ecdysteroids in the EA animals. This suggests that Y organs (YO) require a brief exposure to CHH and MIH in order to maintain the low level of ecdysteroids. Compared to intact animals, the EA crabs did not exhibit the level of peak ecdysteroids, and the major ecdysteroid turned out to be 20-OH E, not PoA. These results further underscore the important actions of MIH and CHH in ecdysteroidogenesis, as they not only inhibit, but also control the composition of output of the YO activity.

Subchronic exposure to nitrite, potassium and their combination on survival, growth, total haemocyte count and gill structure of juvenile blue swimmer crabs, Portunus pelagicus

Among anthropogenic discharges, nitrite and potassium can simultaneously enter aquatic ecosystems at high loading rates which can threaten marine animals. The sensitivity of early juvenile blue swimmer crabs, Portunus pelagicus, to the subchronic exposure to nitrite, potassium and their combination was evaluated by measuring the survival, growth, total haemocyte count (THC) and gill histopathological changes. In all NaNO 2-N and KNO 2-N treatments the survival substantially decreased, many due to “molt death syndrome”, along with significantly lower ( p<0.01) specific growth rates (SGR). Although fewer deaths occurred in the KCl-K treatments, the SGR at the higher concentrations were significantly less ( p<0.05) than the control. The gill histopathological changes following elevated NaNO 2-N, KNO 2-N and KCl-K exposure showed drastic but similar damage. In spite of a long term healing response, indicated by a significant hemolymph THC increase ( p<0.01) and several gill lamellae modifications, early P. pelagicus juveniles are highly sensitive to elevated NO 2-N levels.

Differences in Prey Capture Behavior in Populations of Blue Crab (Callinectes sapidus Rathbun) from Contaminated and Clean Estuaries in New Jersey

Populations living in contaminated environments may exhibit behavioral changes that can alter predator–prey interactions. Blue crabs from the contaminated Hackensack Meadowlands (HM) had reduced ability to capture juvenile blue crabs and adult mummichogs (both active prey) compared with crabs from a reference site (Tuckerton (TK)). However, they consumed equivalent amounts of ribbed mussels and fiddler crabs, which are less active prey. Crabs may have reduced coordination rather than appetite or motivation. The lab data are supported by stomach analysis of field-caught crabs. HM crab stomachs contained ∼60% algae, plant material, detritus, and sediment and much lower weights of crab, fish, and other live food than TK crabs. However, the relative absence of bivalves in their diet may reflect reduced amounts available. When TK crabs were caged in HM or fed food from HM in the lab for 8 weeks, their prey capture ability declined significantly, and mercury in their muscle tissue increased significantly, indicating that environmental factors were responsible for the behavioral differences. When HM crabs were caged in TK or fed fish from TK in the lab for 8 weeks, their prey capture ability improved significantly. Mercury levels were variable and did not show a significant decrease.

Mortality and blood loss by blue swimmer crabs (Portunus pelagicus) after simulated capture and discarding from gillnets

Abstract Uhlmann, S. S., Broadhurst, M. K., Paterson, B. D., Mayer, D. G., Butcher, P., and Brand, C. P. 2009. Mortality and blood loss by blue swimmer crabs (Portunus pelagicus) after simulated capture and discarding from gillnets. – ICES Journal of Marine Science, 66: 455–461. Two laboratory experiments were carried out to quantify the mortality and physiological responses of juvenile blue swimmer crabs (Portunus pelagicus) after simulated gillnet entanglement, air exposure, disentanglement, and discarding. In both experiments, all but control blue swimmer crabs were entangled in 1-m2 gillnet panels for 1 h, exposed to air for 2 min, subjected to various treatments of disentanglement ranging between the forceful removal of none, one, two, and four appendages, then “discarded” into individual experimental tanks and monitored for 10 d. In Experiment 1, mortalities were associated with the number of appendages removed and the occurrence of unsealed wounds. In Experiment 2, live blue swimmer crabs were sampled for blood at 2 min and 6, 24, and 72 h post-discarding to test for the effects of disentanglement and appendage removal on total haemocyte counts, clotting times, protein levels (by refractive index), and blood ion concentrations. Compared with blue swimmer crabs that had sealed or no wounds, those with unsealed wounds had lower total haemocyte counts, protein, and calcium concentrations and increased clotting times and magnesium and sodium levels. Induced autotomy, as opposed to the arbitrary, forceful removal of appendages has the potential to minimize the mortality and stress of discarded, juvenile blue swimmer crabs.