Juvenile Blue Crabs Callinectes Sapidus Research Articles

Assessment of treatment-specific tethering survival bias for the juvenile blue crab Callinectes sapidus in a simulated salt marsh.

The blue crab (Callinectes sapidus) is ecologically and economically important in Chesapeake Bay. Nursery habitats, such as seagrass beds, disproportionately contribute individuals to the adult segment of populations. Salt marshes dominated by smooth cordgrass Spartina alterniflora are intertidal nursery habitats which may serve as a refuge from predation for juvenile blue crabs. However, the effects of various characteristics of salt marshes on nursery metrics, such as survival, have not been quantified. Comparisons of juvenile survival between salt marshes and other habitats often employ tethering to assess survival. Although experimental bias when tethering juvenile prey is well recognized, the potential for habitat-specific bias in salt marshes has not been experimentally tested. Using short-term mesocosm predation experiments, we tested if tethering in simulated salt marsh habitats produces a habitat-specific bias. Juvenile crabs were tethered or un-tethered and randomly allocated to mesocosms at varying simulated shoot densities and unstructured sand. Tethering reduced survival, and its effect was not habitat specific, irrespective of shoot density, as evidenced by a non-significant interaction effect between tethering treatment and habitat. Thus, tethering juvenile blue crabs in salt marsh habitat did not produce treatment-specific bias relative to unvegetated habitat across a range of shoot densities; survival of tethered and un-tethered crabs was positively related to shoot density. These findings indicate that tethering is a useful method for assessing survival in salt marshes, as with other nursery habitats including seagrass beds, algae and unstructured sand.

Biological responses of the predatory blue crab and its hard clam prey to ocean acidification and low salinity

How ocean acidification (OA) interacts with other stressors is understudied, particularly for predators and prey. We assessed long-term exposure to decreased pH and low salinity on (1) juvenile blue crab Callinectes sapidus claw pinch force, (2) juvenile hard clam Mercenaria mercenaria survival, growth, and shell structure, and (3) blue crab and hard clam interactions in filmed mesocosm trials. In 2018 and 2019, we held crabs and clams from the Chesapeake Bay, USA, in crossed pH (low: 7.0, high: 8.0) and salinity (low: 15, high: 30) treatments for 11 and 10 wk, respectively. Afterwards, we assessed crab claw pinch force and clam survival, growth, shell structure, and ridge rugosity. Claw pinch force increased with size in both years but weakened in low pH. Clam growth was negative, indicative of shell dissolution, in low pH in both years compared to the control. Growth was also negative in the 2019 high-pH/low-salinity treatment. Clam survival in both years was lowest in the low-pH/low-salinity treatment and highest in the high-pH/high-salinity treatment. Shell damage and ridge rugosity (indicative of deterioration) were intensified under low pH and negatively correlated with clam survival. Overall, clams were more severely affected by both stressors than crabs. In the filmed predator-prey interactions, pH did not substantially alter crab behavior, but crabs spent more time eating and burying in high-salinity treatments and more time moving in low-salinity treatments. Given the complex effects of pH and salinity on blue crabs and hard clams, projections about climate change on predator-prey interactions will be difficult and must consider multiple stressors.

Resilience of Oxygen Consumption Rates in the Juvenile Blue Crab Callinectes sapidus to Future Predicted Increases in Environmental Temperature and pCo2 in the Mesohaline Chesapeake Bay

Quantifying the physiological impact of environmental stressors on living organisms is critical to predicting the response of any given species to future climate scenarios. Oxygen consumption rates (μmol/g/min) were measured to examine the physiological response of the juvenile blue crab Callinectes sapidus from the Chesapeake Bay (Patuxent River, Maryland) to elevated temperature and dissolved carbon dioxide in water (pCO2) reflective of projected future climate scenarios. Treatment levels were selected to represent current conditions in the Chesapeake Bay (26°C and 800 μatm) and conditions predicted to occur by the year 2100 (31°C and 8,000 μatm). Crabs were exposed in a factorial design to these conditions throughout two successive molts (approximately 30 days). At the end of the exposure, the oxygen consumption rates of individual crabs were determined over at least a 10-h period using a flow-through respiration chamber equipped with optical oxygen electrodes. No significant effect of temperature or pCO2 on oxygen consumption was observed, suggesting the absence of a respiratory impact of these two climate stressors on juvenile blue crabs. Oxygen consumption rates were also determined for crabs that experienced a rapid increase in temperature without prior acclimation. The oxygen consumption rate of crabs may have acclimated to increased temperature during the 30-day exposure period before respiratory measurement. This potential acclimation, combined with high individual variability, and a relatively small difference in temperature treatments are likely the cause for the lack of a statistically significant difference in mean oxygen consumption rates by temperature in the core experiment. The results of this study suggest that the blue crab may be quite resilient to future climate stressors and underscore the need for species-specific studies to quantify the effects of climate change on estuarine crustaceans.

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.

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.

Pre- and post-settlement factors as determinants of juvenile blue crab Callinectes sapidus abundance: results from the north-central Gulf of Mexico

We estimated juvenile abundance and predation potential for young-of-the-year (YOY) blue crabs Callinectes sapidus in upper- and lower-salinity marsh and submerged aquatic vegetation (SAV) 'nursery habitats' of Mobile Bay, Alabama, USA, and surrounding waters, and combined this information with previously published post-larval abundance data from these habitats to evaluate the relative roles of pre- and post-settlement events in determining the abundance of YOY crabs. We found little evidence for a significant relationship between megalopal supply and juvenile abundance, except shortly after a few very large, episodic pulses of postlarvae. However, even after large settlement events, in less than 14 d densities of YOY crabs had declined in a density-dependent manner to prior 'background' levels. These declines in density were probably due to high predation rates that produced as much as 95% mortality on a daily basis. As many previous studies had found, greater predation risk was recorded for crabs on unvegetated substrate than for those in SAV or marsh grass. Overall, losses to predators were high at our study sites compared to those reported in similar studies along the Atlantic coast of the United States. In both years of the study (1990 and 1991), the largest numbers of juvenile crabs were found in poly- and mesohaline SAV and salt marsh habitats. In Year 1, no significant correlations were observed between either marsh stem density or SAV vegetation biomass and YOY crab abundance, although at one location in the second year there was a significant positive correlation between the number of juveniles and mid-bay stem density. No significant differences in crab size (carapace width, CW) were found among locations in either year. In comparison with other, better-studied mid-Atlantic coast estuaries in the US (e.g., the Chesapeake and Delaware bays), very large numbers of megalopae (frequently 1 to 2 orders of magnitude greater) invade the Mobile Bay system, as well as other Gulf Coast estuaries. However, YOY abundance in marsh and SAV 'nursery habitats' was of the same order of magnitude as that reported for mid-Atlantic estuaries. We conclude that although postlarval supply was very large, post-settlement loss to predators was the dominant factor influencing blue crab population dynamics in our north central Gulf of Mexico study area.

Large-scale blue crab recruitment: linking postlarval transport, post-settlement planktonic dispersal, and multiple nursery habitats

A large-scale study of early juvenile blue crab Callinectes sapidus recruitment within a shallow, predominantly wind-driven estuarine system demonstrated that distribution and abundance patterns were jointly influenced by location from oceanic sources of postlarvae, time period, habitat type, and post-settlement planktonic dispersal. The Croatan-Albemarle-Pamlico Estuarine System (CAPES) in North Carolina, USA, is a lagoonal body of water that is separated from the Atlantic Ocean by a chain of barrier islands, which are bisected by Oregon, Hatteras, and Ocracoke Inlets. For sampling purposes, the CAPES was divided into 4 regions that differed in distance and orientation from oceanic sources of postlarvae, as well as available complex benthic habitat types. The Eastern region was closest to oceanic waters, contained 3 major inlets, and harbored extensive seagrass beds. The Northern and Western regions were located along the inland boundary of the CAPES, and con- tained alternative habitat types including the submersed rooted vascular plant Myriophyllum spica- tum and shallow detrital habitats. The Southern region was inshore and contained patchy seagrass. During a period that lacked storm events, virtually all juvenile recruitment occurred within seagrass beds at the Eastern region. Conversely, early juvenile blue crabs were distributed widely throughout the CAPES after the passage of tropical cyclones. The Eastern region appears to act as a relatively consistent initial recruitment site, whereas Northern and Western regions of the CAPES may act as episodic recruitment areas after the passage of tropical cyclones. Similar densities of early juveniles were found in different complex benthic habitats (seagrass, shallow detrital habitat, M. spicatum). A comparison of site-specific, settler-recruit densities (which represent distinct cohorts) suggested that post-settlement juveniles dispersed planktonically throughout the CAPES, most likely due to storm- driven transport. Post-settlement, planktonic dispersal altered the settler-recruit relationship, by both masking and potentially enhancing a density-dependent relationship between settlers and recruits. This study illustrates that ecological processes influencing recruitment, such as post-settlement dis- persal, may be missed when studied at relatively small spatial scales, and that our interpretation of population regulation can vary depending on the scale of study. Studies conducted over broad spa- tial scales can provide a more complete understanding of recruitment dynamics and can elucidate the interconnectedness of subpopulations by identifying potential 'source' areas in species with open populations.

Seasonal abundance, distribution, and habitat selection of juvenile Callinectes sapidus (Rathbun) in the northern Gulf of Mexico

Estimates of juvenile blue crab Callinectes sapidus (Rathbun) abundance within high and low density Cuban shoal grass Halodule wrightii (Ascherson) grassbed microhabitats and adjacent unvegetated substratum were carried out by suction sampling at Ono Island, Alabama, during 1987–1988. Juvenile blue crabs were significantly more abundant in vegetated than in unvegetated substratum for all samples except July and August 1987 and May and July 1988. Newly recruited crabs (⩽5mm CW) exhibited some association with high density grass. Medium-sized individuals (5–10 mm CW) showed a tendency toward association with low density grass. Larger juveniles (>10 mm CW) did not exhibit an association with any particular substratum based on field sampling. Complementary laboratory studies of habitat selection by juvenile blue crabs were carried out at the Dauphin Island Sea Laboratory during July and August 1988. In these laboratory experiments, a preference for high density grass over unvegetated substratum and low density grass was observed for small crabs (11–37 mm CW). In the absence of potential predators (>45 mm CW), female crabs selected low density vegetation more often than other substrata, while male crabs more often selected high density vegetation. In the presence of these potential predators, females always chose vegetated substrata over unvegetated substratum while males increased their selection of high density grass. The larger potential predators selected low density vegetation more often than any other substratum. Field and laboratory studies indicated that habitat partitioning through active selection of substratum by size class of juvenile blue crabs occurred. Laboratory studies indicated habitat partitioning by sex. Substratum selection by crabs was influenced by the presence of larger blue crabs which are potential predators. Although predation may act as the ultimate selection agent for such partitioning, it was not the proximate cause in laboratory studies.