Early Juvenile Crabs Research Articles

Diet shift of Chinese mitten crab (Eriocheir sinensis) during its upstream migration indicated by stable isotopes and fatty acid composition

ABSTRACT The Chinese mitten crab (Eriocheir sinensis) is the largest estuarine invertebrate and one of the most important fishery resources in China. Diet significantly affected survival of the crab. To study the diet of this species during its upstream migration, larvae and juvenile crabs as well as their potential food sources were sampled from three regions along the Changjiang River. Stable isotopes (δ13C and δ15N) in and fatty acid compositions of the samples were analysed to elucidate the crabs’ dietary shift during upstream migration. With upstream migration from the Changjiang Estuary, the average δ13C value in crabs decreased from −20.6 to −25.2‰, and the δ15N value increased from 3.0 to 11.8‰. The fatty acid compositions also changed significantly, with levels of C20:4n-6, C18:2n-6, and C18:3n-3 increasing during upstream migration. However, the levels of C15:0, C17:0, C16:1n-7, and 20:1n-9 exhibited the opposite trend. Pelagic marine prey items (e.g. zooplankton and bacteria) were the dominant food sources of megalopae and early juvenile crabs. The contribution of benthic terrigenous origin carbon sources (e.g. macrophytes and zoobenthos) to the juvenile crab diet sharply increased. The dietary information on the Chinese mitten crab can promote the restoration of wild population and sustainable development of crab aquaculture.

Quality assessment of newly hatched mud crab, Scylla serrata , larvae

Starvation and exposure to formalin were investigated as possible stress tests for evaluating the quality of mud crab, Scylla serrata, larvae. For the starvation stress test, newly hatched zoeae stocked in 150-ml containers were either starved or fed rotifers. Similarly, newly hatched zoeae were stocked in containers with seawater of 0 (control), 20, 30 and 40 mg/L formalin for the formalin stress test. The zoeae from the same batches were used for seed production to monitor their performance and validate the results of stress tests. Starvation was found to be unsuitable for larval quality evaluation. However, the impact of initial food deprivation on the newly hatched larvae indicates that feeding immediately after hatching is necessary for mud crab larvae. Exposure of larvae to 40 mg/L formalin for 3 hr appeared to be a reliable and practical method for larval quality assessment as the survival of larvae in the mass production tanks validated the classification of good and poor quality batches in the stress tests. On this basis, a hatchery operator can decide which batch should be cultured further. Finally, there appears to be a link between the quality of larvae and the performance at the megalopa and early juvenile crabs.

Early juvenile development of Mediterranean Liocarcinus depurator (Crustacea: Decapoda: Brachyura: Portunidae)

The early juvenile crab stages (C1 to C5) of the portunid crab Liocarcinus depurator (Linnaeus, 1758) were obtained in the laboratory from megalopae captured from the plankton in the northwest Mediterranean. Morphological development through these stages is described and compared with previous juvenile descriptions and adult morphology. The definitive adult morphology of carapace teeth is only apparent from stage C4. The incision on the lower orbital margin of the carapace and the mesial lobe (“portunid lobe”) of the endopod of the first maxilliped starts to be evident from C3. Stage C5 is already very similar to that of the typical adult crab. Male and female crabs can be distinguished from stage C2 onwards, based on sexual dimorphism in pleopods. The morphology and size of Mediterranean juvenile crabs was compared with the previous description of individuals reared in the laboratory from ovigerous females from British North Atlantic waters.

Implications of habitat-specific growth and physiological condition on juvenile crab population structure

Post-settlement processes can regulate the size and structure of marine invertebrate and fish populations. Faster growth and better physiological condition generally increase the survival potential of early juveniles, being usually associated with structurally complex habitats. Successive cohorts of early juvenile Carcinus maenas were followed in sandy and seagrass (Zostera noltii) habitats in the Mira Estuary, Portugal, to estimate growth and physiological condition (evaluated by RNA/DNA ratio) of juvenile populations. Mean cohort growth was similar in both habitats. However, in the sandy habitat, population size structure progressed to cohorts of larger carapace width (CW) and the RNA/DNA ratio was always higher than in the Z. noltii habitat. In this habitat, cohorts of low CW prevailed throughout and RNA/DNA ratio only increased after ~5.0 mm CW. Higher densities characterising seagrass areas may result in higher competition for resources, limiting growth and condition and leading to dispersal to less populated habitats. Larger juveniles had the best physiological condition, especially early in the season. Seagrass habitats do not necessarily yield enhanced growth rates and physiological condition of early juvenile crabs in relation to sandy areas. Knowledge of such trends is vital to understand distribution and abundance patterns of fish and marine invertebrate populations.

Spatial dynamics of large-scale, multistage crab (Callinectes sapidus) dispersal: determinants and consequences for recruitment

We assessed determinants and consequences of multistage dispersal on spatial recruitment of the blue crab, Callinectes sapidus, within the Croatan, Albemarle, Pamlico Estuarine System (CAPES), North Carolina, U.S.A. Large-scale sampling of early juvenile crabs over 4 years indicated that spatial abundance patterns were size-dependent and resulted from primary post-larval dispersal (pre-settlement) and secondary juvenile dispersal (early post-settlement). In general, primary dispersal led to high abundances within more seaward habitats, whereas secondary dispersal (which was relatively consistent) expanded the distribution of juveniles, potentially increasing the estuarine nursery capacity. There were strong relationships between juvenile crab density and specific wind characteristics; however, these patterns were spatially explicit. Various physical processes (e.g., seasonal wind events, timing and magnitude of tropical cyclones) interacted to influence dispersal during multiple stages and determined crab recruitment patterns. Our results suggest that the nursery value of different habitats is highly dependent on the dispersal potential (primary and secondary dispersal) to and from these areas, which is largely determined by the relative position of habitats within the estuarine landscape.

Effects of the Stocking Density and Body Size of the Mitten Crab (Eriocheir sinensis) on Aquatic Plant Biomass

ABSTRACT Experiments in tanks and cages were conducted to examine the effects of stocking density and body size of the mitten crab (Eriocheir sinensis) on transplanted submersed macrophyte biomass. The early juvenile crab with 7.0±0.6 mm carapace width (CW) had little effect on plant biomass, regardless of the stocking densities. However, larger crabs (CW: 18.0±2.2, 35.0±3.6, and 60.0±5.7 mm) significantly influenced plant biomass, especially at large stocking densities. Predictive models, using crab body size and stocking density, were generated to demonstrate effect of the mitten crab on the changes of plant biomass. The results indicate that dense mitten crab populations may adversely affect aquatic plant communities, particularly when its animal food resources are scarce.

Pre‐ And Post‐Settlement Determinants of Estuarine Dungeness Crab Recruitment

Abundance of early juvenile Dungeness crab (Cancer magister) is dramatically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat—specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 8 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4—mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post—settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week—long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differneces in wind—driven surface currents, since tidal current speeds in the two locations were similar. More—over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3—5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographical locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48—h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post—settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distribution of juvenile crab, and that predation (including cannibalism) is a key factor regulating local population size of early juvenile crabs in intertidal habitats where postlarval supply is relatively high.

Ontogeny of Hemocyanin Function in the Dungeness Crab Cancer Magister: The Interactive Effects of Developmental Stage and Divalent Cations on Hemocyanin Oxygenation Properties

ABSTRACT Calcium and magnesium ions raise the oxygen affinities of 25S hemocyanins of both first-instar juvenile and adult Cancer magister. A physiologically relevant change in magnesium concentration from 16 to 32mmol l−1 changes first-instar juvenile hemocyanin affinity by 5.6mmHg (0.7kPa) but adult affinity by only 1.1mmHg (0.15kPa). In early juvenile crabs, the higher magnesium sensitivity of the hemocyanin may be compensated for by the lower oxygen affinity, which has been shown previously to be 50% lower than that of the adult under identical experimental conditions. Furthermore, ontogeny of ionic and osmotic regulation occurs during the development of C. magister, with especially high concentrations of magnesium being found in the hemolymph of early juveniles. Intermediate-stage juveniles (fifth to eighth instars) have hemocyanins with subunit stoichiometries and P50 values approaching those of the adult. These findings are significant because they indicate that modulation of C. magister hemocyanin oxygen-affinity during development incorporates differences in intrinsic affinity and differences in divalent cation sensitivity of the stage-specific hemocyanins.

OSMOREGULATION IN THE LARVAE AND ADULTS OF THE GRAPSID CRABSESARMA RETICULATUMSAY

1. Blood osmotic concentrations in the three zoeal stages, megalops stage, and adults were determined over a wide range of salinities for the estuarine, grapsid crab, Sesarma reticulatum Say.2. Larvae are hyper-osmotic over the salinity range 10 to 35‰ and hyper- or isosmotic at 40‰ throughout zoeal and early megalopa life. Older megalops display increased hyper-osmoticity at 10 and 40‰, compared to earlier zoeal stages.3. Adult specimens of S. reticulatum are hyper-osmotic in the salinity range 5 to 27.5‰, and hypo-osmotic in salinities > 27.5‰. The adult response is apparently attained during early juvenile crab stages.4. The effects of molting on blood osmotic concentrations is discussed. The rapid molting cycles of the larvae do not affect the blood osmotic concentration.5. The general larval trend of hyper-osmoticity over wide ranges of external salinities may serve to increase the density of the larva, helping to promote retention within the estuary. Hyper-osmoticity may also act to provide turgor pressure to insure integrity of the thin larval cuticle.