Juvenile Blue Crabs Research Articles

Temporal development of salt marsh value for nekton and epifauna: utilization of dredged material marshes in Galveston Bay, Texas, USA.

Densities of nekton and other fauna were measured inthree created salt marshes to examine habitatdevelopment rate. All three marshes were located onPelican Spit in Galveston Bay, Texas, USA and werecreated on dredged material from the Gulf IntracoastalWaterway. The youngest marsh was planted on 1-mcenters in July of 1992. At the time sampling wasinitiated in fall 1992, the marshes were 9, 5, andless than 1 year in age; sampling continued in thefall and spring through spring 1994. Animaldensities were measured within the vegetation at twoelevations using an enclosure sampler. In the fall of1992, 4 months following the planting of the 92Marsh,densities of most marsh organisms were lower in thismarsh compared with the older two marshes. Significantly lower densities were observed fordominant crustaceans (including three species of grassshrimps, two species of commercially-important penaeidshrimps, thinstripe hermit crabs Clibanarius vittatus,and juvenile blue crabs Callinectes sapidus), adominant fish (Gobionellus boleosoma), and thedominant mollusc (Littoraria irrorata). By the fallof 1993, however, densities of most nekton specieswere similar among the three created salt marshes. Incontrast, reduced densities of less mobile epifauna(C. vittatusand L. irrorata) persisted in the 92Marshthroughout the 2 years of sampling. The patterns ofnekton utilization exhibited in these marshes suggestthat the 92Marsh reached its maximum habitat supportfunction for these animals in less than 1 year. Comparisons of the older marshes with natural marshesin the bay system, however, suggest that all three ofthese created marshes are functioning at lower levelsthan natural marshes in terms of supporting productionof commercially important fishery species such aspenaeid shrimps and C. sapidus.

Carbon and nitrogen sources for juvenile blue crabs Callinectes sapidus in coastal wetlands

We used a combination of field and laboratory techniques to examine the relative importance of food webs based on marsh detritus, benthic algae, or phytoplankton in supporting growth of the blue crab Callinectes sapidus. We conducted a laboratory experiment to compare the growth of newly metamorphosed juveniles fed natural diets from potential settlement habitats such as marshes. The experimental diets consisted of zooplankton, Uca pugnax and Littoraria irrorata tissue, a mixture of plant detritus and associated meiofauna and detritus only. Crabs fed the zooplankton diet showed the fastest growth and reached a mean dry weight of 32.4 mg, from an initial dry weight of 0.8 mg, during a 3 wk period. Based on the isotopic composition, juvenile crabs obtain carbon and nitrogen from various food sources. For example, crabs fed zooplankton obtained their nutrition from phytoplankton-derived organic matter, consistent with zooplankton feeding on phytoplankton. The mean δ 13 C values for juveniles fed detritus and detritus-plus-meiofauna were considerably lighter (δ 13 C = -19‰), than that of their respective diets (δ 13 C = -16%o), suggesting that crabs were selectively ingesting prey items that obtain their nutrition from an isotopically lighter carbon source like phytoplankton. Conversely, crabs fed U. pugnax or L. irrorata had isotopic ratios (δ 13 C = -16 to -14 ‰) consistent with these species feeding on isotopically heavier marsh grass carbon. Isotopic ratios of crabs collected in the field appeared to corroborate the experiment and suggest that either Spartina alterniflora detritus or benthic algae-based food webs supported juvenile crab growth in marsh environments, whereas phytoplankton-based food webs dominate habitats more closely associated with the main estuary.

A food web analysis of the juvenile blue crab, Callinectes sapidus , using stable isotopes in whole animals and individual amino acids

The stable isotope compositions (C and N) of plants and animals of a marsh dominated by Spartina alterniflora in the Delaware Estuary were determined. The study focused on the juvenile stage of the Atlantic blue crab, Callinectes sapidus, and the importance of marsh-derived diets in supporting growth during this stage. Laboratory growth experiments and field data indicated that early juvenile blue crabs living in the Delaware Bay habitat fed primarily on zooplankton, while marsh-dwelling crabs, which were enriched in 13C relative to bay juveniles, utilized marsh-derived carbon for growth. In laboratory experiments, the degree to which juvenile blue crabs isotopically fractionated dietary nitrogen, as well as the growth rate, depended on the protein quality of the diet. The range of δ13C of amino acids in laboratory-reared crabs and their diets was almost 20‰, similar to the isotopic range of amino acids of other organisms. In laboratory studies, the δ13C of nonessential and essential amino acids in the diet were compared to those in juvenile crabs. Isotopic fractionation at the molecular level depended on diet quality and the crabs' physiological requirements. Comparison of whole-animal isotope data with individual amino acid C isotope measurements of wild juvenile blue crabs from the bay and marsh suggested a different source of total dietary carbon, yet a shared protein component, such as zooplankton.

Ontogenetic changes in habitat use by postlarvae and young juveniles of the blue crab

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 186:227-238 (1999) - doi:10.3354/meps186227 Ontogenetic changes in habitat use by postlarvae and young juveniles of the blue crab Renee A. Pardieck, Robert J. Orth*, Robert J. Diaz, Romuald N. Lipcius The College of William and Mary, School of Marine Science, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062, USA *Addressee for correspondence. E-mail: jjorth@vims.edu ABSTRACT: Changing habitat requirements are evident during the developmental cycles of many species. In this field investigation, we attempted to distinguish between depth (shallow vs deep), habitat structure (seagrass species), and study site as factors influencing the distribution and abundance of postlarvae and juvenile blue crabs Callinectes sapidus in the Chesapeake Bay. Deep (>= 70 cm mean low water [MLW]) and shallow (<= 50 cm MLW) suction samples in monospecific Zostera marina and Ruppia maritima beds were taken in the York River, a tributary of the Chesapeake Bay. Our studies revealed ontogenetic changes in habitat use, which suggested that blue crabs are influenced differently by physical and biological factors even during the earliest life stages. Postlarvae through 3rd instar distributions were not related to seagrass species, but their densities increased with distance upriver (regression, p < 0.004, n = 36 to 38, postlarvae: r2 = 0.173, 1st instars: r2 = 0.308, 2nd-3rd instars: r2 = 0.231). This suggests that the smallest instar distributions are related to larval supply and physical forces, such as currents and winds, which determine water-column transport. In contrast, 4th and greater instars were significantly more abundant in Ruppia than in Zostera (ANOVA, df = 1, p < 0.05), possibly because of the high shoot density of Ruppia beds. Habitat use by 4th and greater instars may be related to seasonal changes in seagrass shoot density. Water depth did not influence the distribution of any crab stage. We suggest that habitat selection and differential mortality among habitats influence larger instar distributions more strongly than they influence the distribution of postlarvae and the earliest instars of C. sapidus. KEY WORDS: Blue crabs · Callinectes sapidus · Ontogenetic changes · Habitat structure · Habitat use · Settlement · Recruitment · Shallow-water refuge · Seagrass beds Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 186. Publication date: September 17, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

A comparative study of two sympatric species within the genus Callinectes: osmoregulation, long-term acclimation to salinity and the effects of salinity on growth and moulting

Juvenile blue crabs, Callinectes sapidus Rathbun, and lesser blue crabs, C. similis Williams, were exposed to a range of salinities for 67 days to determine if salinity exerted species specific differential effects on growth and moulting. Growth was measured in terms of carapace width, wet weight, dry weight and ash-free dry weight. Growth rates of the two species (% increase in dry weight/day) were differentially affected by salinity. C. similis juveniles exposed to 5‰ grew significantly slower than those exposed to 10‰, with those at 30‰ exhibiting intermediate growth rates. Salinity had no effect on growth rates of C. sapidus by any measure of weight or carapace width. C. sapidus grew at faster rates than C. similis at low salinity as measured by wet and dry weight, and at all treatments as measured by carapace width. Growth per moult (wet weight) of C. similis was greater than that of C. sapidus. However, intermoult periods of C. similis were longer and exhibited a more pronounced effect of salinity than those of C. sapidus. Feeding rates and hemolymph osmolalities were measured at the beginning and end of the 67 day exposure period. Weight-adjusted feeding rate of C. sapidus increased significantly after 67 days exposure to low salinity, whereas that of C. similis decreased significantly. Each species exhibited a decline in hemolymph osmolality at low salinities by the end of the exposure period. In order to determine if either species exhibits an ontogenetic shift in ability to regulate hemolymph osmolality, juvenile and adult C. sapidus and C. similis were collected and exposed to a range of salinities for measurement of hemolymph osmolalities. These crabs were collected and exposed separately from those used in the 67 day exposure. Adult C. sapidus maintained higher hemolymph osmolalities than juveniles when exposed to low salinities (≤25‰). Hemolymph osmolalities of adult C. similis exposed to low salinity varied with salinity of collection site. Those from a high salinity site (30‰) exhibited hemolymph osmolalities no different than juveniles when exposed to salinities of 2.5 and 10‰. Those from a lower salinity site (22‰) exhibited greater hemolymph osmolalities than juveniles, osmoregulating at levels insignificantly different from adult C. sapidus. Results of this study indicate that although previously published studies may have overestimated the effects of low salinity on C. similis relative to it`s more euryhaline congener C. sapidus, effects of salinity alone are probably sufficient to limit this species' distribution to waters of 10‰ or greater.

Cannibal–prey dynamics in young juveniles and postlarvae of the blue crab

Although cannibalism can act as a density-dependent regulator of population size in terrestrial systems, little is known of its effects in the marine environment. Herein we investigate the influence of cannibalism upon the early life history stages of the blue crab, Callinectes sapidus Rathbun, emphasizing cannibalism between juveniles and postlarvae (i.e. megalopae) of the same year class. In laboratory mesocosms we examined various factors modulating cannibal–prey dynamics, specifically: (1) the effects of habitat and presence of conspecifics on postlarval metamorphosis rate; (2) the effect of metamorphosis rate on the mortality of postlarvae from both intra- and inter-cohort cannibalism; (3) the effects of habitat and predator density on the functional response of young juvenile blue crab predators to varying densities of postlarval prey, and (4) the effects of prey size and habitat on predation mortality.Inter-cohort cannibalism caused significant mortality in every crab size and habitat type combination, and was lower in grass than sand for all prey smaller than fifth instar. Cannibalism between postlarvae was associated with metamorphosis and was density-dependent in sand, but not present in grass. Metamorphosis rates of postlarvae were inversely density-dependent in sand, but density-independent and higher in grass, indicating that habitat and intra-cohort agonism likely affects postlarval metamorphosis rates. Inter-cohort cannibalism was negatively correlated with metamorphosis rates of postlarvae. The functional response of young juvenile cannibalistic blue crabs differed significantly between sand and grass habitats, and between medium and high predator densities. Juvenile crabs displayed a type II, inversely density-dependent functional response in sand, resulting in very high mortality at low densities of postlarval prey. In grass, the crabs displayed a weak type III, density dependent response, yielding significantly lower mortality at low prey densities. Thus, habitat complexity changes the form of the functional response in cannibal–prey interactions and grass provides a relative habitat refuge from cannibalism. Doubling the number of predators in grass decreased the consumption rates per predator significantly and eliminated the density-dependence, indicating that intraspecific density can qualitatively change the form of the functional response. In the crab size experiment, only prey smaller than fifth instars received a habitat refuge from cannibalism in grass, whereas fifth instars received a relative size refuge in sand.Our results demonstrate that intra-year class cannibalism can cause mortality upon settling megalopae and first juvenile instars that is dependent on prey density. We expect inter-cohort cannibalism to cause local extinction of cohorts settling in sand, especially at low settlement densities, and high mortality at moderate settlement densities in grass. Satiation of predators at high settlement densities in grass suggests that episodic settlement can overwhelm predators locally. Furthermore, density-dependent mutual interference within large cohorts in the grass beds likely reduces their predation efficiency, indicating that aggregation of conspecific predators in grass habitats does not necessary lead to an increase in predation pressure. Finally, a relative size-refuge from inter-cohort cannibalism for fifth instar crabs supports an ontogenetic habitat shift around this crab size, which may be influenced by density-dependent agonistic behavior within cohorts. We suggest that intra-year class cannibalism is a major process regulating both survival and dispersal in megalopae and juvenile blue crabs.

Nursery role of seagrass beds: enhanced growth of juvenile blue crabs ( Callinectes sapidus Rathbun)

The role of submerged aquatic vegetation in supporting enhanced growth of Callinectes sapidus was investigated through field and laboratory experiments. In predator-free enclosures (1 m 2) in the lower York River, Virginia, juvenile blue crabs within Zostera marina (L.) beds grew faster than crabs in enclosures deployed outside the beds. First stage crabs were introduced into vegetated or un vegetated enclosures at either 10 or 50 crabs m −2. After the 6 wk experimental period, both survival and growth were significantly higher in vegetated treatments (growth was estimated by change in “volume,” the product of carapace width, length and depth). In fiberglass mesocosms (2.67 m × 1.33 m × 0.67 m) divided into vegetated and unvegetated halves, juvenile blue crabs grew faster in the vegetation, consistent with field findings. Where differences existed between density treatments in field enclosures, juvenile crabs grew faster in high density than in low density treatments. Aggregate crab growth (summed “volumes” of all recaptured individuals) for vegetated enclosures was greater than for unvegetated enclosures. Potential contribution of cannibalism was sufficient to explain some within-habitat density effects, but was not sufficient to account for the entire aggregate differences, suggesting that food may not be limiting within the seagrass beds even at 50 crabs m −2. These results show that early stage blue crabs receive a substantial growth advantage, in addition to the refuge function shown in other studies, from their association with seagrass beds. This trophic advantage may be experienced by juveniles of other species that utilize vegetated nursery areas and may help explain the ontogenetic habitat shifts that characterize many life histories.

Predation on mud crab megalopae, Panopeus herbstii H. Milne Edwards: effect of habitat complexity, predator species and postlarval densities

Laboratory experiments were conducted to evaluate the magnitude of predation on megalopae of the Atlantic mud crab, Panopeus herbstii H. Milne Edwards, in four simulated habitats (salt marsh bottom, shell bottom, seaweed patch and sand bottom). Predators included juvenile blue crabs ( Callinectes sapidus Rathbun), common killifish ( Fundulus heteroclitus (L.)) and grass shrimp ( Paleomonetes pugio Holthius). Experiments were run at two different densities of megalopae. Two-way analysis of variance revealed significant effects of predator type and substratum. Proportional mortality was lowest on shell bottom, and blue crabs were the least effective predators, regardless of habitat. Additionally, proportional mortality was lower at low megalopal densities than at high megalopal densities. Decreased predation at low megalopal abundance may afford density-dependent modulation of mud crab populations.

Effects of Dimilin on the Blue Crab, Callinectes sapidus, in Shallow-Water Habitats

Diffubenzuron (Dimilin) is used as an insecticide for control of the gypsy moth. Because it is a chitin synthetase inhibitor, it can be a potential threat to other arthropods, including crustaceans. We used a static renewal testing paradigm to determine the LC50 of Dimilin WP-25 to juvenile blue crabs (carapace width: 25–60 mm). Both molt stage of dose frequency effect toxicity. When we exposed the crabs at random molt stages, LC50=3.5 mg l−1. When crabs were exposed on the day of molt, LC50=300 μg l−1. If initial exposure occurred on the day of molt and the crabs were subsequently exposed to repeated doses, LC50=18.5 μg l−1. Effects were age and molt-stage sensitive. We include a review of the data available on Dimilin effects in the marine and estuarine environments.

Population regulation of blue crabs Callinectes sapidus in the northern Gulf of Mexico:postlarval supply

Author(s): Morgan, SG; Zimmer-Faust, RK; Heck, KL; Coen, LD | Abstract: Larval supply and early postsettlement mortality are crucial to the maintenance of most marine populations. Larval supply may be especially important in regulating populations of blue crabs Callinectes sapidus because many larvae could be lost during long migrations between estuaries and offshore waters. We examined the relative contributions of differential transport and habitat preferences of megalopae (postlarvae) to 5 potential nursery habitats in Mobile Bay and Mississippi Sound, Alabama, USA. Settlement in each habitat was determined daily during summer and fall for 2 yr and was related to sea surface temperature, wind stress, tides, current velocity and the lunar and tidal amplitude cycles. In 1990, most megalopae settled when winds blew megalopae onshore and tidal amplitudes were minimal. In 1991, onshore winds were light and infrequent, and megalopae settled even more densely during minimum amplitude tides. Semidaily collections of megalopae at dusk and dawn revealed that most megalopae recruit to estuaries during nocturnal flood tides, which only occur during summer and fall in this diurnal tidal regime. Thus, onshore winds facilitated transport of megalopae into estuaries episodically, but megalopae recruited regularly and abundantly during nocturnal minimum amplitude flood tides even when onshore winds were light or absent. Most megalopae were collected in the lower bay although some megalopae settled at the head of the estuary 50 km away from the baymouth. Megalopae required about 2 d to travel this distance as they became increasingly competent to settle from the plankton. Physiological tolerances may explain why most larvae settled in high salinity waters, but differences in current regimes may best explain differences in settlement among sites in the lower bay. Several types of substrate were transplanted from nursery habitats to a sandy area at the baymouth to determine whether megalopae prefer to settle in seagrass beds, marshes or unvegetated substrate once they arrive at a site. Megalopae strongly preferred to settle on vegetation, but it remains unclear whether or not they discriminate among plant types. Thus, passive delivery and habitat preferences of megalopae both may determine the initial distributions of juvenile blue crabs.

Effects of cadmium on survival, osmoregulatory ability and bioenergetics of juvenile blue crabs Callinectes sapidus at different salinities

Juvenile blue crabs, Callinectes sapidus, from the Barataria estuary (Louisiana, USA) were exposed to a range of dissolved cadmium concentrations at 2.5 and 25%. to determine their response to cadmium. 21-Day LC 50s were an order of magnitude lower at 2.5%. (19μg liter −1) than at 25%. (186μg liter −1). Crabs were exposed to Cd levels of 0, 50 and 100μg liter −1 for measurements of feeding, respiration and excretion rates. Respiration was the primary component (x̄ = 86.2%) of the energy expenditure budget in all cases, relative to excretion. Rates of energy expenditure did not vary with cadmium concentration at either salinity, and were much lower than those of energy consumption and absorption. Scope for growth paralleled energy consumption and absorption. Scope for growth did not vary as a function of cadmium concentration at 25%.; scope for growth at 2.5%. was significantly lower in crabs exposed to 50 and 100μg liter −1 Cd relative to control crabs. Cadmium levels of up to 100μg liter −1 do not stress juvenile blue crabs at 25%.; but at 2.5%., these same levels of Cd result in mortality and sub-lethal stress. This finding is important since nursery grounds for juveniles of this species often lie in low salinity, estuarine waters, and high dissolved cadmium levels have been reported in Louisiana estuaries.

Effects of limb autotomy and tethering on juvenile blue crab survival from cannibalism

Effects of limb autotomy and tethering on juvenile blue crab survival from cannibalism

Detection and avoidance of hypoxic water by juvenile Callinectes sapidus and C. similis

Detection and avoidance of hypoxic water by juvenile blue crabs, Callinectes sapidus Rathbun and C. similis Williams were observed under laboratory conditions. Hypoxia avoidance was quantified within an avoidance chamber using response time (time in minutes before a crab's initial entry into the normoxic side of the avoidance chamber after the introduction of hypoxic water into the chamber), total time (amount of time spent by each crab at the hypoxic end of the chamber during the final 50 min of each 60 min treatment), activity (total distance travelled by a crab during a treatment), percent avoidance and preferred oxygen tension as indices of measurement. The mean preferred oxygen tension for C. sapidus was 112 torr oxygen (range = 98 to 125 torr) and for C. similis was 108 torr oxygen (range = 82 to 121 torr). Both species were able to detect hypoxic water and remain at an optimum oxygen tension. When percent avoidance, response time and total time were used as measures of avoidance, C. similis was found to detect and avoid hypoxic water at 0, 25 and 50 torr oxygen tension but not at any higher levels of hypoxia. C. sapidus, on the other hand, did not initiate any significant avoidance behavior even when the water was completely anoxic. Both species of crabs were more active at the higher oxygen tensions than in the more hypoxic levels. Behavioral responses associated with short-term hypoxic exposure included increased activity with the introduction of hypoxic water into the chamber, frequent movement of the eye-stalk, avoidance of hypoxic water by slowly crawling out of the hypoxic zone, restless and erratic movements, and rapid movements of the crabs' antennae. We conclude that C. similis can better detect and avoid hypoxia than C. sapidus.

Effects of tethering on predatory escape by juvenile blue crabs

Effects of tethering on predatory escape by juvenile blue crabs

Hematodinium perezi infections in adult arid juvenile blue crabs Callinectes sapidus from coastal bays of Maryland and Virginia, USA

In coastal bays of Maryland and Virginia, USA, adult and juvenile blue crabs Callinectes sapidus were severely infected with the parasitic dinoflagellate I-lematodinium perezi. Dinoflagellates were observed in the hemocoel of all infected crabs; associated histopathological changes were evident in some tissues. Dinoflagellates could be observed with an inverted microscope through the 5th pleopod of heavily infected juvenile crabs (5 to 29 mm) without invasion. This note documents a high prevalence of H. perezj infections in juvenile blue crabs from coastal bays in Maryland and Virginia. The seasonal infection prevalence cycle reported by previous authors is consistent with observations made during this study.

Digestive carbohydrases of the blue crab Callinectes sapidus (Rathbun): implications in utilization of plant-derived detritus as a trophic resource

Scientific activities of alpha- and beta-galactosidases and alpha-and beta-glucosidases in crude homogenates of the digestive gland and gut tissues of Callinectes sapidus (Rathbun) were measured. Maximal specific activity occurred at pH 4 when homogenates were tested against four artificial substrata. Activity occurred at pH 4–8. Rates of hydrolysis were consistently high (8.8–10.6 nmol substratum hydrolyzed·min −1·mg −1 total protein) for artificial substrata ( p- nitrophenyl-alpha- d-galactopyranoside , p-nitrophenyl-beta- d-galactopyranoside, p-nitrophenyl-alpha- d-glucopyranoside, p-nitrophenyl-beta- d-glucopyranoside ). When natural substrata (melibiose, lactose, sucrose, maltose and cellobiose) were tested with digestive gland homogenates, appreciable enzymatic activity (specific activity 0.041) was detected for maltose. This disaccharide is a component of starch (alpha-1,4-glucan), one of the primary complex storage sugars in vascular plant tissues. These data indicate that C. sapidus may have the physiological capacity to utilize vascular plant material (e.g., starch), a common component of detritus within coastal estuaries and bays. Although detritus has been observed in the guts of both juvenile and adult C. sapidus, the capacity to use detritus may be particularly important to juvenile blue crabs which use vegetated areas as refuge and may have limited abilities to capture and ingest live animal prey.

Tagging Juvenile Blue Crabs, Callinectes sapidus, with Microwire Tags: Retention, Survival, and Growth through Multiple Molts

ABSTRACT During an 80-day laboratory experiment we analyzed the retention of coded microwire tags implanted into 56 juvenile blue crabs, Callinectes sapidus, and compared their growth and mortality rates with those of an equal number of untagged crabs. The initial size of the crabs ranged from 29-67-mm carapace width as measured to the base of the lateral spines; 83% of the crabs molted at least twice and more than one-third molted 3 times. Though some confounding cannibalism occurred, we concluded that 96-98% of the tagged crabs retained the tag through the first molt, with no known losses through subsequent molts. No differences were evident in mortality between tagged and untagged crabs. Growth, expressed as increase in carapace width, and the intermolt period, were not different between the 2 groups. Microwire tags provide an effective long-term marking system for juvenile and adult blue crabs because (1) retention rate is high, (2) tags have no effect on growth or survival, and (3) large numbers of crabs can be batch-marked efficiently under field conditions.

TAGGING JUVENILE BLUE CRABS, CALLINECTES SAPIDUS, WITH MICROWIRE TAGS: RETENTION, SURVIVAL, AND GROWTH THROUGH MULTIPLE MOLTS

ABSTRACT During an 80-day laboratory experiment we analyzed the retention of coded microwire tags implanted into 56 juvenile blue crabs, Callinectes sapidus, and compared their growth and mortality rates with those of an equal number of untagged crabs. The initial size of the crabs ranged from 29-67-mm carapace width as measured to the base of the lateral spines; 83% of the crabs molted at least twice and more than one-third molted 3 times. Though some confounding cannibalism occurred, we concluded that 96-98% of the tagged crabs retained the tag through the first molt, with no known losses through subsequent molts. No differences were evident in mortality between tagged and untagged crabs. Growth, expressed as increase in carapace width, and the intermolt period, were not different between the 2 groups. Microwire tags provide an effective long-term marking system for juvenile and adult blue crabs because (1) retention rate is high, (2) tags have no effect on growth or survival, and (3) large numbers of crabs can be batch-marked efficiently under field conditions.

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.

A chemical defense in Crustacea?

The free-living fourth crab stage of the inquiline pea crab Pinnotheres ostreum (Say) lacks typical antipredator defenses frequently found in planktonic and pelagic crustaceans, such as spines, cryptic coloration and/or diel vertical migration. However, in laboratory feeding trials, four species of estuarine fishes consumed many more first stage juvenile blue crabs Callinectes sapidus Rathbun than P. ostreum even though the prey were similar in size. Observations of predator-prey interactions in the laboratory revealed that, once captured, pea crabs were frequently rejected by mummichogs Fundulus heteroditus (L.). Experiments in which mummichogs were offered six prey items (live, heat-killed and cold-killed crabs of each species above) revealed that live pea crabs were rejected >90% of the time, killed crabs were rejected approximately one-third of the time, and blue crabs, live or dead, were always consumed. These data indicate that the unpalatability of pea crabs is not a consequence of akenisis and suggest an active component to the defense. We argue, on the basis of morphological and behavioral evidence, that mechanical irritation is not the source of pea crab unpalatability. The accumulated evidence strongly suggests that this stage of P. ostreum employs a noxious antipredator substance. The origin and composition of this substance and the prevalence of this defensive system (in other stages and species) remain to be investigated.