Adult Crabs Research Articles

Is the Hatching Clock of Fiddler Crab Larvae (Uca thayeri) Phenotypically Plastic?

Larval release by adult fiddler crabs occurs during the ebbing tides, but its timing relative to the day-night and tidal amplitude cycles depends upon tidal form, e.g., shows phenotypical plasticity. Crabs (Uca thayeri) from Florida's East Coast are exposed to semidiurnal tides and release their larvae at night, whereas crabs from Florida's West Coast are exposed to mixed tides and release their larvae during the afternoon. The purpose of this study was to determine whether the larvae would hatch at other times, specifically those dictated by females from a different coast. To find out, clusters of eggs at similar stages of development, 24-72 h in advance of release, were reciprocally transferred between females from each location. Release of both the transferred and maternal larvae occurred synchronously, at the time dictated by the female's tidal regime. These results indicate that fiddler crab embryos can either advance or delay their hatching clock to match the temporal regime dictated by a brooding female.

Current Trends in Hatchery Techniques and Stock Enhancement for Chinese Mitten Crab, Eriocheir japonica sinensis

Aquaculture of the Native Chinese mitten crab (Eriocheir japonica sinensis) has developed quickly in China to reach 570,000 mt in 2005, which exceeds all other worldwide production of crab from aquaculture and capture fisheries combined. We review the development of aquaculture techniques and stock enhancement that account for this successful production of Chinese mitten crabs. There are two models for mitten crab larviculture. In the first model, intensive larviculture occurs in indoor concrete ponds, with conditions of temperature control, aeration, and a plentiful supply of food (algae, egg yolk, rotifers, Artemia nauplii). In the second model, outdoor extensive larviculture occurs in earth ponds with no temperature control. Grow-out of the mitten crab focuses on release of cultured seed crabs into net enclosures in lakes. As a rule, the production of adult crabs is l50–450 kg/ha and as high as 975 kg/ha. Recently, Chinese researchers noticed deterioration of water quality and the exhaustion of natural food resources in many lakes due to over-stocking crabs, and many studies are now beginning to investigate a model for the sustainable aquaculture of Chinese mitten crab in these systems.

Development and functional morphology of the foreguts of larvae and postlarvae of three crustacean decapods

The development of the foregut structure and the digestive function of the decapods Litopenaeus vannamei, Sesarma rectum and Callichirus major larvae and post larvae were examined. The protozoeal foregut of L. vannamei is simple, lacking a cardiopyloric valve and bearing a rudimentary filter press. In mysis, the filter press is more developed. In the juvenile stage, grooves and a small lateral tooth arise. In S. rectum, the foregut has a functional cardiopyloric valve and a filter press. The megalopal and juvenile stages of this species have a gastric mill similar to those in adult crabs. In C. major, the foregut of the zoeae is specialized, with the appearance of some rigid structures, but no gastric mill was found. Calcified structures are observed in the megalopae and they become more developed in the juvenile stage. The results support suppositions, previously reported in other studies, that feeding behavior of each larval and postlarval stage is directly related to the morphological characteristics of the foreguts.

Stimulation of molting in megalopae of the Asian shore crab Hemigrapsus sanguineus: physical and chemical cues

The Asian shore crab Hemigrapsus sanguineus is a recent invader and abundant mem- ber of rocky intertidal coastlines of the eastern USA. Understanding factors influencing the settlement and metamorphosis of its larvae is important for predict- ing colonization of the species. For many species of brachyuran crustaceans studied to date, chemical cues in the natural environment stimulate molting of megalopae, the final larval stage, to the first crab stage, whereas physical cues alone have little or no effect. However, physical structure can influence the abundance of juvenile and adult crabs. In this study, I tested whether physical/structural and chemical cues stimulate molting of H. sanguineus in the labo- ratory. Molting rates of megalopae exposed to exu- dates from adult crabs (H. sanguineus and the green crab Carcinus maenas) and different physical cues varying in structural complexity (rocks with or with- out biofilms, silt particles, nylon mesh used in aqua- culture) were monitored. Megalopae molted sooner to the first crab stage in the presence of exudates of both crab species, rocks with and without biofilms, and all nylon meshes tested regardless of the size of mesh openings. By separating the effects of chem- ical and physical cues, this study shows both to be important during settlement and metamorphosis of Asian shore crab megalopae, potentially contributing to its successful colonization of North Atlantic coast- lines.

Identification and developmental expression of mRNAs encoding putative insect cuticle hardening hormone, bursicon in the green shore crab Carcinus maenas

Bursicon is the ultimate hormone in insect ecdysis, which is involved in cuticle hardening. Here we show that mRNAs encoding the heterodimeric cystine knot protein bursicon (Burs α, β), are present in crustaceans, suggesting ubiquity of this hormone in arthropods. We firstly report the cloning, sequencing of mRNAs encoding subunits from the water flea, Daphnia arenata and the CNS of the crab, Carcinus maenas, in comparison with insect bursicon subunits. Expression patterns of α and β burs mRNAs were examined by in-situ hybridisation (ISH) and quantitative RT-PCR. In the thoracic ganglion, burs α and β mRNAs were completely colocalised in neurones expressing crustacean cardioactive peptide (CCAP). However, in the brain and eyestalk, bursicon transcripts were never observed, despite a complex expression pattern of CCAP interneurones. Patterns of expression of burs α and β mRNAs were constitutive during the moult cycle of adult crabs, in stark contrast to the situation in insects. Whilst copy numbers of burs β transcripts closely matched those of CCAP, those of burs α mRNA were around 3-fold higher than burs β. This pattern was apparent during embryogenesis, where bursicon transcripts were first observed at around 50% development—the same time as first expression of CCAP mRNA. Transcript ratios ( burs α: β) increased during development. Our studies have shown, for the first time, that bursicon mRNAs are expressed in identified neurones in the nervous system of crustaceans. These findings will now promote further investigation into the functions of bursicon during the moult cycle and development of crustaceans.

RECRUITMENT LIMITATION IN DUNGENESS CRAB POPULATIONS IS DRIVEN BY VARIATION IN ATMOSPHERIC FORCING

Ecologists have long debated the relative importance of biotic interactions vs. abiotic forces on the population dynamics of both marine and terrestrial organisms. Investigation of stock size in Dungeness crab (Cancer magister) is a classic example of this debate. We first tested the hypothesis that adult population size was set by larval success. We found that during a five-year sampling period, adult crab population size from Oregon through central California, USA, as measured by the commercial catch, varied directly with the number of terminal-stage larvae (megalopae) returning to Coos Bay, Oregon, four years earlier; adult population size was largely determined (> 90% of the variation) by success during the larval stage. We then tested whether biotic interactions or abiotic forces caused the variation in larval success. Most of the variation (> 90%) in the number of returning megalopae is explained by the timing of the spring transition, a seasonal shift in atmospheric forcing that drives ocean currents along the west coast of the United States. Early spring transitions lead to larger numbers of returning Dungeness megalopae, while in four other crab taxa, species with very different life history characteristics, early-spring transitions lead to lower numbers of returning megalopae. During the past roughly 30 years, the size of the commercial catch of Dungeness crab is significantly and negatively correlated with the date of the spring transition throughout the California Current system. Long-term variation in the date of the spring transition may explain a major crash in the Dungeness crab fishery in central California, which began in the late 1950s. The data suggest that Dungeness crab population size is determined by variation in larval success and that a significant portion of this variation is due to the timing of the spring transition, a large-scale climatic forcer.

Distribution of Larval Dungeness crabs in Glacier Bay, Southeastern Alaska

Adult Dungeness crabs are restricted primarily in the lower part of Glacier Bay, Alaska, but the interaction of larval dispersion and adult distribution is unknown. To understand the larval occurrence in the upper part of Glacier Bay, Dungeness crab larvae, sea surface temperature (SST), and sea surface salinity (SSS) were collected at 16 near-shore and 12 mid-channel stations in Glacier Bay, southeastern Alaska during six sampling periods from March through August 2000. Each station was visited from one to five times during the entire sampling period. Geographic Information System (GIS) was used to contour SST and SSS distribution in Glacier Bay. Seven to 27 stations were visited during each sampling period. Most larvae (85% were zoeae I) occurred during May 31 to June 14, 2000. Larval density varied from none to between stations. A few later stage larvae occurred during later sampling periods. Overall, no relationship between larval densities, and SSS, and SST existed. Larvae occurring in the upper bay were probably transported by tidal currents from the lower bay; adult Dungeness crabs in Glacier Bay have a relatively high density near the mouth of the bay but decrease sharply around 40km north of the mouth. The lack of adult crabs in the upper 60km of the bay may be related to lower salinity, resulting in sharp haloclines, or colder temperatures which are not conducive to survival or growth of either larvae or adults.

Biotic resistance to green crab, Carcinus maenas, in California bays

The distribution of the introduced European green crab, Carcinus maenas, was investigated in the central California embayments of Bodega Bay Harbor (BBH), Tomales Bay, and Bolinas Lagoon using baited traps and snorkel surveys. Adult green crabs were very spatially limited in all three embayments and occurred primarily in warm, shallow areas that lacked large native Cancer spp. crabs. The green crabs that were found in closest proximity to populations of Cancer spp. exhibited high levels of limb damage and loss; damage was strongly correlated with low ratios of intertidal area: edge, indicative of narrow areas of intertidal that are more easily accessed by large Cancer spp. moving up to forage during periods of tidal inundation. Up to 70% of the green crabs tethered in areas of BBH that are utilized by Cancer spp. experienced limb loss, while those tethered in the marsh, where there are no Cancer spp., were undamaged. The results suggest that the potential distribution of green crabs in the northeastern Pacific will be far less than has been predicted, and that their impacts may be largely attenuated through predation by and competition with native crab species.

Distribution, abundance, and feeding of a disjunct population of lady crab in the southern Gulf of St. Lawrence, Canada

The lady crab (Ovalipes ocellatus) is one of the most common native species of swimming crab (Portunidae) of the Atlantic Coast of North America but most populations occur south of Cape Cod, Massachusetts. There is a disjunct population in Northumberland Strait (southern Gulf of St Lawrence), which was the focus of this study. Adult lady crabs were collected by trawling in water >4 m deep from May to October 1999 to 2005 to determine abundance, distribution, and diet. Lady crab occurred only in a small area (about 2,500 km2) in the central part of Northumberland Strait where bottom water temperature was >18°C during summer, and the substrate was mainly sand or sandy gravel. Male lady crab attained a maximum carapace width (CW) of 112 mm compared to 92 mm CW for females. The summer and autumn diet consisted mainly of infauna. The principal prey (each >5% of diet by weight) were: small bivalve molluscs (primarily Atlantic razor clam Siliqua costata and Macoma sp.; 43%), small rock crab (Cancer irroratus; 13%), polychaetes (11%), fish remains (9%), and small lady crab (9%). All stomachs collected during May (water temperature ≤10°C) were empty. There was little evidence of any difference in feeding intensity between 0700 h and 1900 h.

Breeding Season and Molt Cycle of the Fiddler Crab Uca rapax (Brachyura, Ocypodidae) in a Subtropical Estuary, Brazil, South America

This is the first account of the breeding season and molt cycle of Uca rapax on the Brazilian coast during a one-year period (April 2001-March 2002). At 2 sites, the ltamambuca and Ubatumirim mangrove forests, 2 collectors captured crabs once a month at low tide far 15 minutes by the catch-per-unit-effort procedure, digging into the sediment and removing the crabs. The gonad-development stages of crabs of both sexes were determined by direct observation, and the molt stage was estimated from the hardness of the tegument. Crabs with developed gonads were found mostly in warmer seasons, with ovigerous females occurring mainly in summer and autumn. The reproductive cycle is seasonal. Recently molted individuals were collected in relatively higher numbers for juveniles than for adult crabs.

Histological studies of the ovaries of two tropical portunid crabs, Portunus pelagicus (L.) and Scylla serrata (F.)

Summary The histology of ovaries of adult crabs, Portunus pelagicus and Scylla serrata, were studied and compared using light microscopy. In general, the histological features, the steps of oogenetic cells and the stages of ovarian development appear similar in the two species. The ovary possesses three to four lobes, and each lobe is divided into lobules. A thick capsule surrounds the ovarian lobes and its extensions cover and separate each lobule. The lobule is composed of follicles, each containing a cluster of oocytes at different steps within the confinement of connective tissue trabeculae. There are five steps of oogenetic cells, i.e., oogonia (Og) and four steps of oocytes (Oc1, Oc2, Oc3, Oc4). Og are the earliest germ cells and are located in the center of each lobule. They divide mitotically and move into each follicle where they become Oc1. Oc2 rapidly increase in size, exhibits a clockface nucleus and basophilic cytoplasm. Oc2 possess larger nuclei, non-condensed chromatin, a prominent nucleolus and are intensely basophilic. Oc1 and Oc2 are previtellogenic and do not contain yolk granules. Oc3 are larger in size, with a slate grey cytoplasm and contain a few yolk granules. Oc4 are fully mature oocytes, in possession of completely dispersed chromatin in the nucleus and a very prominent nucleolus. The cytoplasm is eosinophilic due to the presence of numerous yolk granules. The Periodic acid-Schiff reaction and Sudan black stains suggest that Oc4 also contain large amounts of glycoprotein and lipid in their cytoplasm. The ovarian cycle is divided into four stages, i.e., spawn-spent (stage I), proliferative (stage II), pre-mature (stage III) and mature (stage IV). Each stage contains different cellular associations: stage I is characterized by the reconfiguration of connective tissue scaffold, plus remnants of unspawned oocytes; stage II contains numerous dividing Og and a few Oc1; stage III contains primarily Oc1, Oc2, Oc3; and stage IV contains mostly Oc4 and small clusters of Og.

Crescimento somático do caranguejo-uçá Ucides cordatus (Crustacea, Brachyura, Ocypodidae) em laboratório

Em crustáceos, o aumento de tamanho ocorre imediatamente após a muda, quando o animal está com a carapaça mole. O crescimento de Ucides cordatus (Linnaeus, 1763) foi observado em laboratório, através da ocorrência de muda, incremento de muda e período de intermuda. O estudo foi realizado durante os meses de outubro/2000 a março/2002 e um total de 91 caranguejos (15 machos, 33 fêmeas e 43 juvenis) foi coletado no manguezal de Itacuruçá - Coroa Grande, Estado do Rio de Janeiro, Brasil. Os animais foram mantidos em dois tipos de tanques: os caranguejos adultos foram mantidos em tanques de 1000 l, durante 18 meses, enquanto os juvenis foram mantidos em tanques plásticos de 20 l de capacidade durante oito meses. Os animais foram mantidos em sistema aberto de circulação de água do mar e alimentados duas vezes por semana com folhas de Rhizophora mangle (L.) (Rhizophoraceae) e Laguncularia racemosa (Gaertn.) (Combretaceae). A largura da carapaça variou entre 50,1 a 70,0 mm nos machos, 40,2 a 80,0 mm entre as fêmeas e 1,1 a 40,1 mm entre os juvenis. A sobrevivência dos espécimes estudados foi de 46,7% entre os machos, 39,4% entre as fêmeas após dezoito meses e 67,4% entre os juvenis, após oito meses de observação. Os machos e as fêmeas realizaram três mudas durante o experimento, enquanto os juvenis realizaram até duas mudas. As mudas ocorreram entre agosto e abril, mostrando maior freqüência durante a primavera e o verão. O incremento na largura da carapaça diminuiu com o tamanho do indivíduo, com média de 2,21 ± 1,39% para os machos, 1,28 ± 0,84% para as fêmeas e 2,89 ± 2,13% para os juvenis. A relação entre o incremento percentual e a largura da carapaça pode ser expresso pela equação IM = -0.0707LC + 4.645 (r²= 0.40). O período de intermuda foi de 191 ± 140 dias entre os machos, 216 ± 76,2 dias entre as fêmeas e 54 ± 1,41 dia entre os juvenis. O incremento de muda foi estatisticamente significativo (p<0,05) quando comparados machos e fêmeas, e adultos e juvenis.

Metamorphosis of a sesarmid river crab, Armases roberti : stimulation by adult odours versus inhibition by salinity stress

In an experimental laboratory study with the megalopa stage of Armases roberti, a freshwater-inhabiting species of crab from the Caribbean region, we evaluated the combined, potentially antagonistic effects of odours from conspecific adults and of stepwise salinity reductions (simulating upstream migration, reaching within 1 week conditions of 2‰ or freshwater). Neither of these treatments affected the rate of survival, but the duration of development to metamorphosis was significantly (by about 25%) shortened, when odours from conspecific adult crabs were present, regardless of the salinity conditions. Our results indicate that the metamorphosis-stimulating effect of chemical cues from an adult population of A. roberti is far stronger than the potentially retarding effect of increasing hypoosmotic stress. This suggests that the final phase of larval development, including the processes of settlement and metamorphosis, occurs in this species in freshwater habitats, where conspecific populations live.

Assessment of methods to estimate abundance and population structure of the spider crab Maja brachydactyla in soft substrates

A good sampling method for collecting data on the abundance, spatial and demographic structure of a marine population would enable a researcher to: (1) obtain unbiased abundance estimates (absolute or relative), (2) accurately represent the spatial and demographic structure, and (3) obtain enough samples at a reasonable cost to analyze size-frequency distributions and other demographic parameters. Different methods for estimating the abundance and population structure of the spider crab Maja brachydactyla in the Ría de A Coruña (NW Spain) were assessed. Sampling was carried out with different procedures—tangle-nets, beam trawls, diving and traps. The use of tangle-nets caused serious problems as both the specimens sampled and the gear itself were damaged. These nets also provided less spatial resolution of the small-scale distribution of organisms. Sampling by means of diving in this area was unviable due to limited visibility, high turbulence and strong currents. The catches obtained during monthly samplings using traps and beam trawls in the same area during one yearly cycle (June 1998–June 1999) were compared. Trap catches were much higher, in terms of actual quantities and catch per unit of effort (CPUE), although the seasonal pattern was similar with both methods. A significant linear relationship was found between the trap and beam trawl catches: CPUE traps = 3.34 + 3.987CPUE trawl ( R 2 = 0.54, p = 0.02). The beam trawl catches contained a higher proportion of very small crabs (specimens < 60 mm carapace length, CL, comprised 23% of the total catch taken by beam trawling, compared to only 0.7% when traps were used), and very few large specimens (CL > 130 mm), especially adults. The catchability of adult crabs using the beam trawl accounted for only 12% as compared to that of traps. The fishing area of a trap was estimated to be 1440 m 2 for juveniles (CL = 70 mm). A comparison of these two sampling methods in terms of cost-effective sampling (in days) showed that the beam trawl is almost four times as expensive as the traps, for the same number of organisms collected.

Identification and developmental expression of mRNAs encoding crustacean cardioactive peptide (CCAP) in decapod crustaceans

Full-length cDNAs encoding crustacean cardioactive peptide (CCAP) were isolated from several decapod (brachyuran and astacuran) crustaceans: the blue crab Callinectes sapidus, green shore crab Carcinus maenas, European lobster Homarus gamarus and calico crayfish Orconectes immunis. The cDNAs encode open reading frames of 143 (brachyurans) and 139-140 (astacurans) amino acids. Apart from the predicted signal peptides (30-32 amino acids), the conceptually translated precursor codes for a single copy of CCAP and four other peptides that are extremely similar in terms of amino acid sequence within these species, but which clearly show divergence into brachyuran and astacuran groups. Expression patterns of CCAP mRNA and peptide were determined during embryonic development in Carcinus using quantitative RT-PCR and immunohistochemistry with whole-mount confocal microscopy, and showed that significant mRNA expression (at 50% embryonic development) preceded detectable levels of CCAP in the developing central nervous system (CNS; at 70% development). Subsequent CCAP gene expression dramatically increased during the late stages of embryogenesis (80-100%), coincident with developing immunopositive structures. In adult crabs, CCAP gene expression was detected exclusively in the eyestalk, brain and in particular the thoracic ganglia, in accord with the predominance of CCAP-containing cells in this tissue. Measurement of expression patterns of CCAP mRNA in Carcinus and Callinectes thoracic ganglia throughout the moult cycle revealed only modest changes, indicating that previously observed increases in CCAP peptide levels during premoult were not transcriptionally coupled. Severe hypoxic conditions resulted in rapid downregulation of CCAP transcription in the eyestalk, but not the thoracic ganglia in Callinectes, and thermal challenge did not change CCAP mRNA levels. These results offer the first tantalising glimpses of involvement of CCAP in environmental adaptation to extreme, yet biologically relevant stressors, and perhaps suggest that the CCAP-containing neurones in the eyestalk might be involved in adaptation to environmental stressors.

A reaction-diffusion analysis of energetics in large muscle fibers secondarily evolved for aerobic locomotor function

The muscles that power swimming in the blue crab, Callinectes sapidus, grow hypertrophically, such that in juvenile crabs the cell diameters are <60 microm, whereas fibers of the adult crabs often exceed 600 microm. Thus, as these animals grow, their muscle fibers greatly exceed the surface area to volume ratio and intracellular diffusion distance limits of most cells. Previous studies have shown that arginine phosphate (AP) recovery in the anaerobic (light) fibers, which demonstrate a fiber size dependence on anaerobic processes following contraction, is too slow to be restricted by intracellular metabolite diffusive flux, in spite of the fiber's large size. By contrast, the aerobic (dark) fibers have evolved an intricate network of intracellular subdivisions that maintain an effectively small ;metabolic diameter' throughout development. In the present study, we examined the impact of intracellular metabolite diffusive flux on the rate of post-contractile AP resynthesis in the dark muscle, which has a much higher aerobic capacity than the light muscle. AP recovery was measured for 60 min in adults and 15 min in juveniles following burst contractile activity in dark fibers, and a mathematical reaction-diffusion model was used to test whether the observed aerobic rates of AP resynthesis were fast enough to be limited by intracellular metabolite diffusion. Despite the short diffusion distances and high mitochondrial density, the AP recovery rates were relatively slow and we found no evidence of diffusion limitation. However, during simulation of steady-state contraction, which is an activity more typical of the dark fibers, there were substantial intracellular metabolite gradients, indicative of diffusion limitation. This suggests that high ATP turnover rates may lead to diffusion limitation in muscle even when diffusion distances are short, as in the subdivided dark fibers.

Infestation by Carcinonemertes divae (Nemertea: Carcinonemertidae) in Libinia spinosa (Decapoda: Pisidae) from São Sebastião Island, SP, Brazil

Nemerteans of the genus Carcinonemertes are obligate symbiotic egg predators of many decapod crustaceans. This study presents the prevalence and intensity of infestation by Carcinonemertes divae in the crab, Libinia spinosa, from São Sebastião Island, São Paulo State, Brazil. Overall prevalence of infestation was 69.2% and mean intensity of infestation was 20.8±3.4 (range: 1–148). Significant differences in prevalence and mean intensity of infestation were observed between males and females and between ovigerous and non‐ovigerous adult females of Libinia spinosa. Prevalence and mean intensity of infestation did not differ significantly between juvenile and adult crabs or among ovigerous crabs with eggs in different stages of embryogenesis. Carapace width of male crabs was negatively correlated with prevalence of infestation by Carcinonemertes divae. Sexual contact appeared to be a source of transmission of worms between males and females of Libinia spinosa.

HOW TRANSIENT PATCHES AFFECT POPULATION DYNAMICS: THE CASE OF HYPOXIA AND BLUE CRABS

Transient low-oxygen patches may have important consequences for the population dynamics of estuarine species. We investigated whether these transient hypoxic patches altered population dynamics of the commercially important blue crab (Callinectes sapidus) and assessed two alternative hypotheses for the causal mechanism. One hypothesis is that temporary reductions in habitat due to hypoxia increase cannibalism. The second hypothesis is that crab population dynamics result from food limitation caused by hypoxia- induced mortality of the benthos. We developed a spatially explicit individual-based model of blue crabs in a hierarchical framework to connect the autoecology of crabs with the spatial and temporal dynamics of their physical and biological environments. Three primary scenarios were run to examine the interactive effects of (1) hypoxic extent vs. static and transient patches, (2) hypoxic extent vs. prey abundance, and (3) hypoxic extent vs. cannibalism potential. Static patches resulted in populations limited by egg production and recruitment whereas transient patches led to populations limited by the effects of cannibalism and patch interactions. Crab survivorship was greatest for simulations with the largest hypoxic patches which also had the lowest prey abundance and lowest crab densities. In these simulations, nearly all crab mortality was accounted for by aggression, not starvation. In addition, increased prey abundance had little influence on crab abundance and dynamics, and massive reductions in prey abundance (.50%) were necessary to decrease crab abundance, survival, and egg production. Our analyses suggest that cannibalism coupled with decreased egg production determined key aspects of crab demography. Specifically, decreased cannibalism potential resulted in a food-limited crab population with long development times and high adult crab densities whereas increased cannibalism potential led to low adult crab densities with higher individual egg production rates. Our analyses identified several key knowledge gaps, including the nature of crab-crab cannibalism and the role of refuges from predation. Several experiments are suggested to test model predictions and to improve understanding of ecosystem-population linkages for this estuarine species.

Effects of reduced salinities on metamorphosis of a freshwater-tolerant sesarmid crab, Armases roberti: Is upstream migration in the megalopa stage constrained by increasing osmotic stress?

Numerous species of estuarine and freshwater-tolerant crabs show an “export strategy”, i.e. an early larval downstream transport towards coastal marine waters, later zoeal development at higher salinities, and a return of the last larval stage, the megalopa, into estuaries or rivers. The speed and extent of the upstream migration of the megalopa through strong salinity gradients may be constrained by increasing hypo-osmotic stress. In an experimental laboratory study with Armases roberti, a freshwater-inhabiting sesarmid crab from the Caribbean region, we studied in the megalopa stage (after zoeal rearing at 25‰) the tolerance of reduced salinities. In the first experiment, the larvae were exposed directly to various constant salinities (1–25‰). For the second experiment, they were transferred stepwise to strongly diluted media (within 6 days from 25‰ to ≤ 3‰), simulating differential scenarios of upstream migration into brackish or freshwater habitats. When postmoult megalopae were exposed directly to salinities ≤ 3‰, they all died within 24 h. A slightly higher salt concentration (5‰), however, allowed for considerable survival (46%) through metamorphosis to the first juvenile crab stage. In treatments with continuous exposure to 10–15‰, as well as in a control group (25‰), survival to metamorphosis was significantly higher (83–96%), and the average duration of development was shorter compared to 5‰ (12–13 vs. 16 days). In the second experiment, with stepwise salinity reductions, gradual acclimation to decreasing osmotic pressures permitted a successful development to metamorphosis at ≤ 3‰ and even in freshwater (< 0.2‰). This strong physiological adaptability enables the megalopa of A. roberti to cross during its upstream migration, within a short time (6 days), strong osmotic gradients, so that metamorphosis is possible also in freshwater habitats where the conspecific adult crabs live. The speed of migration appears to be limited by physiological constraints related to changes in the capability for osmoregulation occurring during the course of the moulting cycle.

Adult fiddler crabs Uca pugnax (Smith) enhance sediment-associated cues for molting of conspecific megalopae

Adult-associated chemical cues can stimulate settlement and metamorphosis of invertebrate larvae into habitats with an enhanced likelihood of juvenile and adult survival. For example, sediments from adult fiddler crab habitat stimulate fiddler crab megalopae to metamorphose (molt) sooner than sediments without adult cues. A similar stimulation of molting occurs after exposure to waterborne chemical cues from adult habitats and to exudates and extracts of adult crabs. We tested whether sediments from habitats without adult Uca pugnax (Smith), which do not stimulate molting of their megalopae, could become stimulatory through brief exposure to adult crabs. Sediments were collected from tidal flats at several distances (∼ 1 m, ∼ 50 m, and ∼ 5.4 km) from adult habitats, and incubated for 24 h with or without adult crabs. Molting rates of laboratory-reared megalopae exposed for 48 h to adult-conditioned sediments were compared to those for untreated controls. Sediments collected in or within 1 m of adult habitat elicited the highest molting rates, and natural sediments from 50 m and 5.4 km had little or no effect on molting. However, incubating sediments collected away from adult habitat with adult crabs produced a higher molting response, and the magnitude of the enhancement increased with distance from adult habitat. Results suggest that the chemical cues that adult crabs release are retained by sediments and consequently stimulate molting of megalopae, regardless of the nature of the sediments themselves. Lack of chemical cues may retard colonization of newly created or heavily disturbed habitats that are otherwise suitable settlement and adult habitat.