Population characteristics of the deep-sea lobsters Polycheles typhlops and Stereomastis sculpta (Decapoda: Polychelidae) in a bathyal mud community of the Mediterranean Sea

Distribution and abundance as a function of depth, and population parameters such as sex and size structure of the population and aspects of reproductive biology have been studied in the deep-sea polychelid lobsters Polycheles typhlops and Stereomastis sculpta. Samples were taken by otter trawl in the Balearic Sea, a deepsea basin in the northwestern Mediterranean Sea from 1985 to 1989. In both species the largest size classes were dominated by females. In S. sculpta, male and female sizes were very similar. In P. typhlops, ovigerous females and those with external spermatophores were 23 and 25 mm in carapace length (CL), respectively; males with external spermatophores were 17 mm in CL. In S. sculpta, ovigerous females and those with external spermatophores were 24 and 19 mm CL, respectively, and males with external spermatophores 19 mm CL. Highest densities of P. typhlops occurred along the middle slope at depths between 500 and 1000 m. Only small-sized individuals were found at the deepest depths sampled; some recruitment must therefore occur in waters much deeper than those usually inhabited by the adult population. The sex-ratio was 1:1 in most samples, but in some of the shallowest samples females predominated. The depth distribution range of S. sculpta was 981 to 2253 m: densities clearly increased with increasing depth. There were no apparent variations in size distribution as a function of depth. Since very few adult males and females were captured, the population in the survey area would seem to be comprised mainly of juveniles. As a function of depth, females were numerically dominant only in some of the shallowest samples taken in the distribution range of this species. There is strict habitat partitioning between the two species. In both species, the variation in the sex-ratio as a function of depth suggests differential migration between the sexes, probably related to egg incubation and hatching.     

Depth as a factor in abundance and size of juvenile penaeid shrimps in the absence of estuaries and marshes

Triweekly trawling around low tide during daylight at 1, 2, 3, and 4 m depths along north Kuwait Bay's extensive intertidal mudflats from 1 April 1985 through 9 December 1986 showed the catchability ofMetapenaeus affinis (H. Milne-Edwards, 1837),M. stebbingi Nobili, 1904,Parapenaeopsis stylifera (H. Milne-Edwards, 1837), andPenaeus semisulcatus De Haan, 1844 differed significantly with depth and season. All species were significantly more catchable in 1 and 2 m depths than in 3 and 4 m depths.M. affinis showed the clearest and most consistent relationship with depth: catchability in 1 m depths was significantly greater than that in 2 m depths (24.9 vs 5.4 per 5-min tow), and catchability in 1 and 2 m depths was significantly greater than that in 3 and 4 m depths (15.2 vs 1.4 per 5-min tow).M. affinis also showed the clearest relationship of increasing size with depth. Shrimp captured in 1 m depths were significantly smaller (~ 14 mm carapace length, CL) than those captured in 2 m depths in the spring (~23 mm CL) and summer (~ 18 mm CL). In the absence of low-salinity waters and intertidal vegetation, it is believed that the edge of the advancing and receding tide over the mudflats, i.e. the shallowest waters, provides juvenile shrimp, particularlyM. affinis, protection from predation.     

Feeding habits of the dungeness crab Cancer magister as determined by the index of relative importance

Specimens of the Dungeness crab Cancer magister, were collected subtidally and intertidally from an estuary in Washington State, USA in June and September 1980, and January, April, May and July, 1981. Gut contens of freshly collected crabs were analyzed by the Index of Relative Importance; for each prey taxon, this method measured frequency of occurrence, percentage of total biomass, and percentage of total numbers consumed. The most important higher taxon eaten was fish; however, the most important prey genus was the shrimp Crangon spp. There was greater predation on Crangon spp. at night at the intertidal site, and during winter and spring when the shrimp were most abundant there. Feeding activity, as indicated by a weight-specific gut-fullness index, showed no consistent diel pattern. There were significant ontogenetic changes in feeding patterns: first-year crabs preyed primarily on very small bivalves or small crustaceans including their conspecifics; second-year crabs preferred Crangon spp. and fish, and third-year crabs preyed less on Crangon spp. and more on fish. Such changes in feeding habits with ege could be purely due to mechanics of food handling, but might also reduce competition among age groups of crabs, possibly partitioning resources within the estuary. Findings are discussed in terms of optimal foraging and compared to other similar studies.Contribution No. 599, School of Fisheries, University of Washington, Seattle, Washington 98195, USA     

Spread and potential impact of the recently introduced European green crab,Carcinus maenas,in central California

Our study examines the potential impact of the European green crab Carcinus maenas on communities of coastal embayments of western North America. We document the current distribution and range expansion of this species beyond San Francisco Bay, where C. maenas first became established along this coast in 1989–1990, and we test the effect of C. maenas predation on different species and sizes of infaunal invertebrates in field and laboratory experiments. In our samples from eight coastal locations in central California collected between June 1993 and May 1994, we found no green crabs at the two closest embayments south of San Francisco Bay and found the crabs in all four embayments sampled within 120 km north of San Francisco Bay, up to and including Bodega Harbor. C. maenas was not present in samples from sites farther north. This northward range expansion is apparently the result of larval recruitment by a single cohort, corresponding to the predominant northern transport of surface waters and the approximate distance water moves during larval green crab development. At Bodega Harbor, the current northern range limit, the C. maenas population is now well established and reproducing. Females and males became sexually mature within their first year at 40 mm carapace width, molting approximately monthly from summer through fall, and females were ovigerous in late fall of their first year at 50 mm. We expect larvae from this population to recruit locally and to the north, promoting episodic range extensions as new populations are established and reproduce. Enclosure experiments conducted during the summer of 1993 at the intertidal sandflats of Bodega Harbor showed that C. maenas significantly reduced densities of the most abundant taxa, including the bivalves Transennella confusa and T. tantilla, the cumacean Cumella vulgaris, and the amphipod Corophium sp. Furthermore, Carcinus maenas selectively removed larger (>3 mm) rather than smaller (<1 mm) Transennella spp. in both field and laboratory experiments. Based on the available data from this and other studies of green crabs, and our 10 yr study of community dynamics at Bodega Harbor, we predict C. maenas will significantly alter community structure, ecological interactions, and evolutionary processes in embayments of western North America.     

Reproductive biology of commercially exploited deep-water royal red prawns (Haliporoides sibogae,Solenoceridae) in south-east Australia

Deep-water royal red prawns (Haliporoides sibogae de Man, 1907; Solenoceridae: Penaeidea) were sampled from commercial catches off south-east Australia between November 1988 and May 1990. Four stages of sexual development were distinguished macroscopically for each sex. For the females, histology and analysis of oocyte size were used to validate the macroscopic staging of ovaries into undeveloped, developing, early ripe and ripe. Females were larger than males, reaching a maximum size of 46 mm carapace length (CL) compared to 32 mm CL for males. Average size at sexual maturity was 30.8 mm CL for females and 25.8 mm CL for males. Females apparently breed several times in their life, whereas males probably breed only once. Individual fecundity (ranging from about 58 000 to 140 000 oocytes) was relatively low compared with other penaeids, but oocytes were larger (mean diameter=0.41 mm). Two short breeding seasons occurred per year (February to April and July to August). There was a clear size-related latitudinal distribution of the prawns, with largr prawns (spawners) being more abundant in the northern region of the fishing area, and juveniles (CL20 mm) being more abundant in the south. Most of the spawning stock probably lies outside the main fishing ground, while the fishery targets mainly prawns below the size at maturity.     

Epibiotic community of the horseshoe crab Tachypleus gigas

Horseshoe crabs act as moving substrata for simple to complex communities of small marine organisms. Amplexed adult pairs migrate for breeding once every 2 weeks from deep waters towards nearshore waters during highest high tide. Female horseshoe crabs bury themselves to the level of the lateral eyes to deposit eggs while the male crabs fertilize them. Subsequently eggs are buried by the female. Tachypleus gigas (Müller) is the most abundant horseshoe crab species above available along the Orissa coast (India). Adults reach terminal anecdysis once sexually mature and live with their carapace for 4 to 9 years. In spite of this, epibiosis is limited. In the current investigation, differences in the epibiotic community (diatoms and macro-epibionts) present on horseshoe crabs, according to gender, were evaluated, and the macro-epibiont population from different regions of the carapace was mapped. In general, female horseshoe crabs harbored fewer epibionts than the males. Among the diatoms, Navicula spp., Nitzschia spp. and Skeletonema sp. were dominant in both sexes. However, the abundance and diversity of diatoms was greater on the carapaces of male crabs. Among the macro-epibionts, the acorn barnacle (Balanus amphitrite Darwin) and encrusting bryozoan (Membranipora sp.) were the most dominant forms. Barnacles and bryozoans were greater in abundance in the “rough” zone (cardiopthalmic region and anterior region of the opisthosoma). Mapping of the macro-epibionts from different regions of the carapace revealed differential distribution in males and females. Such differentiated distribution of the macro-epibionts can be related to factors such as changing habitat by the horseshoe crabs during breeding, mechanical abrasion and surface availability during mating and nesting periods, requirements of epizootic larvae and surface properties of the carapace (wettability and roughness). In the case of females, mechanical abrasion and surface availability played an important role in the epibiotic community structure and distribution patterns. The surface wettability measurements indicated male carapace to be slightly more hydrophobic than the female carapace. Scanning electron microscopy revealed that the male carapace was comparatively rough compared to the smooth carapace of females. A comparison of surface properties of the carapace indicated that the male carapace is more conducive for epibiosis. Received: 23 August 1999 / Accepted: 25 January 2000     

Données biologiques et biométriques sur les lithodes Lithodes murrayi (Crustacea: Decapoda: Anomura) des îles Crozet (SW océan Idien)

Large numbers of Lithodes murrayi collected by traps and trawls during Cruise MD.03 (1974) of M.S. Marion-Dufresne around Crozet Islands, have been studied, and comparisons made with other species of lithodid crabs. After statistical analysis of data relative to some body parts of males and females, and calculation of length-weight relationships, various aspects of reproduction are discussed: size at sexual maturity, size distribution of females in relation to their reproductive condition, fecundity (which is shown to be low), and egg-size variations. Analysis of the diet revealed a great diversity of prey, including algae, as well as necrophagous and other opportunistic tendencies, which are comparable with similar results obtained for the king crab Paralithodes camtschatica. Four groups of parasites or associated animals have been recorded for the first time from L. murrayi: rhizocephalans (acting as castrating agents), isopods, hydroids and polychaetes (spirorbs). Finally, the prospects of successful commercial fishing of L. murrayi are briefly discussed.     

Growth and longevity of the mud snail Batillaria attramentaria

Crab pots were used to sample a population of Scylla serrata (Forskal) in an estuarine area in Queensland, Australia. Pots were laid 100 m apart at fixed positions for 4 d each month for 1 yr (April 1980–June 1981). Data from recapture of tagged crabs showed that males larger than 140 mm carapace width and females larger than 150 mm had a higher capture probability than did smaller crabs; thus size-frequency distributions based on crab-pot captures are biased. Spacing trials showed that pots positioned 50 m apart fished competitively but that there was no difference in catch between those placed 100 and 200 m apart. Catch distribution indicated that the presence of a crab in a pot reduced the probability of further captures. Temperature and incidence of recently moulted crabs accounted for 66% of variation in monthly catches. Catch-per-unit-effort (CPUE) data can be used as a measure of relative abundance of adults if allowance is made for temperature and the incidence of moulting. The capture-mark-release-recapture method for making population estimates was tested but it is concluded that, because of cost and bias in collecting techniques, it cannot be recommended as a technique for studying S. serrata.     

Biochemical composition of the deep-sea red crab Chaceon quinquedens (Geryonidae): organic reserves of developing embryos and adults

Deep-sea red crabs Chaceon quinquedens (Smith) were collected in traps at depths of 860 and 1043 m in the northern Gulf of Mexico. Ovigerous crabs were maintained in the laboratory and the developing embryos were sampled every 2 wk until hatching. Proximate analysis (lipid, protein, carbohydrate, and ash) of embryos was performed to determine patterns and rates of organic reserve utilization during embryogenesis. Midgut gland, gonads, and clutch (as appropriate) of adult crabs (males, non-ovigerous females and ovigerous females) were analyzed for the same components as the embryos. Red crab embryos exhibited different patterns of yolk deposition and subsequent depletion of yolk components during embryogenesis. There was a range of lipid to protein (L:P) ratios among the different clutches examined, indicating plasticity in the relative proportions of lipid and protein yolk. The energy used for embryogenesis was estimated by converting the amounts of lipid, protein and carbohydrate in the embryos to their caloric equivalents; final values, taken from 9 mo-old embryos whose siblings were hatching as zoeae, were subtracted from the initial values of sibling embryos sampled at the time of collection (2 to 3 mo old). The amount of energy consumed during embryogenesis in the laboratory was relatively constant (0.12 to 0.13 cal egg^-1). There was considerable variability among the concentrations of organic reserves in the midgut gland of adult crabs and in the ovaries of females. Variations in midgut gland L:P ratios and ovaries were related to the reproductive status of the females, but there were no trends related to depth of capture.     

Ontogeny of diet changes in a tropical benthic carnivorous fish, Parupeneus barberinus (Mullidae): relationship between foraging behaviour, habitat use, jaw size, and prey selection

Parupeneus barberinus forages on benthic invertebrates using a wide range of foraging modes, including vigorous digging in the substratum, resulting in considerable disturbance to the benthos. Polychaetes were the most important prey item for all size classes, but fishes less than 120 mm total length consumed more small ostracods and nematodes than did larger fishes. Fishes greater than 120 mm total length consumed mostly bivalves, and fishes over 240 mm total length consumed mostly bivalves and crabs. A morphological examination of the feeding apparatus suggested that the size of important prey items consumed was determined by gape height and jaw width. Prey available to different size classes of fishes was determined by combining information on microhabitat use, foraging behaviours, and prey volumes in the substratum. Small fishes spent more time foraging on the reef flat and slope, compared with larger fishes that foraged mostly on the reef edge and base. In addition smaller fishes foraged mostly in the upper 2 cm of sediment, whereas larger fishes often foraged to depths of 10 cm. Selection ratios showed that different size classes of fishes selectively extracted different prey items from the substratum. Small fishes showed a preference for ostracods whereas large fishes selected for bivalves and crabs. Although polychaetes were the dominant prey item for all size classes, they were consistently selected against.     

Reproductive biology of the spiny lobster, <Emphasis Type="Italic">Panulirus Penicillatus</Emphasis>, in the southeastern coastal waters off Taiwan

The reproductive biology of spiny lobster, Panulirus penicillatus, was studied based on 2,068 lobsters, ranging from 34.28 to 131.60 mm carapace length (CL), sampled in Taitung coastal waters from September 2003 to December 2004. The overall sex ratio approximated 1:1 (χ² = 0.02, P > 0.05), but the monthly sex ratios in 2004 showed significant differences and males were predominant in sizes larger than 80 mm CL. Reproductive activity, assessed using histology, a gonadosomatic index and percentage of ovigerous females, indicated that the mature females could be found in every month and that the major spawning occurred from May to September. The presence of re-developing/re-ripe ovaries by month and size-specific spawning time suggest that larger mature females (>60 mm CL) spawn at least three times a year while smaller new mature females spawn at least once a year. For females, the estimated sizes at 50% physiological and functional maturity were (mean ± SE) 56.46 ± 0.56 mm CL and 66.63 ± 1.07 mm CL. The estimated sizes at functional maturity were between 72 and 74 mm CL for males. The number of eggs per spawning event (brood size, BS) was related to CL by the equation Y _BS = 2.4 × 10^-3CL^4.18 (r ² = 0.902, n = 12). Female lobsters with CL ranging from 60 to 80 mm made the greatest contributions to egg production because of their high brood size and active reproductive activity. A minimum legal size should be established for the fishery to protect egg production potential of lobster population in the southeastern coastal waters off Taiwan.     

Juvenile length-at-age data reveal that spanner crabs (<Emphasis Type="Italic">Ranina ranina</Emphasis>) grow slowly

The spanner crab (Ranina ranina) is a widespread and abundant brachyuran in offshore sand substrata of the Indo-Pacific region. Little is known of this species biology, population dynamics and ecology, despite it being the target of commercial fishing operations in many areas. Previous studies of R. ranina growth using length-frequency analysis of samples collected with commercial fishing gear have derived widely divergent estimates of growth parameters. The estimated time taken to reach 100 mm rostral carapace length (minimum legally exploited size in Queensland, Australia) in those studies has ranged from 1.75 to 8.83 years for females and from 1.08 to 3.58 years for males. Our data show that the commercial fishing apparatus used in those studies is size selective and catches only adult crabs. The resulting size bias in samples collected using that apparatus precludes the application of length-frequency-based techniques to estimate growth parameters from those samples. We devised a new dredge to collect samples of juvenile R. ranina and to calculate juvenile growth rates from modal progression in those samples. We combined those data with estimated mean maximum lengths (L) of 121.7 mm for females and 155.9 mm for males from commercial catch data to model other von Bertalanffy growth parameters using bootstrap methods. Those modelled parameters (K=0.29, T₀=–0.24 for females; K=0.23, T₀=–0.25 for males) indicate that R. ranina grows more slowly than most previous estimates suggest, with females requiring an average of 6.35 years and males 4.31 years to reach 100 mm rostral carapace length. This slow growth is consistent with the slow metabolism of R. ranina, and indicates that this species would be likely to recover slowly from overexploitation.Communicated by G.F. Humphrey, Sydney     

Vertical habitat-partitioning by copepodites and adults of subtropical oceanic copepods

Vertical distributions of the abundant larger copepods, both adults and late copepodites, were observed day and night in the upper 500 m of the North Pacific central gyre in early November, 1971. Densities of the copepodites usually equalled or exceeded those of the adults. Copepod species with maximum densities at or above 100 m (Calanus spp., Nannocalanus minor f. major, Undinula darwini, and Euchaeta rimana) usually had no ontogenetic or diel migration. Neocalanus spp. and Haloptilus longicornis exhibited ontogenetic but not diel migrations. Nannocalanus minor f. minor, Aetideus acutus, Euchaeta media, Scolecithrix spp. and Pleuromamma spp., had both ontogenetic and diel migrations. Adults and copepodites of E. media and Pleuromamma spp. usually had their night modes at the same depth, but the daytime modes were at progressively deeper depths for progressively older stages. Daytime modes for adults and copepodites of A. acutus and Scolecithrix bradyi were at the same depth, but the nighttime modes were at shallower depths for progressively older stages. Night modes of all these migrators were usually in the mixed layer (75 m), where primary production rates were maximal. Congeners usually had similar migratory behavior, but competition probably has been a significant determinant of vertical distribution, since congeners, particularly sibling species, consistently had different depths of maximum occurrence during both day and night.     

Notes on some factors affecting aggressive behaviour in Carcinus mediterraneus

The relationships between aggressive behaviour and animal density and between aggressive behaviour and metabolic excretion have been investigated in Carcinus mediterraneus Czerniavsky. Increasing the crab density by increasing the number of crabs or reducing the available area revealed that the aggressive interactions perindividual did not increase. The frequency of interactions reached a maximum in several minutes following the introduction of crabs. Individuals isolated between 6 and 24 h were more aggressive than non-isolated ones. The water in which a group of crabs had been held had an inhibitory effect (not abolished by commercial aquarium filters) on aggressiveness of pairs of crabs grater than plain water or water in which single crabs had been kept. No home-group effect was observed. The effect of group water was enhanced by increasing the concentration of crabs in the duration of confinement, and it is probably due to the accumulation of catabolites in the medium. Crabs kept in groups had a greater excretion than isolated crabs and the nitrogen oupput decreased with an increase in the number of crabs or whith a decrease in the size of the tank (with a constant volume of water and number of crabs).     

Field and experimental studies on cadmium in the edible crab Cancer pagurus

The distribution of cadmium within captive crabs (Cancer pagurus) exposed experimentally to cadmium-contaminated food and water is described and illustrated by triangular diagrams. Crabs from the Orkney Islands (Scotland) are known to contain relatively high levels of cadmium (up to 62 g g^-1 wet wt) in the hepatopancreas. The distribution of cadmium between the hepatopancreas, gonad, gill, carapace, claw muscle, heart, and haemolymph, is described in crabs collected during 1978, and compared with similar data from crabs exposed to cadmium for ca. 300 d (September 1978 – June 1979) experimentally. It is concluded that the dominant uptake route of cadmium to Orkney crabs is through their diet.     

Feeding,starvation and metabolic rate in the shore crab Carcinus maenas

The relationship between food intake and metabolic rate (as measured by oxygen consumption) of Carcinus maenas (L.) was studied. The metabolic rate of feeding crabs which had been starved for a short period increased, and several days were required for it to return to its original level. During prolonged starvation, the metabolic rate of C. maenas fell in two stages. The first reduction was to about 60% of the feeding level; this occurred during the first week of starvation. There was then a further reduction to about 40% of the feeding level and, at this level of metabolism, the crabs were able to survive for 3 months, with only 50% mortality; most of this occurred in the last 2 weeks. Metabolic rate was found to affect food intake; crabs acclimated to 24°C took 2.4 times as much food as crabs acclimated to 10°C, although the metabolic levels of the two groups differed by a factor of only 1.4. From the results obtained when the crabs were starved and when starved crabs were fed, it is suggested that, during starvation, the metabolic rate of C. maenas first drops from the elevated feeding level to a level at which carbohydrate reserves are utilised, and subsequently to a minimum level at which lipid reserves and proteins are used.     

Biology of penaeid prawns in the Suez Canal lakes

A study was made from January 1988 to March 1989 of the penaid prawns in the Great Bitter Lake and Lake Timsah located in the central part of the Suez Canal. Two species of Red Sea origin were investigated,Metapenaeus stebbingi andTrachypenaeus curvirostris; the former is by far the commoner. Both species displayed seasonal breeding over the period April to October, with peaks in the early and late parts of the season.M. stebbingi had two main cohorts each year. A spring-spawned cohort survived 16 mo, and attained modal sizes of 23 mm carapare length (CL) for females and 16 to 17 mm CL for males. The autumn-spawned cohort survived 13 mo, and reached modal sizes of 20 mm and 15 mm CL for males and females, respectively. The reasons for the success ofM. stebbingi in the Canal lakes, in contrast to other Lessepsian penaeids which have flourished in the Mediterranean Sea, have been considered. The main factors are that it is essentially a shallow-water species, with excellent tolerance of salinity variations and adverse environments. However, the Canal lake populations have smaller maximum sizes, and mature earlier, than those in open seas.     

Diets of four decapod crustaceans (Linuparus trigonus,Metanephrops andamanicus,M. australiensis and M. boschmai) from the continental shelf around Australia

Linuparus trigonus (Palinuridae) was collected from northeast Australia at depths of 220 to 300 m in the austral summer of 1985–1986 and its diet was analysed for diel feeding-periodicity and composition. The sex and size of the lobsters and the depth at which they were caught were recorded. About 35% of the foreguts were less than 10% full. L. trigonus appears to be a predator of slow-moving or nearsessile benthic invertebrates; its diet is mainly bivalves, gastropods, ophiuroids, crustaceans, polychaetes, and foraminiferans. Most dietary items differed significantly between the size classes of lobster. The frequency of crustacean remains increased with depth and more gastropods were consumed by lobsters caught at 260 m than at other depths. No significant periodicity in feeding was found for L. trigonus. The foreguts of three Metanephrops spp. (Nephropidae) collected from north-west Australia in August 1983 (M. andamanicus) and January/February 1984 (M. australiensis and M. boschmai) were nearly empty, and nearly half were less than 10% full. Fish, crustaceans and squids were the most common food items eaten. The main food items varied between the species: M. andamanicus ate nearly equal amounts of fish and crustaceans; M. australiensis ate chiefly crustaceans; M. boschmai mainly ate fish and seldom fed on squid. Metanephrops spp. appear to feed by attacking mobile animals and tearing off appendages or by scavenging.     

Recruitment to adult habitats following marine planktonic development in the fiddler crabs, Uca pugilator, U. pugnax, and U. minax

Three congeneric species of fiddler crabs, Uca pugilator (Bosc, 1802), U. pugnax (Smith, 1870), and U. minax (LeConte, 1855), co-occur in estuaries along the east coast of North America, from Cape Cod to northern Florida. Although U. minax adults are generally found at lower salinities than the other two species, the distributions of all three species overlap to some degree. The distribution of megalopae and juvenile fiddler crabs (from first crab stage to those with a carapace width of 3.0 mm) was examined at four sites along a salinity gradient (from 35.0±2.0 to 3.0±3.0; ) in the North Inlet Estuary, South Carolina, USA, in August 2002. A PCR-RFLP technique was developed to identify individuals from the genus Uca to species from first zoea through the early crab stages. An examination of the distribution of early life stages showed that U. minax reinvades low-salinity adult habitats at settlement, following planktonic larval development in the coastal ocean. Also, juveniles of U. pugilator were found to occupy Spartina alterniflora stands, where adult conspecifics rarely occur. Species frequencies were different for adults compared to early life stages in low-salinity areas of the marsh, where populations overlap. Settlement and survival dynamics of early life-history stages in wet and dry years likely determine the distribution of adult Uca spp. populations along salinity gradients in estuarine ecosystems.Communicated by J.P. Grassle, New Brunswick