Induction of annual cyclical changes in the ichthyofauna of a large microtidal estuary following an artificial and permanent increase in tidal flow

The opening of a deep artificial and second entrance channel into the large Peel‐Harvey Estuary in south‐western Australia in 1994 provided a unique opportunity to determine the characteristics of the ichthyofauna of a microtidal estuary, in which tidal water movement and the rate of recovery of salinities in spring have both increased markedly. Fishes in the original entrance channel and both halves of the two large basins (Peel Inlet and Harvey Estuary), which constitute most of the total area of the estuary, were sampled using a 21·5 m long seine with a fine (3 mm) mesh bunt. The 72 337 fishes that were caught throughout the estuary and represented 43 species, comprised mainly marine species, both in terms of number of species (65·1%) and number of individuals (70·6%). The number of species and density of fishes in each region, and particularly in the basins, showed a pronounced tendency to decline from a maxima in summer to a minima in winter and then to rise in spring. Although the species composition of the fish fauna in the natural entrance channel differed significantly from that in each of the four basin regions, those within the basins were significantly different only in the cases of the southern Harvey Estuary versus both regions of the Peel Inlet. Species composition in the natural entrance channel and four basin regions each underwent conspicuous cyclical changes during the year, which reflected out‐of‐phase changes in the densities of certain species. Although this cyclical pattern was due mainly to changes in the densities of marine species, it was augmented by those of estuarine residents. The cyclicity, which was not evident prior to the opening of the artificial Dawesville Channel, and thus when tidal water movement within the estuary was far less pronounced, parallels the situation recorded for macrotidal northern hemisphere estuaries.     

Nitrogen fixation by Nodularia spumigena Mertens (Cyanobacteriaceae). 1: Field studies and the contribution of blooms to the nitrogen budget of the Peel-Harvey Estuary,Western Australia

Variations in nitrogen fixation (acetylene reduction) by Nodularia spumigena blooms in the Peel-Harvey estuarine system were examined with respect to spatial (sampling station location, and depth) and temporal (seasonal and diurnal) distribution. The annual contributions of nitrogen fixation by the blooms to the nitrogen budget of the estuary were estimated to range from 309 to 713t. Contributions by nitrogen fixation were similar to the riverine inputs in the Harvey Estuary, but lower in the Peel Inlet.The Harvey Estuary had higher biomass and total fixation rates (to 0.4 nmol C₂H₂ · ml^–1 h^–1), but the heterocyst nitrogen fixation rates were greater in the Peel Inlet (to 9 × 10^–1 nmol C₂H₂ · heterocyst^–1 · h^–1). Nitrogen fixation decreased with depth in response to light, though other factors also appeared to be involved. The rates of fixation decreased concurrently with increasing bloom age, total soluble inorganic nitrogen and salinities. Maximum daily fixation rates occurred in the early morning.     

Nodularia (Cyanobacteriaceae) Akinetes in the Sediments of the Peel-Harvey Estuary, Western Australia: Potential Inoculum Source for Nodularia Blooms

The populations of viable Nodularia (Cyanobacteriaceae) propagules in the sediments of the Peel-Harvey Estuary in Western Australia were assessed over location, time, and depth. The sediments of the Harvey Estuary had greater numbers of Nodularia propagules than those of the contiguous Peel Inlet. This was consistent with the distribution of Nodularia blooms in the system. The sediment populations of Nodularia propagules has increased up to 100-fold at the study sites over a 4-year period during which three blooms have occurred. It is considered that the majority of the propagules are akinetes. The significance of the sediment akinetes in providing the inoculum for rapid onset of Nodularia blooms is discussed. The population of akinetes decreased with depth, but viable akinetes were still found at 35 cm, the maximum depth sampled. Bioturbation by polychaete worms is likely to be significant in the distribution of akinetes to these depths.     

Relationship of cellular phosphorus in the cyanobacterium Nodularia to phosphorus availability in the Peel-Harvey Estuarine System

A laboratory-determined critical concentration of phosphorus in the cyanobacterium Nodularia spumigena was used to assess the phosphorus sufficiency of field populations in the Peel-Harvey Estuarine System, Western Australia. Nutrient addition bioassays indicated that phosphorus was the limiting nutrient. Since river flow, and thus phosphorus input, to the estuary ceases 1–2 months before the onset of Nodularia blooms, the phosphorus contained in these blooms was derived from sediment release and nutrient recycling. A progressive increase in the mean cellular phosphorus concentration of Nodularia from 1981 to 1983 indicated that sediments were releasing a greater amount of phophorus each year. Although evidence suggests that the biomass of recent Nodularia blooms is now limited by physical factors, the additional phosphorus contained in each bloom will increase the supply of phosphorus to other organisms. Cellular phosphorus concentrations in Nodularia appeared to be correlated with river phosphorus loadings in the Peel Inlet but not in the Harvey Estuary.     

Dietary compositions of the sparid <Emphasis Type="Italic">Acanthopagrus</Emphasis><Emphasis Type="Italic"> butcheri</Emphasis> in three normally closed and variably hypersaline estuaries differ markedly

We have compared the species composition and diversity of the diets of black bream, Acanthopagrus butcheri, in three normally closed estuaries on the central south coast of Western Australia, which vary markedly in the extents to which they become hypersaline during dry periods. Although black bream was caught seasonally in Stokes Inlet during this 3-year study, it was obtained from the Hamersley and Culham inlets only during the earlier seasons because salinities in those two estuaries subsequently rose to levels that caused massive mortalities of this sparid. Although a wide range of taxa, including macrophytes, polychaetes, molluscs, crustaceans, insects and teleosts, were ingested by A. butcheri in each estuary, the frequencies of ingestion and volumetric dietary contributions of these taxa varied greatly among the fish in these three estuaries. Thus, for example, in comparison with other estuaries, relatively greater contributions were made to the diet by polychaetes and crustaceans in Stokes Inlet, by macrophytes in Hamersley Inlet, and by insects (mainly chironomid larvae) in Culham Inlet. The relatively greater contribution of teleosts to the diets of black bream in the Hamersley and Culham inlets than in Stokes Inlet, and also differences in the main teleost species ingested in the first two estuaries, are consistent with differences in the densities of fish overall and of the main fish species in those estuaries. The diversity of the diet was far greater in Stokes Inlet than in the other two far more variably saline estuaries, presumably reflecting a greater diversity of food. The dietary compositions of black bream in upstream pools in the tributary of Culham Inlet, which offer refuge when salinities increase markedly in the main body of the estuary, differ from those in those downstream regions, further emphasising the opportunistic nature of the feeding behaviour of black bream. The dietary compositions of black bream underwent size-related changes, but the taxa contributing most to those changes varied greatly among estuaries. Size-related changes would be particularly beneficial in reducing intraspecific competition for food in the two estuaries that vary greatly in salinity and would thus be likely to contain a less diverse range of prey.     

A COMPARISON OF THE MOLLUSCAN COMMUNITIES ON INTERTIDAL SAND FLATS IN OYSTER HARBOUR AND PEEL INLET, WESTERN AUSTRALIA

Intertidal molluscs on sandflats in Oyster Harbour and PeelInlet, Western Australia, were compared using transects. Twenty-fourspecies of molluscs were recorded in Oyster Harbour and only7 in Peel Inlet. Two groupings were foundin Oyster Harbour:a midlittoral component and infralittoral fringe. There wasno vertical community structure in Peel Inlet. Molluscs retainedon a 1 mm sieve had about the same mean density and biomassin the two areas but molluscs retained on a 2 mm sieve had amean density of 203/m² and a biomass of 132 g/m² in Oyster Harbourand were absent in Peel Inlet. Several possible reasons forthe absence of large molluscs in Peel Inlet are discussed. PeelInlet is on the west coast in a faunal overlap zone where thenumber of species is reduced. The mean temperature in Peel Inletis 2-3°C higher than in Oyster Harbour and salinity variationsare much more pronounced. (Received 20 November 1979;     

Nutrient levels and the development of diatom and blue-green algal blooms in a shallow Australian estuary

In the Peel-Harvey estuary system, Western Australia, some 90%of riverflow and nutrient loading occurs in three winter months.Diatom blooms follow riverflow, but are replaced by blooms ofthe blue-green Nodularia spumigena Mert., especially in HarveyEstuary. By analysis of time series data from 1977–1983,it is shown that the magnitude of the Nodularia bloom in summeris related to the minimum salinity of the estuary (and hencetotal river flow), maximum phosphate concentration and totalriverine phosphorus loading, in the previous winter. The relationshipshave a predictive capacity. It is argued that diatom bloomstrap phosphorus, which is sedimented largely as faecal pellets;the phosphorus is recycled and supports Nodularia growth underwarmer conditions, and the amount available determines Nodulariabiomass. Nodularia blooms collapse when summer salinities reach30     

Survival,osmoregulation and ammonia-N excretion of blue swimmer crab, Portunus pelagicus, juveniles exposed to different ammonia-N and salinity combinations

Ammonia-N toxicity to early Portunus pelagicus juveniles at different salinities was investigated along with changes to haemolymph osmolality, Na⁺, K⁺, Ca²⁺ and ammonia-N levels, ammonia-N excretion and gill Na⁺/K⁺-ATPase activity. Experimental crabs were acclimated to salinities 15, 30 and 45‰ for one week and 25 replicate crabs were subsequently exposed to 0, 20, 40, 60, 80, 100 and 120 mg L^− 1 ammonia-N for 96-h, respectively. High ammonia-N concentrations were used to determine LC₅₀ values while physiological measurements were conducted at lower concentrations. When crabs were exposed to ammonia-N, anterior gill Na⁺/K⁺-ATPase activity significantly increased (p < 0.05) at all salinities, while this only occurred on the posterior gills at 30‰. For crabs exposed to 20 and 40 mg L^− 1 ammonia-N, both posterior gill Na⁺/K⁺-ATPase activity and ammonia-N excretion were significantly higher at 15‰ than those at 45‰. Despite this trend, the 96-h LC₅₀ value at 15‰ (43.4 mg L^− 1) was significantly lower (p < 0.05) than at both 30‰ and 45‰ (65.8 and 75.2 mg L^− 1, respectively). This may be due to significantly higher (p < 0.05) haemolymph ammonia-N levels of crabs at low salinities and may similarly explain the general ammonia-N toxicity pattern to other crustacean species.     

Parasite modification of predator functional response

Parasite alteration of the host (predator) functional response provides a mechanism by which parasites can alter predator–prey population dynamics and stability. We tested the hypothesis that parasitic infection of a crab (Eurypanopeus depressus) by a rhizocephalan barnacle (Loxothylacus panopei) can modify the crab’s functional response to mussel (Brachidontes exustus) prey and investigated behavioral mechanisms behind a potential change in the response. Infection dramatically reduced mussel consumption by crabs across mussel densities, resulting in a decreased attack rate parameter and a nearly eightfold reduction in maximum consumption (i.e. the asymptote, or inverse of the handling time parameter) in a type II functional response model. To test whether increased handling time of infected crabs drove the decrease in maximum consumption rate, we independently measured handling time through observation. Infection had no effect on handling time and thus could not explain the reduction in consumption. Infection did, however, increase the time that it took crabs to begin handling prey after the start of the handling time experiment. Furthermore, crabs harboring relatively larger parasites remained inactive longer before making contact with prey. This behavioral modification likely contributed to the reduced mussel consumption of infected crabs. A field survey revealed that 20 % of crabs inhabiting oyster reefs at the study site (North Inlet estuary, Georgetown, South Carolina, USA) are infected by the barnacle parasite, indicating that parasite infection could have a substantial effect on the population level crab-mussel interaction.     

Reproductive biology and larval development of the terapontid Amniataba caudavittata, including comparisons with the reproductive strategies of other estuarine teleosts in temperate Western Australia

Samples collected monthly from the Swan Estuary between March 1978 and May 1979, together with environmental data for 1977 to 1980, have been used to elucidate various aspects of the reproductive biology of Amniataba caudavittata in this estuary. The gonads of A. caudavittala started to develop rapidly in the spring, when day length, water temperatures and salinities were increasing markedly. Spawning, which occurred mainly towards the top end of the upper estuary, was initiated in November, when water temperatures and salinities in that region were c. 24° C and 9‰, and it peaked in December/January when they were c. IT C and 17%○, Maturity is attained by at least the majority offish at the end of the second and each subsequent year of life. Although some of the larger 1-year-old fish attained maturity, this occurred in only one of the two tributary rivers, possibly reflecting differences in the salinity regimes in these rivers. Fecundities ranged from 50 000 in a 150-mm fish to 705 000 in a 254-mm fish, with a mean of 310 000. The mature, unfertilized eggs are small and spherical and have a diameter of 560 μm. The larvae are pelagic and characterized by an elongate body, which becomes moderately deep and laterally compressed during development, a short to moderate, tightly-coiled gut, a distinct gap between the anus and the origin of the anal fin and 25 or 26 myomeres. The development of fins and settlement of A. caudavittata larvae occurs at a smaller size than in the larvae of other terapontid species previously described. The success of A. caudavittata in the Swan Estuary can be attributed in part to its production of very large numbers of eggs at a time when, due to low fresh water discharge and a smait tidal range, conditions in the estuary are relatively stable. Comparisons between the biology of A. caudavittata and that of other abundant teleosts that spawn in south-western Australian estuaries show that these species exhibit a wide range of reproductive strategies.     

Comparison of fish impingement by a thermal power station with fish populations in the Ems Estuary

On an annual basis >10 million individuals of c . 35 fish species are impinged at the EC20 unit of the Eems power station located on the Ems Estuary. The most abundant are: herring Clupea harengus , gobies ( Gobiidae ), Nilsson's pipefish Sygnathus rostellatus , three-spined stickleback Gasterosteus aculeatus and sprat Sprattus sprattus . The impingement shows a seasonal pattern which reflects the presence of the fishes in the estuary. The results are compared with other impingement studies at power stations on the Belgian and Dutch estuaries.
In 1995, the cooling water entrance was displaced from the shoreline to 300 m off the coast at the edge of the tidal channel. The effect of this relocation on the level of fish impingement, determined by comparing the ratio of fish density in the cooling water and in the estuary before (1992–1993) and after (1996–1997) the relocation of the intake canal is discussed.     

Availability of salts is not a limiting factor for the land crab Gecarcinus lateralis (Freminville)

The land crab Gecarcinus lateralis (Freminville) is restricted to within ≈ 250 m of the shore on Bermuda. Previous work demonstrated that availability of water did not account for the exclusion of crabs from inland habitats. The crabs' inability to produce dilute urine led to the hypothesis that availability of salts becomes limiting away from the shore. In the laboratory, osmoregulatory abilities of land crabs were tested under several ecologically realistic conditions. Salts were provided in interstitial water of various salinities in damp-sand “burrows”, in “drinking water” of various salinities, or in natural and artificial foods. Crabs maintained hemolymph concentrations very well even when exposed to salinities < 1% seawater (SW), particularly when given a choice of how much contact to have with test salinities (“drinking-water” regime). Rates of ion loss were apparently extremely low, even when low-ion artificial food (flavored filter paper) was passing through the gut. In the field, potential sources of salts were compared with the crabs' osmoregulatory abilities. All available water (rain, dew) was very dilute, and there was no evident gradient in availability of salts at increasing distances from shore that could account for the limited distribution of crabs. Concentrations of ions in forage plants were also low, and also showed no gradient correlated with the crabs' range limit. Crabs that were ion-depleted and then fed vegetation from inland of the normal range showed significant recovery in hemolymph concentrations. We concluded that availability of salts is adequate beyond the existing range of the land crabs, and is not the range-limiting factor. This is paradoxical, given the limited osmoregulatory abilities of crab antennal glands, and raises the question of how land crabs accomplish such extraordinary conservation of ions; the physiological mechanism (extrarenal modification of urine) will be described elsewhere. Factors that limit how far inland land crabs live remain enigmatic; hypotheses for further work are presented.     

Geographic distribution,habitat characterization,and conservation status of <Emphasis Type="Italic">Bolboschoenus</Emphasis> bulrushes (Cyperaceae) in the Hudson River Estuary,USA

Bulrushes of the genus Bolboschoenus are robust, ecologically important sedges occurring in wetlands, including intertidal marshes and mudflats. Despite their importance and multiple serious threats to their habitats, estuarine Bolboschoenus species remain poorly known. We conducted herbarium and field research in order to document historic and current geographic distributions, characterize the habitats, and assess the conservation status of Bolboschoenus species in the Hudson River Estuary, New York, U.S.A. Three species of Bolboschoenus grow in intertidal zones in the Hudson Estuary. Bolboschoenus fluviatilis occurs in the northern, upstream, and freshwater portion of the estuary with multi-year mean surface salinities of 0.078–2.0 ppt. Bolboschoenus robustus occupies the southernmost, downstream, and brackish to saline portion of the estuary with salinities of 4.9–16 ppt. Bolboschoenus novae-angliae occurs in the slightly to strongly brackish region between the other two species with salinities of 1.8–8.0 ppt. The geographic ranges of B. fluviatilis and B. robustus do not overlap, but B. novae-angliae has short zones of sympatry with each of the other two species. Syntopy of B. novae-angliae with each of the other two species is rare. In the Hudson Estuary, B. fluviatilis is secure, but B. novae-angliae and B. robustus are critically imperiled. Threats to future survival of Bolboschoenus species in the Hudson Estuary include competition from invasive plant species (especially Phragmites australis), eutrophication resulting from excess nutrient pollution, and habitat destruction. Our data and analyses provide critical new information for management of existing environmental problems and planned habitat restoration efforts in the Hudson River Estuary.     

The ichthyoplankton of a seasonally closed estuary in temperate Australia. Does an extended period of opening influence species composition?

Fish eggs and larvae were collected monthly between September 1987 and April 1989 from sites throughout the main basin and within the saline regions of the two main tributary rivers of Wilson Inlet, a seasonally closed estuary in south-western Australia. Of the eggs, 43.7% belonged to Engraulis australis (Shaw) and the rest to unidentified teleosts. The larval fish assemblage comprised 17 families represented by 25 species. The Gobiidae contained the highest number of species (four) and contributed approximately 57% of all larvae caught. Pseudogobius olorum (Sauvage) and E. australis were the most abundant species, contributing 43.9 and 27.9% to the total larval catch, respectively. The larvae of species which breed within Wilson Inlet dominated the assemblage, both in terms of number of species (64%) and contribution to total catch (99.9%). The numbers of the eight marine species and one freshwater species represented in the ichthyoplankton were very low. Classification and multi-dimensional scaling ordination showed that the composition of the larval fish fauna at the various sites during a period when the estuary remained open to the sea (December 1988-April 1989) was similar to that of the corresponding sites during the same period in the previous year when the estuary had become closed (December 1987-April 1988). This can be attributed to the spatial distribution, time of occurrence and abundance of estuarine-spawned larvae being similar in the two periods and to the rarity of marine-spawned larvae, even in the spring and summer of 1988/1989 when the estuary was open for the whole time when most marine teleosts spawn in south-western Australia. The low occurrence of marine-spawned larvae in Wilson Inlet reflects the fact that tidal water movement within the basin of the system is so small that it is unable to facilitate the transport and dispersion of larvae. The ichthyoplankton of Wilson Inlet resembles that of other poorly-flushed estuaries in that it is low in species richness and dominated by estuarine-spawned larvae.     

Adaptive significance of hatching rhythms and dispersal patterns of estuarine crab larvae: avoidance of physiological stress by larval export?

Estuarine crabs commonly display two larval dispersal patterns in which larvae are either exported from or retained within estuaries. The semiterrestrial fiddler crab Uca minax (LeConte, 1855) hatches on nocturnal spring high tides in the upper estuary and larvae are rapidly transported downstream. The mud crab Rhithropanopeus harrisii (Gould, 1841) hatches on nocturnal high tides of any amplitude and larvae are retained behaviorally in the upper estuary throughout development. If larvae are exported from the estuary to avoid environmental stress, then exported larvae should be less tolerant of high temperatures and low salinities than retained larvae. Larvae of these two species of estuarine crabs were hatched at 20‰ and 25 °C and subjected to salinities of 0, 5, 10,20, and 30‰, temperatures of 25 and 35 °C, and exposure times of 2, 6, 12, and 48 h. Larvae of both species reared at 30 or 20‰ survived well, while those reared in fresh water all died within 2 h regardless of temperature. Mud crab larvae reared at 5 and 10‰ survived better at the lower temperature (25 °C), higher salinity, and shorter exposure times. There was no significant effect of temperature or salinity on the survival of fiddler crab larvae, although survival decreased with increasing exposure time. Thus, the hypothesis that fiddler crab larvae are exported into stable coastal waters to reduce physiological stress is not supported. However, fiddler crab larvae may have evolved to be very tolerant of extreme temperature and salinity stress because they, unlike mud crabs, often release their larvae into shallow creeks. Most fiddler crab larvae are released on nocturnal spring high tides, which facilitates dispersal from tidal creeks. However, freshwater runoff and heat transferred from the marsh surface to flooding waters may still create stressful conditions for larvae soon after they are released. Larval release on spring high tides may facilitate dispersal from tidal creeks.     

Physiological responses of estuarine phytoplankton to ultraviolet light-induced fluoranthene toxicity

Polycyclic aromatic hydrocarbons (PAHs) are common pollutants associated with urbanization and suburbanization in estuarine systems, but little is known about their effects on the physiological properties of microalgae. We examined the effects of ultraviolet (UV)-activated fluoranthene toxicity on (a) the growth, chlorophyll a content cell⁻¹, and pigment composition of axenic Ankistrodesmus sp. (an estuarine benthic green microalga) and (b) the phytoplankton population growth and pigment composition of natural communities from an urbanized (Murrells Inlet) vs. forested (North Inlet) salt marsh estuary. The zeaxanthin/violaxanthin ratio increased in Ankistrodesmus sp. cultures exposed to UV light in the presence of fluoranthene, supporting the hypothesis that xanthophyll cycling is an energy dissipative response to photoinduced PAH toxicity in this species. Exposure of natural communities to the combination of UV light and fluoranthene resulted in decreased chlorophyll production and increased zeaxanthin violaxanthin⁻¹ in samples from the urbanized estuary (Murrells Inlet), but not North Inlet, suggesting that phytoplankton in the former “fluoranthene-impacted” estuary were more susceptible to fluoranthene toxicity. Consideration of xanthophyll cycling as a microalgal response to UV-activated PAH toxicity has implications to understanding the influence of these contaminants on microbial food web structure and ecosystem production.     

Dietary composition of the blue swimmer crab Portunus pelagicus L. Does it vary with body size and shell state and between estuaries?

The aim of this study was to determine the diets of Portunus pelagicus in the large Peel-Harvey and Leschenault estuaries in south-western Australia in order to ascertain whether the dietary composition of this crab changes with body size during two different moult stages and differs between the two estuaries. Portunus pelagicus, ranging in age and carapace width from ca. 2 months and 12 mm to ca. 112 years and 159 mm, were collected from the shallow basins of the Peel-Harvey and Leschenault estuaries. Examination of the cardiac stomachs of these crabs showed that P. pelagicus does not feed just before or immediately after moulting and that the stomachs of recently-moulted crabs contained significantly more food than those of intermoult crabs. Although the volumetric contribution made by calcareous material to the stomach contents was similarly high in all size classes of recently-moulted crabs, i.e. 47 to 55%, the volumetric contributions made by small bivalves decreased with body size, whereas the reverse occurred with shell fragments of large decapods and, to a lesser extent, polychaetes. The dietary compositions of intermoult crabs were shown by classification and multi-dimensional scaling ordination to differ markedly from those of recently-moulted crabs and to undergo similar progressive ontogenetic changes in both the Peel-Harvey and Leschenault estuaries. Thus, the contribution made by small benthic and epibenthic crustaceans, such as amphipods and tanaids, declined with increasing body size, whereas the reverse occurred with larger prey, such as nereid polychaetes, small decapods and teleosts. The dietary composition of P. pelagicus was influenced more by moult stage, i.e. recently moulted vs. intermoult, than by body size. Although the dietary compositions of P. pelagicus in the two estuaries were not significantly different, they did differ from those recorded from coastal marine waters in the same region, thereby reflecting differences in the potential prey in those two environments.     

Habitat preferences of two estuarine burrowing crabs Helice crassa Dana (Grapsidae) and Macrophthalmus hirtipes (Jacquinot) (Ocypodidae)

The horizontal and vertical distributions of two species of endemic, burrowing mud crabs Helice crassa Dana 1851 (Grapsidae) and Macrophthalmus hirtipes (Jacquinot 1853) (Ocypodidae) are described for the Avon-Heathcote Estuary (43°33′S: 172°44′E), Christchurch, New Zealand. Substratum preference is shown to be the most important factor influencing mud crab distribution, but lack of tolerance to salinities below 4‰ is also a significant factor preventing M. hirtipes from occurring at points close to freshwater input. Both species had similar sediment organic content and particle size requirements. Helice crassa was concentrated in well-drained, compacted sediments above mid-tide level, whilst Macrophthalmus hirtipeswas found in waterlogged areas below mid-tide level. This vertical separation is shown not to be caused by differential desiccation tolerances, but by feeding and burrowing adaptations related to these different substrata.     

Comparison of low salinity tolerance in Callinectes sapidus Rathbun and Callinectes similis Williams postlarvae upon entry into an estuary

Planktonic larvae of estuarine crabs are commonly exported to the continental shelf for development and then return to coastal and estuarine areas as postlarvae (megalopae). Megalopae returning to estuaries must be adapted to survive in brackish water whereas those of coastally distributed species should not need such adaptations. We investigated 1) whether megalopae of the estuarine crab Callinectes sapidus and the coastal crab Callinectes similis undergo changes in salinity tolerance upon entry into an estuary and 2) what factors induce those changes. Megalopae were collected at a coastal site and a nearby estuarine site and exposed to a range of salinities (5, 10, 15, 20 and 30) for 6 h. Percent survival was determined after 24 h reintroduction to the collection site water. We also investigated 1) whether increased salinity tolerance was induced by reduced salinity or estuarine chemical cues, 2) the time to acclimation and 3) the salinity necessary for acclimation. C. sapidus megalopae from the estuarine site were more likely to survive exposure to low salinities than those from the coastal site. C. sapidus megalopae from the coastal site exhibited increased survival after acclimation to salinities of 27 and 23 for 12 h. Estuarine chemical cues had no effect on salinity tolerance. C. similis megalopae were less likely to survive at low salinities and did not exhibit an acclimation response upon exposure to reduced salinities. These results suggest that megalopae of C. sapidus are physiologically adapted to recruit to estuaries whereas megalopae of C. similis are unable to acclimate to low salinity conditions.