Osmoregulation in fiddler crabs (<Emphasis Type="Italic">Uca</Emphasis>) from temperate Atlantic and Gulf of Mexico coasts of North America

Fiddler crabs (Crustacea: Decapoda; genus Uca) were collected from 16 locations along the temperate Atlantic and Gulf of Mexico coasts of eastern North America for osmoregulation studies. Three species, U. pugilator, U. pugnax and U. minax, were taken from habitats between Cape Cod, MA, and St. Mary's River, Nassau Co., GA Seven species, U. panacea, U. speciosa, U. spinicarpa, U. longisignalis, U. rapax, U. pugilator and U. minax, were collected in habitats between the Ochlochonee River, Liberty Co., and Thompson's Bayou, Escambia Co., along the Gulf coast of northwest Florida. To examine differences in osmoregulatory capabilities among the species, specimens were placed in various concentrations of artificial seawater (0-3450 mOsm) for 5 days. The oligohaline or "freshwater" species, U. minax, U. spinicarpa and U. longisignalis, possess the lowest average hemolymph osmolality. They are unable to control hemolymph osmolality above 2000 mOsm. On the other hand, the euryhaline species, U. speciosa, U. panacea, U. pugilator and U. pugnax, have much higher average hemolymph osmolality. They are able to withstand an osmotic challenge of 2200 mOsm or greater. Among the eight species, U. panacea and U. pugilator are able to osmoregulate across the broadest range of seawater concentrations (0-2800 mOsm). After an examination of the osmoregulation in several populations of U. minax, U. pugilator and U. pugnax, physiological adaptation is apparent in the two former species, but not the latter. Experimental evidence for capacity adaptation was examined in the freshwater species, U. minax. If slowly adapted to hypertonic seawater (1800 mOsm) for 14 days, this species is able to withstand much higher osmotic pressures than unadapted crabs. Although hemolymph isosmotic concentration remains the same, the limits for regulation are extended to higher osmolality. Consequently, as a result of studying osmoregulation in several species of Uca captured at various locations across their geographic range in the temperate zone, a clearer view of water-balance physiology is available for the genus. The Uca spp. are not uniform in osmoregulatory abilities. There is considerable inter- and intraspecific physiological variation associated with the ecological distribution of each species, respectively.     

Respiration et excrétion azotée du zooplancton. I. Evaluation des niveaux métaboliques de quelques espèces de Méditerranée occidentale

The effects of sublethal concentrations of mercury in combination with stressful temperature-salinity regimes were considered for larval development of the fiddler crab Uca pugilator (Bosc.). Control organisms were compared to those treated with 1.8 ppb Hg for the following suboptimal regimes: 30°C, 30 S; 30°C, 20 S; 20°C, 30 S, and 20°C, 20 S. As physiological indicators of larval response, the survival rate, the O₂ consumption rate, and phototactic response were measured, following either acute 24 h doses of Hg, or chronic rearing in Hg. All response parameters were modified in larvae maintained under the suboptimal conditions; mercury compounded the effects.Supported by Grant No. 18080 FYI from the Environmental Protection Agency.     

Survival and chloride ion regulation of the porcelain crab Petrolisthes armatus exposed to mercury

Acute toxicity bioassays conducted at various salinities demonstrated that mercury (as mercuric chloride) at low concentrations was lethal to Petrolisthes armatus. Ninety-six hour LC₅₀ values varied from 50 to 64 parts per billion (ppb) of mercury, depending on test salinities. Lower salinities. decreased the time to death of mercuryexposed crabs. Differences in survival after 96 h due to salinity were not statistically significant. Blood chloride concentrations were regulated hyperchloride to the medium at low salinities and hypochloride at high salinities by acclimated crabs. The salinity isochloride to blood was 20 S. Transfer of crabs from 15 S to salinities ranging from 7 to 35 S resulted in new steadystate chloride levels within 12 h. Exposure to 50 ppb mercury did not alter chloride ion regulation of either acclimated crabs or crabs adjusting to new salinities.     

Effects of salinity and temperature on Corophium volutator and C. arenarium (Crustacea: Amphipoda), with particular reference to distribution

The responses of the post-embryonic stages of Corophium volutator (Pallas) and C. arenarium Crawford to the combined effects of salinity and temperature show that gravid females have a wider tolerance than nongravid adult females which in turn are more tolerant than adult males. C. volutator is more tolerant of low salinity (2 to 10) than C. arenarium, but the latter is more tolerant of salinities above 45. The embryos of C. volutator develop normally and hatch at lower salinities and temperatures than those of C. arenarium, in which successful development was recorded at higher temperatures. Females undergoing a pre-copulatory moult failed to lay eggs below salinities of 3 (C. volutator) and 10 (C. arenarium), but in both species the lowest salinity at which all females moulted and laid eggs was 20. The results are discussed in relation to the distribution of both species.     

On the ecology of Heterocypris salinus,H. incongruens and Cypridopsis aculeata (Crustacea: Ostracoda) from Baltic brackish-water rockpools

The ostracod fauna of Baltic brackish-water rockpools is made up of two groups: permanent members of the pools, and occasional guests from the littoral zone. The former group consists of Heterocypris salinus, H. incongruens and Cypridopsis aculeata. These species are characterized by rapid development (which starts when the water temperature approaches 15°C), a short life span, and 2 or 3 separate generations during the summer and autumn. The number of generations is determined by water temperature. Hibernation always takes place as eggs. Hatching and development during the late spring or early summer has been found to be mostly simultaneous. The spawning of the 3 species always starts epidemically. Reproduction is entirely parthenogenetic in the investigated area. Under natural conditions, C. aculeata may be found with either H. salinus or H. incongruens, but these two latter species have never been recorded together. H. incongruens is less tolerant to high salinities than the other 2 species and, even after successive adaptation, it does not resist salinities higher than 16. H. salinus has been found in 35.2 S in the field, and has been kept in 30 S in the laboratory after successive adaptation. The optimum salinity-temperature range for this species is 5 to 10 S and 15°C, when both survival and development are considered. Corresponding figures for C. aculeata are 0.5 to 20 S and 15°C, although this species, like H. salinus, survives longest at 5°C. The very rapid development, parthenogenetic reproduction and short life span of these species must be considered as favourable adaptations to the variable and unstable environment of the rockpool ecosystems.     

Effect of reduced salinity on adenylate energy charge in three estuarine molluscs

The value of adenylate energy charge as a biological indicator of the severity of departure from normal environmental conditions was examined in the gastropod Pyrazus ebeninus (Bruguière, 1792) and the bivalves Anadara trapezia (Deshayes, 1840) and Saccostrea commercialis (Iredale and Roughley, 1933). Mean energy charge for each species decreased by 17% or more when salinity was reduced from 35 to 10. Changes with reduced salinity were also found in the concentrations of individual adenylates and several adenylate ratios other than energy charge. Energy charge was calculated from the relative concentrations of adenosine 5-tri-, di- and monophosphate (ATP, ADP, AMP) in the columellar muscle of P. ebeninus and in the adductor muscle of A. trapezia and S. commercialis. Mean energy charge values for individuals in the low-salinity condition (10) were 0.61 for P. ebeninus, 0. 69 for A. trapezia and 0.53 for S. commercialis, compared with mean energy charge values in the control individuals (35 S) of 0.85 to 0.87 for P. ebeninus, 0.84 to 0.85 for A. trapezia and 0.64 to 0.76 for S. commercialis. The decrease in energy charge occurred within 24 h; no further change was found with exposure to low salinity for 48 h (A. trapezia) or no further change was found with exposure to low salinity for 48 h (A. trapezia) or 20 h (P. ebeninus, S. commercialis). Total adenylate concentrations (ATP+ADP+AMP) differed between the three species, with mean values (kg^-1 wet weight) of 5.0 mmol in P. ebeninus, 3.4 mmol in A. trapezia and 3.0 mmol in S. commercialis. No trends with time were found in total adenylate concentrations in any species. The changes in energy charge that occurred were not consistent with the differing sensitivity of the three species to reduced salinity, and do not support the use of energy charge as an absolute measure of stress in molluscs generally.     

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41N; 76°40W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.     

Influence of extracts of adult crabs on molting of fiddler crab megalopae (<Emphasis Type="Italic">Uca pugnax</Emphasis>)

Larvae of many benthic invertebrates metamorphose in response to habitat cues, which include the presence of adult conspecifics. Prior research showed that fiddler crab [Uca pugnax (Smith)] megalopae advance molting to the first crab stage in seawater in which conspecific adult crabs were maintained. In the present study, extracts of adult crabs were prepared and the specificity and protein content were characterized. U. pugnax megalopae were reared in the laboratory to minimize their prior exposure to potential molting cues. Then they were presented with extract solutions (in 1995, 1996, 1997, and 2000) to determine the specificity of the molting response to extracts of several crab species and the effects of the protein concentration of the extract, age of the megalopae at exposure to extract, and the duration of exposure on the molting response. Megalopae of U. pugnax molted sooner in seawater containing extract from adult conspecifics than in filtered seawater without extract or in seawater containing extract from the congener U. minax. Extract from the mud crab Dyspanopeus sayi retarded molting of U. pugnax megalopae. The stimulatory effect of U. pugnax extract on molting of megalopae was dependent on extract concentration. U. pugnax megalopae did not respond to extract until 6 days after molting, and only 1 day of exposure was sufficient to stimulate molting. The boiled extract was effective after being frozen (–15°C) for 2 years. The preparation of stable extract provides a uniform stimulus for multiple experiments examining the specificity of molting cues for brachyuran crustacean larvae and the onset of receptivity to cues.Communicated by J.P. Grassle, New Brunswick     

Salinity tolerance,salinity preference and temperature tolerance in the high-shore harpacticoid copepod Tigriopus brevicornis

Tigriopus brevicornis (O. F. Müller) were collected in 1992 from rock pools close to U.M.B.S. Millport, Isle of Cumbrae, U.K. and acclimated to various combinations of salinity and temperature for at least 1 wk prior to laboratory experiments. Higher salinities of acclimation enhanced tolerance to high salinity stress, while tolerance of low salinities was hardly affected by acclimation salinity. Acclimation to low temperature (10°C) extended the survivable salinity range for T. brevicornis. High-salinity acclimation enhanced the survivable temperature range. Copepods acclimated to 60 survived significantly lower and higher temperatures than did 34-acclimated individuals. At high temperature, 75-acclimated female copepods had the highest median lethal temperature, 38.9°C. Females were significantly more resistant to high temperatures than males. The copepods were seen to have a very low median lethal temperature when frozen into solid ice for 2 h; 50% mortality occurred at-16.9°C in 10°C, 34-acclimated T. brevicornis. Salinity preference experiments demonstrated an ability to discriminate between salinities differing by as little as 3. Copepods acclimated to 34 chose salinities near their acclimation salinity; individuals acclimated to 5 favoured slightly higher salinities, while copepods acclimated to 60 chose rather lower salinities.     

Invertebrate introductions in marine habitats: two species of hydromedusae (Cnidaria) native to the Black Sea,Maeotias inexspectata and Blackfordia virginica,invade San Francisco Bay

The hydrozoans Maeotias inexspectata Ostroumoff, 1896 and Blackfordia virginica Mayer, 1910, believed to be native to the Black Sea (i.e. Sarmatic) and resident in a variety of estuarine habitats worldwide, were found as introduced species in the Petaluma River and Napa River, California, in 1992 and 1993. These rivers are mostly-estuarine tributaries that flow into north San Francisco Bay. Both species appeared to be well-established in this brackishwater habitat. Salinities at the collection sites were about 11 during the summer, rising to nearly 20 in the early autumn and falling to near 0 in the winter. Large numbers of all sizes of both species of medusae were observed and collected, indicating that the hydroid stages of the life cycles of the two are also well-established in these rivers. In the Petaluma River, populations of both species were at maximum in late July, with numbers of individuals declining through August and into September; the Napa River was sampled only in October, and at that time only B. virginica was found. Examination of full guts of M. inexspectata and B. virginica medusae revealed that both species had fed nearly exclusively on small crustaceans, principally barnacle nauplii, copepods and their eggs and nauplii, and crab zoea larvae (M. inexspectata only). All the M. inexspectata medusae were males, indicating that the population has probably developed from the introduction of perhaps only a single male polyp or polyp bud. In spite of its inability to reproduce sexually, this population appears to be maintained by the prodigious ability of the polyp to bud and reproduce asexually, and is fully capable of invading additional low-salinity habitats from its present Petaluma River site. Male and female B. virginica medusae were collected in both the Petaluma River and the Napa River, indicating that B. virginica may have been introduced by either the polyp or medusa stage (or both), but that multiple individuals (of both sexes) must have arrived from another port in one or more invasions. As indicated for M. inexspectata, the B. virginica population will also probably seed new populations in San Francisco Bay and elsewhere. Based on its cnidome as well as the morphology of both medusa and polyp, M. inexspectata has been reclassified by moving it from the family Olindiidae, Limnomedusae, to the family Moerisiidae, Anthomedusae.     

Effects of changes in water salinity upon exercise and cardiac performance in the European seabass (<Emphasis Type="Italic">Dicentrarchus labrax</Emphasis>)

The European seabass is an active euryhaline teleost that migrates and forages in waters of widely differing salinities. Oxygen uptake (M_O2) was measured in seabass (average mass and forklength 510 g and 34 cm, respectively) during exercise at incremental swimming speeds in a tunnel respirometer in seawater (SW) at a salinity of 30 and temperature of 14°C, and their maximal sustainable (critical) swimming speed (U_crit) determined. Cardiac output (Q) was measured via an ultrasound flow probe on their ventral aorta. The fish were then exposed to acute reductions in water salinity, to either SW (control), 10, 5, or freshwater (FW, 0), and their exercise and cardiac performance measured again, 18 h later. Seabass were also acclimated to FW for 3 weeks, and then their exercise performance measured before and at 18 h after acute exposure to SW at 30. In SW, seabass exhibited an exponential increase in M_O2 and Q with increasing swimming speed, to a maximum M_O2 of 339±17 mg kg^–1 h^–1 and maximum Q of 52.0±1.9 ml min^–1 kg^–1 (mean±1 SEM; n=19). Both M_O2 and Q exhibited signs of a plateau as the fish approached a U_crit of 2.25±0.08 bodylengths s^–1. Increases in Q during exercise were almost exclusively due to increased heart rate rather than ventricular stroke volume. There were no significant effects of the changes in salinity upon M_O2 during exercise, U_crit or cardiac performance. This was linked to an exceptional capacity to maintain plasma osmolality and tissue water content unchanged following all salinity challenges. This extraordinary adaptation would allow the seabass to maintain skeletal and cardiac muscle function while migrating through waters of widely differing salinities.Communicated by S.A. Poulet, Roscoff     

The ecology of hydromedusae from the Cochin area

A survey has been made of medusae found in plankton samples taken fortnightly over a period of a year at 2 fixed stations in the waters near Cochin, Kerala, India. Twenty two species were identified, 8 of them were recorded for the first time in this region. The 2 stations were found to be controlled by different hydrographical regimes; that at Aroor is entirely characterised by backwater conditions, with salinity afternating from fresh water to 32 S according to the monsoon cycle; the station at Fairway Buoy is also dominated by the monsoon cycle, with mixed, low-salinity (11 to 13) waters at the surface, and upwelled Arabian Sea water in the bottom layer during the Southwest monsoon Life cycles are discussed and the population dynamics of Eutima commensalis at Aroor are analysed. A mathematical model is proposed for more detailed population dynamics studies of plankton species in the Cochin Backwaters, taking E. commensalis as an example.     

Routine oxygen consumption of Mugil cephalus,Liza dumerili and L. richardsoni at Different temperatures and salinities

Oxygen consumption studies were undertaken with 3 mullet species to determine b, the exponent of w, as well as a, as indices of metabolic rate in the equation M=aw ^bwhere M=metabolic rate, a=the intensity of metabolism, W=body weight, and b=the exponent of w. This was done under 5 experimental temperatures (13°, 18°, 23°, 28°, 33°C) for Mugil cephalus and Liza dumerili at 1 and at 35 S, and for L. richardsoni at 35S only. Mean b values were approximately 0.85. The a values depended on temperature, and increased according to Van't Hoff's law except for L. dumerili (1 S) and L. richardsoni (35 S) for a temperature increase from 23° to 28°C. It was found that handling had a profound influence on metabolic rate and led to considerably increased consumption rates during the first 8 h after introduction into the respiration chambers. Fasting in L. dumerili resulted in a total drop of 27% in oxygen consumption over a period of 6 days, of which 10% occurred over the first 24 h. Oxygen consumption displayed diurnal rhythms during the 6 day period, with lowest consumption rates at midday and midnight and highest just after sunrise and sunset.     

Zooplankton of the Bristol Channel and Severn Estuary. The distribution of four copepods in relation to salinity

The seasonal variations in distribution and abundance of the common zooplankton species in the Bristol Channel and Severn Estuary were related to the salinity regimes observed over the period November 1973 to February 1975. The dominant constituents in all regions were the calanoid copepods, which reached maximum densities in July: approximately 100 times their winter levels. Four zooplankton assemblages were recognised using an objective classification program which computed similarity coefficients and used group-average sorting. The assemblages existed along the salinity gradient observed from the Severn Estuary to the Celtic Sea. The assemblages were classified as true estuarine, estuarine and marine, euryhaline marine and stenohaline marine and were characterized by the copepods Eurytemora affinis (Poppe) (<30S), Acartia bifilosa var. inermis (rose) (27 to 33.5S), Centropages hamatus (Lilljeborg) (31 to 35S) and Calanus helgolandicus (Claus) (>33S), respectively.     

Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13 S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30 S to 3.5 at 15 S. Oxygen consumption rates (QO₂) of both species held at 30 S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO₂ of both species held at 30, 20 and 15 S varied directly with salinity. Perivisceral fluid PO₂ varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20 S than at 30 S due to reduced PO₂ and extracellular fluid volume at the lower salinities. The QO₂ of both species varied directly with ambient salinity during a 30-10-30. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO₂ of either species over a 15-30-15. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO₂ declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15 S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.     

Response to perceived predation threat in fiddler crabs: trust thy neighbor as thyself?

Prey require information if they are to respond to predation threat in a risk-sensitive manner. One way that individuals can obtain this information is through the predator-mediated, threat-induced behavior of conspecifics. We examined such a possibility in a refuge-seeking species, the sand fiddler crab (Uca pugilator). Crabs were either exposed directly to a simulated predation threat (a moving cylinder) or the threat-induced response of a near neighbor. We found that fiddler crabs responded to the flight of their neighbors even when they, themselves, were not privy to the stimulus that induced their neighbors response. However, the wider range of behaviors exhibited by these crabs—which included no reaction, freezing, running back to the burrow entrance, and burrow retreat—suggest that non-threatened crabs either (1) perceived the gravity of the predation threat differently from their directly threatened neighbors and/or (2) engaged in behaviors that allowed them to acquire further information in the face of uncertainty. Conspecific behaviors also had an effect on the hiding duration of crabs, with individuals hiding longer if they saw both the predation threat and the flight of their neighbor. Our results suggest that cues provided by conspecifics can play an important role in guiding the antipredator response of refuge-seeking prey.     

Effects of acute changes in temperature and salinity on the oxygen uptake of larvae of herring (Clupea harengus) and plaice (Pleuronectes platessa)

Routine oxygen uptake (QO₂) by yolk-sac and firstfeeding larvae of herring (Clupea harengus L.) and plaice (Pleuronectes platessa L.) was studied after acute change of temperature (8°, 13°, 18°C) and salinity (5, 12.7, 32, 40). In both species, QO₂ (l mg^-1 dry wt h^-1) of both larval stages increased with increasing temperature. Salinity effect on QO₂ varied: for yolk-sac larvae of both species a lower QO₂ was found at lower combined salinities (5 and 12.7); for feeding larvae a lower QO₂ was observed at 12.7 for both species, possibly due to the relatively smaller size of larvae used at this salinity. For both species, oxygen uptake increased as larvae grew and weight regression coefficients were between 0.74 and 1.33. At 32 S, no difference was found in oxygen consumption between species as a function of temperature.Based on a dissertation submitted in partial fulfillment of the requirements for the degree of Master of Science at the University of Stirling, Stirling, Scotland. The work was performed at the Dunstaffnage Marine Research Laboratory, Oban, Scotland     

Mangrove forest structure and productivity in the Fly River estuary,Papua New Guinea

In April, July and August 1989 and February 1990, the delta region of the Fly River was surveyed to establish the aerial extent of mangrove forests, their species composition, tree densities and basal areas, and potential net primary production. Mangrove forests cover 87 400 ha, mainly on islands within the delta. Twentynine mangrove plant species were recorded, but there were only three major forest types in the delta.Rhizophora apiculata-Bruguiera parviflora forests (hereafterRhizophora-Bruguiera forests) predominated in regions where river water salinities were >10. These forests covered 31 500 ha and had mean total tree densities and basal areas of 2027 stems ha^–1 and 21 m² ha^–1, respectively. Forests of the palmNypa fruticans (hereafterNypa forests) covered 38 400 ha, mainly in regions where river salinities were ~1 to 10, and had mean total densities and basal areas of 4431 stems ha^–1 and 38 m² ha^–1, respectively. Forests dominated byAvicennia marina and/orSonneratia lanceolata (hereafterAvicennia-Sonneratia forests) predominated on accreting banks of sediment and covered 17 500 ha. In very low-salinity (< 1) regions there are large monospecific stands ofS. lanceolata. Mean total densities and basal areas forAvicennia-Sonneratia forests were 7036 stems ha^–1 and 22 m² ha^–1, respectively. Mean net primary productivity (kg C ha^–1 d^–1) was estimated to be 26.7, 27.1 and 19.0 forRhizophora-Bruguiera, Nypa andAvicennia-Sonneratia forests, respectively. Total daily net primary production by all mangrove forests was estimated at 2214 t carbon. Using assumptions based on work in tropical Australia, it was estimated that ~678 t carbon (or 31% of primary production) were exported daily from mangrove forests to the waters of the delta.Contribution No. 550 from the Australian Institute of Marine Science     

Salinity tolerance of benthic estuarine diatoms as tested with a rapid polarographic measurement of photosynthesis

Measurements of net photosynthesis of benthic estuarine diatoms were made by polarographic registration of oxygen saturation. A measuring cell was constructed in which media with salinities of 2.0 to 100.7 were pumped over the algae between measurements. Diatoms from unialgal cultures and mixed populations from intertidal flats appeared to be highly tolerant of extreme salinities. During short-term exposures (20 min) the net photosynthesis of the algae did not drop below 70% of the initial values, within the salinity range 4.0 to 60.0. Prolonged exposure (up to 6 h) gave essentially the same results. Populations of benthic diatoms, sampled from field stations with mean salinities of about 30, 12, and below 5, showed only gradual differences in their tolerance of salinities between 2.0 and 33.7. Two species, Navicula arenaria and Nitzschia sigma, were cultured in media ranging in salinity from 8.0 to 45.0 and from 2.0 to 45, respectively. The tolerance to changing salinity was only slightly affected by the salinity of the medium in the preculture. The role of salinity in the production and distribution of intertidal diatoms is discussed.     

Interactions of methylmercury,cadmium, and salinity on regeneration in the fiddler crabs Uca pugilator,U. pugnax and U. minax

After multiple autotomy in the fiddler crabs Uca pugilator, U. pugnax and U. minax, both methylmercury and cadmium retard limb regeneration and ecdysis. When crabs in sea water are exposed to a mixture of both metals, the effect is increased, indicating that the two are interacting in an additive way. In 50% sea water (=15% S), the effects of cadmium are greatly intensified so that growth of limb buds is extremely slow, if it occurs at all. When methylmercury is present in the water at the same time, the severe effects of cadmium are somewhat ameliorated, indicating an antogonistic interaction of the two metals under these conditions. Adding additional calcium to the 50% sea water also decreased the severity of the cadmium effect, thus supporting the idea of a calcium-cadmium competition.