Behavioural adaptations of the fiddler crabs Uca vocans borealis (crane) and Uca lactea lactea (De Haan) for coexistence on an intertidal shore

The distribution and coexistence of the fiddler crabs U. vocans borealis and U. lactea lactea was investigated in the upper shore of a sandy beach with respect to particle size, water content and total organic carbon content of the sediments. A clear segregation of habitats between the two species was apparent. U. lactea lactea were only found in sediments with significantly lower total organic carbon content. U. lactea lactea had a higher ratio for the size of third maxilliped to body size than U. vocans borealis. There was no statistical difference in the median sediment particle size of the habitats where both species were found on the study shore. U. lactea lactea was larger on shores where U. vocans borealis were absent. Smaller U. vocans borealis individuals occupied sediments with higher water content and finer particles. Smaller individuals of U. vocans borealis also possessed fewer spoon-tipped setae on second maxilliped than their larger conspecifics, which were found mostly on coarser sediments. U. lactea lactea spent significantly more time on the surface than in the burrow during low tide when compared with U. vocans borealis. Both Uca species exhibited similar desiccation resistance. U. lactea lactea tends to keep its frontal region in close contact with sediments to maintain moisture when being exposed to air.     

The response of meiofauna and microphytobenthos to engineering effects of fiddler crabs on a subtropical intertidal sandflat

Fiddler crabs are key bioturbators on tidal flats. During their intense bioturbation process, they manipulate large amounts of sediment, altering the physical state of existing materials. We investigated whether different types of sediment bioturbation produced by fiddler crabs modulate meiofaunal assemblages and microphytobenthic content. We hypothesized that sedimentary structures produced by burrowing (the burrow itself and the excavation pellets) and feeding (feeding pellets) generate different microenvironments compared with areas without apparent signs of fiddler crab disturbance, affecting both meiofauna and microphytobenthos, independent of the sampling period. Our results indicate that the engineering effects of burrow construction and maintenance and the engineering effects of fiddler crab foraging modulate meiofaunal assemblages in different ways. Overall, meiofauna from burrows and excavation pellets was more abundant and diverse than at control sites, whereas feeding pellets contained poor meiofaunal assemblages. By contrast, only foraging effects were detected on microphytobenthos; independent of the sampling period, Chl a and phaeopigment content were higher in the feeding pellets, but similar among burrows, excavation pellets and control sites. The present study demonstrates that the different engineering effects of fiddler crabs are an important source of habitat heterogeneity and a structuring agent of meiofaunal assemblages on subtropical tidal flats.     

The relative importance of substratum characteristics and recruitment in determining the spatial distribution of the fiddler crab Uca uruguayensis Nobili

The roles of sediment characteristics and the pattern of recruitment in influencing the abundance of the fiddler crab Uca uruguayensis on Argentinean mudflats were evaluated. The density of adult crabs showed a patchy distribution related to the sediment thickness (depth at which a layer of fossil shells are buried), but the density of juvenile crabs was not coupled with the density of adult crabs. In a field experiment, fossil shells were removed and the density of crabs significantly increased, which demonstrates that the presence of the layer of shells is a structure that may hinder the establishment of burrows. The density of crabs was related to sediment thickness, sediment torque and organic matter content. The importance of each of these variables was different for adult and juvenile crabs, indicating that the distribution of adult crabs may be caused by mechanisms affecting adult crabs themselves and is not established by the recruitment pattern. Moreover, in a field experiment, the density of juveniles decreased when adult crabs were added, and increased when adult crabs were removed.The morphology of burrows was related to sediment characteristics. Burrows were deeper, longer and more voluminous when sediment thickness was high. The volume of burrows decreased with increasing sediment torque. These results suggest that the morphology of burrows is related to the space available and the ease with which sediment it can be excavated. However, an important amount of variability remained unexplained, suggesting the presence of additional environmental variables or behavioural plasticity not considered by this study. Together, these results demonstrate that the spatial heterogeneity in the environmental factors can be translated to a spatial heterogeneity in the distribution of fiddler crabs.     

Habitat partitioning between prey soldier crab Mictyris brevidactylus and predator fiddler crab Uca perplexa

Interaction and habitat partition between the soldier crab Mictyris brevidactylus (prey) and the fiddler crab Uca perplexa (predator) were examined at a sandy tidal flat on Okinawa Island, Japan, where they co-occur. Both live in dense colonies. When the soldier crabs were released in the densely populated habitat of the fiddler crab, male fiddler crabs, which maintain permanent burrows in hard sediment, preyed on small soldier crabs and repelled large ones. Thus, the fiddler crabs prevented the soldier crabs from trespassing. It was also observed whether soldier crabs burrowed successfully when they were released 1) where soldier crab burrows just under the sand were abundant, 2) in a transition area between the two species, 3) an area without either species, and 4) where artificial tunnels simulated soldier crabs' feeding tunnels were made by piling up sand in the area lacking either species. In contrast to the non-habitat area, many soldier crabs burrowed in the sediment near the release point in the tunnel, transition and artificial tunnel areas. This indicates that the feeding tunnels on the surface attracted other crabs after emergence. When the large male fiddler crabs were transplanted into the artificial burrows made in soft sediment of the soldier crab habitat, all left their artificial burrows by 2 days. In the fiddler crab habitat, however, about one-third of the transplanted male fiddler crabs remained in the artificial burrows after 3 days. The soldier crabs regularly disturb the sediment by the up and down movement of their burrow (small air chamber) between tides. This disturbance probably prevents the fiddler crab from making and occupying permanent burrows. Thus, it appears that these crabs divide the sandy intertidal zone by sediment hardness and exclude each other by different means.     

Behavioural evidence for body colour signaling in the fiddler crab Uca perplexa (Brachyura: Ocypodidae)

In the reproductive season, mature females of the fiddler crab Uca perplexa leave their burrows and wander about their habitat for mating. To clarify whether the fiddler crabs respond to colour or luminosity, I examined the behavioural responses of the males to the wandering females before and after the females were painted white, red, black or blue. The behaviours of the males were categorized into three types: lateral–circular wave and lateral–straight wave for courting, and repelling. Before painting, almost all of the males courted the females. After painting, significantly fewer males courted the red-, black- and blue-painted females than courted the white-painted females. These results mean that the fiddler crabs can discriminate colours or luminosity. The role of body colour as a visual signal in crab society is discussed.     

Large‐scale ecosystem engineering by flamingos and fiddler crabs on West‐African intertidal flats promote joint food availability

Although the ecosystem engineering concept is well established in ecology, cases of joint engineering by multiple species at large scales remain rare. Here, we combine observational studies and exclosure experiments to investigate how co‐occurring greater flamingos Phoenicopterus roseus and fiddler crabs Uca tangeri promote their own and each other's food availability by creating a spatially complex mosaic of depressions (bowls, gullies) and hummocks (plateaus, mounds) in the intertidal zone. This results in a mosaic of microhabitats with different tidal inundation regimes. These microhabitats are spatially organized with labyrinth‐like patterns in the high intertidal zone and spotted patterns in the lower intertidal, both of which likely arise from biophysical interactions between these organisms and hydrodynamic forces. We show that the resulting spatial complexity is vital for biofilm production. The depression microhabitats were wetter and richer in organic matter and biofilms compared with hummocks. Excluding flamingos and crabs resulted in an increase in biofilm biomass over the shorter term (six months), but a decrease over the longer term (after one year). Moreover, our results strongly suggest that these biogeomorphological microhabitats in the mosaics were maintained by the feeding activities of flamingos and to a lesser extent crabs. During a period of flamingo exclusion, all the spotted patterns filled up with sediment, while the exclusion of crabs led to gradual sediment accumulation in the labyrinth‐like patterns. Collectively, these findings provide empirical evidence for large‐scale joint promotion of food availability by multiple taxa in a marine ecosystem.     

Impact of fiddler crab foraging and tidal inundation on an intertidal sandflat: season-dependent effects in one tidal cycle

Intertidal sandflats inhabited by fiddler crabs are ideal systems in which to study the effects of physical and biological processes. This study addressed two questions: (1) Do fiddler crab feeding and tidal inundation have measurable effects on the sandflat over one tidal cycle? (2) Does the sandflat change over the course of a year? In field exclusion/inclusion experiments, fiddler crabs reduced sediment organic content by 40%, Chlorophyll a levels by 20% and meiofaunal density by 60% in one tidal cycle. Effects were most pronounced in the spring when organic content and meiofaunal densities were maximal. Effects of foraging were not erased by the tide and accumulated over time. The sandflat had highest levels of all variables in spring and minimal levels during summer and fall. Crabs graze the sandflat to minimum levels in the spring. Due to crab foraging, the flat is barren during the summer and fall, and recovers during the winter when crabs are minimally active.     

Morphological and behavioural adaptations to the rocky substrate by the fiddler crab Uca panamensis (Stimpson, 1859): preference for feeding substratum and feeding mechanism

The fiddler crab Uca panamensis (Stimpson, 1959) inhabits rocky shores. We examined its preference for feeding substratum—sand or rock—and its manner of feeding. The crab made its burrow in the sand among rocks but preferred to feed on rocks. The feeding time decreased as the distance between the burrow and the rock increased. We consider this to be a result of exclusive interaction among the crabs because they defended their feeding area on the rocks against others.The crab wetted a small area of rock with water held in the branchial chambers before and during feeding. It pinched up the wetted surface in the minor chelipeds, which have bundles of setae on the posterior tips of the dactyl and pollex, and put the material into its buccal cavity. It never expelled sand pellets while feeding on rock, which indicates that it swallowed the food particles directly, without sorting. The bundles of setae retained water by capillary attraction, which suggests that they capture the suspended fine food particles scraped from the rock. The wetting action may prevent the fine materials from dispersing. We consider that morphological alteration of the minor chelipeds, the application of water from the branchial chambers, and direct swallowing permit the fiddler crab to feed on fine materials attached to rocks.     

Composition and abundance of shorebird and macroinvertebrate communities differ according to densities of burrowing fiddler crabs in tropical intertidal flats

The burrowing activity of fiddler crabs inhabiting intertidal flats creates visually distinct patches within these habitats. However, differences in the composition and abundance of shorebirds and their macroinvertebrate prey between areas inhabited or not by crabs are yet to be studied. Here, we compare the macroinvertebrate and shorebird assemblages in low and high crab density areas in the intertidal flats of the Bijagos archipelago, Guinea-Bissau. High crab density areas are associated with lower richness and densities of macroinvertebrates. Shorebird assemblages were also less rich at high crab density areas and the differences in species composition occurred according to prey type preferences. Fiddler crab density was the most important variable explaining macroinvertebrate abundance, after accounting for the effects of fine fraction of sediment and distance to coast. Nonetheless, a controlled experimental setup would be required to attribute differences found to the engineering activity of fiddler crabs rather than other unaccounted habitat features. Our findings suggest that crab patches should be taken into account when assessing the distribution and abundance of macroinvertebrates and shorebirds in intertidal areas. Since low and high crab density areas differ markedly in terms of shorebird carrying capacity, monitoring variations in their extent will be important to interpret past and present population trends.

     

Fatty acid compositions of gonadal material and diets of the sea urchin, Psammechinus miliaris: trophic and nutritional implications

The fatty acid compositions of gonadal material was examined for the sea urchin Psammechinus miliaris (Gmelin) held in aquaria and fed either salmon feed pellets or the macroalga, Laminaria saccharina for 18 months. Gonadal material was also examined from P. miliaris collected from four field sites, including commercial scallop lines encrusted with the mussel, Mytilus edulis, sea cages stocked with Atlantic salmon Salmo salar and two intertidal sea-loch sites, characterised by either a fine mud or a macroalgal substratum. The fatty acid compositions of known and potential dietary material was examined. The proportions of certain fatty acids in the gonads of P. miliaris were significantly affected by diet type and location. Docosahexaenoic acid (DHA) 22:6 n-3 was significantly higher in the gonads of the sea urchins fed salmon feed in aquaria and collected from the salmon cages and scallop lines than in the gonads of the sea urchins fed L. saccharina in aquaria and collected from the intertidal sea loch sites. The salmon feed and the mussel tissue also contained a high proportion of this fatty acid. Stearidonic acid 18:4 n-3 and arachidonic acid 20:4 n-6, however, were found in significantly higher proportions than DHA in the gonads of the sea urchins fed L. saccharina and collected from the two intertidal sea-loch sites. L. saccharina was also found to contain high proportions of stearidonic and arachidonic acid. The gonads of the sea urchins collected from the intertidal site, characterised by a mud substratum, and from the scallop lines were found to contain a lower 18:1 n-9/18:1 n-7 ratio and a higher proportion of branched and odd-chained fatty acids, signifying a high dietary bacterial input, than the sea urchins held in the aquaria and collected from the salmon cage. 20:2 and 22:2 non-methylene-interrupted dienoic fatty acids (NMIDs) were found in P. miliaris fed diets lacking these fatty acids suggesting de novo biosynthesis. These results, therefore, suggest that the proportions/ratios of certain fatty acids in the gonads of P. miliaris could be used to give an indication of the predominant diet type of this species in the wild.     

Addition of dissolved nitrogen and dissolved organic carbon from wild fish faeces and food around Mediterranean fish farms: Implications for waste-dispersal models

Leaching of ammonium (NH₄⁺) and dissolved organic carbon (DOC) from food pellets used at three fish farms in the Mediterranean Sea and the faeces of four different species of farm-associated wild fish (Trachurus mediterraneus, Mugil cephalus, Trachinotus ovatus and Boops boops) were determined. They were placed in seawater and agitated slowly (5 cm s^− 1) to reflect natural conditions during their fall to the sediment. Two temperatures were tested, 25 °C and 15 °C, to assess the influence of seasons on leaching rates. Leaching from fish faeces was generally higher compared to food pellets. T. mediterraneus faeces leached more NH₄⁺ and DOC than M. cephalus, T. ovatus and B. boops. The results showed that there is an important addition of NH₄⁺ and DOC to the water column during sinking of the faeces and that this is species-dependent. Water turbulence and faeces composition seemed to have a higher influence than temperature on the leaching process. Due to the high abundance and biomass of farm-associated fish in the Mediterranean and their capacity to remove waste, they appear to be an important component for models that predict the impact of aquaculture. Large biomasses of wild fish at fish farms may reduce the impact on benthic systems but increase the nitrogen and carbon loads into the water column, affecting the pelagic system and modifying the spatial dispersion of wastes.     

Nutritional ecology of thalassinidean shrimps constructing burrows with debris chambers: The distribution and use of macronutrients and micronutrients

Four different approaches were combined to determine the nutritional relevance of debris chambers in the burrows of two thalassinidean shrimps: (1) the natural abundance of carbon and nitrogen stable isotopes in potential food sources, (2) their nutritional value based on the content and composition of essential nutrients, (3) a dual labelling experiment with shrimp in aquaria employing ¹⁵N- and ¹³C-labelled seagrass debris and (4) ration estimates using the acquisition rate of plant debris by the shrimps. The results of the four approaches confirmed the use of plant debris as a food source. Based on the natural abundance of stable isotopes, Corallianassa longiventris apparently relies on the chamber content and the burrow wall as sources of carbon and nitrogen, whereas Pestarella tyrrhena probably relies on ambient debris and on benthic foraminiferans and microphytobenthos in the surface sediment. Corallianassa longiventris obtains its essential nutrients predominantly from chamber debris and to a lesser extent from its burrow wall, P. tyrrhena from chamber debris, the burrow wall and the surface sediment. Among the essential nutrients, those amino acids commonly deficient to deposit feeders were particularly enriched in the burrow environments of the two shrimps. Highly unsaturated fatty acids (HUFAs) were lacking in all of C. longiventris potential food sources; this species may either be able to synthesize them de novo from linolic acid or may use another unknown source. For P. tyrrhena, surface sediment and chamber debris represent potential HUFA sources. The most probable thiamine and β-carotene supplier for C. longiventris is the chamber debris, for P. tyrrhena again the surface sediment. In both species, the rate of debris introduction into the burrow is sufficient to meet the nutritional demand.     

Polar lipid ester-linked fatty acid composition of Lake Vechten seston: an ecological application of lipid analysis

Abstract In order to relate the benthic lipid composition to possible sources in the water column, the sestonic communities of a monomictic lake were profiled using their saponifiable polar lipid fatty acids, which were identified by capillary gas chromatography-mass spectrometry (GC-MS). The epilimnion, dominated by the dinoflagellate alga Ceratium hirundella , was characterized by C_20:5 and C_22:6 polyunsaturated fatty acids. The photic anoxic metalimnion supported a radically different community, dominated by photosynthetic sulfur-oxidizing bacteria ( Chromatium and Chloronema spp.) and a Synechococcus -like cyanobacterium, and was characterized by high concentrations of C₁₆ and C₁₈ monounsaturated fatty acids. The fatty acid compositions of the hypolimnetic seston and the sediment were qualitatively similar to that of the metalimnion. Methyl-branched acids, commonly found in eubacteria, increased with depth in the water column. The concentrations of several unusual fatty acids found in Desulfovibrio spp. Desulfobacter spp. and Desulfotomaculum spp. were inversely related to sulfate concentration in the metalimnion. After the water column mixed in the winter, steep gradients of respiratory terminal electron acceptors developed in the surface sediment and were reflected in the polar lipid fatty acid compositions. The results show that fatty acids derived from the membranes of epilimnetic phytoplankton were efficiently metabolized in the oxic portion of the water column. The fatty acids synthesized by prokaryotic microorganisms at and below the oxycline dominated the sediment. The polar lipid fatty acid composition of the sediment showed seasonal changes which can be associated with concentrations of terminal electron acceptors of microbial respiration, and thus with physiologically distinct bacterial groups.     

Sexual selection and the physiological consequences of habitat choice by a fiddler crab

In mid-Atlantic salt marshes, reproductively active male sand fiddler crabs, Uca pugilator, use a single greatly enlarged major claw as both a weapon to defend specialized breeding burrows from other males and an ornament to attract females for mating. During the summer breeding season, females strongly prefer to mate with males controlling burrows in open areas high on the shore. Food availability decreases while temperature and desiccation stress increase with increasing shore height, suggesting that the timing and location of fiddler crab mating activity may result in a potential trade-off between reproductive success and physiological condition for male crabs. We compared thermal preferences in laboratory choice experiments to body temperatures of models and living crabs in the field and found that from the perspective of a fiddler crab, the thermal environment of the mating area is quite harsh relative to other marsh microhabitats. High temperatures significantly constrained fiddler crab activity on the marsh surface, a disadvantage heightened by strongly reduced food availability in the breeding area. Nevertheless, when the chance of successfully acquiring a mate was high, males accepted a higher body temperature (and concomitantly higher metabolic and water loss rates) than when the chances of mating were low. Likewise, experimentally lowering costs by adding food and reducing thermal stress in situ increased fiddler crab waving display levels significantly. Our data suggest that fiddler crabs can mitigate potential life history trade-offs by tuning their behavior in response to the magnitude of both energetic and non-energetic costs and benefits.     

A Spatially Explicit Model of Synchronization in Fiddler Crab Waving Displays

Fiddler crabs (Uca spp., Decapoda: Ocypodidae) are commonly found forming large aggregations in intertidal zones, where they perform rhythmic waving displays with their greatly enlarged claws. While performing these displays, fiddler crabs often synchronize their behavior with neighboring males, forming the only known synchronized visual courtship displays involving reflected light and moving body parts. Despite being one of the most conspicuous aspects of fiddler crab behavior, little is known about the mechanisms underlying synchronization of male displays. In this study we develop a spatially explicit model of fiddler crab waving displays using coupled logistic map equations. We explored two alternative models in which males either direct their attention at random angles or preferentially toward neighbors. Our results indicate that synchronization is possible over a fairly large region of parameter space. Moreover, our model was capable of generating local synchronization neighborhoods, as commonly observed in fiddler crabs under natural conditions.     

Taxonomy and biogeography of the fiddler crabs (Ocypodidae: Genus Uca) of the Atlantic and Gulf coasts of eastern North America

Fifteen species of fiddler crabs are reported for eastern North America between Massachusetts and Quintana Roo, Mexico. Thirteen occur in the United States and 11 in Mexico, with eight in common to the two countries. Of 13 species in the Gulf of Mexico, five are endemic and a sixth is restricted largely to the peninsulas of Florida and Yucatan. The status of U. rapax in the northern Gulf remains to be resolved. Range limits of most species approximate one or the other of two sets of intersecting thermal and geological boundaries that subdivide the Gulf of Mexico along north-south and east-west axes. Species belonging to subgenus Minuca tend to replace one another at the thermally-controlled Carolinian-Caribbean marine biotic boundary across the Florida peninsula and northern Gulf. However, only U. minax of all the North American fiddler crabs exhibits the classical disjunct Carolinian distribution, and this appears basically to reflect the discontinuous distribution of temperate salt marshes that are the habitat of the species. Distributions of species belonging to subgenus Celuca adhere for the most part to the subdivision of the Gulf into western terrigenous and eastern carbonate sedimentary provinces. The northern transition occurs in the vicinity of Apalachee Bay and the southern at Laguna de Terminos. A third distributional pattern is shown by U. subcylindrica , a specialized endemic species of the hypersaline Laguna Madre system of the western Gulf. The level of endemism in the fiddler crabs is relatively high in comparison with that of other marine groups within the Gulf of Mexico. This may be a consequence of the adaptations of fiddler crabs as specialized deposit feeders to regional differences in climatic and edaphic characteristics of a marginally marine upper shore habitat. The distributional patterns of the endemics could prove useful in reconstructing palaeoecological events of evolutionary significance within the Gulf of Mexico.     

Fatty acid compositions of arthropod and cephalopod photoreceptors: interspecific,seasonal and developmental studies

Fatty acid compositions of the compound eyes of insects (soldier-bug, Hemiptera, and silk moth, Lepidoptera), crustaceans (crayfish and grapsid crab, Decapoda) and inner and outer segments of visual cells of a squid (Cephalopoda, Mollusca) were analyzed by gaschromatography for interspecific comparison. Fatty acid compositions showed great variation among species. In insect compound eyes, 16:0 and 18:0 were the main saturated fatty acids, and 18:1 was the dominant unsaturated fatty acid. Silk moth eyes contained, in addition, considerable amounts of 18:2 and 20:5. In crustacean compound eyes, the main saturated fatty acids were 16:0 and 18:0, and 14:0 (5.0%) was only detected in grapsid crabs; the main unsaturated fatty acids were 20:4, 20:5 and 22:6. Both whole eyes and rhabdom fraction of crayfish showed similar profiles of fatty acid compositions. Both inner and outer segments of squid retinae were characterized by high amounts of unsaturated fatty acids, especially 22:6. Compound eyes of grapsid crabs were used for the experiments on seasonal changes of fatty acid compositions. UFA/SFA ratios (weight in % of unsaturated fatty acids saturated fatty acids) were lowest (1.0) in July and highest (2.5) in March, and unsaturation indexes (average number of double bonds per molecule) were lowest (1.5) in July and highest (2.3) in March. Fatty acids 18:0 and 20:1 showed a significant correlation with the changes of seasonal temperature. Fatty acid analysis of the developing compound eyes of silk moths during the pupal stage revealed that eicosapentanoic acid (20:5) increased remarkably in parallel with the development of photoreceptive membranes, the rhabdoms. This suggests that eicosapentaenoic acid may play an important role in formation and function of rhabdoms.     

Periphyton density and shading in relation to tidal depth and fiddler crab activity in intertidal seagrass beds of the Banc d'Arguin (Mauritania)

Periphyton development was studied on microscopic glass slides and leaves of Zostera noltii Hornem. in an intertidal area in the Banc d'Arguin (Mauritania). The effects of shading, tidal depth and grazing activities by the fiddler crab Uca tangeri Eydoux were evaluated. For all experiments, periphyton ash content was high (52–93%) and ash-free dry weight ranged between 0.10–0.63 mg cm^–2. Slides accumulated more periphyton than leaves.Artificial shading (62–99%) for 13 days had no effect on periphyton densities on leaves. Increased tidal depth resulted in higher ash-free dry weight on slides, but in lower ash-free dry weight on leaves. Significant variation along the coastline also existed. The effect of fiddler crabs was studied using exclosures. Presence of fiddler crabs reduced periphyton density on slides, whereas light transmittance was increased. On leaves, no significant fiddler crab effect was found. This difference between leaves and slides was probably caused by a storm at the day before the end of the experiment, and by the higher periphyton density on slides as compared with leaves. As visual inspection during the experiment showed clear differences in appearance of leaves inside and outside the exclosures, the storm probably sloughed off mainly the older leaves, i.e. those on which the differences in periphyton cover were the highest.It is hypothesized that periphyton accumulation is higher with increased tidal depth, whereas fiddler crab grazing pressure also increases in this direction. The result is a decreased periphyton density with increased tidal depth.The presently found light extinction coefficients (mean 0.8 m^–1) and periphyton light attenuance (up to 25%) in Banc d'Arguin are not likely to affect seagrass leaf growth.     

The effect of abrupt salinity change on oxygen consumption in crab megalopae of Chincoteague Bay,USA

Variability in salinity is an environmental stressor that crab megalopae encounter as they are carried by tides and currents throughout Chincoteague Bay. We exposed blue crab (Callinectes sapidus) and fiddler crab (Uca spp.) megalopae to abrupt salinity changes from 10 to 31 ppt and measured their oxygen usage. It was hypothesized that the megalopae would cope with the changes in a manner reflective of the documented abilities and tolerances of adult crabs. It was also hypothesized that lower salinities would have a particularly detrimental effect on the megalopae reflected by both increased oxygen usage and mortality. The megalopae of both species did exhibit an increase in oxygen use at lower salinities, although the effect was more pronounced during the initial transition and decreased during acclimation. The megalopae mirrored the adult responses, with blue crab larvae consuming more oxygen per mg of wet weight at lower salinities, whereas fiddler crab larval oxygen consumption was relatively uniform at all salinities. Mortality of some blue crab postlarvae was observed at 10 ppt while all larval fiddler crabs survived. Coupled with the introduction of additional fresh water into the global water system, these results indicate that further investigation into this subject is necessary.     

Organic matter mineralization in an organic-rich sediment: Experimental stimulation of sulfate reduction by fish food pellets

Abstract The combined effects of organic matter additions and temperature on short chain fatty acid (SCFA) turnover, sulfate reduction and nutrient accumulation were examined in an organic-rich fish farm sediment. Fish food pellets, which contribute significantly to the organic matter loss from fish farms, were added to surface sediment at three loadings (2.8; 14.0; 28.0 mg ww g⁻¹ ww sediment; equivalent to organic matter loadings measured during fish farming) and incubated for 30 days in anaerobic bags at 5°C and 15°C. SCFA accumulated to high levels (acetate up to 85 mM, propionate up to 17 mM, butyrate up to 25 mM) in sediments amended with food pellets, and sulfate reduction was stimulated up to 30 times relative to unamended sediments. Sulfate reducers appeared saturated with substrates (SCFA) even in the lowest additions. A low C/N ratio (0.4–1.8) of the major mineralization products (TCO₂ and NH₄⁺) indicated preferential nitrogen mineralization in amended sediment compared with the total particulate pool (C/N = 8.8–11.9) and added food pellets (C/N = 8.4).