Biochemical influence of amphidromous shrimps on stream ecosystems in Japan

Not only through species interactions but also through excretion of nutrients, consumers can have profound effects on the ecosystem structure. While many studies in lentic ecosystems address both effects in combination, little is known about their linkages in lotic ecosystems. By a combination of field manipulative experiment, excretion measurements of consumes, and field surveys, we evaluated biochemical effects of amphidromous shrimps on stream ecosystems. The field manipulative experiment showed that the presence of shrimp suppressed the total aquatic insect biomass by 9% but increased the total benthic macroinvertebrate biomass including the shrimps by 196%. The biomass-specific NH4+ excretion rate by shrimp was similar to aquatic insects, and the calculated mean NH4+ excretion by benthic macroinvertebrate assemblage was 144% higher in the presence of shrimps. On the other hand, shrimps excreted much less PO43− than aquatic insects, and the PO43− excretion by the benthic macroinvertebrate assemblage did not change by the presence of shrimps. The field survey showed a positive correlation between NO3− concentration in stream water and shrimp density, suggesting that the excess NH4+ was nitrified and raised NO3− concentration. In contrast, PO43− concentration in stream water was negatively associated with the shrimp density, indicating that the dominance of shrimps that excrete little PO43− decreased the PO43− concentration. While nutrient concentration of stream water is often attributed to the condition of the watershed area, the results of this study indicate downstream connectivity to the ocean can also influence the nutrient dynamics of the stream through the density of amphidromous shrimps.

Density-dependent condition of juvenile penaeid shrimps in seagrass-dominated aquatic vegetation beds located at different distance from a tidal inlet

Seagrasses are critical habitats for the recruitment and growth of juvenile penaeid shrimps within estuaries and coastal lagoons. The location of a seagrass bed within the lagoon can determine the value of a particular bed for shrimp populations. Consequently, differences in the abundance of shrimp can be found in seagrasses depending on their location. As shrimp density increases, density-dependent effects on biological parameters are more likely to occur. However, knowledge about density-dependent processes on shrimp populations in nursery habitats remains limited. The present investigation was undertaken to examine the effects of population density on shrimp condition in two selected seagrass beds, located at different distance from a tidal inlet, one 25 km away (distant) and the other 1 km away (nearby), in a subtropical coastal lagoon. The study was based on monthly samplings during one year in Laguna Madre (Mexico), performing a total of 36 shrimp trawls (100 m2 each one) within each seagrass bed (n = 3 trawls per bed per month for 12 months). Shrimp density was related to the proximity to the tidal inlet (higher density was consistently observed in the nearby seagrass bed), which in turn adversely affected the condition of both species studied (Penaeus aztecus and P. duorarum). In this regard, the present study provides the first evidence of density-dependent effects on shrimp condition inhabiting a nursery habitat. Both shrimp species exhibited a negative relationship between condition and shrimp density. However, this pattern differed depending on the proximity to the tidal inlet, suggesting that shrimp populations inhabiting the nearby seagrass bed are exposed to density-dependent effects on condition; whereas, such effects were not detected in the distant seagrass bed. Shrimp density within the distant seagrass bed was probably below carrying capacity, which is suggested by the better shrimp condition observed in that area of the lagoon. Intra and interspecific competition for food items is surmised to occur, predominantly within seagrass beds near the tidal inlet. However, this hypothesis needs to be tested in future studies.

Effect of stingray (Hemitrygon akajei) foraging on a ghost shrimp population (Nihonotrypaea harmandi) on an intertidal sandflat, western Kyushu, Japan

Callianassid shrimp residing in deep burrows have large bioturbating effects on marine soft-bottom communities. A few predators that excavate deep pits could have substantial effects on shrimp populations, as well as knock-on effects. Processes and consequences of such effects on shrimp populations are poorly understood. On a 300-m-wide intertidal sandflat area between tide marks in western Kyushu between 1989 and 1994, shrimp population densities were stable, reaching >1300individualsm–2. Dasyatid stingray feeding pits reaching depths up to 20cm occurred abruptly in large numbers in 1994, after which shrimp densities decreased yearly to hundreds of individuals per square metre in 2001. The densities of ray feeding pits formed per day were monitored every or every other spring tide between 2000 and 2001. Schools of rays were enclosed during submerged times and their body sizes recorded alive to determine size-frequency distribution. The body-size frequency distributions of shrimp were compared among the gut contents of several rays, ray feeding pits and intact sandflat. Reductions in the shrimp density per ray feeding bout compared with the density on the intact sandflat were recorded. A model of daily predation at different seasonal rates was used to simulate the yearly change in shrimp density. The result was consistent with the actual change.

Water Quality and Sediment Profile in Shrimp Culture with Different Sediment Redox Potential and Stocking Densities Under Laboratory Condition

Sediment quality has been considered as one of the prime factors influencing the environment quality that support maximum shrimp production.The aim of the study was toevaluate the effects of sediment redox potential and shrimp stocking density on the profile of some sediment and water quality parameters. Two factors randomized factorial design was applied, with stocking density (60 and 120 shrimps.m-2) as the first variable and sediment redox potential (-65 mV, -108 mV and -206 mV) as the second variable. Some significant changes in TP, total Mn, and total S concentrations in the sediment were observed after the experimentation (P<0.05). Sediment redox potential significantly affected the dissolved oxygen, TAN, NO2, NO3, and H2S concentrations in the water. Whereas shrimp stocking density affected all water quality parameters except H2S concentration. Significant interactions between redox potential and stocking densities were observed in the nitrite and alkalinity concentrations. The significant effects of both shrimp density and redox potential on the sediment and water parameters in particular those that are known to directly affect the shrimp welfare (e.g. oxygen, ammonia, nitrite and H2S) indicate that these variables are of important aspects in shrimp pond management. Furthermore, the results clearly showed that -206mV redox potential significantly reduced the dissolved oxygen concentration in the sediment-water interface and increased the generation of H2S in water column. Thereby, this redox potential level is not advisable for shrimp culture system. Keywords: redox potential, stocking density.

Ecological correlates of trophic polyphenism in spadefoot tadpoles inhabiting playas

Polyphenism in larval amphibians has been related to several factors, including wetland hydroperiod, food availability, competition, and predation. Spadefoot toads (genus Spea Cope, 1866) often exhibit a trophic polyphenism by developing distinct carnivore and omnivore larval morphotypes. Using a multimodel selection approach, we investigated the influence of land use (cropland vs. grassland type) and differences in annual precipitation on morphotype expression in Plains spadefoot ( Spea bombifrons (Cope, 1863)) and New Mexico spadefoot ( Spea multiplicata (Cope, 1863)) toads in playas. We also examined the relative importance of tadpole density, tadpole age, water-loss stress, wetland size, density of larval mole salamanders (genus Ambystoma Tschudi, 1838; a predator on Spea tadpoles), and food resources on morph occurrence. The carnivore morphotype developed almost exclusively in S. bombifrons and rarely in S. multiplicata regardless of land use. Habitat availability measured by water-loss rate, as well as predation risk and tadpole age, were the most important factors influencing carnivore proportions. Ambystoma density was positively associated, whereas water-depth loss and tadpole age were negatively associated with the proportion of carnivores. The greatest proportion of carnivores was observed in grassland playas, which had the highest density of Ambystoma predators, longest hydroperiods, and experienced water-depth gain. Fairy shrimp density was not correlated with the proportion of carnivores. Upland land use through cultivation-associated erosion is altering wetland trophic structure, which further influences morphotype expression in Spea tadpoles and playa amphibian community structure.

Effect of foraging activity on the shoal structure of cod (Gadus morhua)

Acoustic methods used to quantify the shoaling dynamics of Atlantic cod (Gadus morhua) during their shoreward migration across the northeast Newfoundland shelf (spring 1992) showed that shoal structure and horizontal displacement were associated with the density distribution of small pelagic scatterers, for example, shrimp (Pandalus borealis). Cod speeds (to 20 km/d) were lower after encounters with shrimp (4–7 km/d). The relationship between several characteristics of the cod shoal (density, vertical spread, height off the bottom) and shrimp density was dome shaped. Internal shoal densities declined and shoal spread and height increased until shrimp densities surpassed a critical level. Above this threshold, these trends were reversed. The vertical spread of the cod shoal matched that of shrimp up to, but not beyond, a shrimp spread of 85 m. At this observed maximum in the expansion of the shoal, fish were eight body lengths apart. At all times during our survey, cod stayed within the warmer waters (> 2 °C) found at depths greater than 250 m. Our results are the first field verification of theoretical predictions and laboratory findings that shoals expand and contract in response to foraging activity and demonstrate the potential importance of cod aggregation dynamics to interpretations of trawl data.