Tropical Freshwater Species Research Articles

The developing zebrafish (Danio rerio): A vertebrate model for high‐throughput screening of chemical libraries

The zebrafish, Danio rerio, a small, tropical freshwater species native to Pakistan and India, has become a National Institutes of Health-sanctioned model organism and, due to its many advantages as an experimental vertebrate, it has garnered intense interest from the world's scientific community. Some have labeled the zebrafish, the "vertebrate Drosophila," due to its genetic tractability, small size, low cost, and rapid development. The transparency of the embryo, external development, and the many hundreds of mutant and transgenic lines available add to the allure. Now it appears, the zebrafish can be used for high-throughput screening (HTS) of drug libraries in the discovery process of promising new therapeutics. In this review, various types of screening methods are briefly outlined, as are a variety of screens for different disease models, to highlight the range of zebrafish HTS possibilities. High-content screening (HCS) has been available for cell-based screens for some time and, very recently, HCS is being adapted for the zebrafish. This will allow analysis, at high resolution, of drug effects on whole vertebrates; thus, whole body effects as well as those on specific organs and tissues may be determined.

Dissolved Organic Carbon Reduces Uranium Bioavailability and Toxicity. 2. Uranium[VI] Speciation and Toxicity to Three Tropical Freshwater Organisms

The influence of dissolved organic carbon (DOC) on the toxicity of uranium (U) to three Australian tropical freshwater species, the Northern Trout Gudgeon (Mogurnda mogurnda), green hydra (Hydra viridissima) and unicellular green alga (Chlorella sp.) was assessed. Exposures were conducted in synthetic soft water without DOC and with DOC added in the form of standard Suwannee River Fulvic Acid (SRFA). Organisms were exposed to a range of U concentrations at a range of DOC concentrations (0-20 mg L(-1)). U toxicity was up to 20 times less in water containing 20 mg L(-1) DOC, relative to DOC-free test waters. U toxicity was also assessed using natural water from a tropical Australian billabong containing 10 mg L(-1) DOC. U toxicity was up to ten times less in the billabong water, relative to DOC--free test waters. SRFA was twice as effective at reducing U toxicity as the billabong water at equivalent DOC concentrations. Geochemical speciation modeling confirmed the decreased U toxicity that resulted from both DOC sources was primarily due to a decrease in the free uranyl ion (UO2(2+)) through complexation with DOC. A predictive model is presented for each of the organisms that can be used to predict U toxicity at a given U and DOC concentration.

Uranium Exposure to the Tropical Duckweed Lemna aequinoctialis and Pulmonate Snail Amerianna cumingi: Fate and Toxicity

The discharge of catchment-management water from the Ranger uranium (U) mine into Magela Creek upstream of the Ramsar-listed Magela Floodplain in Kakadu National Park is an important part of the mine's water-management system. Because U is one of the primary toxicants associated with this water, a receiving-water trigger value (TV), based on chronic toxicity data from five local native species, was derived for U. To strengthen the data set underpinning the derivation of the TV, the chronic toxicity of U to two additional tropical freshwater species, duckweed Lemna aequinoctialis (96-hour growth rate), and pulmonate gastropod, Amerianna cumingi (96-hour reproduction), was determined. The fate of U within the test systems was an important component of the study because analysis of U concentrations during the snail tests indicated that a substantial proportion of U (approximately 25%) was being lost from the test solutions when integrated during the entire test duration. Analysis of the snails and their food for U indicated that only a small proportion that was lost from solution was being taken up by the snails. Therefore, the majority of U that was lost was considered unavailable to the snails, and thus the exposure concentrations used to calculate the toxicity estimates were adjusted downward. Integrating the loss of U from the L. aequinoctialis test solutions over time showed that only a small proportion (6% to 13%) was lost during the test: Of that, almost half (2-5%) was taken up by the plants (constituting exposure). Uranium was only moderately toxic to L. aequinoctialis, with no observed-effect concentrations, lowest observed-effect concentrations, and inhibition concentrations causing 10% and 50% effects (IC10 and IC50) values of 226, 404, 207, and 1435 microg/l, respectively. A. cumingi was found to be more sensitive to U than L. aequinoctialis, with NOEC, LOEC, IC10, and IC50 values of 60, 61, 15, and 278 microg/l, respectively. The data for these two additional species will be used to revise the current TV for U in Magela Creek.

The handling of salt by the neotropical cultured freshwater catfish Rhamdia quelen

The jundiá ( Rhamdia quelen) is a Siluriform native to South America, intensely cultivated in Southern Brazil, and which has been increasingly considered as a potential substitute for the culture of exotic species. Due to this interest, there is a constant need of methods to reduce parasite infestations common to the species. The use of sodium chloride (NaCl) is one of such practices. However, not much is known about the effects of NaCl on the osmotic homeostasis of this and other commercially relevant tropical freshwater species. This study thus aimed at evaluating the effect of adding NaCl (5, 15, or 25 g/L) to the water for 1 h, on the osmoregulation, and capacity for muscle tissue water regulation of the jundiá. In addition, muscle slices were subjected in vitro to isosmotic (control, animal in fresh water) and hyper-osmotic (120, 150, and 180% of control) salines, for evaluation of their capacity to control their water content. Some changes have been observed, when fishes submitted to 25 g/L were compared to freshwater controls: plasma ions and osmolality increased, the last from 260 ± 5 to 419 ± 12 mOsm/kg H 2O; plasma glucose increased from 47.8 to 92.8 mg/dL, and muscle water content decreased from 80 to 76.7%; plasma cortisol levels remained unchanged (~ 150 ng/mL). Branchial carbonic anhydrase activity also increased in 25 g/L, probably as a response to the metabolic acidosis that develops upon severe saline challenges. In vitro experiments indicated a high capacity for water content regulation, even when muscle slices were exposed to an 80% increase in osmolality. Exposure of jundiás for 1 h to 25 g/L was indeed harmful to their osmotic homeostasis, also affecting sensory and motor functions (observed as loss of barbel function, and swim impairment). However, the high degree of euryhalinity shown both in vivo and in vitro by this freshwater siluriform is coherent with its occasional presence in dilute estuarine waters and to the positioning of its family within a group of seawater-related Siluriforms. Thus, a reasonable amount of salt (15 g/L) for an intermediate time period (1 h) could be employed against parasites of the jundiá, while still being harmless to both the jundiás and humans that consume its flesh.

Comparison of tropical and temperate freshwater animal species' acute sensitivities to chemicals: Implications for deriving safe extrapolation factors

AbstractToxicity data for tropical species are often lacking for ecological risk assessment. Consequently, tropical and subtropical countries use water quality criteria (WQC) derived from temperate species (e.g., United States, Canada, or Europe) to assess ecological risks in their aquatic systems, leaving an unknown margin of uncertainty. To address this issue, we use species sensitivity distributions of freshwater animal species to determine whether temperate datasets are adequately protective of tropical species assemblages for 18 chemical substances. The results indicate that the relative sensitivities of tropical and temperate species are noticeably different for some of these chemicals. For most metals, temperate species tend to be more sensitive than their tropical counterparts. However, for un‐ionized ammonia, phenol, and some pesticides (e.g., chlorpyrifos), tropical species are probably more sensitive. On the basis of the results from objective comparisons of the ratio between temperate and tropical hazardous concentration values for 10% of species, or the 90% protection level, we recommend that an extrapolation factor of 10 should be applied when such surrogate temperate WQCs are used for tropical or subtropical regions and a priori knowledge on the sensitivity of tropical species is very limited or not available.

Nutritional regulation of hepatocyte fatty acid desaturation and polyunsaturated fatty acid composition in zebrafish (Danio rerio) and tilapia (Oreochromis niloticus)

The desaturation and elongation of [1-14C]18:3n-3 was investigated in hepatocytes of the tropical warm freshwater species, zebrafish (Danio rerio) and Nile tilapia (Oreochromis niloticus). The hepatocyte fatty acid desaturation/elongation pathway was assayed before and after the fish were fed two experimental diets, a control diet containing fish oil (FO) and a diet containing vegetable oil (VO; a blend of olive, linseed and high oleic acid sunflower oils) for 10 weeks. The VO diet was formulated to provide 1% each of 18:2n-6 and 18:3n-3, and so satisfy the possible EFA requirements of zebrafish and tilapia. At the end of the dietary trial, the lipid and fatty acid composition was determined in whole zebrafish, and liver, white muscle and brain of tilapia. Both zebrafish and tilapia expressed a hepatocyte fatty acid desaturation/elongation pattern consistent with them being freshwater and planktonivorous fish. The data also showed that hepatic fatty acid desaturation/elongation was nutritionally regulated with the activities being higher in fish fed the VO diet compared to fish fed the FO diet. In zebrafish, the main effect of the VO diet was increased fatty acid Δ6 desaturase activity resulting in the production of significantly more 18:4n-3 compared to fish fed the FO diet. In tilapia, all activities in the pathway were greater in fish fed the VO diet resulting in increased amounts of all fatty acids in the pathway, but primarily eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). However, the fatty acid compositional data indicated that despite increased activity, desaturation of 18:3n-3 was insufficient to maintain tissue proportions of EPA and DHA in fish fed the VO diet at the same level as in fish fed the FO diet. Practically, these results indicate that manipulation of tilapia diets in commercial culture in response to the declining global fish oil market would have important consequences for fish fatty acid composition and the health of consumers. Scientifically, zebrafish and tilapia, both the subject of active genome mapping projects, could be useful models for studies of lipid and fatty acid metabolism at a molecular biological and genetic level.

Spectral transmission and short-wave absorbing pigments in the fish lens—I. Phylogenetic distribution and identity

Fish lens transmission was found to vary depending on the type and concentration of short-wave absorbing compounds present within the lens. Pigments extracted from lenses of ten species were identified as mycosporine-like amino acids (mainly palythine, palythene and asterina-330, λ maxs around 320–360 nm) which are also thought to be present in the majority of the 120 species examined here. A novel mycosporine-like pigment with λ max 385 nm was isolated from the lens of the flying fish, Exocoetus obtusirostris, while lenses of several closely related tropical freshwater species were found to have high concentrations of the tryptophan catabolite 3-hydroxykynurenine (λ max 370 nm). The type of lens pigment a species possesses and its concentration depends upon both the animal's phylogenetic group and its “optical niche”.

Comparative ecology of submersed grass beds in freshwater, estuarine, and marine environments1

Worldwide, there are 500–700 species of submersed angiosperms adapted to freshwater and estuarine environments compared with 50 species adapted to marine waters. In their evolution from freshwater ancestors, seagrasses have undergone extensive anatomical changes (e.g. reduction in floral and leaf structures, reduction of xylem tissue with a lacunal gas transport system), as well as physiological adaptations (bicarbonate utilization in photosynthesis). Seagrasses appear to have more annual production than do their freshwater counterparts because they develop greater standing crops and have the capacity to store photosynthetic products in extensive rhizome systems in the sediments. For example, maximum productivity of >10 g C m−2 d−1 has been reported for tropical seagrass species (Cymodocea nodosa and Thalassia testudinum), but the maximum productivity of temperate freshwater species such as Myriophyllum or tropical freshwater species such as Hydrilla is usually <5 g C m−2 d−1. In addition, the marine environment provides ample supplies of inorganic carbon (C) and increased mixing energies, making CO2 limitation less likely. One calculation suggests that marine macrophytes impact the global C budget by sequestering as much as 109 t of C per year.Secondary productivities of seagrass communities can also be high. For example, stable isotopic ratios suggest that macrophytic C is important in sustaining several species of commercial fish species in Australia, accounting for >50% of their diets. Also, sea urchins (Diadema antilarum) consume plant material, creating bare halos around tropical patch reefs in the Caribbean Sea. It is difficult to generalize regarding brackish submersed aquatics in estuaries because their coverage is variable due to light limitation and algal overgrowth from eutrophication. Freshwater macrophytes seem rarely grazed by fish (except via exotic introductions of Tilapia or carp), but waterfowl use is often significant at the end of the growing season. Thus, trophic relations in freshwater macrophyte beds may be qualitatively different and much more pulsed than in seagrass systems, with more re‐selection in lakes and more K‐selection in marine environments.

Cold shock reactions in iced tropical fish

The development of a rigor mortis‐like stiffening and the biochemical changes associated with it were investigated in tilapia (Oreochromis aureus/niloticus hybrid), a tropical freshwater species, and common carp (Cyprinus carpio), a temperate freshwater fish, during storage in ice (0°C) and at ambient temperature (22°C). Onset of stiffening in carp occurred between 16 and 17 hr after death at both temperatures but full stiffness developed much later and was a longer duration at 0°C. In tilapia, onset occurred after 7 hr at 22°C and full stiffness was established after 19 hr. However, at 0°C, tilapia experienced a cold shock reaction such that they stiffened within minutes of being placed in ice and were fully rigid within 8 hr. Resolution of stiffness in this species also occurred later at 0°C. The rate of ATP degradation was similar under both storage conditions in tilapia but more rapid at ambient temperature in carp. Although the rate of lactic acid accumulation was faster at the higher temperature in tilapia, it was not nearly so marked as for carp. Objective measurement of contractions in excised muscle fibres from trout (Salmo gairdnerii) and tilapia indicated that reducing the temperature delayed the occurrence of the contraction and reduced its intensity. It was concluded that cold shock stiffening and rigor mortis stiffening are different.