Hydrobiology

Tannins are plant secondary compounds that leach into soil and water. Tannin concentrations can be higher in lentic freshwater than in other aquatic ecosystems, which can result in toxicity to freshwater organisms. While the amount of plant material and the plant tissue concentrations affect aqueous tannin concentrations, little is known about which environmental conditions alter the removal and breakdown of tannins in freshwater, altering the exposure of freshwater organisms. We investigated the effects of soil, light, and temperature on aqueous tannin concentrations. Tannins degraded faster in the presence of and/or sorbed to wetland soils, silt, and clay but not sand. It is unclear whether finer soil particles or the chemical makeup of soils facilitated sorption and/or degradation. Contrary to previous work, we found no effect of ambient light over a 14-day period or sunlight over a 7-day period on tannin degradation, but tannins degraded faster in warmer water. Warmer freshwater ecosystems with greater silt or clay sediments are likely to have lower aqueous tannin concentrations with lower toxicity to aquatic organisms. This research used tannic acid, which contains gallotannins. Further research is needed to determine if similar patterns are found with other tannin types and the complex mixtures of different tannins found in most plants. Full article

An important top-down predator, the northern pike (Esox lucius), faces harsh environmental conditions in the northern boreal ecoregion. They are often managed for recreational fishing and, more recently, to create environmental offsets; strategies aimed at balancing ecological impacts by enhancing or restoring habitats. Our study examines northern pike populations in two remote boreal lakes in northern Alberta: Steepbank and Wappau. The lakes differ in size, vegetation cover, and trophic status, providing a natural experiment for investigating northern pike growth, condition, diet, and population density. Over three years (2018–2020), northern pike were sampled using gill nets. Population metrics, including growth, condition, and stomach contents, were compared between the lakes. Steepbank, a smaller, oligotrophic lake with low vegetation cover, showed lower prey fish densities compared to the larger, eutrophic Wappau, but it did not differ in northern pike catch per unit effort. Growth rates and body condition varied significantly between the lakes, with the northern pike in Wappau exhibiting faster growth and a better condition in the older age groups, while the younger northern pike in Steepbank had higher relative weights. A diet analysis revealed significant differences in prey consumption: Steepbank northern pike displayed higher rates of conspecific predation and invertebrate consumption, particularly in the younger age classes. These findings highlight how lake characteristics and prey availability shape northern pike population dynamics, offering valuable insights for lake management approaches in northern Alberta. Full article

The previously available coding region for the spiny dogfish (Squalus acanthias) AQP3-2 gene was amplified from cDNAs using PCR. Agarose gel electrophoresis gave a band of the AQP3-2 coding region, as well as multiple smaller splice variant bands. The main AQP3-2 band and the largest and most fluorescently intense pair of these splice variant bands were cloned and sequenced. Amplifications were performed on a range of tissue cDNAs, but AQP3-2 was only expressed in the kidney and brain. Quantitative PCR amplifications using pre-existing kidney cDNA from an environmental salinity acclimation experiment showed that the abundance of mRNA from both the main AQP3-2 transcript and the largest splice variant (Splice Variant 1) was lower in 120% seawater (SW) acclimated fish, although only the values for Splice Variant 1 were statistically significant. A custom-made affinity-purified rabbit polyclonal AQP3-2 antibody was produced, and this gave four bands of around the correct sizes (which were 27 and 32 kDa) for the complete AQP3-2 and Splice Variant 1 proteins. Two of the bands may have been N-glycosylated forms of these proteins. Other bands were also present on the Western blot. No bands were present when the antibody was pre-blocked by the peptide antigen. In tissue sections of the dogfish kidney, immunohistochemical localization experiments showed that AQP3-2 was expressed in the early distal tubule (EDT) and late distal tubule (LDT) nephron segments. The results suggest that AQP3-2 may be involved in cell volume regulation in the EDT and water and urea absorption in the LDT nephron segment. Full article

S. sphyraena is a widely distributed species with low commercial value and no sufficient scientific knowledge of its biology. In the present study, the age, growth, weight–length relationship, otolith morphometry, and reproduction of the species were investigated in the Eastern Ionian Sea for the first time. The von Bertalanffy growth function parameters were L_∞ = 63.65 cm, k = 0.14 year⁻¹ and t₀ = −2.01 years and Φ′ = 2.75. The negative allometric growth in weight was found. Slope b of the weight–length relationship was 2.634. For the otolith moprhometry, the variables radius, length, width, area, perimeter, roundness, circularity, form factor, rectangularity, and ellipticity were examined, which showed that the otolith shape is elongated with an elliptical and rectangular form. Six otolith variables (radius, length, width, area, perimeter, and ellipticity) showed a significant relationship with size. The sex ratio (females/males) was 1:0.74 (no statistically significant difference from 1:1), and the spawning season extended from April to June with peak values of GSI in May for females and April for males. The results of this work improve our knowledge of the species life cycle and provide basic information for species stock identification and fisheries management. Full article

The sea bivalve clam Tridacna crocea inhabiting the shallow sea of tropical and subtropical zones lives with the symbiotic alga zooxanthella in its mantle. Zooxanthellae algae perform photosynthesis and supply nutrients to T. crocea. Recently, the abundance of T. crocea has decreased rapidly due to overfishing in coastal areas in Okinawa, Japan. T. crocea culture systems for mass production will contribute to the conservation of T. crocea and thus marine ecosystems. Environmental control methods for T. crocea culture have not been established because of a lack of knowledge about the appropriate environmental conditions for T. crocea growth. The present study was initiated to obtain basic data for developing environmental control methods for T. crocea land-based aquaculture. The effects of water temperature, dissolved oxygen concentration, and photosynthetic photon flux density (PPFD) on the O₂ exchange rates of the symbiotic system of T. crocea and zooxanthella, which are indicators of photosynthesis and respiration in the system, and the effect of daily integrated PPFD on T. crocea growth were investigated. Basic knowledge was obtained for the development of optimal environmental control technology for T. crocea clam culture. The optimum water temperature and dissolved oxygen concentration for photosynthesis in this symbiotic system were 28 °C, 5–6 mgO₂ L⁻¹ and 500 μmol m⁻² d⁻¹, respectively. The optimum daily integrated PPFD for clam growth was 20 mol m⁻² d⁻¹. Full article

The evaluation of mitochondrial DNA and genetic analysis is helpful for economically significant species. Clarias gariepinus is a critical species in aquaculture. This study investigates the genetic diversity and population differentiation of C. gariepinus from 19 countries using 164 sequences of the mitochondrial DNA’s Cytochrome c oxidase I (COI) gene. The haplotype analysis revealed a total of 17 haplotypes, with a nucleotide diversity (π) of 0.012 and a haplotype diversity (Hd) of 0.87. The results of an AMOVA and fixation index indicated significant genetic variation and structure among the populations. Additionally, neutrality tests and mismatch distribution analysis supported the hypothesis of under-purifying selection in C. gariepinus. The findings suggested that the population did not experience expansion. In conclusion, the genetic analysis highlighted substantial variation among C. gariepinus populations from different locations, providing valuable insights for the global management of this species. Full article

Over the course of industrialization in the 20th century, vast emissions of air pollutants have occurred. The exhaust gasses contain sulfur and nitrogen oxides, which are converted to sulfuric acid and nitric acid in the atmosphere. This causes acid rain to enter aquatic and terrestrial ecosystems, the most serious consequence of which is large-scale forest dieback across Europe and North America. However, through various political measures, the exhaust gasses have been reduced and, thus, acid rain and forest dieback were stopped. Nevertheless, the lingering effects of this pollution are still present today and are reflected in hydrochemistry. More recently, fluctuating precipitation regimes are causing additional stress to ecosystems in Central Europe. Climatic extremes are becoming more pronounced with climate change. Substantial differences between drought years and years with regular precipitation are directly altering the discharge of springs. Now, two overlapping and interacting syndromes of environmental pressures can be studied in these small catchments at a landscape scale: (1) acidification and (2) climate change. In this long-term study, the waters of 102 forest springs, located in two neighboring forest landscapes in north-eastern Bavaria, Germany (Frankenwald and Fichtelgebirge), were investigated over 24 years (1996 to 2020). By linking changes in pH values with changes in precipitation and spring discharge, we found that pH increases with decreasing discharge and decreasing precipitation. This effect was strongest in the Frankenwald compared to the Fichtelgebirge. We hypothesize that this temporal pattern reflects the longer residence time and, in consequence, the increased buffering of acidic interflow in small catchments during periods of drought. However, this should not be misinterpreted as rapid recovery from acidification because this effect fades in times of enhanced precipitation. We recommend that fluctuations in weather regimes be considered when investigating biogeochemical patterns throughout forest landscapes. Full article

Salmonid fishes provide an important indicator of climate change given their reliance on cold water. We evaluated temporal changes in the density of stream-dwelling brook trout (Salvelinus fontinalis) from surveys conducted over a 36-year period (1988–2023) by the Maryland Department of Natural Resources in Eastern North America. Nonparametric trend analyses revealed decreasing densities of adult fish (age 1+) in 19 sites (27%) and increases in 5 sites (7%). In contrast, juvenile fish (age 0) densities decreased in 4 sites (6%) and increased in 10 sites (14%). Declining adult brook trout trends were related to atmospheric warming rates during the study period, and this relationship was stronger than the effects of land use change or non-native brown trout. In contrast, juvenile fish trends generally increased with elevation but were not related to air temperature trends or land use change. Our analysis reveals significant changes in several brook trout populations over recent decades and implicates warming atmospheric conditions in population declines. Our findings also suggest the importance of temperature for adult survival rather than recruitment limitation in brook trout population dynamics. Full article

Freshwater slugs are scarce and belong exclusively to panpulmonate Acochlidimorpha. There is a radiation of eight species of large-sized slugs living benthically in rivers on tropical Indo-Pacific Islands. In the Western Atlantic, only one small interstitial slug, Tantulum elegans Rankin, 1979, is known from the Caribbean island of St. Vincent. We recently discovered a novel species of freshwater slugs in Cuba. Here, we describe Potamohedyle espinosai n. gen. n. sp., which is the first freshwater slug in the region of the Western Atlantic with a benthic lifestyle, in 3D-microanatomical and histological detail using light and scanning electron microscopy. It shows a mix of characters from different freshwater acochlidimorph genera, such as a medium body size, the presence of an osphradial ganglion, a distal gonoduct with a muscular sphincter, a penis with a solid thorn and cuticular comb, and a basal finger with a hollow stylet. Morphological adaptations to a life in freshwater include multiplicated renopericardioducts. The taxonomic character mix justifies the establishment of a novel genus within the herein diagnostically modified freshwater family Tantulidae. A molecular phylogenetic hypothesis of riverine slugs including the first Caribbean representatives suggests that the transition to freshwater occurred once along the stemline of limnic Acochlidiidae, secondarily marine Pseudunelidae and limnic Tantulidae. Full article

Leaf litter is an important input to freshwater systems. Leaves provide carbon, nutrients, and secondary compounds. We examined the effects of tree leaf species on chlorophyll a concentration—a proxy for phytoplankton biomass. We found that an input of Chinese tallow (Triadica sebiferum, invasive in the southeastern USA) and red maple (Acer rubrum) leaves resulted in lower chlorophyll concentrations than controls and other native species. These leaf species also leached tannins, resulting in a darker water color, and either may have caused the patterns observed. To separate these potential mechanisms (darker water leading to light limitation and tannin toxicity), we conducted a second experiment with a fully factorial design manipulating tannins and water color. We found that darker water resulted in the lowest chlorophyll concentration, suggesting light limitation. In the clear-water treatment, the addition of tannic acid lowered chlorophyll concentrations but also resulted in moderately darker water by the end of the experiment. The tannic acid may have been toxic to the algae, or there may have been some light limitation. Our results suggest that tannins that darken water color may substantially suppress phytoplankton and that tree species composition may influence both phytoplankton and the brownification of freshwater. Full article

Harmful algal bloom events occur in salt, brackish, and fresh water. In bodies of water such as oceans and estuaries, diatoms or dinoflagellates form “tides” that produce toxins associated with seafood poisoning, including paralytic shellfish poisoning, or respiratory distress from inhalation of aerosolized toxins. Cyanobacteria predominantly bloom in fresh water; they can produce microcystins; cylindrospermopsin; and other toxins that humans or animals might be exposed to through water contact, inhalation, or ingestion. Animals that become ill or die can be sentinels for harmful algal bloom events. In a One Health approach, information about harmful algal bloom exposures and health effects support efforts to detect these events and mitigate and prevent associated illnesses. Human, animal, and environmental health partners can work together to document the occurrence and impacts of harmful algal bloom events and characterize associated illnesses. Full article

Winter ulcer disease (WUD) is widely recognized as a serious threat to animal welfare and a major contributor to revenue loss within the aquaculture sector, particularly affecting the salmon-farming industry. This highlights the significant impact of WUD on both animal well-being and the economic sustainability of fish farming. WUD causes hemorrhagic signs and results in dermal lesions and ulcers. This disease can lead to higher mortality rates and a considerable decline in the fish’s market value. Moritella viscosa, a Gram-negative bacterium, is predominantly, but not exclusively, correlated with the emergence of WUD, mostly during the colder seasons. Waterborne transmission is the primary way for spreading the bacterium within a population. However, there is remarkable variation in the prevalence and characteristics of WUD in different regions. In Europe, this disease often occurs in the winter, and the intensity and occurrence of outbreaks are influenced by water temperature and salinity. In contrast, outbreaks are typically observed in the summer and mid-autumn in Eastern Canada. Despite the administration of various polyvalent vaccines, outbreaks of skin ulcers have been documented in Canada, and studies have highlighted the possible roles of other bacterial pathogens in Atlantic salmon. This review discusses the etiology, pathogenesis, and potential mitigation or prevention strategies for WUD, mainly in Atlantic salmon. Moreover, it underscores the necessity of conducting further investigations to discover the potential unknown causative agents of ulcerative disease and design appropriate vaccines or preventive strategies for these pathogens. Full article

Large rivers are difficult to sample due to their size yet critical to monitor because humans heavily rely upon and alter them. Aquatic invertebrates are commonly used to assess the ecosystem quality of streams, but methods to sample large rivers are underdeveloped. We sampled aquatic invertebrates using a Hess sampler and rock baskets in the Snake River near Jackson, Wyoming, USA. Hess samples collected more aquatic invertebrate taxa and a higher proportion of Ephemeroptera, Plecoptera, and burrowing taxa. Rock baskets collected a higher proportion of Trichoptera, filterers, and clinging taxa. Bioassessment metrics differed between sampling methods; richness, diversity, evenness, Ephemeroptera, Plecoptera, and Trichoptera (EPT), and Hilsenhoff’s biotic index produced higher values in Hess samples, and percent EPT was higher in rock baskets. Non-metric multidimensional scaling and analysis of similarity indicated that the samplers collected different assemblages (p < 0.001). The standard error of total invertebrate density was smaller and most taxa were collected with seven replicate samples. Understanding how sampling methods alter the aquatic invertebrates collected will help managers develop monitoring protocols that are best suited to the river and collect the most unbiased invertebrate assemblages. Full article

In the Mediterranean, the bluespotted cornetfish Fistularia commersonii Rüppell, 1838, presents a minor socioeconomic impact and the assessment of any environmental impact requires more relevant data. The congeneric red cornetfish Fistularia petimba Lacepède, 1803, has expanded its distribution range within the basin but only small numbers have been reported to date. A total of 207 individuals of F. commersonii were collected between April 2021 and March 2022 from the Levantine coast of Rhodes and 92 more from various locations. Additionally, 13 individuals of Fistularia petimba were caught in March 2024 from the Aegean coasts of the island. We aim to assess the current progression of the population of the two cornetfish, the possible further exploitation of F. commersonii to boost the local fishing economy, their possible dietary overlaps and to add valuable biological and ecological data. In F. commersonii, male to female ratio (1:1.33) significantly departed from 1:1, with length–weight relationships exhibiting positive allometric growth. Six age groups were identified. The highest reproductive intensity was observed during summer. The onset of sexual maturity was estimated at 65.52 cm in total length (1.8 years). Longevity was estimated at 11.1 years with females growing larger than males. The exploitation rate (E = 0.47) indicated that the population is underexploited. The optimum and target fishing mortality were higher in comparison with the present fishing mortality (F = 0.48), indicating a potential for commercial exploitation of the species. For F. petimba, the sex ratio was 1:2.25. The species preys on crustaceans, followed by fish and molluscs. The finding of Vanderhorstia mertensi (Klausewitz, 1974) in the stomach content of both cornetfishes constitutes the second published record for Hellenic waters and the first for the Dodecanese Islands. Full article

Many swimming protists travel by actuating whip-like flagella to generate thrust. While many organisms’ flagella have been observed to have hair-like protrusions called mastigonemes, the function of these mastigonemes is not known. In particular, however, although theory, numerics, and some experiments suggest that mastigonemes can initiate the reversal in direction of thrust generated relative to the direction of traveling waves propagated along flagella, other experiments have found that they do not have hydrodynamic effects. Thus, it remains unclear whether mastigonemes have a hydrodynamic effect and function; additionally, any hydrodynamic effects may be species-dependent, which calls for the investigation of additional species. In this work, we report experimental observations of the cryptophyte C. paramecium that obtain their cell body, flagellar, and mastigoneme geometries, as well as their swimming kinematics and behavior. We then use the observed geometries and kinematics to numerically simulate swimming trajectories for a particularly well-characterized reorientation event, with and without various configurations of the mastigonemes. The comparison of numerical and experimental results shows that a configuration of mastigonemes in the beating plane best reproduces the observed reorientation dynamics, suggesting that in C. paramecium, although mastigonemes do not lead to gross changes in motility, such as thrust reversal, they do exert important quantitative effects. Full article

Urban and agricultural runoffs can transport contaminants and pesticides into freshwater ecosystems, particularly in the developing tropics. For instance, organophosphate and pyrethroids pesticides, such as Roundup, Malathion, and Permethrin, have been found in tropical streams. The uncontrolled application of these pesticides has become a growing concern due to their adverse effects on various non-targeted organisms. Unfortunately, most studies have focused on a few selected model species, ignoring the effects on other non-target organisms, which may play an important role in tropical lotic ecosystems. In addition, the biological characteristics of aquatic crustaceans, including their morphology, physiology, and behavior, make them susceptible to toxic chemicals. For this reason, this study used the widely distributed freshwater shrimp Xiphocaris elongata as a model organism to determine the acute toxicity of Permethrin, Malathion, and Roundup. Our results show that the proportion of mortality of X. elongata in each concentration group became progressively higher as the concentration of exposure increased. We also found that the synthetic pyrethroid Permethrin was the most toxic pesticide tested, with a median lethal concentration (LC₅₀) value for 96 h of 3.96 × 10⁻⁶ µg·L⁻¹, followed by organophosphate Malathion (8.87 µg·L⁻¹) and Roundup (748.92 µg·L⁻¹). Experiments with this freshwater shrimp showed a good control performance and reproducibility for the tested pesticides. This study demonstrated that X. elongata is a suitable test organism that can be a representative bioindicator of pesticide toxicity in tropical streams. Full article

The family Eirenidae is one of the major taxa of the order Leptothecata, comprising approximately 80 species from ten genera. In this study, taxonomic investigations, including morphological observations and molecular 16S phylogenetic analyses, were conducted on unknown Eirenidae specimens collected off the coast of Iwaki, Fukushima Prefecture, eastern Japan, in June 2022. The specimens had the following morphological characteristics: marginal warts and tentacular bulbs with lateral cirri and without adaxial papillae, a mouth with simple lips, four simple radial canals, and eight statocysts common to the genus Eutima. However, this species can be distinguished from other species of Eutima by the number of tentacles, number and shape of marginal warts, position of the gonads, and gastric peduncle length. Moreover, the monophyly of the species was evident in the 16S rRNA phylogenetic tree (as indicated by the high bootstrap value of 100%), thereby supporting the validity of the new species. Based on these results, we describe it as a new species, Eutima onahamaensis, for taxonomic stabilization. We also made detailed observations of the morphology and molecular phylogenetic analyses of one of the species newly recorded in Japan: Eutima diademata. A comparative table of the primary diagnostic characteristics of Eutima has been provided. This study provided taxonomic keys for identifying species in the genus Eutima. Full article

Individual identification is an important ability for humans and perhaps also for non-human animals to lead social lives. It is also desirable for laboratory experiments to keep records of each animal while rearing them in mass. However, the specific body parts or the acceptable visual angles that enable individual identification are mostly unknown for non-human animals. In this study, we investigated whether artificial intelligence (AI) could distinguish individual medaka, a model animal for biological, agrarian, ecological, and ethological studies, based on the dorsal view. Using Teachable Machine, we took photographs of adult fish (n = 4) and used the images for machine learning. To our surprise, the AI could perfectly identify the four individuals in a total of 11 independent experiments, and the identification was valid for up to 10 days. The AI could also distinguish eight individuals, although machine learning required more time and effort. These results clearly demonstrate that the dorsal appearances of this small spot-/stripe-less fish are polymorphic enough for individual identification. Whether these clues can be applied to laboratory experiments where individual identification would be beneficial is an intriguing theme for future research. Full article

This comprehensive review explores the escalating challenge of nutrient enrichment in aquatic ecosystems, spotlighting the dire ecological threats posed by harmful algal blooms (HABs) and excessive particulate organic matter (POM). Investigating recent advancements in water treatment technologies and management strategies, the study emphasizes the critical need for a multifaceted approach that incorporates physical, chemical, and biological methods to effectively address these issues. By conducting detailed comparative analyses across diverse aquatic environments, it highlights the complexities of mitigating HABs and underscores the importance of environment-specific strategies. The paper advocates for sustainable, innovative solutions and international cooperation to enhance global water quality and ecosystem health. It calls for ongoing advancement, regular monitoring, and comprehensive research to adapt to emerging challenges, thus ensuring the preservation of aquatic biodiversity and the protection of communities reliant on these vital resources. The necessity of integrating technological innovation, ecological understanding, and global cooperation to safeguard aquatic ecosystems for future generations is paramount. Full article