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In nature, living organisms evolve unique functional components with mechanically adaptive compatibility to cater dynamic change of interface friction/lubrication. This mechanism can be used for developing intelligent artificial lubrication-regulation systems. Inspired by the muscle hardening-triggered lubrication of longsnout catfish , here we report a modulus adaptive lubricating hydrogel prototype consisting of top mucus-like hydrophilic lubricating layer and muscle-like bottom hydrogel that can stiffen via thermal-triggered phase separation. It exhibits instant switch from soft/high frictional state (~0.3 MPa, μ ~0.37) to stiff/lubricating state (~120 MPa, μ ~0.027) in water upon heating up. Such switchable lubrication is effective for wide range of normal loads and attributed to the modulus-dominated adaptive contact mechanism. As a proof-of-concept, switchable lubricating hydrogel bullets and patches are engineered for realizing controllable interface movements. These important results demonstrate potential applications in the fields of intelligent motion devices and soft robots. Stimuli responsive materials are interesting for applications in different technological fields, but realizing controllable surface friction-control based on the change of the modulus of a material is less investigated. Here the authors demonstrate a lubrication regulating strategy and prototype based on thermally triggered changes of the modulus of a hydrogel.

Little is known about the genetic basis of convergent traits that originate repeatedly over broad taxonomic scales. The myogenic electric organ has evolved six times in fishes to produce electric fields used in communication, navigation, predation, or defense. We have examined the genomic basis of the convergent anatomical and physiological origins of these organs by assembling the genome of the electric eel (Electrophorus electricus) and sequencing electric organ and skeletal muscle transcriptomes from three lineages that have independently evolved electric organs. Our results indicate that, despite millions of years of evolution and large differences in the morphology of electric organ cells, independent lineages have leveraged similar transcription factors and developmental and cellular pathways in the evolution of electric organs.

Light microscopy, scanning electron microscopy, and proteomics analysis using high-resolution accurate mass spectrometry exhibit significant adaptive structural modifications and characteristic proteins in the adhesive organ (AO) of Pterygoplichthys disjunctivus, that could assist the adhesion of the fish to the substratum . The free surface of the epithelium of AO showed characteristic mound-like tubercles separated by deep furrows. These could enhance surface grip, and assist in creation of vacuum facilitating adhesion effectively. Spine-like unculi on the surface of the epithelium appear to assist clinging of the fish to the substratum as well as to scrape food particles for feeding. Taste buds located on the summit of mound-like tubercles could serve as adaptations to enhance their sensitivity in food selection and in environmental perception. Mucous and serous goblet cell secretions are believed to function as biological adhesives and protective surface lubricants. Proteomic analysis identified 285 differentially expressed proteins in the AO compared to those in the ventral and the dorsal skin. Out of these proteins in the AO, 80 proteins were significantly abundant. These were Periplakin, Desmoplakin, and Filamin A-like (adhesion related proteins); KRT8 and KRT19 (keratinization associated proteins); Myosin-7, Myosin light chain 13, and Tropomyosin 1 (proteins involved in cytoskeletal organization); and Apolipoprotein A1 and Complement Component 9 (proteins related to immune defense). Gene ontology analysis using Enrichr software revealed the enrichment of unique biological functions and pathways. This study provides a comprehensive understanding of the adaptive strategies that enable P. disjunctivus to thrive in turbulent hillstream environments. Additionally, the proteomic profile established in this study serves as a foundation for characterizing and comparing skin proteomes across teleost species.

Plant protein (PP) sources are generally used in high levels in fish diets. Mostly, PP sources are deficient in taurine; hence, there is a need for its supplementation to fish fed high PP diets. Therefore, effects of dietary taurine were examined on growth performance, feed utilization, immunity, and antioxidant parameters of African catfish, Clarias gariepinus (B.). Fish (10.3 ± 0.4 g) were fed on diets (40% crude protein) containing different taurine levels of 0 (control), 10, 20, 30, or 40 g/kg diet for 12 weeks. Fish fed a taurine-free diet (the control) with high PP sources showed poor growth as compared with these fed taurine-enriched diets where taurine stimulatory effects were observed on fish growth and feed intake. Feed conversion ratio and fish survival rate were not significantly differed among different treatments. Fish fed taurine-enriched diets showed also higher levels of serum glucose, cholesterol, total protein, albumin, globulin, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, urea, and creatinine over that fed the control diet. Furthermore, lysozyme and respiratory burst activities as well as superoxide dismutase and catalase activities were significantly elevated in fish fed taurine-enriched diets (P < 0.05) and their highest levels were observed in fish fed 30 g/kg diet. Additionally, taurine deposition in fish muscles was positively correlated with dietary taurine levels (P < 0.05). The present study concludes that taurine is a limiting factor for growth, immunity, and antioxidants responses of African catfish fed high PP-based diets and it should be incorporated in its diets with an optimum level of 20 g/kg diet.

In the present study, juvenile striped catfish (Pangasianodon hypophthalmus), a freshwater fish species, have been chronically exposed to a salinity gradient from freshwater to 20 psu (practical salinity unit) and were sampled at the beginning (D20) and the end (D34) of exposure. The results revealed that the intestinal microbial profile of striped catfish reared in freshwater conditions were dominated by the phyla Bacteroidetes, Firmicutes, Proteobacteria, and Verrucomicrobia. Alpha diversity measures (observed OTUs (operational taxonomic units), Shannon and Faith’s PD (phylogenetic diversity)) showed a decreasing pattern as the salinities increased, except for the phylogenetic diversity at D34, which was showing an opposite trend. Furthermore, the beta diversity between groups was significantly different. Vibrio and Akkermansia genera were affected differentially with increasing salinity, the former being increased while the latter was decreased. The genus Sulfurospirillium was found predominantly in fish submitted to salinity treatments. Regarding the host response, the fish intestine likely contributed to osmoregulation by modifying the expression of osmoregulatory genes such as nka1a, nka1b, slc12a1, slc12a2, cftr, and aqp1, especially in fish exposed to 15 and 20 psu. The expression of heat shock proteins (hsp) hsp60, hsp70, and hsp90 was significantly increased in fish reared in 15 and 20 psu. On the other hand, the expression of pattern recognition receptors (PRRs) were inhibited in fish exposed to 20 psu at D20. In conclusion, the fish intestinal microbiota was significantly disrupted in salinities higher than 10 psu and these effects were proportional to the exposure time. In addition, the modifications of intestinal gene expression related to ion exchange and stressful responses may help the fish to adapt hyperosmotic environment

Water quality in land-based fish production can be controlled through either instantaneous water exchange or costly wastewater treatment followed by recirculation. Agricultural-aquaculture integration is an excellent alternative technique for reducing nutrient discharge levels, boosting profitability, and converting fish culture wastewater into valuable products. The current study employed a solar energy system to power two separate IMTA-aquaponics systems (Nutrient Film Technique, NFT, and Floating Raft Systems, FRS) for the cultivation of Nile tilapia, African catfish, thin-lipped grey mullet, freshwater crayfish, freshwater mussels, and a variety of vegetables. Tilapia and catfish were fed exclusively on diets under the IMTA system. All wastewater from tilapia and catfish ponds, both dissolved and solid, flows sequentially to ponds containing other cultivated species. The water then flows through the IMTA system's terminal point to the NFT and FRS systems before returning to the tilapia and catfish ponds, allowing complete control of the nutrient flow throughout this entire circular system. Two 147-day production cycles were concluded. The results from the second production cycle are reported. Total biomass gain for aquatic species in the IMTA system was 736.46 kg, compared to 145.49 kg in the tilapia and 271.01 kg in the catfish monoculture systems. The current IMTA system had a cumulative feed conversion ratio (FCR) of 0.90, while the FCRs for tilapia and catfish were 1.28 and 1.42, respectively. Nile tilapia and catfish consumed 571.90 kg of feed containing 25.70 kg of nitrogen (N) and 9.70 kg of phosphorus (P), reflecting, and gaining 11.41 and 3.93 kg of dietary N and P, representing 44.40 and 40.46% dietary N and P retention, respectively. In the IMTA system, the addition of mullet and prawn as detrivores aquatic animals improves dietary N and P utilization efficiency to 59.06 and 51.19%, respectively, while the addition of mussels as herbivore animals improves dietary N and P utilization efficiency to 65.61 and 54.67%, respectively. Finally, using FRS and NFT as hydroponic systems increased dietary N and P efficiency to 83.51% N and 96.82% P, respectively. This study shows that the IMTA-Aquaponic system, as a bio-integrated food production system, can convert the majority of fish-fed residues into valuable products suitable for desert, rural, and urban areas in impoverished and developing countries.

Phytochemicals and essential oils have been widely used as growth promoters in aquaculture. However, the optimal dose of a blend of essential oils for promoting the growth and health of African catfish ( Clarias gariepinus ) remains largely unknown. Therefore, the present study aimed to assess the dose-response effects of dietary supplementation with a blend of clove and peppermint oils (CPO) on growth indices, feed utilization, physio-metabolic responses, immunity, and organ histology in African catfish. Catfish (105.1 ± 0.5 g) were fed diets containing 0.0 (control; CPO0), 1.0 (CPO1), 2.0 (CPO2), or 3.0 mL CPO/kg diet for two months. The results displayed that fish performance and feed intake significantly improved with increasing CPO levels in a linear manner ( P  < 0.01). Additionally, the specific growth rate and hemoglobin (R 2  = 84.6%) levels showed quadratic improvements with the optimal doses at 1.12 mL and 1.6 mL/kg diet, respectively. The inclusion of dietary CPO linearly affected aspartate aminotransferase (AST; R 2  = 93.1%), alanine aminotransferase (ALT; R 2  = 97.7%), alkaline phosphatase (ALP; R 2  = 97.0%), and glucose (R 2  = 85.9%) levels in the plasma of catfish ( P  < 0.001). Increasing the concentrations of dietary CPO led to a quadratic decrease in creatinine (R 2  = 98.7%), uric acid (R 2  = 94.8%), and cortisol (R 2  = 87.1%; P  < 0.001), with optimal doses confirmed at 1.75, 2.7, and 1.3-mL CPO per kg of diet, respectively. Dietary CPO has a significant modulatory impact on immune-antioxidant variables in African catfish in a dose-dependent manner. Increasing CPO in the fish diets resulted in a quadratic increase in the levels of immunoglobulin G (IgG; R 2  = 98.8%), lysozyme activity (R 2  = 93.9%), and total antioxidant capacity (R 2  = 84.9%). The corresponding dose–response curves displayed that the optimal doses were at 1.85, 2.1, and 1.8 mL/kg of diet. Furthermore, superoxide dismutase (SOD), and reduced glutathione (GSH) levels were significantly improved in all CPO-treated groups in a quadratic manner, with optimal doses at 1.25 mL (R 2  = 90.6%) and 1.55 mL (R 2  = 89.7%) per kg of diet. Catfish fed diets supplemented with CPO showed gradual improvements in hepatocytes (HP), and blood vessels (BV) with an increased number of binucleated cells. Moreover, catfish fed diets containing CPO had improvement in the tubular epithelium, Bowman’s capsule, and a few melanomacrophage areas. This study revealed that CPO supplementation at an optimal dose of 1–2 mL/kg of diet significantly improved growth performance, feed utilization, metabolic processes, and immune and antioxidant functions in African catfish.

Background Walking catfish ( Clarias batrachus ) is a freshwater fish capable of air-breathing and locomotion on land. It usually inhabits various low-oxygen habitats, burrows inside the mudflat, and sometimes “walks” to search for suitable environments during summer. It has evolved accessory air-breathing organs for respiring air and corresponding mechanisms to survive in such challenging environments. Thereby, it serves as a great model for understanding adaptations to terrestrial life. Results Comparative genomics with channel catfish ( Ictalurus punctatus ) revealed specific adaptations of C. batrachus in DNA repair, enzyme activator activity, and small GTPase regulator activity. Comparative analysis with 11 non-air-breathing fish species suggested adaptive evolution in gene expression and nitrogenous waste metabolic processes. Further, myoglobin, olfactory receptor related to class A G protein-coupled receptor 1, and sulfotransferase 6b1 genes were found to be expanded in the air-breathing walking catfish genome, with 15, 15, and 12 copies, respectively, compared to non-air-breathing fishes that possess only 1–2 copies of these genes. Additionally, we sequenced and compared the transcriptomes of the gill and the air-breathing organ to characterize the mechanism of aerial respiration involved in elastic fiber formation, oxygen binding and transport, angiogenesis, ion homeostasis and acid-base balance. The hemoglobin genes were expressed dramatically higher in the air-breathing organ than in the gill of walking catfish. Conclusions This study provides an important genomic resource for understanding the adaptive mechanisms of walking catfish to terrestrial environments. It is possible that the coupling of enhanced abilities for oxygen storage and oxygen transport through genomic expansion of myoglobin genes and transcriptomic up-regulation of hemoglobin and angiogenesis-related genes are important components of the molecular basis for adaptation of this aquatic species to terrestrial life.

Size relationships between prey and predators are a key topic in ecology. A positive correlation often exists between the sizes of prey and predators. Although predators can consume much smaller prey, some preferentially target larger prey and avoid very small ones. Prey size preferences can be shaped by prey quality, alternative prey availability, and handling costs. Additionally, the defensive behaviour of small prey may prevent ingestion after capture, although this hypothesis has not been experimentally tested. In this study, I observed the behavioural responses of the predatory fish Silurus asotus (Siluriformes: Siluridae) to eight aquatic beetle species (Coleoptera: Gyrinidae, Dytiscidae, Hydrophilidae) of varying body sizes under aquarium conditions. The rate of successful predation varied from 20% to 90% among beetle species. Prey size significantly affected predation success: small beetles were more frequently spat out by fish after capture. When the locomotion of the small beetle Regimbartia attenuata was experimentally restricted by leg amputation, most individuals were readily consumed. These results demonstrate that small beetles use their legs to move rapidly within the fish’s mouth or cling to the internal surfaces, thereby preventing ingestion and eliciting rejection. Thus, post-capture defences of small prey can influence prey size preferences in whole-swallowing predators.

Mimic prey species keep their distance Müllerian mimics are poisonous or unpalatable potential prey species that are not closely related to the species they mimic but have evolved similar warning coloration to discourage shared predators, gaining in the process through strength in numbers. It is not clear if this effect is sufficient to maintain coexistence when competitive exclusion would be expected to favour one mimic at the expense of the others. Martin Taylor and colleagues address that question in a study of Müllerian mimicry in a species-rich group of tropical catfish. Using morphometric and stable isotope analysis, they find that mimics do not occupy identical niches so are not in direct competition for food, thus explaining their ability to coexist. Müllerian mimics have convergently evolved similar warning colouration because of the advantage of strength in numbers. However, it is not clear if this effect is sufficient to maintain coexistence when competitive exclusion would be expected to favour one mimic at the expense of the others. Here, Müllerian mimicry in catfish is characterized, and it is shown through morphometric and stable isotope analysis that mimics do not occupy identical niches, so are not in direct competition, thus explaining their coexistence. Until recently, the study of negative and antagonistic interactions (for example, competition and predation) has dominated our understanding of community structure, maintenance and assembly 1 . Nevertheless, a recent theoretical model suggests that positive interactions (for example, mutualisms) may counterbalance competition, facilitating long-term coexistence even among ecologically undifferentiated species 2 . Müllerian mimics are mutualists that share the costs of predator education 3 and are therefore ideally suited for the investigation of positive and negative interactions in community dynamics. The sole empirical test of this model in a Müllerian mimetic community supports the prediction that positive interactions outweigh the negative effects of spatial overlap 4 (without quantifying resource acquisition). Understanding the role of trophic niche partitioning in facilitating the evolution and stability of Müllerian mimetic communities is now of critical importance, but has yet to be formally investigated. Here we show that resource partitioning and phylogeny determine community structure and outweigh the positive effects of Müllerian mimicry in a species-rich group of neotropical catfishes. From multiple, independent reproductively isolated allopatric communities displaying convergently evolved colour patterns, 92% consist of species that do not compete for resources. Significant differences in phylogenetically conserved traits (snout morphology and body size) were consistently linked to trait-specific resource acquisition. Thus, we report the first evidence, to our knowledge, that competition for trophic resources and phylogeny are pivotal factors in the stable evolution of Müllerian mimicry rings. More generally, our work demonstrates that competition for resources is likely to have a dominant role in the structuring of communities that are simultaneously subject to the effects of both positive and negative interactions.