Differential Transcriptomic Response of Rainbow Trout to Infection with Two Strains of IPNV

The IPN virus (IPNV) causes a highly contagious disease that affects farmed salmonids. IPNV isolates have been phylogenetically classified into seven genogroups, of which two are present in Chile, genogroups 1 and 5. This study aimed to compare the transcriptomic response of rainbow trout fry challenged with two Chilean isolates of IPNV, RTTX (genogroup 1), and ALKA (genogroup 5). Tissue samples from challenged individuals and controls were taken at 1, 7, and 20 days post-challenge and analyzed by RNA-Seq. The results revealed that infection with RTTX elicited a greater modulation of the trout transcriptome compared to ALKA infection, generating a greater number of highly differentially expressed genes in relation to the control fish. Gene Ontology enrichment indicated that functions related to the inflammatory and immune responses were modulated in fish challenged with both isolates throughout the trial, but with different regulation patterns. On day 1 post challenge, these functions were activated in those challenged with ALKA, but suppressed in RTTX-challenged fish. These results suggest that rainbow trout exhibit a differential transcriptomic response to infection with the two genetically distinct IPNV isolates, especially at early times post-infection.

Gill Transcriptomic Responses to Toxin-producing Alga Prymnesium parvum in Rainbow Trout

The gill of teleost fish is a multifunctional organ involved in many physiological processes, including protection of the mucosal gill surface against pathogens and other environmental antigens by the gill-associated lymphoid tissue (GIALT). Climate change associated phenomena, such as increasing frequency and magnitude of harmful algal blooms (HABs) put extra strain on gill function, contributing to enhanced fish mortality and fish kills. However, the molecular basis of the HAB-induced gill injury remains largely unknown due to the lack of high-throughput transcriptomic studies performed on teleost fish in laboratory conditions. We used juvenile rainbow trout (Oncorhynchus mykiss) to investigate the transcriptomic responses of the gill tissue to two (high and low) sublethal densities of the toxin-producing alga Prymnesium parvum, in relation to non-exposed control fish. The exposure time to P. parvum (4–5 h) was sufficient to identify three different phenotypic responses among the exposed fish, enabling us to focus on the common gill transcriptomic responses to P. parvum that were independent of dose and phenotype. The inspection of common differentially expressed genes (DEGs), canonical pathways, upstream regulators and downstream effects pointed towards P. parvum-induced inflammatory response and gill inflammation driven by alterations of Acute Phase Response Signalling, IL-6 Signalling, IL-10 Signalling, Role of PKR in Interferon Induction and Antiviral Response, IL-8 Signalling and IL-17 Signalling pathways. While we could not determine if the inferred gill inflammation was progressing or resolving, our study clearly suggests that P. parvum blooms may contribute to the serious gill disorders in fish. By providing insights into the gill transcriptomic responses to toxin-producing P. parvum in teleost fish, our research opens new avenues for investigating how to monitor and mitigate toxicity of HABs before they become lethal.

Potential Efficacy of Chitosan-Poly (Lactide-Co-Glycolide)-Encapsulated Trivalent Immersion Vaccine in Olive Flounder (Paralichthys olivaceus) Against Viral Hemorrhagic Septicemia Virus, Streptococcus parauberis Serotype I, and Miamiensis avidus (Scuticociliate)

Olive flounder (Paralichthys olivaceus) is the most valuable aquaculture species in Korea, corresponding to ~60% of its total production. However, infectious diseases often break out among farmed flounders, causing high mortality and substantial economic losses. Although some deleterious pathogens, such as Vibrio spp. and Streptococcus iniae, have been eradicated or contained over the years through vaccination and proper health management, the current disease status of Korean flounder shows that the viral hemorrhagic septicemia virus (VHSV), Streptococcus parauberis, and Miamiensis avidus are causing serious disease problem in recent years. Furthermore, these three pathogens have differing optimal temperature and can attack young fingerlings and mature fish throughout the year-round culture cycle. In this context, we developed a chitosan-poly(lactide-co-glycolide) (PLGA)-encapsulated trivalent vaccine containing formalin-killed VHSV, S. parauberis serotype-I, and M. avidus and administered it to olive flounder fingerlings by immersion route using a prime-boost strategy. At 35 days post-initial vaccination, three separate challenge experiments were conducted via intraperitoneal injection with the three targeted pathogens at their respective optimal temperature. The relative percentages of survival were 66.63%, 53.3%, and 66.75% in the group immunized against VHSV, S. parauberis serotype-I, and M. avidus, respectively, compared to the non-vaccinated challenge (NVC) control group. The immunized fish also demonstrated significantly (p < 0.05) higher specific antibody titers in serum and higher transcript levels of Ig genes in the mucosal and systemic tissues than those of NVC control fish. Furthermore, the study showed significant (p < 0.05) upregulation of various immune genes in the vaccinated fish, suggesting induction of strong protective immune response, ultimately leading to improved survival against the three pathogens. Thus, the formulated mucosal vaccine can be an effective prophylactic measure against VHS, streptococcosis, and scuticociliatosis diseases in olive flounder.

Positive feedback promotes terrestrial emergence behaviour in an amphibious fish

Major ecological transitions such as the invasion of land by aquatic vertebrates have been hypothesised to be facilitated by positive feedback between habitat choice and phenotypic plasticity. We tested whether aquatic hypoxia, emergence behaviour, and plastic changes in gill surface area could create such a positive feedback loop and drive an amphibious fish to spend increasing amounts of time out of water. We found that terrestrially acclimated amphibious mangrove rivulus Kryptolebias marmoratus were more sensitive to, and less tolerant of, aquatic hypoxia relative to water-acclimated fish, which are necessary trade-offs for positive feedback to occur. Next, we acclimated fish to normoxic or hypoxic water with the opportunity to emerge for 7d to test the predictions that fish in hypoxic conditions should regularly leave water, reduce gill surface area, and become less hypoxia tolerant. Consistent with these predictions, fish in severe hypoxia spent almost 50% of the time out of water and coverage of the gill lamellae by an inter-lamellar cell mass almost doubled. Hypoxia acclimated fish were also more sensitive to acute aquatic hypoxia (emergence at higher oxygen levels), and lost equilibrium faster in hypoxic water compared to control fish. Thus, we show that a positive feedback loop develops in amphibious fish where emergence behaviour begets further emergence behaviour, driven by gill remodelling which reduces aquatic respiratory function. Such a scenario may explain how amphibious behaviour has repeatedly evolved in fishes that occupy hypoxic aquatic habitats despite the associated challenges of life on land.

Blood Performance: A New Formula for Fish Growth and Health

Monitoring fish health in a repeatable and accurate manner can contribute to the profitability and sustainability of aquaculture. Haematological and blood biochemistry parameters have been powerful tools and becoming increasingly common in aquaculture studies. Fish growth is closely related to its health status. A fish with a higher growth rate is more likely to be a healthy one. Any change in the physiological status of the fish, from pollution to nutritional stress, can cause changes in the blood parameters. Various aquaculture studies have measured the following components: red blood cells, white blood cells, haemoglobin, haematocrit, and total protein. However, because these parameters do not always follow the same trend across experimental fish, it is difficult to draw a firm conclusion about which parameter should be considered. Therefore, Blood Performance (BP) as a new formula is introduced, which is a more reliable indicator. This formula is simple and sums up the natural logarithm of the five above-mentioned parameters. More than 90 published peer-reviewed articles that measured these five parameters in the last six years confirmed the reliability and validity of this formula. Regardless of which supplements were added to the diets, the fish with a higher growth rate had higher BP as well. In addition, in 44 studies out of 53 articles, there was a significant positive correlation between specific growth rate and BP. Under different stressful situations, from pollution to thermal stress, the fish under stress had a lower BP than the control. Fish meal and fish oil replacement studies were further evidence for this formula and showed that adding excessive alternative proteins decreased growth along with BP. In conclusion, BP can be a reliable indicator of fish health and growth when it is compared between groups in the same experiment or farm. Although there was a positive correlation between specific growth rate and BP, comparing BP between experiments is not recommended. Standardising the haematological assays can improve the reliability and accuracy of BP across experiments.

Market Infrastructure and Species Availability of Four Wholesale Fish Markets in Dhaka City, Bangladesh

The study was conducted on species availability and infrastructure analysis of four different wholesale fish markets in Dhaka City. Two markets were selected from Dhaka South City Corporation, Showari Ghat and Jatrabari fish market, and two from Dhaka North City Corporation, Karwan Bazar and Abdullahpur fish market. The investigation was carried out from November 2018 to October 2019. Questionnaire based survey, FGDs (Focus Group Discussion) and KIIs (Key Informant Interviews) were conducted to get quantitative and qualitative data for present study. A total number of 62 fishes and 6 crustacean species were recorded during the study period. Among all species, 39 freshwater, 10 marine and 13 exotic fishes were recorded. Infrastructure facilities were very poor for all four markets in the matter of hygiene and quality control. Fish shades were not enough, improper sanitation and drainage system indicates the possibilities of disease transmission and deteriorate fish quality. Electricity and water supplies were good but no system was present to determine the distillation of those supplied water. Thus, infrastructural condition of the markets does not support a hygienic condition for fish marketing to provide quality fish and fish byproducts. The study highlighted the need for national initiatives, awareness building, and law implementation for improving the wholesale markets infrastructure to a minimum standard so that fish trade could provide good quality to the people of Dhaka City Bangladesh J. Zool. 49 (2): 289-300, 2021

Survival and Axonal Outgrowth of the Mauthner Cell Following Spinal Cord Crush Does Not Drive Post-injury Startle Responses

A pair of Mauthner cells (M-cells) can be found in the hindbrain of most teleost fish, as well as amphibians and lamprey. The axons of these reticulospinal neurons cross the midline and synapse on interneurons and motoneurons as they descend the length of the spinal cord. The M-cell initiates fast C-type startle responses (fast C-starts) in goldfish and zebrafish triggered by abrupt acoustic/vibratory stimuli. Starting about 70 days after whole spinal cord crush, less robust startle responses with longer latencies manifest in adult goldfish, Carassius auratus. The morphological and electrophysiological identifiability of the M-cell provides a unique opportunity to study cellular responses to spinal cord injury and the relation of axonal regrowth to a defined behavior. After spinal cord crush at the spinomedullary junction about one-third of the damaged M-axons of adult goldfish send at least one sprout past the wound site between 56 and 85 days postoperatively. These caudally projecting sprouts follow a more lateral trajectory relative to their position in the fasciculus longitudinalis medialis of control fish. Other sprouts, some from the same axon, follow aberrant pathways that include rostral projections, reversal of direction, midline crossings, neuromas, and projection out the first ventral root. Stimulating M-axons in goldfish that had post-injury startle behavior between 198 and 468 days postoperatively resulted in no or minimal EMG activity in trunk and tail musculature as compared to control fish. Although M-cells can survive for at least 468 day (∼1.3 years) after spinal cord crush, maintain regrowth, and elicit putative trunk EMG responses, the cell does not appear to play a substantive role in the emergence of acoustic/vibratory-triggered responses. We speculate that aberrant pathway choice of this neuron may limit its role in the recovery of behavior and discuss structural and functional properties of alternative candidate neurons that may render them more supportive of post-injury startle behavior.

Toxicity Evaluation of Arsenic Nanoparticles on Growth, Biochemical, Hematological, and Physiological Parameters of Labeo rohita Juveniles

The present study aims to assess the induced nanotoxicity of arsenic nanoparticles (AsNPs) on different organs of fresh water fish Labeo rohita. AsNPs were synthesized by chemical reduction method using sodium arsenite as precursor, ice-cold sodium borohydride as reducing agent, and sodium hydroxide to adjust the solution pH. The synthesized AsNPs were characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) for optical, structural, and morphological investigations. The UV-Vis absorption peaks occurring at around 300 nm indicated the presence of AsNPs in colloidal sample. The rhombohedral crystalline nature and metallic purity of AsNPs with crystallite size of 30 ± 1 nm were confirmed by characteristic peaks of XRD pattern. The SEM micrograph revealed the almost spherical shape and 40 ± 10 nm average size prepared AsNPs. For assessment of induced nanotoxicity, juveniles of Labeo rohita (L. rohita) were exposed to three different concentrations of AsNPs (namely, 1, 10, and 20 mg/L) for 30 days (n = 15 per group), and the control fish was kept untreated. It was observed that the routine behavior activities (such as swimming, mutual interactions, and feed intake) were affected by AsNPs. The growth of AsNPs treated fish was found retarded as compared to the control fish. Total erythrocyte count, total leukocyte count, and hemoglobin and hematocrit values were low in the AsNPs treated fish. Immunobiochemical assays revealed that protein level was altered in the AsNPs treated fish. The levels of antioxidant enzymes catalase and superoxide dismutase were low in the treated fish. The histological alteration induced by AsNPs in liver, gills, and kidneys demonstrated the damage in form of glomerulus shrinkage, vacuolation, inflammation, necrosis, lamellar disorganization, and hemorrhage in comparison with untreated fish. The results of the present study indicate that AsNPs exposure causes behavior, growth, hematology, immunobiochemical, and histological shortcomings in L. rohita.

Effect of salinity changes on growth, survival and biochemical parameters of freshwater fish Gibelion catla (Hamilton, 1822)

Aim: The present investigation was conducted to access the effect of salinity stress on growth performance, survival and biochemical parameters of Gibelion catla under different salinity conditions with an idea to assess the potentiality of this species in inland saline affected areas. Methodology: A 30-day-experiment was conducted in 1000 l (n=10) FRP tanks to study the effect of different salinity levels on the growth and biochemical parameters of catla species. A total of 120 fish (10.4 g) was randomly distributed into four treatments (0, 3, 6 and 9 ppt) with three replicates. The water quality parameters such as pH, temperature, nitrite, nitrate, ammonia, alkalinity and dissolved oxygen were recorded for each treatment on weekly basis, whereas fish sampling was carried out at 0th and 30th day of the experimental period to ascertain survival, weight gain, specific growth rate and feed conversion ratio of the fishes from each treatment. Results: The highest growth rate was found in control subsequently in 3 and 6 ppt and the survival rate was 96.67, 83.33, 76.67 and 0% against 0, 3, 6 and 9 ppt, respectively. The red blood cells (3.65×106 μl) and haemoglobin concentration, (8.17 gm dl-1) were also higher in control fish, followed by 3 and 6 ppt. However, white blood cells (24.40×106 μl) and glucose level (123.23 mg dl-1) were higher at 6 ppt, compared to control. Plasma protein level of fish at control (0 ppt salinity) was significantly higher than those of fish exposed to 3 and 6 ppt salinities. Interpretation: The present investigation revealed that an increase in salinity level had a significant impact on the growth and physiology of Gibelion catla. However, this species can be reared in low saline areas for some time which will not only help in the utilization of salt affected areas but will also help in the generation of employment and income.

The Influence of an Enriched Environment in Enhancing Recognition Memory in Zebrafish (Danio rerio)

Environmental enrichment is used to increase social and physical stimulation for animals in captivity which can lead to enhanced cognition. Fundamental to the positive effect enrichment has on the brain is that it provides opportunities for captive animals to recognize and discriminate between different stimuli in the environment. In the wild, being able to discriminate between novel or familiar stimuli has implications for survival, for example finding food, hiding from predators, or even choosing a mate. The novel object recognition (NOR) test is a cognitive task that is used extensively in the rodent literature to assess object recognition and memory, where the amount of time an animal spends exploring a novel vs. familiar object is quantified. Enrichment has been shown to enhance object recognition in rodents. More recently, the use of the NOR test has been applied to another animal model, zebrafish (Danio rerio), however, the effects of enrichment have not yet been explored. In the current study we looked at the effects of enrichment on object recognition in zebrafish using the NOR test. Adult zebrafish were housed in either enriched conditions (gravel substrate, plastic plants, shelter, heater and a filter) or plain conditions (heater and filter only) for 6 months before behavioral NOR tests were conducted. Enriched fish showed a preference for a novel object over a familiar one at a distance but did not show a preference during close inspection. Control fish did not show a preference at either distance. Our results suggest that enrichment can enhance zebrafish ability to discriminate between novel and familiar objects, but distance from the object may be an important factor. Future research is needed to determine whether any enhancements in object recognition are a result of an increase in sensory stimulation from being reared with enrichment, or whether it is due to a reduction in stress reactivity.