International Journal of Aquatic Biology

Breaking the cold-blooded rule: Opah’s secret to whole-body endothermy

Sandipan Gupta — 2026-08-09

Endothermy represents a significant evolutionary advancement that improves physiological efficiency and ecological adaptability; yet, among fishes, it is typically limited to regional heat retention. The opah (Lampris spp.) is exceptional as the only known teleost to exhibit true whole-body endothermy, sustaining elevated body temperatures regardless of external conditions. This review synthesizes current knowledge on the anatomical, physiological, and molecular mechanisms underlying this rare adaptation. At the core of opah endothermy is a unique counter-current heat-exchange system within the gill arches, where retia mirabilia retain metabolically produced heat that would otherwise be lost during respiration. The continuous movement of the pectoral fins generates significant metabolic heat, which is effectively conserved and distributed via insulated circulatory pathways, thereby maintaining higher temperatures in cardiac, neural, and visceral tissues. This systemic thermal elevation enhances aerobic performance, sensory function, and ecological competitiveness in cold, mesopelagic habitats. Comparative analyses indicate that this strategy is evolutionarily distinct from regional endothermy observed in lamnid sharks and scombrid fishes, highlighting a striking case of convergent thermal adaptation. Emerging genomic and mitochondrial evidence further suggests enhanced oxidative capacity and specialized muscle bioenergetics underlying sustained heat production. In summary, the opah challenges the conventional limits of fish physiology and serves as an intriguing model for exploring the evolutionary shifts between ectothermy and endothermy. Future comprehensive research integrating functional genomics, bioenergetics, and ecological modeling will be crucial to understanding the selective pressures and constraints that shape this rare and extraordinary characteristic.

At the core of opah endothermy is a unique counter-current heat exchange system located within the gill arches, where retia mirabilia retain metabolically produced heat that would otherwise be lost during respiration. The continuous movement of the pectoral fins generates significant metabolic heat, which is effectively conserved and distributed through insulated circulatory pathways, allowing for the maintenance of higher temperatures across cardiac, neural, and visceral tissues. This systemic thermal elevation enhances aerobic performance, sensory function, and ecological competitiveness in cold, mesopelagic habitats. Comparative analyses indicate that this strategy is evolutionarily distinct from regional endothermy observed in lamnid sharks and scombrid fishes, highlighting a striking case of convergent thermal adaptation. Emerging genomic and mitochondrial evidence further suggests enhanced oxidative capacity and specialized muscle bioenergetics underlying sustained heat production. In summary, the opah challenges the conventional limits of fish physiology and serves as an intriguing model for exploring the evolutionary shifts between ectothermy and endothermy. Future comprehensive research that integrates functional genomics, bioenergetics, and ecological modeling will be crucial in understanding the selective pressures and limitations that shape this rare and extraordinary characteristic.

Aquarium fish invasions in Türkiye’s lakes and rivers: Assessing risks and consequences

Burak Secer — 2026-08-09

The introduction of non-native aquarium fish into freshwater ecosystems has become an increasing problem in Türkiye. Owing to the rapid growth in the introduction of non-native aquarium fish, there is a need to assess their dispersal, monitor their ecological impacts, and develop management methods to prevent further introductions. Recent findings have confirmed the introduction of 14 species of aquarium fish into some of the country’s rivers and lakes. Eleven of which are invasive, and the Climate Change Assessment (CCA) and Basic Risk Assessment (BRA) scores of these species are high. Ornamental fishes are distributed throughout the water basins of Türkiye. Introductions are mainly human-induced, often due to pet owners dumping unwanted aquarium fish into the wild without regard for their ecological implications. The species that have been introduced have been found in aquatic ecosystems such as slow-flowing rivers, lakes, and dams, and have the potential to develop sustainable populations. The ecological risks that result from introductions include competition and predation among local fauna and species, interbreeding, habitat modification, and the transfer of diseases and parasites. This study aims to investigate the dispersal of introduced aquarium fish in Türkiye’s freshwater aquatic ecosystems, considering their ecological impacts.

Effects of pine pollen as a dietary feed additive on growth performance, survival, and stress response of postlarval giant freshwater prawn (Macrobrachium rosenbergii)

Jhelena Rose R. Laudan — 2026-08-09

The rising costs and environmental concerns associated with fishmeal-based feeds have spurred efforts to identify sustainable, viable alternatives in freshwater prawn farming. The present study was performed to investigate the effects of supplementation of pine pollen meal (PPM) on growth performance, length development, survival, microbial characteristics, stress tolerance, and feed cost in postlarval Macrobrachium rosenbergii. A 45-day feeding experiment was conducted in a completely randomized design with three dietary treatments: a control diet without PPM and diets supplemented with 25% and 50% PPM. Growth parameters, including weight gain, specific growth rate, final length, and length increment, were evaluated alongside survival rate, water quality, microbial counts, and stress response. The 25% PPM diet provided the maximum growth and length increment for post-larval prawns, whereas the 50% PPM diet showed the highest survival and stress resistance. Water quality conditions were within acceptable limits during the experiment. Pine-pollen supplemented treatments resulted in enhanced lactic acid bacterial populations in the culture water. These findings suggested that pine pollen meal could be a promising functional dietary additive to improve growth performance, length development, survival, and stress tolerance of juvenile M. rosenbergii as it supports its potential application in sustainable freshwater prawn production.

The population dynamics and stock status of African catfish Clarias gariepinus (Burchell, 1822) in Ribb Reservoir, Lake Tana Sub-Basin, Ethiopia

Alamrew Eyayu — 2026-09-11

The African catfish, Clarias gariepinus, is of great economic importance in Ethiopian fisheries. The species is hardy and fast-growing in any aquatic environment. This study aimed to investigate growth parameters and exploitation rates of C. gariepinus to support effective fisheries management. For this study, a twelve-month gillnet catch was employed. The FiSAT II software was used to examine the population dynamics of C. gariepinus in Ribb Reservoir using length–frequency structured data. The VBGF parameters were estimated as: L_? = 99.75 cm, K = 0.98/year, and t₀= -0.148 year. The total, natural, and fishing mortality coefficients were 3.23, 1.17, and 2.06 per year, respectively. The relative yield-per-recruit (Y’/R) analysis predicted Emax = 0.406 and revealed that C. gariepinus is overexploited, with E_0.5 = 0.267, which is lower than the current exploitation rate (E_{current =}0.64). The analysis of the probability of length at first capture indicated 71.35 cm, implying that the species is on the verge of overexploitation. Therefore, management measures need to target protecting growth from overfishing. To ensure the effectiveness of any fisheries management regulation, all stakeholders and policymakers need to establish a robust co-management program to conserve essential fish stocks.

Phenotypic and genotypic antibiotic resistance of dominant bacterial isolates from cultured Whiteleg shrimp, Penaeus vannamei

Mary Ann Gito — 2026-09-11

Antibiotic resistance in aquaculture poses significant threats to aquatic animal health and may contribute to the spread of antimicrobial resistance in the environment and human populations. This study characterized the phenotypic and genotypic antibiotic resistance profiles of dominant bacterial isolates recovered from cultured whiteleg shrimp (Penaeus vannamei) obtained from shrimp farms in Iloilo, Philippines. A total of 150 shrimp samples were processed for bacterial isolation, and dominant isolates were screened for multidrug resistance (MDR) using the Kirby–Bauer disk diffusion method. Based on the antibiotic resistance index (ARI), nine MDR isolates were selected for further analysis. Phenotypic susceptibility testing showed that all MDR isolates were resistant to ampicillin and rifampin. Resistance to ciprofloxacin (55.5%), erythromycin (44.4%), tetracycline (22.2%), chloramphenicol (11.1%), and streptomycin (11.1%) was also observed. Polymerase chain reaction (PCR) detected antibiotic resistance genes associated with chloramphenicol, tetracycline, and quinolone resistance, namely catB (44.4%), tetE (22.2%), and gyrA (11.1%), respectively. Discrepancies between phenotypic resistance patterns and detected resistance genes were observed, indicating phenotypic–genotypic discordance. These findings suggest that additional mechanisms beyond the targeted genes may influence the expression of resistance. The study highlights the complexity of antimicrobial resistance in aquaculture and underscores the value of integrating phenotypic and molecular approaches for effective surveillance and management.

Protective effect of Ocimum gratissimum leaf aqueous extract on antioxidant capacity, immunocompetence, and disease resistance of Nile tilapia (Oreochromis niloticus) fingerlings against Vibrio parahaemolyticus infection

Rasaq Ibrahim — 2026-09-11

Infectious diseases are major constraints on the aquaculture industry, and they frequently arise from stress factors associated with intensive aquaculture practices. This study was carried out to assess the protective effect of Ocimum gratissimum leaf aqueous extract on immunological responses, antioxidant capacity, and disease resistance of Oreochromis niloticus fingerlings against Vibrio parahaemolyticus infection. Five iso-nitrogenous (30.02%) diets containing 0 (control), 2, 4g, 8, and 16 g Ocimum-leaf extract (g/kg) were fed to triplicate groups of fish (initial weight 3.31±0.05g) for 90 days. After a 90-day feeding trial, test fish were injected with pathogenic V. parahaemolyticus, and survival was recorded over the next 14 days. The white blood cell (12.29±0.35×10³/µl), red blood cells (3.60±0.21×10³/µl), packed cell volume (27.76±0.08 g/dl), and haemoglobin (7.21±0.47 g/dl) were all highest in fish fed 8 g/kg of Ocimum leaf extract. The biochemical parameters, such as alkaline phosphatase (ALP), total protein, albumin, and globulin contents, increased significantly in Ocimum leaf extracts fed groups compared with the control; these groups also registered the lowest values of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and malondialdehyde (MDA). Following a challenge with V. parahaemolyticus, the study indicated that Ocimum leaf extract reduced metabolic stress in infected fish compared with the control. There is a significant increase in glucose (101.00±2.89 mg/dl) levels and liver enzymes ALT (74.14±2.31 UI/ L) and AST (49.00±1.73 UI/L) in the control group (0 g/kg), whereas the opposite was true for diets supplemented with Ocimum leaf extract. The Ocimum leaf extract group significantly enhanced immune parameters, including lysozyme, immunoglobulin (IgM), and Interleukin-1? (IL-1?), in the spleen, skin, and kidney. After infection, the supplemented diets containing Ocimum leaf extract showed significantly higher lysozyme and IgM activities in the spleen, skin, and kidney. The result of the challenge test with V. parahaemolyticus revealed that the highest survival (80%) was recorded in the group fed Ocimum-supplemented diets at 8 and 16 g/kg, and the lowest skin lesion morbidity (15%) was observed in fish fed 8 g/kg. This study revealed that the inclusion of O. gratissimum leaf extract in the diet of O. niloticus enhanced biochemical parameters and immune response, and improved resistance to V. parahaemolyticus.

Sea-based cage culture of red tilapia (Oreochromis spp.) juveniles: Effects of different cyclic feeding regimes on compensatory growth, feed utilization efficiency, biochemical composition, and water quality

Juliebee C. Rosos — 2026-09-11

Red tilapia (Oreochromis spp.) is a valuable aquaculture species due to its rapid growth, salinity tolerance, and adaptability to diverse environmental conditions. A 60?day feeding trial was conducted in sea?based cages (2× 2× 2 m) at 37 fish?m^-3 (equivalent to 296 fish?cage^-1) to assess the effects of cyclic feeding regimes (CFRs) on growth performance, feed utilization efficiency, biochemical composition, condition factor, survival, and water quality of red tilapia juveniles. Tilapia juveniles (10.67±0.50 g; mean ±SEM) were subjected to three feeding strategies: continuous daily feeding (control), 1:6 CFR (1?day starvation followed by 6 days refeeding), and 2:5 CFR (2?days starvation followed by 5 days refeeding). The results showed that fish subjected to 1?day starvation exhibited compensatory growth (CG), with final body weight (FBW), weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER), feed conversion ratio (FCR), and condition factor (CF) comparable to the control group. In contrast, 2?day starvation (2:5 CFR) significantly reduced growth performance compared to the control group. FCR, PER, and CF of red tilapia juveniles were not significantly affected by CFRs. Correlation analysis revealed a strongly significant negative relationship between CFRs and FBW (r = ?0.91), WG (r = ?0.91), and SGR (r = ?0.90), while it showed a moderately significant negative correlation with FBL (r = ?0.65), LI (r = ?0.65), and PER (r = ?0.62). A strong positive correlation was also observed between CFR and survival rate (r = 0.90), indicating that cyclic feeding schedules significantly influence the survival of red tilapia juveniles. Biochemical analysis indicated decreased crude protein and increased lipid content with longer starvation periods. Salinity showed a moderate positive correlation with CFRs (r = 0.76) and dissolved oxygen (DO; r = 0.79), and a strong negative correlation with pH (r = ?0.82). Hyperphagia during refeeding was identified as the primary mechanism for CG. Overall, short-term cyclic feeding (1:6 CFR) offers a viable management strategy to reduce feed costs while maintaining growth and sustainability in sea-based tilapia farming.

Quantifying microplastics in native and farmed fish: Evidence from Orestias agassii and Oncorhynchus mykiss in Lake Titicaca

Angel Canales-Gutiérrez — 2026-09-11

Orestias agassii (black carachi) and Oncorhynchus mykiss (rainbow trout) are key fish species in the diet of local populations surrounding Lake Titicaca. However, recent studies have raised concern about the presence of microplastics (MPs) in this high-altitude aquatic ecosystem. Hence, this study aimed to quantify MPs in the gastrointestinal tracts of black carachi and rainbow trout from the Chucuito and Uros Chulluni areas of Lake Titicaca. The research was conducted between May and July 2024. A total of 100 O. agassii and 50 O. mykiss individuals were dissected, and gastrointestinal contents were digested using 10% KOH to isolate MPs. The results showed an average of 6.02 MPs/individual in black carachi from Uros Chulluni and 5.34 MPs/individual in Chucuito, providing the first documented evidence of MP ingestion in this native species in Lake Titicaca. In rainbow trout, MP concentrations were significantly higher, with averages of 21.1 MPs in Capachica and 44.6 MPs in Chucuito, and maximum values of 71 and 135 MPs/individual, respectively. A positive relationship was found between MP ingestion and digestive tract morphometry in both native and farmed species.

Effects of different carbon sources on the growth of the monkey river prawn, Macrobrachium lar, and nitrogen–phosphorus uptake by plants in a biofloc aquaponics system

Gamal M. Samadan — 2026-09-11

This study evaluated the effects of different carbon sources on the growth performance of Monkey River prawn, Macrobrachium lar, and on nitrogen and phosphorus uptake by leafy vegetables in a biofloc aquaponics system on Ternate Island. A completely randomized design was used with four treatments: molasses, rice bran, tapioca, and a carbon?free control, each with three replicates. Juvenile M. lar (0.035±0.001 g) were stocked at 30 prawns per 100 L tank and reared for 6 weeks, while water spinach, lettuce, and pakcoy were grown in media beds receiving recirculating effluent. Molasses significantly increased absolute growth and survival of M. lar compared with other treatments, although specific growth rate was similar to the control, and feed conversion ratio did not differ among treatments. Water quality variables remained within acceptable ranges for freshwater prawns and were not affected by the carbon source. Carbon source and plant species significantly influenced plant nitrogen content, with the highest N percentages generally observed in mustard green and celery under the control treatment, whereas molasses tended to reduce tissue N, indicating stronger microbial competition for dissolved inorganic nitrogen. Phosphorus content was less responsive to main effects but showed significant carbon–plant interactions, with pakcoy under rice bran and control exhibiting the greatest P accumulation. These results demonstrate that molasses?driven biofloc enhances M. lar production, while careful selection of plant species and carbon sources can improve nutrient recovery, supporting the development of sustainable biofloc aquaponics for freshwater prawns in small?island environments.

Environmental gradients and physicochemical properties of water as determinants of mangrove gastropod community structure in Sarawak, Borneo

Mohammad Saufi Mohammad Ramli — 2026-09-17

Mangrove ecosystems are highly dynamic ecological boundaries where water physicochemical properties influence benthic community structure, including gastropod assemblages. This study investigated the relationship between environmental gradients and gastropod community structure in a coastal mangrove system in northern Sarawak, Malaysia. Field sampling was conducted from September 2023 to March 2024 across three stations covering downstream to upstream. Gastropods were collected using quadrats during low tide and identified to species level, while in situ and ex situ water parameters, including salinity, pH, temperature, dissolved oxygen (DO), turbidity, conductivity, ammonia, nitrite, nitrate, and phosphate, were measured. Diversity indices, one-way ANOVA, cluster analysis, Principal Component Analysis (PCA), and Canonical Correspondence Analysis (CCA) were used to evaluate spatial variation. A total of eight species from four families were recorded, with the highest species richness and diversity observed at Station 2 (H' = 1.556), whereas the upstream Station 3 exhibited the greatest dominance (D = 0.3211). Significant spatial differences were detected in the concentrations of temperature, turbidity, DO, ammonia, nitrite, and phosphate, indicating a clear environmental gradient along the river course. PCA and cluster analysis revealed distinct separation of stations, reflecting variation in gastropod community structure. Pearson correlation analysis revealed that pH was significantly and negatively correlated with species dominance; ammonia and nitrite were positively correlated with dominance; conductivity was negatively associated with Ellobium aurismidae abundance; and nitrate and salinity were associated with Cerithidea quoyii abundance. Although CCA suggested tendencies between species distribution and variables such as salinity, DO, and nutrient concentrations, model significance was not supported by permutation testing. These findings provide baseline information on gastropod diversity and water quality characteristics in Sarawak's mangroves and highlight the influence of spatial environmental gradients on community patterns. The study contributes to understanding ecosystem responses to physicochemical change and supports future biomonitoring and conservation of tropical mangrove habitats.

Differential sensitivity of diatom and macrophyte indices to road-construction-related physical disturbance in two Mediterranean intermittent rivers (Sicily, Italy)

Lorenzo Maria Ruggeri — 2026-09-19

Ecological status assessment in Mediterranean intermittent rivers is routinely challenged by natural hydrological variability, and the coherence between diatom and macrophyte responses to transient anthropogenic pressures such as road construction remains poorly resolved. This study compared the sensitivity of the Intercalibration Common Metric index (ICMi, diatoms) and the Macrophyte Biological Index for Rivers (IBMR, macrophytes) at four permanently marked stations, paired upstream/downstream of an active road-building site ("Lotto 4"), on two intermittent watercourses of the Hyblaean Plateau (San Leonardo River and Margi Stream, SE Sicily, Italy). Sampling was conducted across three hydrological phases (winter recharge, spring transition, and summer recession; December 2024–September 2025). Turbidity and total suspended solids (TSS) were measured alongside standard physicochemical LIMeco supporting elements. The ICMi declined sharply at the Margi downstream station in spring (ICMi = 0.239, "Bad"; reference site ICMi = 0.363) coinciding with elevated turbidity (38.4±4.1 NTU downstream vs. 9.2±1.3 NTU upstream) and TSS (64.7±6.0 vs. 14.1±1.8 mg L?¹) under stable, "Good" nutrient conditions (LIMeco = 0.530), while the IBMR remained comparatively stable across stations (range 0.645-0.730). Diatom assemblages downstream shifted from adnate/pedunculate taxa dominated by Achnanthidium minutissimum (28%) towards motile, silt-tolerant taxa dominated by Nitzschia palea (38%) and Navicula gregaria (27%), while macrophyte cover and composition changed only modestly. The results support a hypothesis that diatoms provide a faster, higher-resolution signal of localized physical-mechanical disturbance than macrophytes in these systems. Given the constraints of a four-station, three-season design without a pre-construction baseline, we present this as a hypothesis-generating case study rather than as a definitive general rule. Implications for environmental impact monitoring of linear infrastructure projects in Mediterranean intermittent rivers are discussed.

Modeling habitat suitability for Trichiurus lepturus Linnaeus, 1758 under climate change scenarios using ensemble approaches

Abouzar Habibi — 2026-10-10

The impact of climate change on fish distribution has drawn increasing attention worldwide. Studying habitat suitability trends of Trichiurus lepturus, a commercially valuable circum?global species, is significant for climate?adaptive fisheries management. Using an ensemble species distribution model with 11,632 occurrence records and six environmental predictors (Sea bottom temperature, Salinity, Current Velocity, Depth, Primary production, pH) from Bio?ORACLE (v3.0), we assessed global habitat suitability of T. lepturus under current (2010-2020) and future (2040-2050, 2090-2100) scenarios (SSP2?4.5 and SSP5?8.5). Ensemble Model performance was high (AUC = 0.964, TSS = 0.830). Primary production (39.0%), sea bottom temperature (32.3%), and pH (17.1%) were the most influential drivers. Currently, suitable habitat covers 8.80 million km², concentrated in the tropical/subtropical shelves of the western Pacific and Indian Oceans. Future projections show a slight contraction: total suitable habitat declines by 0.1-0.2% under SSP2?4.5 and by 1.4% under SSP5?8.5 by 2100. Paired t?tests on 20 ensemble layers showed no significant changes in total suitable area (P>0.05). Classifying habitat suitability into three quality classes (high, moderate, and low) shows that highly suitable areas remain largely stable (from 8.41 to 8.33 million km²), while moderate?suitability areas decrease by 15.4% and low?suitability areas increase correspondingly. Under the moderate SSP2?4.5 pathway, habitat quality remains stable throughout the century, whereas under the high-emission SSP5-8.5 pathway, the shift by 2100 indicates increased vulnerability of T. lepturus to future climate change. This study underscores how climate change affects species' habitat distributions and provides a scientific basis for future habitat protection.

Keywords: Species Distribution, Climate Change, Fisheries, Habitat, Ensemble Model