Betta Research Articles

Masculinization of beta fish larvae Betta splendens through the different treatment immersion of honey solution and larval age

Betta fish Betta splendens have morphological differences between males and females. Male betta fish have more attractive body colors and shapes than females. This causes the selling value and market demand for males to be higher than that of females. Therefore, it is necessary to make efforts to get enough male fish through masculinization. So far masculinization or the sex reversal into males using androgen hormones before sex differentiation occurs. However, the use of steroid hormones has been banned and reduced, because they cause environmental and public health problems and are carcinogenic. This study used natural honey solution to determine the effect of length immersion time in the honey solution and to obtain a suitable larval age for masculinization on the early gonadal formation and larval survival after treatment. Masculinization was carried out using larvae in three treatments immersion time as follows 10 hours, 11 hours, 12 hours, and control. After that, the treatment was continued with the best results of the treatment using 7-day old larvae (D7), D9, and D11. Each treatment using 30 larvae was immersed in a honey solution of 3 liters with a concentration of 5 ml / L. The treatments were kept until 60 days before sex observation. Early observations of gonad formation were carried out after the larvae were D20 with HE-stained histology. The data obtained were analyzed using descriptive statistics and a chi-square test. The results showed that the treatment for 12 hours of immersion had the highest male sex ratio (93%). While the treatment on D7 larvae age in honey solution had the highest percentage in males (96%). It can be concluded that the increase in the percentage of male betta fish is influenced by the length of time immersion in the honey solution and the age of the larvae in the first week after hatching. All treatments did not affect the survival of betta fish larvae.

Timing of isolation from an enriched environment determines the level of aggressive behavior and sexual maturity in Siamese fighting fish (Betta splendens)

BackgroundTeleost fish are known to respond to environmental manipulation, which makes them an ideal model animal for testing relationships between the environment and behavior. The Siamese fighting fish, Betta splendens, is a solitary, highly territorial fish that displays fierce stereotyped aggressive behavior toward conspecifics or members of other species. Adult fish, especially males, are generally housed in isolation in captivity. Here we report evidence that an enriched rearing environment can decrease the level of aggression in bettas and enable adults to be housed in groups.ResultsB. splendens individuals were hatched in our laboratory and raised in groups in an enriched environment. At the juvenile or subadult stage, some individuals were relocated to a poor environment and kept in isolation. To evaluate aggression, a mirror-image test was conducted at the juvenile, subadult, and adult stages for each fish, and body parameters as well as plasma concentrations of 11-ketotestosterone, estradiol, and cortisol were evaluated. Male and female adult bettas raised in a group showed lower levels of aggression than other adult fish. The magnitude of threatening behavior was greater in adult bettas isolated as subadults, whereas the magnitude of fighting behavior was grater in adult bettas isolated as juveniles. The influence of rearing conditions on behavior was greater in females than in males. Plasma cortisol concentrations of adult bettas isolated as subadults after the mirror-image test were higher than those in other experimental groups. Adult males isolated as subadults had significantly higher plasma concentrations of 11-ketotestosterone than males raised in a group and isolated as juveniles. Females isolated as subadults had a higher gonadosomatic index than females raised in a group and females isolated as juveniles.ConclusionsThese results indicate that bettas can be kept in a group under enriched environments and that the timing of isolation influences the aggression and sexual maturity of bettas. Female and male bettas responded differently to environmental manipulation. Judging from their level of sexual maturity, bettas isolated as subadults show proper development.

Androgens and corticosteroids increase in response to mirror images and interacting conspecifics in males of the Siamese fighting fish Betta splendens

The role of hormones as modulators of aggressive behavior in fish remains poorly understood. Androgens and corticosteroids, in particular, have been associated with aggressive behavior in fish but it is still not clear if animals adjust the secretion of these hormones to regulate behavior during ongoing fights, in response to fight outcomes in order to adjust aggressive behavior in subsequent fights, or both. With its stereotyped displays and high aggression levels, the Siamese fighting fish Betta splendens is an excellent model to investigate this question. Here, we compared the behavioral and endocrine response of male B. splendens to fights where there is no winner or loser by presenting them with a size-matched live interacting conspecific behind a transparent partition or with a mirror image. The aggressive response started with threat displays that were overall similar in frequency and duration towards both types of stimuli. Fights transitioned to overt attacks and interacting with a live conspecific elicited a higher frequency of attempted bites and head hits, as compared with the mirror image. There was a pronounced increase in plasma androgens (11-ketotestosterone and testosterone) and corticosteroids (cortisol) levels in response to the aggression challenge, independent of stimulus type. Post-fight intra-group levels of these hormones did not correlate with measures of physical activity or aggressive behavior. A linear discriminant analysis including all behavioral and endocrine data was a poor classifier of fish from the conspecific and mirror trials, showing that overall the behavioral and endocrine response to mirror images and conspecifics was similar. The results show that fight resolution is not necessary to induce an evident increase in peripheral levels of androgens and corticosteroids in B. splendens. However, the function of these hormones during present and future aggressive contests remains to be clarified.

Genomic Basis of Striking Fin Shapes and Colors in the Fighting Fish.

Resolving the genomic basis underlying phenotypic variations is a question of great importance in evolutionary biology. However, understanding how genotypes determine the phenotypes is still challenging. Centuries of artificial selective breeding for beauty and aggression resulted in a plethora of colors, long-fin varieties, and hyper-aggressive behavior in the air-breathing Siamese fighting fish (Betta splendens), supplying an excellent system for studying the genomic basis of phenotypic variations. Combining whole-genome sequencing, quantitative trait loci mapping, genome-wide association studies, and genome editing, we investigated the genomic basis of huge morphological variation in fins and striking differences in coloration in the fighting fish. Results revealed that the double tail, elephant ear, albino, and fin spot mutants each were determined by single major-effect loci. The elephant ear phenotype was likely related to differential expression of a potassium ion channel gene, kcnh8. The albinotic phenotype was likely linked to a cis-regulatory element acting on the mitfa gene and the double-tail mutant was suggested to be caused by a deletion in a zic1/zic4 coenhancer. Our data highlight that major loci and cis-regulatory elements play important roles in bringing about phenotypic innovations and establish Bettas as new powerful model to study the genomic basis of evolved changes.

Histological Study of Suprabranchial Chamber Membranes in Anabantoidei and Clariidae Fishes.

Simple SummaryAir-breathing fish constitute a broad evolutionary group of fish, which are generally characterized by distinctive phenotypical plasticity. These fishes usually inhabit waters where oxygen deficiency occurs periodically, which is why they have developed a variety of accessory respiratory organs (AROs) that may be used in an obligatory or a facultative manner. Knowledge of the structure of these organs is important for both the breeding and the conservation of these fish species. The aim of this study was to conduct a comparative histological analysis of two types of AROs found in the Anabantoidei suborder and the Clariidae family, both of which are freshwater fish taxa of high ecological and commercial importance.Accessory respiratory organs (AROs) are a group of anatomical structures found in fish, which support the gills and skin in the process of oxygen uptake. AROs are found in many fish taxa and differ significantly, but in the suborder Anabantoidei, which has a labyrinth organ (LO), and the family Clariidae, which has a dendritic organ (DO), these structures are found in the suprabranchial cavity (SBC). In this study, the SBC walls, AROs, and gills were studied in anabantoid (Betta splendens, Ctenopoma acutirostre, Helostoma temminckii) and clariid (Clarias angolensis, Clarias batrachus) fishes. The histological structure of the investigated organs was partially similar, especially in relation to their connective tissue core; however, there were noticeable differences in the epithelial layer. There were no significant species-specific differences in the structure of the AROs within the two taxa, but the SBC walls had diversified structures, depending on the observed location. The observed differences between species suggest that the remarkable physiological and morphological plasticity of the five investigated species can be associated with structural variety within their AROs. Furthermore, based on the observed histology of the SBC walls, it is reasonable to conclude that this structure participates in the process of gas exchange, not only in clariid fish but also in anabantoids.

A unique neurogenomic state emerges after aggressive confrontations in males of the fish Betta splendens.

Territorial defense involves frequent aggressive confrontations with competitors, but little is known about how brain-transcriptomic profiles change between individuals competing for territory establishment. Our previous study elucidated that when two fish Betta splendens males interact, transcriptomes across their brains synchronize in a way that reflects a mutual assessment process between them at the gene expression level. Here we aim to evaluate how the brain-transcriptomic profiles of opponents change immediately after shifting their social status (i.e., the winner/loser has emerged) and 30min after this shift. We showed that changes in the expression of certain genes are unique to different fighting stages and the expression patterns of certain genes are transiently or persistently changed across all fighting stages. These brain transcriptomic responses are in accordance with behavioral changes across the fight. Strikingly, the specificity of the brain-transcriptomic synchronization of a pair during fighting was gradually lost after fighting ceased, leading to the emergence of a basal neurogenomic state in which the changes in gene expression were reduced to minimum and consistent across all individuals. This state shares common characteristics with the hibernation state that animals adopt to minimize their metabolic rates to save energy. Interestingly, expression changes for genes related to metabolism, autism spectrum disorder, and long-term memory still differentiated losers from winners. Together, the fighting system using male B. splendens provides a promising platform for investigating neurogenomic states of aggression in vertebrates.

Bioinspired Tunable Structural Color Film with Janus Wettability and Interfacial Floatability towards Visible Water Quality Monitoring

AbstractSuperwettability materials from existing natural creatures have been widely studied to enable artificial manufacture. Variable wettability states, especially Janus wettability, have attracted particular interest because of the applications in various intelligent systems. However, to date, most of these existing Janus wettability surfaces lack stimuli‐response visualization, which requires the connection of electrical instruments to process and display external stimulus signals. Inspired by the functional performance of lotus leaf and Betta splendens, a multifunctional asymmetric film is designed by using the superhydrophobic/superhydrophilic binary cooperative strategy and tunable structural color feature. Thus, it is demonstrated that the Janus membrane could not only timely report the arrival of the environmental variables via directional migration induced by Marangoni effect, but also quantitively feedback the stimuli through visible structural color variations. These features indicate that the Janus wettability structural color film may open a potential chapter in designing and fabricating the multifunctional robotic environmental detector.

Effect of feeding mosquito larvae on the coloration of Siamese fighting fish (Betta splendens) during grow-out

Coloration is one of the most valued features in aquaculture or ornamental fish, and the Siamese fighting fish (Betta splendens) (Regan 1910) is an excellent model to study coloration. Carotenoids are one of the pigments that express colour in fish. Microalgae synthesize these pigments, which can be transferred through feeding first to mosquito larvae, then to fish when they feed on mosquito larvae. We tested the effect of feeding mosquito larvae on coloration and growth in Siamese fighting fish. Over a 60-day period, 52 individual Siamese fighting fish (32 males and 20 females) were fed with commercial micro pellets (control diet) or mosquito larvae (experimental diet). We expected that fish fed with mosquito larvae would be more colourful and larger than fish fed with commercial micro pellets. Consistent with this prediction, Siamese fighting fish were more colourful when they were fed the mosquito larvae diet than when they were fed a commercial micro pellet diet. We, therefore, recommend the use of mosquito larvae for Siamese fighting fish production. Additionally, since the Siamese fighting fish is an efficient predator of the mosquito larvae, we suggest the use of this live food as a high quality alternative food and a colour bio-capsule with numerous carotenoid pigments.

A new record of Betta splendens (Anabantiformes: Osphronemidae) for the northeastern Brazil

The introduction of exotic species, although harmful to the natural environment, still occurs accidentally, intentionally or as a strategy of biological control. In this work, a specimen of Betta splendens was collected and recorded for Northeastern Brazil, a fish of the family Osphronemidae, originally from Asia. This species has the potential to compete with the native species, causing alterations in the natural community. The record of this species in Brazilian environments and the lack of diagnostic works on this region makes this work of special importance and as an incentive for future investigations.

BettaNet: A Deep Learning Architecture for Classification of Wild Siamese Betta Species

Fish classification is a mix of animal sciences and artificial intelligence. With the advent of machine learning in artificial intelligence, classification has been done using computer vision algorithms and now deep learning is gaining prominence. Betta fish classification is not much explored. The wild species of Betta Splendens which are native to the Kingdom of Thailand are taken in the research reported in this paper. BettaNet architecture, a modified version of ResNet 152 is used to classify 6 species of wild species of betta. The experimental results show that the proposed BettaNet architecture holds better in performance in terms of accuracy and F1-scores. Two different datasets were used and the performance obtained by the proposed architecture reduced the cross-entropy loss over different experimental configurations.

Overview of the betta fish genome regarding species radiation, parental care, behavioral aggression, and pigmentation model relevant to humans.

The Siamese fighting fish (Betta splendens, also known as the betta) is well known in aquarium markets, and also presents an exciting new research model for studying parental care, aggressive behavior, and cryptically diverse pigmentation. However, concentrated efforts are required, both in the context of conservation biology and in its genetics, to address the problems of ongoing outbreeding depression, loss of biodiversity, and lack of scientific biological information. The evolutionary dynamics of the betta must be better understood at the genomic scale in order to resolve the phylogenetic status of unrecognized species, develop molecular markers to study variation in traits, and identify interesting sets of genes encoding various bioresource functions. The recent revolution in multi-omics approaches such as genomics, transcriptomics, epigenomics, and proteomics has uncovered genetic diversity and gained insights into many aspects of betta bioresources. Here, we present current research and future plans in an ongoing megaproject to characterize the betta genome as de novo assemblies, genes and repeat annotations, generating data to study diverse biological phenomena. We highlight key questions that require answers and propose new directions and recommendations to develop bioresource management to protect and enhance the betta genus. Successful accomplishment of these plans will allow the creation of a reference annotated genome and provide valuable information at the molecular level that can be utilized to sustain biodiversity and eco-management of the betta to improve breeding programs for future biomedical research.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

Extrato de orégano como aditivo na ração para Betta splendens

O objetivo desse trabalho foi avaliar o extrato de orégano (EO) (Origanum vulgare) como aditivo em rações para Betta splendens. Utilizou-se 40 juvenis de B. splendens, com peso médio inicial de 0,30 ± 0,02g, num delineamento inteiramente casualizado com quatro tratamentos e dez repetições, por 45 dias. Os tratamentos consistiram em uma ração isenta de extrato de orégano (0% de EO) e outras três rações adicionadas de EO (0,25%; 0,50% e 0,75%). As variáveis analisadas foram: peso inicial (PI) e final (PF), ganho de peso médio (GP), taxa de eficiência proteica, comprimento da cabeça (CCAB), índice de perfil (IPERFIL), índice de cabeça (ICAB), comprimento total (CTOT) e padrão (CPAD)e altura. Como variável de avaliação do estresse oxidativo foi verificado a atividade da enzima catalase (CAT) no tecido muscular. Os dados foram submetidos à ANOVA, teste de Tukey e regressão a 5% de probabilidade. Não se observou efeito (p>0,05) dos tratamentos sobre as variáveis avaliadas de: PF, CCAB, ALT, IPERF, ICAB e TCE. No entanto, verificou-se efeito linear crescente à medida que se aumentou adição de EO nas rações sobre o GP, CPAD, CTOTAL e CAT. Recomenda-se assim o uso de rações com 0,75% de EO para Betta splendens.

Masculinization of betta fish (Betta splendens) through natural honey immersion with different concentration

Male betta fish is more popular and expensive compared to female betta fish because it has a more beautiful body shape and color. One effort to increase the population of male fish is the method of masculinization to reverse the sex of fish into males, so that the profit value is higher. Generally, sex directives or sex reversal usually use the steroid hormone: 17α-metiltestosterone (MT) by the method: immersion, injection or orally through feed. However, this MT hormone has side effects which can cause pollution, liver damage in test animals and can cause mortality. This study was conducted to determine the survival rate of larvae and increase the percentage of male betta fish through natural honey immersion with different concentrations. The method used in this study was a laboratory experiment, using 3 treatments namely A (5ml/L), B (6ml/L), C (7ml/L) and 3 replications. Betta fish larvae as test animals are 7 days old, with natural honey immersion for 12 hours and then reared for 60 days. The results of this study indicate that the survival rate of betta fish larvae after honey immersion treatment (12 hours) averaged 100% and the percentage of survival of betta fish during rearing (60 days) ranged from 25.55 - 30.00%. And the average sex percentage of male sex larvae in the treatment of natural honey immersion is 5 ml / L (85.00%), 6 ml / L (95.24%) and 7 ml / L (100%). Thus it can be concluded that proper honey concentration plays an important role in the process of masculinization.

The effect of enriched Cladocera on growth, survivability and body coloration of Siamese fighting fish

The effect of enriched Cladocera on growth, survivability and body coloration of Siamese fighting fish

Forest before the trees in the aquatic world: global and local processing in teleost fishes.

BackgroundThe study of illusory phenomena is important to understanding the similarities and differences between mammals and birds’ perceptual systems. In recent years, the analysis has been enlarged to include cold-blooded vertebrates, such as fish. However, evidence collected in the literature have drawn a contradictory picture, with some fish species exhibiting a human-like perception of visual illusions and others showing either a reversed perception or no susceptibility to visual illusions. The possibility exists that these mixed results relate to interspecific variability in perceptual grouping mechanisms. Therefore, we studied whether fish of five species exhibit a spontaneous tendency to prioritize a global analysis of the visual scene—also known as global-to-local precedence—instead of focusing on local details.MethodsUsing Navon-like stimuli (i.e., larger recognisable shapes composed of copies of smaller different shapes), we trained redtail splitfin, zebrafish, angelfish, Siamese fighting fish and three spot gourami to discriminate between two figures characterized by congruency between global and local information (a circle made by small circles and a cross made by small crosses). In the test phase, we put global and local cues (e.g., a circle made by small crosses) into contrast to see whether fish spontaneously rely on global or local information.ResultsLike humans, fish seem to have an overall global-to-local precedence, with no significant differences among the species. However, looking at the species-specific level, only four out of five species showed a significant global-to-local precedence, and at different degrees. Because these species are distantly related and occupy a broad spectrum of ecological adaptations, we suggest that the tendency to prioritize a global analysis of visual inputs may be more similar in fish than expected by the mixed results of visual illusion studies.

Effects of different feeding frequency on Siamese fighting fish (Betta splenden) and Guppy (Poecilia reticulata) Juveniles: Data on growth performance and survival rate.

Optimal feeding frequency in aquaculture is vital for the sustainable and economical production of healthy, high-quality fish. This article described the growth performance and survival rate data in the juvenile phase of two commercially important ornamental fish species; Betta splendens (Siamese fighting fish) and Poecilia reticulata (Guppy) reared at different feeding frequency. Thirty days old juveniles of both species were randomly distributed into 12 3 L rectangular plastic tank (n = 10 fish/tank; three replicates per feeding frequency) where they were subjected to four different feeding frequency treatments (1 meal/day (T1), 2 meal/day (Control), 3 meal/day (T2) and four meal/day (T3)) using commercial ornamental fish micropellets for 60 days. The juvenile's weight and length were measured once a week while the number of live fish were recorded daily. The amount of feed intake was also recorded by weighing in the weight of micropellet left after feeding. At the end of the experimental period, the specific growth rate (SGR), food conversion ratio (FCR) and condition factor (K) were calculated for growth performance using the weight, length and feed intake data while the survival rate was calculated using the number of live fish data. Normality test, One-way ANOVA analysis followed by Tukey pos-hoc test were then performed on the data obtained from the calculation of SGR, FCR, K an survival rate. The data presented in this article will aid the rearing process of both species’ juveniles for commercial, experimental and personal usage purpose.

Improving the Chromosome-Level Genome Assembly of the Siamese Fighting Fish (Betta splendens) in a University Master's Course.

Ever decreasing costs along with advances in sequencing and library preparation technologies enable even small research groups to generate chromosome-level assemblies today. Here we report the generation of an improved chromosome-level assembly for the Siamese fighting fish (Betta splendens) that was carried out during a practical university master’s course. The Siamese fighting fish is a popular aquarium fish and an emerging model species for research on aggressive behavior. We updated the current genome assembly by generating a new long-read nanopore-based assembly with subsequent scaffolding to chromosome-level using previously published Hi-C data. The use of ∼35x nanopore-based long-read data sequenced on a MinION platform (Oxford Nanopore Technologies) allowed us to generate a baseline assembly of only 1,276 contigs with a contig N50 of 2.1 Mbp, and a total length of 441 Mbp. Scaffolding using the Hi-C data resulted in 109 scaffolds with a scaffold N50 of 20.7 Mbp. More than 99% of the assembly is comprised in 21 scaffolds. The assembly showed the presence of 96.1% complete BUSCO genes from the Actinopterygii dataset indicating a high quality of the assembly. We present an improved full chromosome-level assembly of the Siamese fighting fish generated during a university master’s course. The use of ∼35× long-read nanopore data drastically improved the baseline assembly in terms of continuity. We show that relatively in-expensive high-throughput sequencing technologies such as the long-read MinION sequencing platform can be used in educational settings allowing the students to gain practical skills in modern genomics and generate high quality results that benefit downstream research projects.

Behavioral and brain- transcriptomic synchronization between the two opponents of a fighting pair of the fish Betta splendens.

Conspecific male animals fight for resources such as food and mating opportunities but typically stop fighting after assessing their relative fighting abilities to avoid serious injuries. Physiologically, how the fighting behavior is controlled remains unknown. Using the fighting fish Betta splendens, we studied behavioral and brain-transcriptomic changes during the fight between the two opponents. At the behavioral level, surface-breathing, and biting/striking occurred only during intervals between mouth-locking. Eventually, the behaviors of the two opponents became synchronized, with each pair showing a unique behavioral pattern. At the physiological level, we examined the expression patterns of 23,306 brain transcripts using RNA-sequencing data from brains of fighting pairs after a 20-min (D20) and a 60-min (D60) fight. The two opponents in each D60 fighting pair showed a strong gene expression correlation, whereas those in D20 fighting pairs showed a weak correlation. Moreover, each fighting pair in the D60 group showed pair-specific gene expression patterns in a grade of membership analysis (GoM) and were grouped as a pair in the heatmap clustering. The observed pair-specific individualization in brain-transcriptomic synchronization (PIBS) suggested that this synchronization provides a physiological basis for the behavioral synchronization. An analysis using the synchronized genes in fighting pairs of the D60 group found genes enriched for ion transport, synaptic function, and learning and memory. Brain-transcriptomic synchronization could be a general phenomenon and may provide a new cornerstone with which to investigate coordinating and sustaining social interactions between two interacting partners of vertebrates.

The Delboeuf illusion's bias in food choice of teleost fishes: an interspecific study

An optimal foraging strategy often requires identifying and choosing the larger amount of food in the presence of multiple options, in order to maximize food intake. Food quantity estimation frequently depends on the perceptual ability to segregate food from the surrounding background. In human and nonhuman animals, it has recently been shown that the perception of food size is affected by the size of the background on which food is presented, with a tendency to overestimate food items encircled by a small background. This perceptual bias resembles an illusory effect known as the Delboeuf illusion. We investigated whether this bias occurs in five fish species: zebrafish, Danio rerio, redtail splitfin, Xenotoca eiseni, angelfish, Pterophyllum scalare, Siamese fighting fish, Betta splendens, and three-spot gourami, Trichopodus trichopterus. In control trials, we observed their spontaneous preference for choosing the larger of two amounts of food. In test trials, two same-sized food amounts were presented on a small and a large plate, a pattern that causes the Delboeuf illusion in humans. Similarities in subjects’ choices between the five species were greater than differences, with a significant bias towards selecting the food amount on the large plate. This suggests the existence of a perceptual bias, although one operating in the opposite direction from that of humans, which might interfere with the accurate food size estimation necessary for an optimal foraging strategy.