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Morphological descriptions of 13 wild fighting fish species were revised by examining specimens collected as near as possible to the type specimen localities. The description has been revised for certainty and consistency. Principal Component Analysis (PCA) and Kruskal–Wallis with Dunn’s post hoc tests were analyzed and revealed previously described morphological characteristics were evaluated for distinguishing features. The PCA showed that the high loading factor of the principal component 1 (PC1) indicated that head length (0.221) was beneficial for the fish species identification. The Kruskal–Wallis test indicated the significant differences in morphometric characteristics among the fish species (p < 0.05). Then the significant differences in Dunn’s post hoc test (p < 0.001) in head length distinguished the mouth-brooders (i.e., Betta apollon, B. ferox, B. kuehnei, B. pallida, B. pi, B. prima, B. pugnax, and B. simplex) from bubble-nesters (i.e., B. imbellis, B. mahachaiensis, B. siamorientalis, B. smaragdina, and B. splendens) with a head length range of 29.27–36.42 vs. 24.14–28.31% of standard length, including the symmetrical caudal fin. Some characteristics distinguish the fish into subgroups and some species, such as a greater number of anal soft fin rays and lateral line scales of B. pi among the mouth-brooding species (28–29 vs. 22–26 fin rays and 33–35 vs. 26–32 scales) or of B. smaragdina among the bubble-nesting species (26–28 vs. 22–25 fin rays and 31–33 vs. 27–30 scales). Additionally, descriptive characteristics such as anal and caudal fin shape and melanophore pigmentation on the head, body, and fins, including the coloration of live specimens, are useful for distinguish among species and generating a dichotomous identification key.

Background Fishes are the one of the most diverse groups of animals with respect to their modes of sex determination, providing unique models for uncovering the evolutionary and molecular mechanisms underlying sex determination and reversal. Here, we have investigated how sex is determined in a species of both commercial and ecological importance, the Siamese fighting fish Betta splendens . Results We conducted association mapping on four commercial and two wild populations of B. splendens . In three of the four commercial populations, the master sex determining (MSD) locus was found to be located in a region of ~ 80 kb on LG2 which harbours five protein coding genes, including dmrt1 , a gene involved in male sex determination in different animal taxa. In these fish, dmrt1 shows a male-biased gonadal expression from undifferentiated stages to adult organs and the knockout of this gene resulted in ovarian development in XY genotypes. Genome sequencing of XX and YY genotypes identified a transposon, drbx1 , inserted into the fourth intron of the X-linked dmrt1 allele. Methylation assays revealed that epigenetic changes induced by drbx1 spread out to the promoter region of dmrt1 . In addition, drbx1 being inserted between two closely linked cis -regulatory elements reduced their enhancer activities. Thus, epigenetic changes, induced by drbx1 , contribute to the reduced expression of the X-linked dmrt1 allele, leading to female development. This represents a previously undescribed solution in animals relying on dmrt1 function for sex determination. Differentiation between the X and Y chromosomes is limited to a small region of ~ 200 kb surrounding the MSD gene. Recombination suppression spread slightly out of the SD locus. However, this mechanism was not found in the fourth commercial stock we studied, or in the two wild populations analysed, suggesting that it originated recently during domestication. Conclusions Taken together, our data provide novel insights into the role of epigenetic regulation of dmrt1 in sex determination and turnover of SD systems and suggest that fighting fish are a suitable model to study the initial stages of sex chromosome evolution.

The presence of an audience alters male—male interactions in a number of social species. Additionally, whether individuals have interacted previously may influence the current encounter. Although it is clear that previous exposure influences communication and that the presence of an audience affects behavior in a number of contexts, how these phenomena might interact to impact interactions is underexplored. This study examined the effects of audience familiarity on male—male interactions in Siamese fighting fish (Betta splendens). Treatments that varied based on familiarity with and sex of the audience as well as length of time after initial audience exposure were used to investigate whether male aggressive interactions were affected by these factors. Males first encountered an individual that would later become an audience. Pairs of males then interacted with each other in either the presence of this familiar individual or a new, unfamiliar audience conspecific or with no audience at all. Opponent-directed gill flaring and tail beats were influenced by familiarity with the audience individual, although they were affected differently by the sex of the audience present. Gill flaring was affected predominantly by the presence of a male audience, whereas tail beat rate was influenced by the presence of a female audience. Gill flaring rate did not differ regardless of whether 4 or 24 h had passed between the initial exposure to the audience and the testing phase. In contrast, the familiarity effect disappeared after 4 h for tail beats. These results stress the importance of considering the familiarity of individuals within a network when examining communication networks.

Background Aggression is an evolutionarily conserved behavior critical for animal survival. In the fish Betta splendens , across different stages of fighting interactions, fighting opponents suffer from various stressors, especially from the great demand for oxygen. Using RNA sequencing, we profiled differential alternative splicing (DAS) events in the brains of fish collected before fighting, during fighting, and after fighting to study the involvement of alternative splicing (AS) in the response to stress during the fight. Results We found that fighting interactions induced the greatest increase in AS in the ‘during-fighting’ fish, followed by that of the ‘after-fighting’ fish. Intron retention (IR) was the most enriched type among all the basic AS events. DAS genes were mainly associated with synapse assembly, ion transport, and regulation of protein secretion. We further observed that IR events significantly differentiated between winners and losers for 19 genes, which were associated with messenger RNA biogenesis, DNA repair, and transcription machinery. These genes share many common features, including shorter intron length and higher GC content. Conclusions This study is the first comprehensive view of AS induced by fighting interactions in a fish species across different stages of those interactions, especially with respect to IR events in winners and losers. Together, these findings facilitate future investigations into transcriptome complexity and AS regulation in response to stress under the context of aggression in vertebrates.

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.

Metformin, the medicine most commonly prescribed for treatment of Type II diabetes, is among the most abundant pharmaceuticals being introduced into the environment. Pharmaceuticals are increasingly found in wastewater and surface waters around the world, often due to incomplete metabolism in humans and subsequent excretion in human waste. Risk analyses and exposure studies have raised concerns about potential negative impacts of pharmaceuticals at current environmental levels. Results of the present study indicate that metformin at concentrations in the range of what has been documented in freshwater systems and waste-water effluent (40 μg/L) affects aggressive behavior in adult male Betta splendens. Subjects exhibited less aggression toward a male dummy stimulus after four weeks exposure to metformin-treated water when compared to behavior measured immediately prior to their exposure, and in comparison to a separate cohort of un-exposed control fish. This effect persisted after 20 weeks exposure as well. Subjects exposed to metformin at a concentration twice that currently observed in nature (80 μg/L) exhibited an even more substantial reduction in aggressive behaviors compared to controls and pre-exposure measurements than those observed in the low-dose treatment group. Such changes in behavior have the potential to affect male fitness and possibly impact the health of natural populations of aquatic organisms exposed to the drug.

Gene duplication is one of the most important sources of novel genotypic diversity and the subsequent evolution of phenotypic diversity. Determining the evolutionary history and functional changes of duplicated genes is crucial for a comprehensive understanding of adaptive evolution. The evolutionary history of visual opsin genes is very dynamic, with repeated duplication events followed by sub- or neofunctionalization. While duplication of the green-sensitive opsins rh2 is common in teleost fish, fewer cases of multiple duplication events of the red-sensitive opsin lws are known. In this study, we investigate the visual opsin gene repertoire of the anabantoid fishes, focusing on the five lws opsin genes found in the genus Betta. We determine the evolutionary history of the lws opsin gene by taking advantage of whole-genome sequences of nine anabantoid species, including the newly assembled genome of Betta imbellis. Our results show that at least two independent duplications of lws occurred in the Betta lineage. The analysis of amino acid sequences of the lws paralogs of Betta revealed high levels of diversification in four of the seven transmembrane regions of the lws protein. Amino acid substitutions at two key-tuning sites are predicted to lead to differentiation of absorption maxima (λmax) between the paralogs within Betta. Finally, eye transcriptomics of B. splendens at different developmental stages revealed expression shifts between paralogs for all cone opsin classes. The lws genes are expressed according to their relative position in the lws opsin cluster throughout ontogeny. We conclude that temporal collinearity of lws expression might have facilitated subfunctionalization of lws in Betta and teleost opsins in general.

The behaviour of animals towards their mirror image (“mirror test”) is routinely used as a proxy to measure aggression levels, especially in fish. The lack of evidence for visual self-recognition in fish supports this method. However, recent work points towards different hormonal and gene expression responses when fish are exposed either to conspecific opponents or to their mirror image, urging for validation of this widespread method. Here, we test the predictive value of mirror tests in three sympatric cichlid species from Lake Tanganyika: the cooperative breeder Neolamprologus pulcher, the polygamous shell brooder Telmatochromis vittatus and the monogamous, biparental piscivore Lepidiolamprologus elongatus. In particular, we compare differences in restrained and overt aggression levels for individuals of each species when confronted with a mirror or a live conspecific. The three species differed in response to the two contest situations. While in N. pulcher both aggressive responses were correlated between the mirror test and the live opponent fight, there was no such relationship in T. vittatus and L. elongatus. Thus, the mirror test appears to be a suitable surrogate for intraspecific aggression in N. pulcher, while aggression against a mirror image has limited predictive value for intraspecific aggression in the other two species. These results underline the importance of validating the mirror test’s predictive value in a study species before drawing conclusions from mirror tests about aggressiveness under natural, social conditions.

Before the heritable basis or fitness consequences of differences in decision-making can be explored, consistent variation in decision-making among individuals must be established. The presentation of conflicting stimuli in the form of a female and a male provides a unique situation that has not been widely explored in terms of consistent individual variation. To assess consistency and plasticity in decision-making, male Siamese fighting fish were presented with a single female, single male and paired male and female presented simultaneously both when they did and did not have nests. Having a nest had a marked effect on male response, but only towards the female. Males differed from one another in the strategies they used when faced with conflicting stimuli; some males focused on the male, others on the female, and still others divided their response between the two. Most importantly, males behaved consistently yet differed from one another when context and reproductive state were constant. Finally, some males exhibited consistent plasticity as a result of social environment and/or reproductive state, the first finding of this kind. This study demonstrates the presence of consistent individual variation in decision-making, providing the groundwork for further studies into the evolution of this trait.

The Betta broodfish supply in Indonesia majorly comes from wild populations and fetches a higher price compared to hatchery cross-breeds, especially Betta rubra. This condition exerts pressure on the wild populations. Therefore, this study aims to examine the diversity, distribution, and conservation status of Betta fish species list in the study area. A total of 422 Betta fishes were collected from January 2019 to January 2021 from 19 of 59 sampling locations surveyed, consisting of four species, namely, Betta imbellis (TL: 14.09-31.98 mm; BW: 0.03-0.39 g), B. splendens (TL: 21.09-40.14 mm; BW: 0.05-0.70 g), B. rubra (TL: 11.35 to 47.80 mm; BW: 0.02-0.90 g), and B. dennisyongi (TL: 20.09-47.81 mm; BW: 0.07-0.91 g). There are four Betta species occurred in Aceh waters, namely B. dennisyongi, B. rubra, B. splendens, and B. imbellis. The results showed that B. dennisyongi was found at 8 sampling locations and had higher local distribution (13.56% LD) followed by B. rubra (8.47% LD), B. splendens (6.78% LD) and B. imbellis which had the lowest distribution and was only found in two locations (3.39% LD). Furthermore, field measurements (in situ) of water quality in the wild habitat showed temperature ranging from 23.7 to 31.9°C while pH ranged from 5.24 to 8.51. Based on the IUCN Redlist data, B. rubra is categorized as being critically endangered.