Life in a bubble: the role of the labyrinth organ in determining territory, mating and aggressive behaviours in anabantoids

Terrestrial locomotion characteristics of climbing perch (Anabas testudineus)

The evolution and utilization of limbs facilitated terrestrial vertebrate movement on land, but little is known about how other lateral structures enhance terrestrial locomotion in amphibian fishes without terrestrialized limb structures. Climbing perch (Anabas testudineus) exhibit sustained terrestrial locomotion using uniaxial rotating gill covers instead of appendages. To investigate the role of such simple lateral structures in terrestrial locomotion and the motion-generating mechanism of the corresponding locomotor structure configuration (gill covers and body undulation), we measured the terrestrial kinematics of climbing perch and quantitatively analysed its motion characteristics. The digitized locomotor kinematics showed a unique body postural adjustment ability that enables the regulation of the posture of the caudal peduncle for converting lateral bending force into propulsion. An analysis of the coordination characteristics demonstrated that the motion of the gill cover is kinematically independent of axial undulation, suggesting that the gill cover functions as an anchored simple support pole while axial undulation actively mediates body posture and produces propulsive force. The two identified feature shapes explained more than 87% of the complex lateral undulation in multistage locomotion. The kinematic characteristics enhance our understanding of the underlying coordinating mechanism corresponding to locomotor configurations. Our work provides quantitative insight into the terrestrial locomotor adaptation of climbing perch and sheds light on terrestrial motion potential of locomotor configurations containing a typical aquatic body and restricted lateral structure.

The reference genome of Macropodus opercularis (the paradise fish)

Amongst fishes, zebrafish (Danio rerio) has gained popularity as a model system over most other species and while their value as a model is well documented, their usefulness is limited in certain fields of research such as behavior. By embracing other, less conventional experimental organisms, opportunities arise to gain broader insights into evolution and development, as well as studying behavioral aspects not available in current popular model systems. The anabantoid paradise fish (Macropodus opercularis), an "air-breather" species has a highly complex behavioral repertoire and has been the subject of many ethological investigations but lacks genomic resources. Here we report the reference genome assembly of M. opercularis using long-read sequences at 150-fold coverage. The final assembly consisted of 483,077,705 base pairs (~483 Mb) on 152 contigs. Within the assembled genome we identified and annotated 20,157 protein coding genes and assigned ~90% of them to orthogroups.

The paradise fish, an advanced animal model for behavioral genetics and evolutionary developmental biology

Paradise fish (Macropodus opercularis) is an air-breathing freshwater fish species with a signature labyrinth organ capable of extracting oxygen from the air that helps these fish to survive in hypoxic environments. The appearance of this evolutionary innovation in anabantoids resulted in a rewired circulatory system, but also in the emergence of species-specific behaviors, such as territorial display, courtship and parental care in the case of the paradise fish. Early zoologists were intrigued by the structure and function of the labyrinth apparatus and a series of detailed descriptive histological studies at the beginning of the 20th century revealed the ontogenesis and function of this specialized system. A few decades later, these fish became the subject of numerous ethological studies, and detailed ethograms of their behavior were constructed. These latter studies also demonstrated a strong genetic component underlying their behavior, but due to lack of adequate molecular tools, the fine genetic dissection of the behavior was not possible at the time. The technological breakthroughs that transformed developmental biology and behavioral genetics in the past decades, however, give us now a unique opportunity to revisit these old questions. Building on the classic descriptive studies, the new methodologies will allow us to follow the development of the labyrinth apparatus at a cellular resolution, reveal the genes involved in this process and also the genetic architecture behind the complex behaviors that we can observe in this species.

Coordination and persistence of aggressive visual communication in Siamese fighting fish

Animals coordinate their behavior with each other during both cooperative and agonistic social interactions. Such coordination often adopts the form of "turn taking", in which the interactive partners alternate the performance of a behavior. Apart from acoustic communication, how turn taking between animals is coordinated is not well understood. Furthermore, the neural substrates that regulate persistence in engaging in social interactions are poorly studied. Here, we use Siamese fighting fish ( Betta splendens ), to study visually-driven turn-taking aggressive behavior. Using encounters with conspecifics and with animations, we characterize the dynamic visual features of an opponent and the behavioral sequences that drive turn taking. Through a brain-wide screen of neuronal activity during coordinated and persistent aggressive behavior, followed by targeted brain lesions, we find that the caudal portion of the dorsomedial telencephalon, an amygdala-like region, promotes persistent participation in aggressive interactions, yet is not necessary for coordination. Our work highlights how dynamic visual cues shape the rhythm of social interactions at multiple timescales, and points to the pallial amygdala as a region controlling engagement in such interactions. These results suggest an evolutionarily conserved role of the vertebrate pallial amygdala in regulating the persistence of emotional states.

Degenerative and Regenerative Actin Cytoskeleton Rearrangements, Cell Death, and Paradoxical Proliferation in the Gills of Pearl Gourami (Trichogaster leerii) Exposed to Suspended Soot Microparticles

The effect is studied of water-suspended soot microparticles on the actin cytoskeleton, apoptosis, and proliferation in the gill epithelium of pearl gourami. To this end, the fish are kept in aquariums with 0.005 g/L of soot for 5 and 14 days. Laser confocal microscopy is used to find that at the analyzed times of exposure to the pollutant zones appear in the gill epithelium, where the actin framework of adhesion belts dissociates and F-actin either forms clumps or concentrates perinuclearly. It is shown that the exposure to soot microparticles enhances apoptosis. On day 5, suppression of the proliferation of cells occurs, but the proliferation increases to the control values on day 14. Such a paradoxical increase in proliferation may be a compensatory process, maintaining the necessary level of gill function under the exposure to toxic soot. This process may occur until the gills' recovery reserve is exhausted. In general, soot microparticles cause profound changes in the actin cytoskeleton in gill cells, greatly enhance cell death, and influence cell proliferation as described. Together, these processes may cause gill dysfunction and affect the viability of fish.

The reference genome of the paradise fish (Macropodus opercularis)

Over the decades, a small number of model species, each representative of a larger taxa, have dominated the field of biological research. Amongst fishes, zebrafish (Danio rerio) has gained popularity over most other species and while their value as a model is well documented, their usefulness is limited in certain fields of research such as behavior. By embracing other, less conventional experimental organisms, opportunities arise to gain broader insights into evolution and development, as well as studying behavioral aspects not available in current popular model systems. The anabantoid paradise fish (Macropodus opercularis), an "air-breather" species from Southeast Asia, has a highly complex behavioral repertoire and has been the subject of many ethological investigations, but lacks genomic resources. Here we report the reference genome assembly of Macropodus opercularis using long-read sequences at 150-fold coverage. The final assembly consisted of ≈483 Mb on 152 contigs. Within the assembled genome we identified and annotated 20,157 protein coding genes and assigned ≈90% of them to orthogroups. Completeness analysis showed that 98.5% of the Actinopterygii core gene set (ODB10) was present as a complete ortholog in our reference genome with a further 1.2 % being present in a fragmented form. Additionally, we cloned multiple genes important during early development and using newly developed in situ hybridization protocols, we showed that they have conserved expression patterns.

Siamese fighting fish

Steven Portugal introduces the Siamese fighting fish (Betta splendens).

Short-range hunters: exploring the function and constraints of water shooting in dwarf gouramis

Ballistic predation is a rare foraging adaptation: in fishes, most attention has focused on a single genus, the archerfish, known to manipulate water to shoot down prey above the water surface. However, several gourami species also exhibit apparently similar 'shooting' behaviour, spitting water up to 5 cm above the surface. In a series of experiments, we explored the shooting behaviour and aspects of its significance as a foraging ability in the dwarf gourami (Trichogaster lalius). We investigated sex differences in shooting abilities to determine whether gourami shooting is related to the sex-specific bubble nest manufacture where males mix air and water at the surface to form bubbles. We found that, actually, both sexes were equally able to shoot and could learn to shoot a novel target. In a second experiment, we presented untrained gouramis with opportunities to shoot at live prey and found they successfully shot down both fruit flies and crickets. Finally, we explored the effect of target height on shooting performance to establish potential constraints of shooting as a foraging ability. The frequency of attempted shots and success of hitting targets decreased with height, whereas latency to shoot increased. We also observed that repeatable individual differences account for variation in these measures of shooting performance. Together, our results provide evidence that gourami shooting has a foraging function analogous to that of archerfish. Gourami shooting may serve as an example of convergent evolution and provide opportunities for comparative studies into the, as yet unexplored, ecology and evolution of shooting in fishes.

Cooling of Siamese fighting fish Betta splendens (Teleostei, Osphronemidae) embryos at low temperatures

The Siamese fighting fish (Betta splendens) has great importance as an ornamental aquarium fish as well as laboratory model species. Due to its rapid development, a cooling-embryo protocol could provide some advantages in their transportation, embryonic synchronization, and optimization of hatcheries. In this context, this work aimed to develop a protocol to storage B. splendens embryos at two temperatures (5 and 14 °C), testing three cryoprotective solutions (S1: 0.5 M sucrose, 1.5 M methanol; S2: 0.25 M sucrose, 0.75 M methanol; and S3: 0.125 M sucrose, 0.375 M methanol) and evaluating the quality of the larvae obtained. Moreover, a method to isolate the embryos from the bubble nest constructed by the male and to incubate them without parental care was applied in this study. The cooling assays were done using embryos of 24-h-post-fertilization at 26 °C and the results demonstrated that it is possible to store these embryos deprived of cryoprotectants at 5 °C for at least 6-h without negative effects. Meanwhile, S2 and S3 were the most suitable solutions for its storage for 9-h at 5 °C or 24-h at 14 °C, obtaining 77% hatching and 52% normal larvae in the first case or 88% hatching and 81% larvae with mild abnormalities in the second one. Indeed, type and frequency of larval abnormalities were evaluated and, remarkably, a partial recovery was described on malformed larvae from embryo cooled at 14 °C. Finally, this work is the first report about the cooling of B. splendens embryos and establishes the conditions for further studies on this field with this species.

Extrinsic stressors modulate resource evaluations: insights from territoriality under artificial noise

Background

Competition is considered to rely on the value attributed to resources by animals, but the influence of extrinsic stressors on this value remains unexplored. Although natural or anthropogenic environmental stress often drives decreased competition, assumptions that this relies on resource devaluation are without formal evidence. According to theory, physiological or perceptual effects may influence contest behaviour directly, but motivational changes due to resource value are expected to manifest as behavioural adjustments only in interaction with attainment costs and resource benefits. Thus, we hypothesise that stressor-induced resource devaluations will impose greater effects when attainment costs are high, but not when resource benefits are higher. Noise may elicit such effects because it impacts the acoustic environment and imposes physiological and behavioural costs to animals. Therefore, we manipulated the acoustic environment using playbacks of artificial noise to test our hypotheses in the territorial male Siamese fighting fish, Betta splendens.

Results

Compared to a no-playback control, noise reduced defense motivation only when territory owners faced comparatively bigger opponents that impose greater injury costs, but not when territories also contained bubble nests that offer reproductive benefits. In turn, nest-size decreases were noted only after contests under noise treatment, but temporal nest-size changes relied on cross-contest variation in noise and comparative opponent size. Thus, the combined effects of noise are conditional on added attainment costs and offset by exceeding resource benefits.

Conclusion

Our findings provide support for the hypothesised modulation of resource value under extrinsic stress and suggest implications for competition under increasing anthropogenic activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00397-x.

Housing, Husbandry and Welfare of a "Classic" Fish Model, the Paradise Fish (Macropodus opercularis)

Simple Summary

Paradise fish (Macropodus opercularis) has been a favored subject of behavioral research during the last decades of the 20th century. Lately, however, with a massively expanding genetic toolkit and a well annotated, fully sequenced genome, zebrafish (Danio rerio) became a central model of recent behavioral research. But, as the zebrafish behavioral repertoire is less complex than that of the paradise fish, the focus on zebrafish is a compromise. With the advent of novel methodologies, we think it is time to bring back paradise fish and develop it into a modern model of behavioral and evolutionary developmental biology (evo-devo) studies. The first step is to define the housing and husbandry conditions that can make a paradise fish a relevant and trustworthy model. Here, we define the relevant welfare parameters for keeping a healthy population of paradise fish and provide a detailed description of our recent experience in raising and successfully breeding this species under laboratory conditions.

Abstract

Thanks to its small size, external fertilization and fecundity, over the past four decades, zebrafish (Danio rerio) has become the dominant fish model species in biological and biomedical research. Multiple lines of evidence, however, suggest that the reliance on only a handful of genetic model organisms is problematic, as their unique evolutionary histories makes them less than ideal to study biological questions unrelated to their historically contingent adaptations. Therefore, a need has emerged to develop novel model species, better suited for studying particular problems. The paradise fish (Macropodus opercularis) has a much more complex behavioral repertoire than zebrafish and has been a favored model animal in ethological research during the last decades of the previous century. We believe that with currently available, easily adaptable genetic toolkits, this species could be easily developed into a popular model of behavioral genetics. Despite its earlier popularity, however, the description of a detailed housing and husbandry protocol for this species is still missing from scientific literature. We present here a detailed description of how to raise and breed paradise fish successfully under laboratory conditions, and also discuss some of the challenges we faced while creating a stable breeding population for this species in our facility.

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.

Agonistic encounters induce changes in the brain and behavior, but their underlying molecular mechanisms remain poorly understood. The fighting fish Betta splendens are small freshwater fish that are well known for their aggressiveness and are widely used to study aggression. Here, by measuring aggressive behavior displays (bite/strike/surface-breathing) between two opponents during fighting, we demonstrate that the two opponents in each fighting pair showed similar fighting configurations by influencing each other. In addition, we compared brain gene expression between opponents and showed synchronization of gene expression within a fighting pair, leading to pair-specific synchronization in genes associated with ion transport, synapse function, and learning and memory. This study presents the possibility that similar behaviors in pairs of animals under similar conditions may trigger synchronizing waves of transcription between the individuals, providing a hint to support the idea that fighting behaviors contain cooperative aspects at the molecular level.

Multiplicity and Polymorphism of Fish Hemoglobins

The diversity of fish hemoglobins and the association with oxygen availability and physiological requirements during the life cycle has attracted scientists since the first report on multiple hemoglobin in fishes (Buhler and Shanks 1959). The functional heterogeneity of the fish hemoglobins enables many species to tolerate hypoxic conditions and exhausting swimming, but also to maintain the gas pressure in the swim bladder at large depths. The hemoglobin repertoire has further increased in various species displaying polymorphic hemoglobin variants differing in oxygen binding properties. The multiplicity of fish hemoglobins as particularly found in the tetraploid salmonids strongly contrasts with the complete loss of hemoglobins in Antarctic icefishes and illustrates the adaptive radiation in the oxygen transport of this successful vertebrate group.

Patterns of Fish Reproduction at the Interface between Air and Water

Although fishes by nature are aquatic, many species reproduce in such a way that their embryos are exposed to air either occasionally or constantly during incubation. We examine the ecological context and review specific examples of reproduction by fishes at the air-water interface, including fishes that do and do not breathe air. Four modes of reproduction at the air-water interface are described across 18 teleost orders, from fresh water, estuaries, and sea water. Mode 1, the most common type of reproduction by fishes at the air-water interface, includes 21 families of mostly marine teleosts that spawn in water onto a substrate surface, on vegetation, or into hollow objects such as shells that will later be continuously or occasionally exposed to air. Although the eggs are emerged into air, many of these species do not emerge into air as adults, and only about half of them breathe air. Mode 2 involves six families of freshwater fishes setting up and guarding a nest and guarding on the water surface, either with bubbles or in vegetation. Most of these species breathe air. In Mode 3, annual killifishes in at least two families in seasonally dry habitats bury eggs in mud in temporary pools, then die before the next generation emerges. These species neither guard nests nor breathe air. Mudskippers (Gobiidae) breathe air and use Mode 4, excavating burrows in a soft substrate and then storing air in a subterranean chamber. In a variation of Mode 4, eggs are placed on bubbles within a nesting burrow by swamp eels (Synbranchidae). No fishes from basal taxa are known to place their embryos where they will be exposed to air, although most of these species breathe air as adults. The widespread but still rare, diverse forms of fish reproduction at the air-water interface across a broad taxonomic spectrum suggest repeated independent evolutionary events and strong selection pressure for adult fishes to protect their embryos from hypoxic waters, aquatic predators, pathogens, and UV radiation. Air-breathing by adult fishes appears to be de-coupled from air exposure of developing embryos or aerial emersion of adults during spawning.

Behavioral consequences of dietary exposure to crude oil extracts in the Siamese fighting fish (Betta splendens)

Uptake by fishes of crude oil and its polycyclic aromatic hydrocarbons (PAHs) components occurs via gills, dietary intake, or diffusion through the skin. Dietary exposure to crude oil and its components is environmentally relevant, and induces physiological and morphological disruptions in fish. However, the impacts of crude oil on fish social and reproductive behaviors and thus the possible influences on reproductive success are poorly understood. As a part of their intraspecific interactions, male Siamese fighting fish (Betta splendens) exhibit highly stereotypic behavioral and territorial displays. This makes this species a tractable model for testing crude oil effects on behavior. After 2 weeks of acclimation at 29 °C, male adult betta fish were divided into three groups and fed for 4 weeks with food spiked with water (control), low oil concentrations or high oil concentrations (∑Total PAH concentrations 340, 3960 or 8820 ng/g dw, respectively) to determine subsequent alterations in behavioral displays. Compared with control fish, the aggressive display of "opercular flaring" was significantly increased (P < 0.03, n = 14-16) in oil-exposed fish. Bubble nest building, as well as testis and brain mass, were significantly reduced in treated fish (P < 0.05). Hematocrit of treated groups was increased significantly (P < 0.02) from 21% in control fish to ∼27% in both oil exposure groups. Dietary exposure over a 4-week period to low, relevant levels of crude oil thus leads to an increase in aggressive behavioral displays, a decrease in reproductive activity and additional morphological changes.

Very high blood oxygen affinity and large Bohr shift differentiates the air-breathing siamese fighting fish (Betta splendens) from the closely related anabantoid the blue gourami (Trichopodus trichopterus)

The Siamese fighting fish, Betta splendens, and the blue gourami, Trichopodus trichopterus, are two closely related air-breathing anabantoid fishes. B. splendens is a sedentary facultative air breather frequenting often hypoxic waters, while T. trichopterus is a more active obligatory air-breather inhabiting better oxygenated waters. Despite their close taxonomic relationship, previous studies have shown inter-specific differences in both physiological and morphological plasticity. Consequently, we hypothesized that B. splendens would have the higher blood oxygen affinity characteristics typical of more hypoxia-tolerant fishes. Whole blood oxygen equilibrium curves were determined at 27 °C and pHs of 7.62, 7.44 and 7.25. At a pH of 7.62, the blood O₂ affinity (P₅₀) of B. splendens was just 2.9 mmHg, while that of T. trichopterus was ~5 times higher at 14.7 mmHg. There were no significant differences in P₅₀ between males and females in either species. The Bohr coefficient in B. splendens and T. trichopterus was -1.79 and - 0.83, respectively. B. splendens, unlike T. trichopterus, showed a large Root effect. Hills cooperatively coefficient, n, was ~2 in both species, indicating a significant binding cooperative between oxygen and hemoglobin. Collectively, these differences in blood O₂ transport characteristics in these two closely related species are likely correlated with the differing habitats in which they breed and inhabit as adults, as well as different activity levels. Finally, the very high blood O₂ affinity of B. splendens is not extraordinary among air-breathing fish, as revealed by a review of the literature of blood oxygen affinity in air-breathing fishes.