For decades, the zebrafish (Danio rerio) has stood as the preeminent aquatic model in laboratories worldwide, dominating research in neurobiology, genetics, and behavioral science. Its popularity stems from its small size, transparent larvae, and the ability to conduct large-scale genetic and pharmacological screens. However, while zebrafish have unveiled numerous insights into brain function and development, their natural social behaviors impose certain experimental constraints. As a schooling species, the zebrafish’s cognitive and exploratory patterns are profoundly influenced by the presence of conspecifics, which clouds the analysis of solitary cognition and memory processes. This inherent social dependency has led researchers to seek complementary model organisms that could illuminate alternative aspects of brain function.
Enter the paradise fish (Macropodus opercularis), a once extensively studied species in behavioral biology that has recently resurged in scientific interest. Unlike zebrafish, paradise fish are territorially inclined and more solitary in nature. This behavioral ecology suggests they can serve as an ideal complementary model for studying cognitive processes such as working memory and problem-solving in isolation. A pioneering collaboration between neuroscientists at Eötvös Loránd University (ELTE) and the HUN-REN Institute of Experimental Medicine (HUN-REN IEM) has meticulously compared the two fish species. Their results challenge established assumptions by revealing that paradise fish exhibit cognitive strategies previously attributed predominantly to mammals.
A central finding of this research is the demonstration of a uniquely effective exploratory strategy employed by paradise fish, known as spatial alternation. This behavioral pattern entails the fish systematically choosing to explore previously unvisited areas, indicating the operation of working memory in navigating novel environments. Until now, such strategic alternation has been primarily documented in mammalian models, particularly rodents in maze navigation tasks, leading to the prevailing view that this form of cognitive mapping is a hallmark of higher vertebrates. The discovery of spatial alternation in paradise fish calls for a reassessment of the evolutionary origins of working memory and suggests that certain cognitive faculties might be more widespread across vertebrate taxa than formerly appreciated.
The experimental design leveraged the inherent social distinctions between zebrafish and paradise fish to evaluate exploratory behaviors in both group and solitary contexts. Zebrafish showed enhanced exploration when accompanied by conspecifics, implying that their social nature accelerates novelty-seeking behavior and cognitive engagement. Conversely, paradise fish displayed remarkable aptitude for solitary exploration, investigating new environments with greater consistency and persistence, even in complete isolation. This contrast underscores the importance of species-specific behavioral ecology in shaping cognitive strategies and challenges the uniform application of zebrafish findings across different contexts.
Neuroethological investigations suggest that the cognitive differences observed between these species are underpinned by distinct neural circuitries engaged during spatial exploration. Zebrafish’s social facilitation of exploration likely involves integrative sensory processing within brain regions specialized for social cognition, such as the dorsomedial telencephalon, homologous to mammalian amygdaloid complexes. Paradise fish, on the other hand, may recruit neural substrates linked to spatial working memory and decision-making, including the homologous structures to the mammalian hippocampus and prefrontal cortex. Ongoing electrophysiological and neuroanatomical studies aim to map these divergent circuitries to better understand the neural basis of solitary versus social cognition in fish.
This comparative approach also holds promise for preclinical drug discovery and neuropharmacology. Zebrafish have been instrumental in high-throughput screening due to their prolific breeding and genetic tractability but remain limited by the confounding effects of social context. Paradise fish, representing a solitary model, could extend the translational relevance of behavioral assays by providing a robust system to test compounds affecting working memory and executive function without social modulation. This dual-model strategy could thus enrich the pharmacodynamic profiling pipeline and uncover molecules with differential efficacy in social versus solitary cognitive domains.
Interestingly, the paradise fish is not an entirely new organism in the annals of behavioral research. Historically studied at ELTE’s Department of Ethology under Professor Vilmos Csányi, this species had been somewhat neglected in recent decades amidst the surge of zebrafish-centric investigations. The current renaissance of paradise fish research illustrates the cyclical nature of scientific inquiry, where “forgotten” models can regain significance in light of novel experimental questions and technological advancements. This revival aligns with a broader trend in neuroscience advocating for diversity in model organisms to better capture the complexities of brain function and behavior.
The implications of these findings extend beyond neuroscience into evolutionary biology and cognitive psychology. They provoke intriguing questions about how ecological niches shape cognitive adaptations. The solitary, territorial lifestyle of paradise fish may have favored the evolution of more robust working memory systems to navigate spatial challenges alone, while sociality in zebrafish may prioritize rapid information flow and collective decision-making. Such distinctions offer a window into how environmental pressures sculpt neural architectures and behaviors across vertebrate evolution, bridging gaps between comparative psychology and neurobiology.
Another critical insight from this research is the methodological importance of accounting for species-specific behaviors when interpreting neurobehavioral data. Using only one model, such as zebrafish, risks overlooking vital cognitive phenomena or misattributing behaviors influenced by sociality to the species as a whole. The paradisiacal fish model offers a counterbalance, emphasizing that experimental isolation can uncover latent cognitive capacities obscured in socially dependent species. This enhances the rigor and ecological validity of behavioral assays and facilitates translational relevance to solitary human conditions, such as social anxiety or autism spectrum disorders.
The research team, led by postdoctoral neuroscientist Dr. Zoltán K. Varga in conjunction with developmental geneticist Máté Varga and ethologist Ádám Miklósi, has established a robust comparative framework. This system evaluates distinct but overlapping parameters of sociability, anxiety, and cognition, leveraging behavioral assays tailored for each species. Such integrative methodology ensures that differences observed are not artifacts of divergent sensory or motor capabilities but reflect genuine cognitive strategies. The project exemplifies interdisciplinary collaboration bridging genetics, ethology, and neuroscience, setting a precedent for future translational research paradigms.
Moreover, this work invites the scientific community to rethink the criteria used to select animal models in neurobiological research. While zebrafish remain indispensable due to their genetic and experimental advantages, alternative models like the paradise fish can provide complementary perspectives necessary for a holistic understanding of brain function. Embracing model diversity enhances the ecological and construct validity of studies, which is crucial for developing therapies targeting complex neuropsychiatric conditions marked by deficits in working memory and cognition.
In conclusion, the rediscovery and systematic evaluation of paradise fish as a complementary translational model represents a significant advance in the field of behavioral neuroscience. Its unique solitary lifestyle and corresponding cognitive repertoire contribute novel insights into working memory and problem-solving strategies within vertebrate taxa. Coupled with the established zebrafish model, paradise fish provide a powerful dyad for unraveling the multifaceted neural mechanisms underlying cognition. This research not only broadens our conceptual frameworks but also holds promise for advancing neuropharmacological interventions by better representing the diversity of brain functions across species.
Subject of Research: Comparative cognitive and behavioral neuroscience focusing on working memory and exploratory strategies in fish models.
Article Title: Paradise fish (Macropodus opercularis) as a complementary translational model for emotional and cognitive function
News Publication Date: 29-Jul-2025
Web References:
https://www.nature.com/articles/s42003-025-08556-0
http://dx.doi.org/10.1038/s42003-025-08556-0
Image Credits: Eötvös Loránd University
Keywords: Memory, Memory formation, Working memory, Evolutionary biology, Fish, Ethology
