Two fish species, two strategies – a new model to study working memory

For decades zebrafish has eclipsed all other fish species in laboratories, but lately other, non-traditional model fish species are also in the spotlight. Amongst them is a once well-studied species of behavioral biology, the paradise fish. According to recent findings, its unique working memory and consistent exploratory strategy reveal behaviors previously thought to be characteristic mainly of mammals. Results published in Communications Biology by researchers from Eötvös Loránd University (ELTE) and the HUN-REN Institute of Experimental Medicine (HUN-REN IEM) suggest that the two species, taken together, can provide complementary insights into the workings of the brain.

For over three decades, the zebrafish has revolutionized preclinical biomedical research: hundreds of individuals can be studied simultaneously, and even whole-brain activity can be recorded in living animals. Its limitation, however, lies in its social nature: as a schooling species, its behavior is always influenced by the presence—or absence—of conspecifics. This means that when studied in isolation, results may be skewed by its innate social tendencies.

To overcome this, researchers at ELTE and HUN-REN IEM turned to a more solitary, territorial species: the paradise fish. A unique set of behaviors make this species a particularly intriguing complementary model.

Comparative experiments between zebrafish and paradise fish revealed striking differences. Zebrafish benefit from the presence of their peers, exploring new environments more quickly when not alone. Paradise fish, in contrast, confidently and effectively investigate novel areas even in complete isolation—often outperforming zebrafish.

“What surprised us was

that paradise fish outperformed zebrafish not because they were faster or more curious, but because they used a special strategy.

They employed what is known as ‘alternation’: always swimming toward areas they had not yet explored, as if they remembered exactly where they had already been. This behavior engages working memory, and until now has been described mostly in mammals, not in fish,” explains Zoltán K. Varga, postdoctoral neuroscientist at HUN-REN IEM and first author of the study.

Zoltán K. Varga, along with developmental geneticist Máté Varga (ELTE Department of Genetics) and ethologist Ádám Miklósi (ELTE Department of Ethology), developed a systematic comparative framework to evaluate sociability, anxiety, and cognition in both species.

Their results show that zebrafish and paradise fish, used together, provide complementary opportunities to study brain function.

While zebrafish are ideal for examining social behavior, paradise fish may prove crucial for understanding solitary problem-solving and memory mechanisms.

Interestingly, this is not the first time paradise fish have drawn scientific attention. At ELTE’s Department of Ethology, the species was studied extensively decades ago under the leadership of Professor Vilmos Csányi. Now, modern neuroscience is rediscovering the potential hidden in this “forgotten” fish.

“We now have a better understanding of why solitary and schooling species show different skills and strategies in the same contexts, such as when exploring an unfamiliar environment. Both neuroscience and pharmacological research stand to benefit from recognizing these species-specific differences,” Zoltan K. Varga conclude.