Depth sensing in larval zebrafish prey capture

• Applicant: Professor Aristides Arrenberg, Ph.D.
• Subject Area: Cognitive, Systems and Behavioural Neurobiology ; Sensory and Behavioural Biology ; Developmental Neurobiology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 537563569

Depth perception – the computation of distance from the 2D input of both retinae – is essential for survival in a 3D world. Much is known about such mechanisms in primate cortex. However, this ability is not restricted to primates, nor is a cerebral cortex required. For example, even invertebrates like praying mantis possess binocular stereopsis. Stereopsis is also not the only mechanism available for depth perception, e.g. mice strongly depend on monocular information like motion parallax. However, despite a plethora of behavioral studies in many species, we know surprisingly little about their neural implementations of non-cortical mechanisms underlying depth perception. Revealing such mechanisms would broaden our perspective on alternative evolutionary solutions to depth perception in animals with smaller brains lacking a cortex. Larval zebrafish hunt microorganisms starting 5 days post fertilization and rely on precise distance estimation for successful capture strikes. This stereotyped prey capture behavior is highly visual and follows a sequential order of behavioral motifs. While the field is beginning to understand monocular visuomotor transformations during hunting initiation, the circuits underlying the final capture strike decision remain unknown. This decision relies on knowledge about distance, as most capture strikes occur, when fish are about 500 µm away from their prey. Strike preparation involves eye convergence and likely relies on binocular stereopsis as behavioral observations in one-eyed larvae suggest. A very recent study demonstrated the importance of UV light for capture strikes, however, in this and other studies stimuli were presented millimeters away from the animal. These distances were outside the 500 µm range of natural behavior and precluded investigation of binocular processing. A hypothetical pathway based on intertectal, binocular neurons has recently been proposed. To conclusively determine the depth sensing mechanism for capture strikes, we will modify our visual stimulation arena to simulate prey at short distances. While tracking eyes and tail, we will record neuronal activity through two-photon calcium imaging to identify depth-encoding neurons. By performing receptive field mapping in three spatial dimensions, we will then test neuronal selectivity for particular stimulus depths, and whether these neurons are restricted to prey capture behavior or subserve a more general depth perception capacity in zebrafish. Finally, we will target the identified monocular and binocular neurons with holographic optogenetic manipulation to confirm the causal involvement of identified neuron types in triggering capture strikes. We seek not only to explain the neuronal implementation of depth perception in zebrafish prey capture, but also to shed some light on the evolution of depth perception mechanisms in other species that do not have a cortex.

• DFG Programme: Research Grants