Density-dependent emergence of collective phototaxis in a microcrustacean revealed by automated tracking

Light is a fundamental environmental cue influencing movement, habitat selection, and aggregation in aquatic organisms. While phototaxis has traditionally been studied at the level of individual responses, how social context and density modulate light-driven behavior in non-marine microcrustaceans remains poorly understood. Here, we investigated the phototactic response of the ostracod Heterocypris incongruens across three density regimes (single individuals, pairs, and groups of ≥30 individuals). We combined automated video tracking with quantitative behavioral analysis to extract spatial position, locomotor activity, and directional metrics across a light-off phase following a brief light pulse (<320 ms), a prolonged light stimulus, and a post-stimulus phase. This approach allowed us to resolve individual and emergent group-level dynamics within a unified analytical framework. Spatial attraction toward the light source and directional alignment were significantly enhanced in high-density groups, whereas isolated individuals and pairs did not exhibit comparable stimulus-dependent changes in spatial organization. Larger groups, but not isolated individuals, already oriented toward the brief onset pulse, suggesting that higher densities enhance responsiveness to transient directional cues. In contrast, locomotor variables such as velocity were largely independent of the light stimulus and reflected baseline differences between solitary and grouped conditions, indicating a decoupling between locomotor activity and directional control, with phototaxis expressed as a reorganization of movement direction rather than an increase in movement intensity. These findings suggest that the expression of light-guided movement is modulated by density-dependent effects, rather than being an individual-level trait.