Extreme heat events are predicted to be more frequent and intense over the next few decades. Ectotherms are particularly sensitive to such events due to their reliance on the external environment to perform their daily activities and maintain body temperature. Exposure to higher temperatures during fish development can accelerate growth rates, which directly impacts growth and mortality rates. Yet very little is known about the impact of rearing density and social cues on the behavior of developing embryos exposed to thermal stress. In this study, we measured the startle response of 1 day post fertilization zebrafish embryos at different rearing densities (10, 20, and 40 eggs), and with and without regular social and heat-induced chemical cues. Our results show that activity increases with higher embryo densities, and heat-induced social cues modulate activity both with and without application of thermal stress. Since the startle response is a proxy for how well sensory information and motor actions are coordinated, rearing density and social cues emerge as important factors for understanding the response of developing fish to environmental stressors.