Light sensing enhances thermotolerance and competitive fitness via serotonergic signaling in an eyeless organism.

Organisms leverage environmental cues to anticipate and adapt to changing conditions. Here, we show that Caenorhabditis elegans, despite being eyeless, utilizes photoperception as an anticipatory cue to enhance survival under thermal stress. Exposure to low-intensity light activates a heat-shock response via the photoreceptor LITE-1, triggering serotonin signaling that promotes thermotolerance through the serotonin receptor SER-5 in the intestine and muscle. Beyond acute stress protection, light perception modulates reproductive behavior by delaying egg laying under unfavorable conditions and induces intergenerational thermotolerance, enhancing progeny survival at elevated temperatures. Photoperception also improves population competitiveness, providing a competitive advantage in dynamic environments. These findings reveal a previously unrecognized role for photoperception in a non-photosynthetic animal and establish it as a key mechanism for thermal adaptation and fitness.