Conspecific alarm cues induce chemical but not morphological defenses in Pseudo‐nitzschia

This study investigates the chemical and morphological responses of two domoic acid (DA)‐producing diatoms, Pseudo‐nitzschia multiseries and P. fukuyoi , to conspecific chemical cues released from mechanically damaged cells. Cell homogenate filtrate (CHF) was added to batch cultures via single or repeated doses to simulate acute and sustained cell disruption associated with grazing. Results showed that CHF exposure did not affect growth rate, cell abundance, and morphological traits, including chain density, average chain length, and frustule silicification, in either species. In contrast, cellular DA content increased significantly in response to CHF addition, reaching up to 2.16‐fold higher in P. multiseries and 1.44‐fold in P. fukuyoi , particularly during the decline phase. The magnitude of these toxicogenic responses depended on species identity, growth stage, and cue delivery regime, with repeated or persistent cue exposure eliciting stronger responses. Algal physiological parameters and environmental conditions (pH, nutrients, bacterial abundance) remained largely unchanged, indicating that toxin induction was primarily driven by direct sensing of algal‐derived infochemicals rather than indirect environmental effects. Notably, enhanced DA production occurred without detectable reductions in growth or cell abundance. These findings demonstrate that Pseudo‐nitzschia can deploy efficient, chemically mediated defenses in response to conspecific alarm cues, with important implications for toxin regulation, grazer‐prey interactions, and the ecological dynamics of harmful algal blooms.