Trans‐Generational Morphological Trait Plasticity in Parthenogenetic Offspring of Two Brachionus dorcas Morphotypes Induced by Asplanchna Kairomones

ABSTRACT We compared trans‐generational (F0–F12) morphological trait plasticity induced by Asplanchna kairomones between two Brachionus dorcas morphotypes (long‐posterior spines, LS; short‐posterior spines, SS) along with life‐table parameters of the non‐induced morphotypes. Under control conditions, SS rotifers tended to show higher fertility and smaller body size than LS rotifers. Low kairomone concentrations (50 and 200 ind./L) tended to increase body size in SS offspring, while exposure to 50, 200, and 800 ind./L kairomones induced spine elongation in both morphotypes, with posterolateral spine (PS) length increasing with kairomone concentration. Compared to the F0 generation, offspring of both morphotypes in unexposed controls showed generational fluctuations in body size; LS offspring exhibited shortening or no change in anteromedian spine (AMS) and anterolateral spine (ALS) lengths, while SS offspring showed elongation or no change in these spine lengths and PS length. Across all kairomone treatments, significant elongation of AMS and ALS in LS offspring was typically observed only in later generations, whereas SS offspring exhibited significant elongation from F1 through F12; LS offspring showed significant PS elongation from F2 through F12, with maximum lengths in the later generations (F5–F12), while SS offspring showed significant PS elongation from F1 through F12, peaking in early generations (F2–F4). Notably, the multi‐generational mean PS length in SS offspring remained significantly shorter than that in LS offspring under each kairomone treatment. Overall, SS offspring appeared to employ a synergistic defense combining increased body size and spine elongation favoring a “rapid and moderate response,” whereas LS offspring exhibited a “slow and extreme defense” strategy. These divergent strategies may result from evolutionary trade‐offs involving resource allocation, environmental predictability, and genetic constraints.