Agam, O.; Braun, E.

The primary morphological transition in Hydra regeneration, from an initially quasi-spherical tissue fragment to an elongated body, the hallmark of a mature Hydra, is preceded by a prolonged period of modest shape changes. Here, we ask whether this early stage already contains signatures of morphogenetic organization consistent with canalization toward the main morphological transition. We analyzed shape fluctuations during this period in tissue fragments with different initial and physiological conditions. Using principal component analysis, we quantified the effective dimension of the dynamical morphological fluctuation modes. We find that this effective dimension decreases progressively during the preparatory stage, well before the onset of significant elongation, indicating a progressive restriction of the accessible fluctuation manifold. This decrease is not explained by a single global measure of shape and persists when early and late states are compared at approximately matched shapes. We further show that calcium activity is associated with both the visible morphological changes and this hidden dynamical state. Tissues retaining positional cues from the parent Hydra exhibit lower effective dimensions, whereas tissues lacking such cues or subjected to mechanochemical perturbation maintain higher effective dimensions. These results identify an early, hidden dynamical phase of canalization in Hydra regeneration.