Two physical features—summarized by two morphological length scales—determine cell stability: the asymmetry in shape, quantified by LH/a, and the mass distribution, quantified by LW/a, where a is the semi-major axis, LH quantifies the distance between the centre of buoyancy and the centre of hydrodynamic stress, and LW the distance between the centre of buoyancy and the centre of mass (Fig. 3). Colours denote the cell rotation rate ω following an orientational perturbation (equation (4): ω > 0 denotes negatively gravitactic cells (stable upward), ω < 0 denotes positively gravitactic cells (stable downward), and ω = 0 (white dashed line) denotes neutrally stable cells. Sample asymmetry configurations corresponding to different locations on the regime diagram are illustrated by the black-and-white schematics. Filled circles denote experimental data (see Extended Data Table 2). The morphological adaptation of HA452 cells in response to overturning causes the population stability to switch (red arrow crossing the white dashed line). The original upward swimming population splits into a subpopulation swimming downward HA452(↓) and a subpopulation swimming upward, HA452(↑).