(A) Schematic of hydrostatic climbing mechanics. Like a rocket, a larva generates thrust in the direction it points (top), enabling it to generate upwards trajectories by rotating upwards to adopt nose-up postures. Complementarily, it may generate lift like a helicopter (bottom), creating an attack angle between trajectory and posture. (B) Trajectory of individual swim bouts as a function of posture, for control (2912 bouts) and finless larvae (1890 bouts). The unity line corresponds to 0 attack angle. Example postures and corresponding trajectories (inset) are indicated with red circles. (C) Representative epochs of climbing by one control and one finless larva, depicting posture and trajectory at the times of sequential bouts. Relative positions are to scale, but the body schematic is smaller than actual size to better highlight the trajectory. (D) Mean attack angles for control and finless siblings from six clutches (pairwise t-test, t5=4.55, p=0.0061). (E) Cumulative fractions of postures during climbs with trajectories greater than 20°, for control and finless siblings, plotted as mean ± S.D. across clutches. (F) Absolute deviation of posture from horizontal during climbs in (D) for control and finless siblings (t5=5.02, p=0.0040).

Data from individual bouts (gray lines) and their mean (black) were aligned to peak speed at time 0 and baseline subtracted at −0.75 s. (B) Angular speed as a function of time during bouts (bottom), plotted as mean and S.D., for control and finless siblings. Durations when speed exceeded 5 mm·s⁻¹ are plotted for control and finless larvae (top) as median and 95% confidence interval.

(A) Schematic of hydrostatic forces acting on larvae absent lift during bouts (top) and between bouts (bottom). (B) Trajectory as a function of swim speed, plotted as means of equally-sized bins for four clutches (gray lines) and their mean (black). Swim bouts (light gray band, speeds faster than 5 mm·s⁻¹) tended slightly upwards, while larvae sank at slow speeds, particularly slower than 0.1 mm·s⁻¹ (dark gray band). (C) Polar probability distributions of trajectories during swim bouts (light gray, top) and at speeds slower than 0.1 mm·s⁻¹ (dark gray, bottom). (D) Vertical displacement during the interval between two bouts (when speed decreased below 5mm·s⁻¹) as a function of interval duration, for individual bouts and mean of equally-sized bins.

(A) Schematic of fin-body coordination for climbing. Positive posture changes are paired with positive attack angles and negative body rotations with no attack angle, reflecting exclusion of the fins. (B) Mean linear and angular acceleration during swim bouts at 1, 2, and 3 weeks post-fertilization (wpf), temporally aligned to peak linear speed (time 0). The window used to compute posture change is highlighted in gray. (C) Attack angle as a function of posture change for bouts at 1, 2, and 3 wpf, with cropped attack angle probability distributions (right). Data plotted as means of equally sized bins (black lines) and superimposed with best-fit sigmoids and their bootstrapped S.D. (D) Fin-body ratio, defined as the maximal slope of best-fit sigmoid to attack angle and posture change, is plotted with 95% confidence intervals as a function of age. (E,F) Mean attack angle (E) and absolute deviation from horizontal (F) for each clutch and age, evaluated over 48 hr, are plotted as functions of fin-body ratio with Pearson’s correlation coefficients (r; p=5.6E-6 for attack angle and p=6.3E-3 for deviation from horizontal). Small values convey body-dominant synergies, while large values convey fin-dominant synergies. Developmental trajectories for the four individual clutches are plotted on identical axes (right).

(A) Grayscale, dorsal-perspective photomicrographs of representative larvae at 1, 2, and 3 wpf, with rostrocaudal axis labeled (R–C). Pectoral fins are indicated with arrowhead. Gamma was adjusted and images scaled to comparable head length (scale bars 0.25 mm). (B) Rostrocaudal distance from base of the pectoral fins to the estimated center of mass (COM, see Materials and methods), in body lengths (BL), as a function of age (n = 15 larvae). (C) Pectoral fin length as a function of age. (D) Pectoral fin length normalized to body length as a function of age.

Attack angle as a function of posture change for individual clutches (columns) at each age (rows), plotted as means of equally-sized bins, superimposed with four parameter sigmoid fits. Empirical fin bias (α^), computed as an index of maximal sigmoid slope (slope/(1+slope)), is listed.

(A) Representative lateral photomicrographs, one of a larva with typical development of utricular (anterior) otoliths (top, control: wild-type or heterozygous for otogelin) and another of its sibling lacking utricular otoliths (bottom, otogelin -/-). Utricle position is encircled in red. (B) Attack angle as a function of posture change for bouts by control larvae (4767 bouts) and otogelin -/- siblings (3656 bouts). Data plotted as means of equally sized bins (gray lines) superimposed with best-fit sigmoids and their bootstrapped S.D. Marginals show cropped probability distributions, with otogelin -/- marginals superimposed on control data as dashed lines. (C) Fin-body ratio, or the maximal slope of best-fit sigmoid, plotted with 95% confidence interval. (D) Median posture change during bouts by individual clutches (gray) and their means. Pairwise t-test, t₄ = 3.13, p = 0.035. (E) Mean deviation of posture from horizontal during steep climbs (> 20°). Pairwise t-test, t₄ = 3.02, p = 0.039.

(A) Attack angle as a function of posture change for bouts by control larvae (602 bouts) and siblings with lesioned Purkinje cells (408 bouts). Data plotted as means of equally-sized bins (gray lines) superimposed with best-fit sum of two sigmoids and their bootstrapped S.D. Marginals show cropped probability distributions, with marginals from lesioned larvae superimposed on control data as dashed lines. (B) Proportion of bouts with attack angles more positive than 1 wpf baseline (−1.59°) given nose-down posture change (< −1°), with bootstrapped 95% CI. (C) Proportion of bouts with attack angles more positive than 1 wpf baseline (−1.59°) given nose-up posture change (> 1°), with bootstrapped 95% CI. (D,E) Largest magnitude slopes of the nose-down (D) and nose-up (E) best-fit sigmoids to data in (A), with bootstrapped 95% CI.