The Picasso triggerfish Rhinecanthus aculeatus is common in Indo-Pacific coral reefs, yet quantitative data on intraspecific morphological variation and developmental stability of its caudal peduncle spines remain limited, especially in the South China Sea, where reef degradation has been extensively documented in the literature. To address this, we examined 31 individuals from the Yongle Islands (Xisha Islands) using traditional morphometrics, truss distances, and angular cosine parameters for body shape, and developed a multi-level fluctuating asymmetry (FA) framework to dissect asymmetry in the caudal spine complex. Eye diameter showed strong negative allometry and low FA (FA5ₚct = 1. 56% ± 1. 57%), supporting its suitability as a reference trait. Angular cosine values captured body shape variation more sensitively (mean CV = 14. 78%) than traditional ratios (7. 10%) or truss ratios (6. 66%). Principal component analysis revealed continuous morphological dispersion without discrete clustering. Total spine FA5ₐbs was 2. 13 ± 1. 75, per-row difference FA5ₐbs was 3. 10 ± 3. 30, and within-row standard deviation FA5ₐbs was 0. 59 ± 0. 44. Per-row absolute differences declined numerically from anterior to posterior rows, although this trend was not statistically significant (Jonckheere–Terpstra test, p = 0. 0955). A significant negative partial correlation between eye FA and total spine FA (partial ρ = −0. 498, p = 0. 0048; FDR-adjusted p = 0. 0397) emerged after controlling for standard length, which we tentatively interpret as possible evidence of inter-organ developmental compensation. These findings document substantial developmental instability in the caudal spine complex of R. aculeatus from the Xisha Islands and illustrate the utility of multi-level FA analysis for functionally partitioned structures.
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