(A) Experimental design. Adult male GRZ-strain turquoise killifish were sacrificed at 39, 56, 73, and 128 days post-hatching, flash-frozen and homogenized. (B–E) Alpha-diversity spectra, indicating average within-individual repertoire diversity for each age group and diversity order (*: 0.05 ≤ 0.01, **: 0.01≤ p ≤ 0.001, Kruskal-Wallis permutation test, Appendix 1—note 7). (B) Clonal alpha-diversity spectra. (C) VJ alpha-diversity spectra, all clones. (D) VJ alpha-diversity spectra, large clones (>4 unique sequences) only. (E) VJ alpha-diversity spectra, small clones (<5 unique sequences) only. (F) VJ alpha-diversity ratios for old vs. young killifish at each diversity order, for small (dashed lines) or large (solid lines) clones. Color indicates the older age group being compared to young (39 days) fish. (G) Normalized VJ beta-diversity spectra, indicating between-individual variability in repertoire composition for each age group and diversity order. (H) Distributions of pairwise repertoire dissimilarity index (RDI) distances between individuals in each age group (***: p ≤ 0.001, Mann-Whitney U tests for pairwise age differences), based on the VJ composition of each individual’s repertoire. (I) Principal coordinate analysis (PCoA) of pairwise RDI distances for each age group, visualizing the progressively greater dispersion seen at later ages. Each curve in (B–G) represents the mean across 2000 bootstrap replicates (Appendix 1—note 7); shaded regions indicate 95% confidence intervals over the same.