The tattoo composition is the pinwheel/sun motif extended from Arlong’s own bandanna/shoulder tattoos (see Arlong Tattoo Sun Mark). The crew-brand-as-skin-mark canonization extends the Ch. 69 Sun Pirates emblem visual pre-load to Nami as a crew member. The village’s reaction — “That’s the mark of Arlong’s pirates!!” — is the chapter’s structural-evidence beat for the Nami Is An Officer In Arlongs Crew Ch. 69 reveal.
Forward-payoff: the arc-climax (Ch. 93-95 expected) — when the tattoo is removed/replaced — will fully discharge the seed. The post-arc Nami-with-replacement-pinwheel-tattoo (windmill, currents-symbol) is the long-term visible-on-skin payoff.
Partial payoff — Ch. 81 pages 11-12 (Nami stabs her own tattoo)