Striped marlins have elongated bodies with a long, thin, pointed bill. They have dark blue or black backs, with 12‑16 blue vertical bars on the lower sides. When the fish is excited, the colour of these bars rapidly changes from blue to lavender via the contraction or expansion of chromatophores (special pigmented cells). The first dorsal fin is dark blue, with the other fins being dark brown and sometimes tinged with blue.