Iridophores and sexual dimorphism in the squid Doryteuthis gahi (Loliginidae) from the southwestern Atlantic

Light reflecting structures (iridophores) were examined in live Doryteuthis gahi with mantle lengths (ML) between 50 and 420 mm, both during trawl surveys onboard ship and in shallow-water spawning grounds (10–15 m depth) by SCUBA divers around the Falkland Islands. Several types of iridophores were distinguished based on their shape, colour reflection and position on the body. Ventrally, silvery iridophores cover pigmented organs such as the eyeballs and ink sac and might be used for countershading the squid body. Pink iridophores (splotches) cover the whole mantle, head, fins and arm bases. Green-red iridophores on the dorsal side of the eyeballs, as well as pink splotches on the dorsal and lateral mantle, head and arm bases can be either uncovered or masked by overlaying chromatophores. Two specific bright green stripes of iridophores develop under the fins only in females with ML .80 mm; these may be involved with distant sex recognition, both in forming unisex schools and in ‘preliminary mating’ in the twilight feeding grounds located at the shelf edge. During solitary egg laying in shallow waters, females mask these green stripes almost completely using the overlying chromatophores.