Mating behavior and its morphological correlates in two color morphs of Girardinus metallicus (Pisces: Poeciliidae), a species previously thought not to exhibit courtship display

• We describe sexually dichromatic morphs of the Cuban fish, Girardinus metallicus.
• Black but not yellow morph males exhibit courtship display involving the gonopodium.
• Larger males with longer gonopodia courted and attempted copulations more often.
• Blacker males were more aggressive and dominant males attempted more copulations.
• Black may be a badge of status in aggressive encounters among males.

Girardinus metallicus is a Cuban poeciliid fish whose social behavior has been little studied. The only account involves a colorless morph that is sexually monochromatic and does not exhibit courtship display. We describe the behavior of two other morphs (black and yellow) that exhibit sexual dichromatism. We observed courtship displays in black but not yellow males. Contrary to the pattern in most poeciliids, black males exhibit long gonopodia and courtship; typically, longer gonopodia evolve in species without courtship, because they facilitate coercive mating but circumvent female choice. We focused on the black morph to address whether morphological traits are favored by sexual selection. Larger males with longer gonopodia courted and attempted copulations more often. Black area was not associated with intersexual interactions, but was positively associated with aggressiveness. Dominant males attempted more copulations, consistent with the idea that black coloration may be a badge of status. Black males may possess long gonopodia because the gonopodium itself is a target of female choice. However, there was no difference in gonopodium length between black and yellow males, although the latter do not court. We discuss processes that may maintain the polymorphism and prospects for future studies in this intriguing system.