Gonadal tissue color is not a reliable indicator of sex in rocky intertidal mussels

Gonadal tissue coloration was previously thought to be a reliable indicator of sex (male vs. female) in intertidal mussels. However, no investigations have been performed to determine whether color is an accurate representation of sex and to evaluate how this relationship varies throughout the environment. Patterns of gonadal tissue coloration were examined in the mussel Mytilus californianus along 2 environmental axes during the summer of 2004: (1) a food-availability gradient across 4 sites on the central Oregon coast and (2) a vertical (tidal height) stress gradient within each of the sites. Gonadal tissue color was unrelated to food availability. Both male and female mussels at the high edge of the mussel bed had orange gonadal tissue, contrary to conventional wisdom that mussel sex can be determined visually by gonadal color (males = white, females = orange). Field classification based on tissue color is therefore an unreliable indicator of sex in mussels. Environmental stress appears to influence patterns of tissue color (carotenoid pigment concentration) in the intertidal zone.