As adults, Fundulus heteroclitus heteroclitus range between 12.7 and 17.8 centimeters in length, the females growing larger than the males. They have flattened heads and the mouth is turned upward, clearly an adaptation to feeding at the surface of the water. This attractive fish is dimorphic, meaning that males and females have different physical characteristics. The males are darker in color than the females and exhibit blue or orange markings during the breeding season (Save the Bay, date unknown). Males are dark olive green on the dorsal side and lighter yellow on the ventral side. They also display vertical stripes along their sides. Females are silverish yellow on the ventral side and that color gradually fades to a more distinct yellow on the dorsal side. They also lack the stripes that male Fundulus heteroclitus heteroclitus display. All Fundulus heteroclitus heteroclitus have a single soft dorsal fin and their pelvic fins are located close to the rear fin (National Oceanic and Atmospheric Administration Coastal Services Center, 2001).