When I first started examining surfperch it was difficult for me to tell males from females, but now I’ve got it down. There are differences in body and fin color, pattern, and the shape of the anal fin.
The top panels above shows a female and male redtail surfperch, Amphistichus rhodoterus. The male has a bolder, darker pattern with more contrast. The fins of males are frequently redder than the same fins on females. The two sexes seem to be abut the same size but the largest ones I have observed myself are females.
One of the most striking differences between the sexes is the anal fin. This makes a lot of sense because surf perch have internal fertilization. Males have an intromittent organ on the anal fin that gives it structure not found on the female anal fin. Above you can see the simple anal fin of the female, and the complex anal fin of the male.
Internal fertilization has major implications for the kinds of questions we ask about the natural history of surfperch. Because they have internal fertilization, I’m always thinking about their courtship, and the role of vision and color preferences, and what they eat. All of this has to do with questions about how three or more similar species sharing time and space on the outer shores keep from getting mixed up. Maybe they do get mixed up. The orange spot on the anal fin of the male red-tailed surfperch (above) is usually only seen in the silver surfperch.
Note: I updates the images and lightly edited the text on August 25, 2017. I extended this post a little in Boy Meets Girl Revisited. And, you can see more images comparing males and females in Redtail Rush.