Boy Meets Girl Revisited

Earlier in the year I wrote about how to tell the difference between male and female redtail surfperch in my post, Boy Meets Girl. Separating the sexes comes down to recognizing differences in the anal fin, color, and pattern. This works well for mature fish but what about juveniles?

Juvenile in shallow water on sand
Juvenile redtail surfperch, Amphistichus rhodoterus

I caught the small fish above is from May. We can’t tell what sex it is; it’s too immature for that. Details to notice about immature redtails are distinct bars on a silvery background, and no color or pattern between the bars. The anal fin is simple, resembling that of a mature female. Contrast this juvenile with the mature male below.

Mature redtail surfperch; note the spots of color between the main bars

The mature fish above is from January. Compared to the juvenile, notice wider bars and inter-bar spots. Adults don’t look very silvery compared to juveniles. They have less space available for the silvery background color prominent in juveniles. One more piece of the puzzle is seen in the fish below, which I observed in February.

Subadult fish on sand in shallow water
Subadult redtail surfperch

The fish that is starting to mature. It’s a subadult male. What you notice here are widening bars and a bit more inter-bar pattern than we see in the juvenile and less than we see in the adult male. One thing you can’t see in the photo is the first hint of a maturing anal fin.  The anal fin starts to mature at the same time the inter-bar pattern becomes apparent. Thus, you should be able to determine the sex of any redtail old enough to have an inter-bar pattern.


  1. Ok, so now I’m curious about juvenile –> female differences. Does the female retain juvenile characters? Is it based on a lack of anal fin changes?

    In comparing the female from Boy Meets Girl with the juveniles in this post, it seems the female coloration (shade and intensity) seems to be quite similar to the juvenile, except for the spots on the female. But this could be individual variation or lighting issues.

    Maybe it isn’t really easy to distinguish between juveniles and mature females. I assume the males can figure it out, or maybe they waste some time sexually assaulting juveniles with their fancy spotted anal fins.

    1. Hi Megmi,
      You are correct. Compared to juveniles, adult males are quite divergent in coloration, pattern and anal fin shape. Adult females are more like juveniles in coloration and anal fin shape. Adult females will have some color between the bars, so there is some pattern difference. Finally, adults of both sexes seem to have a slightly different body shape than juveniles; maybe I can explore that in some future post. Thanks for asking this great question.

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