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Bizarre Facts: Sheephead Can Change Sex When Necessary.

California Sheephead fish are a funny-looking species. The males rock a white chin, large protruding canine teeth, and a very organized red and black body, while the females almost look like a large, slender goldfish. They are fascinating fish that play a vital role in maintaining the balance of kelp forests. Although male and female sheepheads look different, there is something about the fish’s sex that the two have in common— all sheephead are born female. Sheephead are sequential protogynous hermaphrodites meaning they begin life as females and will eventually become males.

Even though they are born female, a sheephead can develop into a male when the ratio of males to females becomes unbalanced in a local population. According to the Monterey Bay Aquarium, as female sheephead get larger, they can produce more and more eggs. At some point, however, they could fertilize more eggs as a male than they could make themselves as a female. This reproductive pressure is potentially how sequential hermaphroditism evolved—as a case of biological economics. This may be why the switch is delayed when resources are plentiful— females can keep getting bigger and making more eggs.

Teasing out the effects of social versus genetic control in gender change has been tricky, according to the Monterey Bay Aquarium. In a healthy and historic kelp forest, many males can be found in close proximity, with perhaps a few dominant individuals holding a territory. When resources are plentiful, females switch genders later in life, around 13 years. When resources are scarce, or male-to-female ratios are small due to overfishing, females change into males around seven to eight years.

In 1990, Robert Cowen studied sheephead at four sites where food availability varied. In the area with the most food, females changed sex at about 13 years old and lived for nearly 21 years. In the area with the least food, females changed sex at five to six years old and lived about nine years. At least in these two areas, the females changed sex about two-thirds of the way through their life spans.

Male sheephead are highly prized by sport fishermen and anglers. However, due to their hermaphroditic life history, the removal of males can drastically affect the genetics of a population and cause females to switch genders younger and smaller.

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