Approximately 2% of the world’s fish species, or about 500 species, are known to change sex at some point during their adult life.

Some species, like the black-spotted fish (as shown above), switch from female to male periodically. Others, such as clownfish, can change from male to female, while species like coral-dwelling gobies switch genders based on environmental conditions.

This phenomenon is distinct in fish because, unlike mammals and birds, many fish species do not have their sex determined by sex chromosomes.

Environmental cues trigger changes in gene activity, influencing the production of essential hormones and enzymes. A key enzyme, aromatase, plays a critical role by converting male hormones into female ones and changing gonads into ovaries.

Social dynamics can also act as environmental signals. Clark clownfish, for instance, live among sea anemones in small groups during the breeding season. If a breeding female passes away, the largest subordinate male is known to change sex and assume her role.

Changes in water quality can signal a shift in gender as well.

Research indicates that pollutants entering rivers can induce male fish to exhibit female traits, such as spawning behaviors.

Furthermore, a 2008 study found that a mere 1 to 2 degrees Celsius increase in water temperature could skew the sex ratio of certain fish towards a higher male count.

Some sex changes are advantageous; for example, clownfish evolve to switch genders as a survival strategy to enhance reproduction. However, human activities are disrupting natural sex change processes.

Polluting rivers or warming oceans presents severe risks to future aquatic species.

This article addresses the question posed by Alex Jackson via email: “How can animals switch gender?”

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