Fish Sex Change & Blue Planet: Nature’s Adaptation on Display

The “Blue Planet II” series brought viewers face to face with one of the more fascinating traits in the animal kingdom: sequential hermaphroditism — where fish can change sex during their lifetime. It’s a phenomenon that challenges human assumptions about gender norms in Nature.

What Happens & Why

• A prime example shown in the series is the kobudai (Asian sheepshead wrasse). A fish born and identified as female later transforms into a male, developing male behaviours, testes, and even physical changes like a bulbous forehead.

• This transformation is not rare among fish. Scientists estimate that about 2% of fish species have the ability to switch sex.

• The direction of change can be:

  • Female → Male (called protogyny), often in species where larger males dominate breeding.

  • Male → Female (protandry) in other species.

  • In some rare cases, fish can change back and forth depending on environmental or social conditions.

Adaptive Advantages

• Length of life and size matter: for some fish, being female when young and then male when older (or vice versa) increases chances of reproduction in different social scenarios.

• Social hierarchy triggers change: in many wrasse species, the absence or death of a dominant male can prompt the largest female in the group to change sex to take over mating duties.

Mechanism Behind the Transformation

• The process involves hormonal shifts — for instance, changes in levels of estrogen and androgens, influenced by enzymes such as aromatase, which helps convert some hormones in ways that prompt sex change.

• Physical changes often accompany internal ones: alterations in behaviour, colouration, sometimes even physical features like body shape or fin structure.

Environmental Impacts & Concerns

• Environmental factors such as water temperature, pollution, acidity, presence of chemicals or parasites can influence fish sex ratios and trigger intersex conditions.

• These changes have ecological implications: shifting sex ratios can affect reproduction, population stability, and ultimately ecosystem health.