SCIENCE
Gender Benders
Text by Chetana Purushotham
Images by Umeed Mistry
It’s strange but it’s true: Underwater, gender is more often than not a shifting phenomenon. Get the low-down on the how and why sex is fluid in the sea
Sex is a strangely shifting phenomenon in the oceans, and this gender bending is more common than you might ever have realised!
IN 2003, a surprise event transformed our perception of life in the ocean.
This transformation came rather unexpectedly from a heart-warming animated children’s movie, called Finding Nemo. The story of a widowed clownfish’s search for his abducted son, Nemo, changed the general understanding of the ocean as a vast monotonous blue, to a colourful world where Nature’s imagination seems to have run wild.
The surprising level of scientific detail in the film taught a generation a lesson in marine biology and conservation, with every laugh and tear.
However, the makers of the film could only go so far in accurately depicting the life-history of its protagonists (the clownfish family) without severely convoluting the plot. Clownfish, like several other fish families, are “hermaphrodites”, which means that they can and will manipulate their sexuality at least once in their lifetime. How would the audience have responded if they knew that in reality Nemo would have been born, not as a male, but in fact, genderless? “He” would eventually have transformed into a male, once his father had turned female following Nemo’s mother’s death.
SHIFTING SEXUALITY
Can we imagine a world where our gender and sexuality is almost guaranteed to change during the course of our lives? Where sexuality is determined, less by our genes at the time of conception, and more by the number of men and women living in our peer group? To think that an individual could change from one sex to another after
attaining maturity or to take the place of a lost member of its society, seems absolutely bizarre. Yet this is the norm in the world of fish (and some other marine invertebrates). An adaptation that has evolved independently at different points in fish ancestry, sex change offers its adopters a strong reproductive advantage.
The question is, how beneficial can it actually be, to go to the trouble of switching between sexes? And how exactly does it all take place?
1 HAREMS AND HEIRARCHIES
Hermaphroditism is the ability of an individual to behave as a male or a female, either simultaneously or sequentially (at different points in time).
• Simultaneous hermaphrodites are relatively rare among fish, typically found in deeper seas, where finding mates of the opposite sex is a challenge. Growing and maintaining both male and female reproductive organs allows fish to mate successfully with practically any individual they encounter.
• Sequential hermaphrodites, on the other hand, come in various shades. They can change from male to female (protandry) or female to male (protogyny). It could happen just once or repeatedly – the variations are numerous! At least 15 to 20 fish families (clownfish, wrasses, parrotfishes, groupers, angelfishes, damselfishes, gobis, to name a few) have at least one kind of hermaphrodite in their midst. The type and extent of sex change is tied very closely with the kind of social setting the fish finds itself in.
Clownfish (Amphiprion) societies are based on strict hierarchies, where a dominant large female forms a monogamous pair with the next biggest fish living in her anemone, which is a male. All the other clownfish in the group (the Nemos) are significantly smaller in size and are, in fact, of indeterminate sex. In the event that the female dies or disappears, the lone male immediately starts to transform into the dominant female and the most competitive of the indeterminate small ones grows into a male. The hierarchy is hence restored.
Blue-streak cleaner wrasse (Labroides dimidiatus) live in male dominated harems where one or more males defend a group of females over whom they have complete mating rights. If one of the dominant males dies, one of the larger females transforms into a male, taking the spot of the deceased, and mating resumes.
In both situations, we might wonder why the widowed fish didn’t choose to wait around for another individual to come along or why it didn’t venture out on its own to look for a mate – was a sex change really necessary? By leaving the group and hunting for a new mate, fish become extremely vulnerable to predation, a cost too high to pay for sex. Instead, by changing its sex, the fish’s society remains intact, the harem is not lost and all parties are able to mate successfully.
2 FLIPPING THE SWITCH
In evolution-speak, changing sex helps a fish maximise its chances of passing on its genes in its lifetime. A fish has the option of switching its sex when the odds of finding a mate and producing offspring are higher after the switch. To help wrap our heads around this, it is important to remember that fish, like all organisms, live in a sexually competitive world; size and numbers matter. Sex changing is a clever adaptation which enables them to turn potential biological disadvantages into advantages. • Parrotfish (family Scaridae) females prefer big males. So if you were a parrotfish, you would be better off staying a female when you are small and take your pick from among the large males and successfully mate. Once you grow large enough, you can switch over to being a male and still retain your chances of ample mating success.
• Hawkfishes (Cirrhitichthys falco) take the flexible nature of sex change to a new level, switching between being male and female multiple times depending on which sex gives it a better chance of mating at a given time.
3 UNDERGOING THE TRANSFORMATION
Fish living in harems and hierarchies often recognise their biological cue to make the sex switch when common interactions with their mate, like aggressive nudges and displays, suddenly stop. This triggers hormonal changes in the fish’s body and in its sex organs accordingly. The bodily transformation can take up to two weeks, during which period the fish wastes no time assuming its new role. Clownfish males begin to put on weight, parrotfish start to change body colour, and most baffling of them all, female blue-streak cleaner wrasses begin to court other females in the harem even while still carrying eggs.
4 TRANSFORMING PERCEPTIONS
It seems quite understandable why the true nature of Nemo’s gender was not revealed in the animated movie. It would have been rather strange to see the movie end with Nemo and his father both undergoing sex changes and becoming the next mating pair of their anemone!
Female blue-streak cleaner wrasses begin to court other females while still carrying eggs
It does however bring to light the rainbow of complexities in the marine world that may be difficult for some humans to relate to, so much so that we often miss them completely.
Perhaps a few moments with those incredible clownfish on the reef would help to broaden our horizons.
For starters, a good life lesson from these clever fish – when an opportunity presents itself, seize it and let it transform you!