If a dominant bluehead wrasse male is removed from his coral reef home, the biggest female in his harem takes charge to ensure the group's survival.
Within minutes, she begins a stunning transformation. In a little over a week, the once-yellow female becomes a blue male.
In 2019, scientists mapped out the "genetic cascade" that sets off the transformation.
First, the genes that regulate the ovaries and estrogen production switch off, while other genes associated with androgen production ramp up activity. Levels of the neurotransmitter and hormone isotocin increase — causing the once-female fish to "act male" (i.e. become more aggressive and territorial).
As a result, the wrasse ensure their reproductive future.
Much like in the bluehead wrasse, California sheephead live in harem-style groups, with one male controlling a large group of females. When the male dies — or as can be the case, hunted by humans due to his larger size — the largest female in the group transitions from female-to-male.
In a study published this month in the journal Ecological Applications, scientists discovered that in species fished by humans, sex-switching combined with harem-style reproductive patterns may ensure the long-term reproductive survival of the group.
That matters for fish in marine protected areas, the researchers say. Fish like the California sheephead appear to recover faster than do fixed-sex fish.
Gag groupers are "protogynous hermaphrodites" — they have sex-switching built in to their biology, so unlike the California sheephead, environmental pressures aren't necessary to trigger the change.
The location and time-dependent sex switch is a clever adaptation. Male gag grouper appear to stay in the spawning grounds all year according to researchers at Florida State University. As a result, that can make them vulnerable to over-fishing. By switching sex, the older female gag grouper ensure there is a steady turnover of males for younger females to spawn with.
If you live at 8,000 feet below the surface of the ocean, you need to get creative when it comes to mating if you want your species to survive. Opportunities to mate are few and far between — but as Inverse reported in 2017, if you are a deepsea lizardfish, you can create your own opportunities.
Deepsea lizardfish are the dominant predator in their dark, deep ecosystem, according to researchers from the National Environmental Science Program in Australia. They thrive in scarcity by being able to eat almost anything that comes their way — and the same applies to sex.
The lizardfish are born with both female and male sexual organs, meaning that if another deepsea lizardfish happens along their way, the two can procreate, bringing more adorable baby lizardfish into the world.
When hamlets mate, they engage in an intriguing trade — rather than one fish performing the male role and one fish performing the female role, both fish exchange eggs, with each fertilizing the other's eggs.
To seal the deal in such equitable fashion, the fish have to be under certain conditions, a paper published this month in the journal The American Naturalist suggests.
1: Eggs have to be costly to produce.
2: It can't be too hard to find a mate.
3: It can't be too easy to find a mate either. Otherwise, the fish might cheat and pretend they are going in for a fair trade, only to play the easier male part and bounce.
4: The fish have to know whether their potential mate is going to try and cheat them.
"It only makes sense to wait for someone with whom a fair trade is possible if there's a relatively high probability that I'll encounter someone like that in the first place."
These five fish species demonstrate how sex-switching can guarantee population survival under pressure — but there is another fish worth mentioning for its unusual mating habits, too: anglerfish.
Like the deepsea lizard fish, anglerfish inhabit the deep sea, where they take romantic attachment to a new level.