Note: This program first aired on August 11, 2012.
Once upon a time there was an entertainment company, and
they set out to make a film for children that was accurate about life in the
ocean, but entertaining at the same time. They chose a clown fish for their
main character, and featured several other real fish and animal species as
supporting characters. They went scuba diving on real reefs, to see how real
fish move, in an attempt to get their animation as accurate as possible. The
film went on to be one of the top grossing kids movies of all time, and win
four Academy Awards. You can guess by now I am talking about Disney’s Finding
Nemo.
They got a lot right in finding Nemo, but there’s one thing
they got wrong, really wrong. Here’s the thing about clownfish. Most of us know
that they live in a symbiotic relationship with a host anemone, a creature that
would sting and possibly kill most reef animals. What most of us don’t know is
that clown fish live in strict social
groups, of usually 5-6 animals. At the top of the hierarchy is the breeding
female. She is the largest of the group, and calls the shots. The next in command is the breeding male.
After that, the three to four smaller, non breeding, male fish. This is how it works, the female is in
charge, but if anything happens to her, everyone below her gets a promotion and
bumps up in the hierarchy. The male becomes the female, and the biggest of the
asexual fish becomes the male. So, when Nemo’s mom got eaten by that barracuda
at the beginning of the movie, Nemo’s dad should have switched it up and become
Nemo’s new mom, because clownfish are sequential hermaphrodites.
In clownfish, it appears that stress hormones control this
process. Clownfish are born male, but switch to female when they fully mature.
In a social group the breeding female keeps all the other fish in her group
stressed by harassing them and limiting their access to recourses. This keeps
them from developing fully and with their resources going towards stress
hormones instead of growth, they remain male. If something happens to the
female, the stress pressure is temporarily released, and everyone in the group
has a bit of respite. They actually grow, and the largest male, the one that
was the breeder actually gets to mature into a female, at which point, he
starts harassing all the other fish again, and the cycle starts over. This is
the sex change suppression model, the normal course of development is impeded
by strict social control. Its important to note here that the normal course of
development is a switch from male to female, the system in question just
controls when that happens. The other main model of sequential hermaphrodism in
fishes at least, is the sex induction model. This requires an awareness on the
part of the individual fish of sex ratios or size ratios in a large group. When
the ratio becomes too skewed, a sex change is initiated to rectify the
situation. For you salt water aquarists, Sea Goldies and various species of
wrasse are subject to this kind of sex change.
Sequential hermaphrodism is common in many gastropods as
well as a few flat worms, the random crustacean and even an echinoderm. None of
these species are as cute, and screen friendly as Nemo though. Though I
understand why Disney ducked this issue when it made the film, I think the
world would be a more interesting and open minded place if parents had to explain
sequential hermaphrodism to their children, and kids grew up understanding that
when it comes to gender (and most everything else for that matter) the world
doesn’t necessarily work the same way we humans do.
References:
Interested in the challenges of animating a fish? http://news.nationalgeographic.com/news/2003/05/0530_030530_findingnemo.html
Personal communication, Dr. Ann Cleveland, Maine Maritime
Academy, Castine Maine (once again, having a clownfish researcher for a boss
has its perks!)
Interesting perspective on just how unique the clownfish
transformation is: http://rhodeslab.beckman.illinois.edu/fish/Fish%20Lab.htm
Abstract only online: Environmental Biology of Fishes
Volume 29, Number 2 (1990), 81-93, DOI: 10.1007/BF00005025
http://www.springerlink.com/content/m806xlu045751u21/
outlines the models of different mechanisms for sex change in fishes
