Sunday, February 24, 2013

Oh, Those Wild, Sexy Reef Fish!

WARNING: If you're easily offended by "unnatural" phenomenon like sex-changes this is NOT the post for you to read! It's all about the all-too-common occurrence on the reef of fish undergoing a sex change.

For the rest of you, read on. This is fascinating stuff!

Out in the real world, meaning the one we humans haven't artificially insulated ourselves from with technology, where the realities of life and death are governed by chemical and physical laws, animals must stick to a strict budget. It isn't a financial budget, but an energy budget that must be balanced. Every living organism, for its survival and the survival of the species, does whatever it takes to optimize energy use.

Energy InWork to Survive 


Energy In = the number of useable calories an organism takes in, and

Work to Survive = using those calories for growth, finding food, shelter, defending your shelter, finding a mate, the process of mating, and repair of tissue damage or injury incurred from any of the above activities.

If you don't take enough energy in, you can't do all of the activities needed to survive. The whole point of survival in the animal kingdom is to reproduce. The more offspring you have, the more likely your genes are to survive through subsequent generations, thereby making you somewhat immortal.

As anyone on a budget can tell you, making sure that what you take in can cover all your expenses, you have to make some trade-offs. The more you spend on having fancy things and defending them, the less you have to wine and dine potential mates. But, without those accoutrements that announce your relative fitness, even your superiority, to prospective mates you won't get that opportunity.

No surprise that the perfect balance is different for males and females. In most species, females do the majority of the work in the offspring department. Whether they carry those embryos internally, or scatter them to the currents in the ocean, they still have the greater energy investment in egg production. Their eggs provide all the initial nutrition and organelles, as well as DNA, to the newly fertilized offspring. Sperm are packs of DNA and nothing more. It doesn't take much energy to make them.

This difference in energy expenditure for reproduction is one of the driving forces behind sexual dimorphism: differences in physical appearance between the males and females of a species. In particular, this accounts for size differences: When males have to protect territory or a harem from other males, spending energy to get bigger makes sense. When you can swim in, release lots of sperm, and leave, being small and quick is beneficial. Likewise, if a female produces many eggs, a far more energy intensive undertaking than sperm production, being large is beneficial.

Since "survival of the fittest" doesn't really mean not dying, but actually reproducing, "success" in the natural world means producing healthy offspring that go on to be successful themselves, the more offspring you produce, the better. If a female with a larger body cavity can produce more eggs per reproductive episode, and the more eggs produced increases her potential reproductive success, it's to her benefit to be a big female. Conversely, if larger males have greatly increased reproductive success (i.e. gain control of a harem), it would be beneficial to be a big male.

This is known as the size advantage model, developed by Ghiselin in 1969, that states "if an individual could significantly increase its reproductive success after reaching a certain size by being a different sex, it would be to their advantage to switch to that sex." And on the coral reef, they do!

Here are some of the brilliant reef fish that can change sex--sometimes in a matter of a few hours.
The Blueheaded Wrasse
The cleaner wrasse, Labroides dimidiatus. Typically, live in harems with a dominant male around cleaning stations. When the male is removed, the largest female becomes a functional male. The Blueheaded wrasse, Thalassoma bifasciatum, do not form harems, but the dominant or terminal males take temporary control of spawning sites. Removal of terminal males results in the largest females transforming.

Moonheaded Wrasse
When the largest female turns into a male, it is called Protogyny, Almost all the sex-changing species in the Caribbean are protogynous hermaphrodites, switching from females to males after reaching a certain size, age, or when the harem's male is removed. Some other examples are the parrotfish, Spanish hogfish, Holacanthus tricolor (the rock beauty angelfish), and some damselfish.

Rock Beauty

Clownfish in their anemone
Anemone fish, like the skunk anemonefish Amphiprion akallaopisos, and clownfish, like Nemo, are protandrous. They change from male to female. Anemonefish live in monogamous pairs composed of a large female and a smaller, functional male. The pair may share their anemone-home with other, small, stunted (not sexually developed) males (juveniles; all anemonefish are born male). When the female is removed, the functional male changes sex and the largest juvenile becomes a functional male. Snook (Centropomus undecimalis) are one of the few known Caribbean protandrous fish.

Some reef fish, like gobies, can actively change sex in either direction. This lets them maximize their genetic fitness under any environmental situation.

There are many amazing things in the natural world. We'd do well to study them, be amazed by them, and learn from them. Our lesson from these fish? Embrace change, especially when it benefits us!

For More Info on Sex Changing Fish:
DeLoach, Ned and Paul Humann (1999). Reef Fish Behavior: Florida, Caribbean, Bahamas. New World Publications, Inc. Jacksonville, FL. 359 pp.
Hendrickson, Robert (1984). The Ocean Almanac. Doubleday. New York, NY. 446 pp.
The National Audobon Society (1997). Field Guide to Tropical Marine Fishes: Caribbean, Gulf of Mexico, Florida, Bahamas, Bermuda. Alfred A. Knopf, Inc. New York, NY. 720 pp.
Rice, Aaron N. (undated). Physiology of Sex-Change in Reef Fish. Available at:


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