Helping you keep your aquarium beautiful and healthy


Fish, Sex, and Stephen Colbert

by Robert W. Bly, founder,

If you want to see some kinky sex, look no further than your fish tank!

All animals, including humans, exhibit unusual sexual behavior, and nowhere is this more prevalent than among fish.

Let's start with a popular tropical fish, the swordtail, so-named because of a sword-like fin extending from the tail fin in the males.

But here's something you might not know about your swordtails: up to 30 percent of older females grow the sword, switch their sex, and become males after they have used up their supply of eggs. The changing of a fish from female to male is called protogyny.

The reverse process, protandry, occurs when a male fish changes into a female. For instance, anemonefish live in groups, and only two of them mate: the largest male and the largest female, because she can produce the most eggs.

When she dies, the largest male changes sex, and becomes a female - also of large size to ensure maximum egg production.

The fish Thalassoma swings both ways. The females can turn into males and vice versa.

Members of the coral reef family of fish Pseudochromidae can also switch gender from male to female and female to male. They do so when they cannot find a partner of the opposite sex.

Next, let's look at a less-common aquarium fish: the four-eyed fish. It is so-named because each eye is divided into an upper and lower cornea.

The bottom half of the eye sees underwater. The top half is held above the water to watch for falling insects to eat.

These four-eyed fish have strange sex, too: the genitals point in one direction, either left or right. Right-pointing males mate more easily with left-pointing females and vice versa.

Parrot fish are the studs of the freshwater aquarium: they form harems, with one large male enjoying the exclusive company of a group of smaller females.

There are a few species of fish in which natural hermaphrodites occur. A hermaphrodite has both male and female characteristics.

Contamination of water with estrogen and estrogen-like molecules is causing hermaphrodites to occur in species, such as white perch in the Great Lakes, in which it does not normally take place. These estrogen-like compounds - called "estrogen mimics" -- are used in everything from industrial products to plastics and pesticides.

Even Stephen Colbert is talking about kinky fish sex. On a recent program, he had a segment on pollution during which his guest said that male small-mouth bass in the Potomac River have been found carrying eggs, because estrogen-like pollutants are causing them to change sex.

Squid have the oddest mating ritual: at night, they begin a nuptial dance, circling each other in a spawning area about 600 feet in diameter. When the sun rises, they begin to have sex and continue all day long - breaking only to allow the female to dive down and deposit her eggs.

Anglerfish have the most dangerous mating ritual: when the male finds a female, he bites her in the rear. The bite releases an enzyme partially digesting the skin of his mouth and her flank.

The partial digestion fuses the two fish down to the blood vessel level. The male atrophies until he is essentially a pair of gonads releasing sperm into the female and fertilizing her eggs.

Paleontologists believe that fish may have actually invented sex some 380 million years ago. Fossils discovered in Australia show that sexual intercourse was common among armored placoderms, a shark-like species now extinct.