Nov 12, 2003


Octopus can't hide his desire: Field researcher 1st to document invertebrate erection

By John Biemer, Tribune staff reporter, Published November 12, 2003

As the Field Museum's Dr. Janet Voight watched a male octopus get rebuffed in his attempt to mate with a female, she noticed something striking: One of the sea creature's eight arms was longer than usual.

This was noteworthy because the two-spot octopus uses the tip of one tentacle to pass spermatophores, or tubular packets filled with millions of sperm, to females during mating. Later study concluded that the arm was engorged because it contains erectile tissue--the first documentation, scientists say, of an invertebrate erection.

The discovery seems to demonstrate an evolutionary quirk that unites man and mollusk, said co-researcher Dr. Joseph Thompson, a biology professor at St. Joseph's University in Philadelphia.

"Here's an example of two very different organisms, mammals and octopuses, that do not share a recent common ancestry, but both have settled on a similar solution to a common problem," said Thompson, who specializes in morphology and physiology.

Though research on the subject is still in its beginning stages, understanding how that erectile tissue works on an animal so different from mammals has an outside chance of helping to reveal a potent chemical for control of blood pressure, or a new Viagra, Voight said. The study appears in a recent issue of the Journal of Zoology .

However, unlike in mammals, the two-spot's tissue is not continuous with the reproductive tract, because for octopuses it's located at the end of its modified tentacle. Still, the tissue in the ligula, the organ at the tip of the octopus' mating arm, has similarities with that on a mammalian penis.

Erectile tissue is defined by its ability to inflate. Penises and the two-spot octopus' ligula both have abundant blood vessels, large internal cavities divided by networks of collagen fibers that provide the support to constrain the distended organ so that it elongates rather than simply growing larger.

However, humans and octopuses have different means of controlling blood flow to the tissues, said Voight, an octopus specialist, and the octopuses may have a distinct blood pressure control chemical.

"Because octopuses are so very distantly related to humans and this character has arisen separately, there could be a radically different mechanism controlling it," she wrote in an e-mail Tuesday from a research vessel from the Pacific Rim, off the coast of Mexico.

"It might be a long shot," Voight concedes, but studying such mechanisms could potentially lead to a new medicine to combat erectile dysfunction in humans.

Dr. Greg Bales, a urologist at the University of Chicago Hospital, called the phenomenon "intriguing" and worthy of further exploration. "Perhaps there could be a translation into a human model," he said. "How plausible that is, I think it's difficult to say."

The erectile tissue is not common to all octopuses, Thompson said, but appears to have been developed as a survival trait among some species. The ligula is bright white and does not have the color-changing cells that allow two-spots to blend in with any background. Because the species hunts during the day, the white ligula could attract predators, so the ability to contract it may reduce the risk.

Scientists say octopus mating habits are difficult to observe, so many aspects are still clouded in mystery. In some cases, females will prey on males. In the case of the frisky male in the Texas tank, the "female was literally fighting him off and in the end, he bit and envenomed her," Voight wrote. "I won't go into the details, but I separated the two at that point."

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