Photograph courtesy MBARI

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A female O. deletron squid carrying sperm packets—seen as white dots—in 2007.

Photograph courtesy MBARI

Squid Males "Bisexual"—Evolved Shot-in-the-Dark Mating Strategy

Mating with anything with eight arms pays off in dim depths, study says.

When it comes to mating, some male squid aren't very picky: They copulate just as often with other males as with females, a new study says.

That's because would-be suitors of the hand-size species Octopoteuthis deletron, which live in the murky depths of the eastern Pacific Ocean, can't easily tell the males from the females, the research shows.

"They can see each other, but they are not able very well to distinguish between the sexes at the distance at which they decide, 'I'm going to mate' or 'I'm not going to mate,'" said study leader Hendrik Hoving, of the Monterey Bay Aquarium Research Institute in California.

So "males mate with basically any member of the same species. ... They just take a chance."

It's also hard to tell he from she: A female squid's defining feature is a patch of wrinkled skin. (See squid pictures.)

The result is a strategy that the study authors call "a shot in the dark"—it's just not worth it to males to make sure their partner is the right gender.

Same-Sex Mating Rare in Nature

For the study, Hoving and colleagues recorded squid via robotic submarines in the dark, 1,300 to 2,600 feet (400 to 800 meters) underwater. The scientists observed more than a hundred male and female squid, and found that just as many male squids as female squid bore sperm packets on their bodies—showing that males slap a sperm packet on just about anything with eight arms.

When he finds a suitable partner, the male uses his large penis to transfer multiple sperm packets to the male or female. These break open into smaller sperm sacs that attach to his partner's mantle, fins, and arms.

But the "love affair" ends there: The squid, which lead a solitary existence, die shortly after mating.

Nathan Bailey, of the U.K.'s University of St. Andrews, said the study team "makes a pretty good case" for their claims about the male squid's lack of choosiness.

Very few species show such high levels of what biologists call same-sex sexual behavior, Bailey, who wasn't involved in the research, said by email. "Some primates or dolphins do, but this study puts O. deletron on the higher end of the scale."

Hoving acknowledged that his research results can become fodder for jokes.

"But I don't really care," he said. "I'm interested in deep-sea animals and how they're capable of living in that environment, and one of the challenges is finding the opposite sex."

The study appears in this week's issue of the journal Biology Letters.