Procreating is serious business in the animal kingdom, and Mother Nature has gone out of her way to cleverly ensure the survival of her many, many denizens. From sex organs with multiple heads to prodigious shafts exceeding an animal's own body length, here are 12 of the weirdest (and often scariest) animal penises on the planet:

1. The sea slug's disposable, regenerating penis

The Chromodoris reticulata sea slug has the peculiar ability to discard its penis after copulation. But what makes this soft-bodied marine mollusk particularly different is its ability to grow a new penis within 24 hours. Sea slugs are "simultaneous hermaphrodites," meaning they possess both male and female genitals and use both during intercourse. Once the slugs complete copulation, they "crawl away with their elongated penises still dangling," and then the organs suddenly sever from their bodies and float away. Within 24 hours, the slugs regenerate the penises and are ready to do it all over again.

2. The fish penis with four sharp hooks

At first glance, this unremarkable freshwater-dweller from Mexico looks harmless. Yet the newly discovered llanos mosquitofish (Gambusia quadruncus) is in possession of some of the most "fearsome" genitalia on the planet, says Megan Gannon at LiveScience. North Carolina State University Researchers found a reproductive shaft equipped with four sharp hooks, which males use to grab onto resistant females during mating. It's definitely an "unpleasant looking genital barb," says Ian Chant at Geekosystem, but "when they're trying to break through the tissue that blocks their mates' genital pores, male mosquitofish are probably less than concerned with winning any beauty contests."

3. The sea turtle's massive, terrifying penis

Male sea turtles are "horrifically" well-endowed, says Darren Naish at Scientific American. Softshell varieties like the leatherback have penises that, when erect, extend to nearly half the animal's 8-foot body length. The endpoint culminates in a five-lobed head that discharges semen from four different branches (think: Ridley Scott's Aliens franchise). Scientists surmise that males evolved these "innovative penises" in order to inseminate females from long distances, namely to get past their protective shells and bulky, swatting tails.

4. The Argentine bluebird duck's long corkscrew

Most male birds don't have penises. But a few, such as the Argentine lake duck, more than make up for it, says Miss Cellania at Mental Floss. The duck's penis is shaped like a corkscrew and can extend to more than 17 inches. (The bird is only 16 inches tall.) Females are often observed trying to fly away from an eager mate, leading experts to believe that the long penis could be an evolutionary response that makes forced copulation easier. "Conversely, the long penis could be the reason the females try to escape."

5. The fish with a penis on its head

Most animals have their reproductive organs located close to their tail-ends. But that's not the case for a newly discovered species of river fish identified in Vietnam's Mekong Delta. Phallostethus culling's penis sprouts, oddly enough, out of its head. More specifically, the reproductive organ shoots out from just under the fish's throat, giving the fish the unique ability to grab onto a female with its mouth while fertilizing her eggs at the same time.

6. The warehouse pirate bug's dagger-like shaft

The warehouse pirate bug is typically used to guard grain storage warehouses, where it chows down on threatening moths and larvae. The tiny 3mm (0.11 inch) creature also practices one of the animal kingdom's most dangerous copulation methods: Its spiny penis is razor-sharp, and instead of inseminating females by traditional means, the bug uses its sex organ to violently stab through her exoskeleton in order to ejaculate. This leaves "gaping, seeping wounds" in female pirate bugs, who often die shortly after giving birth.

7. Dolphins' retractable "hand"

The good-natured swimmers have retractable penises that are kind of like multi-tools. Not only does the male dolphin's penis swivel around, but it's often also used to feel out other objects, kind of like a human hand. This gives the dolphin a "ravenous sexual appetite," says Neatorama, and could help explain why males are often seen trying to "hump inanimate objects" and "other animals like sea turtles."

8. Flatworms' fencing penises

Flatworms are hermaphrodites, meaning they have both male and female sex organs, which triggers some seriously odd reproductive behavior. When trying to impregnate another worm, for example, a flatworm tries to pierce the skin of the other using its penis. Experts term this heated competition "penis fencing": The first to successfully impregnate the other while fending off advances becomes the de facto male, who wins because he won't have to expend the energy required to carry eggs.

9. The echidna's turn-taking heads

The echidna, also known as the spiny anteater, is native to Australia and New Guinea. Like its cousin the platypus, echidna females lay eggs instead of giving birth like other mammals. Scientists, however, have long been perplexed by the male anteater's mysterious sex organ, which secretes semen from four different heads. Getting all four of these into a female is impossible when fully engorged, so the echidna only insert two at any given time. Studies suggest that going halfsies may make the individual sperm swim faster.

10. The orb spider's burdensome load

Male orb spiders detach their penis and insert them in female spiders to impregnate them, says Jennifer Welsh at LiveScience. Having penis-detached intercourse allows the male spiders to not only escape the hungry female — who successfully eats the male 75 percent of the time — but also gives them a better chance of fending off competing males. New research suggests that without the extra weight bogging them down, the new eunuchs become "superior fighters," with enhanced endurance that gives them a better shot at surviving.

11. The barnacle's accordion-like organ

Though they don't appear to do much more than stick to the underside of boats, barnacles actually possess one of the longest penises in the world — at least relative to their body size. The filament-like penis extending from its shell has an exoskeleton with "accordion-like folds" that stretch out to inseminate nearby females. Barnacles that live near shore breaks often develop thicker, heartier manparts to survive the crash of oncoming waves.

12. The argonaut octopus' detachable worm

Male argonauts are much smaller than females — only 3/4 of an inch versus a gal's 4 inches. That's why when attempting to mate, the dimunitive male argonaut octupus tries to maintain its distance, using a "special tentacle" that detaches from its body. The swimming penis squirms its way over to the female to deliver semen to her waiting eggs. The practice is so deceptive, in fact, that when Italian scientists first discovered it in the 1800s, they thought it was a new kind of parasitic worm.

This article — originally published on June 22, 2012 — was last updated on February 13, 2013.