This Sea Slug Uses Its Penis To Scrape Out Rival Sperm

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Underwater close-up photography of Chromodoris reticulata (Getty)
Underwater close-up photography of Chromodoris reticulata (Getty Images)

This Sea Slug Uses Its Penis To Scrape Out Rival Sperm

By: Emily Willingham

* This article is a repost which originally appeared on Forbes.com.

The sea slug Chromodoris reticulata shot to glorious reproductive fame in 2013 with the news that it not only mated using a disposable penis but also had a couple of spares on hand. Now it turns out that the animal uses its thorny penis to scoop sperm out of its mating partner. And no, it’s not their own sperm they’re removing.

These sea slugs, also known as nudibranchs, are simultaneous hermaphrodites. They have both male and female sex organs and produce both types of gametes: sperm and eggs. When C. reticulatamate, each member of the pair inserts a bit of its penis into the other’s vagina. When the deed is done, each partner can autotomize–spontaneously break off–that bit of its penis, and yet live to mate another day. As in, literally within a day, because behind that bit that broke off are two more bits, waiting to enter into service.

Such features make these little white-speckled red sea creatures fascinating enough. Adding to their allure, they look like squishy underwater bulls wearing flounces. They have a pair of horn-like rhinophores at the front for scent detection and a set of fluffy secondary gills perched like a bustle at the rear.

But such graceful accouterments notwithstanding, this nudibranch gives every sign of living a life full of brutal mating challenges. The fact that it can dispense with a penis and be ready with another within 24 hours suggests that it must mate often to keep up with neighboring sea slug competition.

As with many organisms who don’t have time to waste, the penises of this species have a very specific set of features. Some snakes have cactus-like, terrifying-looking hemipenes, but the C. reticulata penis has backward-pointing spines. And that whole breaking-off-the-penis thing isn’t just shuffling off something old to be able to mate anew. Along with its own bit of penis, the sea slug is also discarding rival sperm that it scraped from its partner during copulation.

Researchers publishing in the Journal of Ethology confirmed this bit of copulatory tool use by analyzing the DNA adherent to the broken off penis after their lab sea slugs copulated. Sure enough, those backward-oriented spines are dragging out rival sperm as the sea slug ditches its own penis. This tactic circumvents storage of sperm from an earlier copulation and helps give the most recent contender an edge in the mating Olympics. It also explains why this nudibranch ditches the penis rather than trying to retract it. The study authors don’t make it sound terribly comfortable:

“Though a long and thorny penis is advantageous in scraping out allo-sperm at copulation, such a penis is difficult to pull back into the body again … and the backward-pointing spines on the penis covered with sperm … will not remove allo-sperm efficiently at the next copulation, like a  VelcroTM tape. Such morphological and functional inconveniency may have made C. reticulata develop a cheap and fragile penis and dispose of it, rather than a robust but expensive one and reuse it.”

Wouldn’t want to go to all the effort of an expensive and resuable penis when cheap replacements look more functional and offer more comfort.

All of this work to find out what a sea slug does with its spiny disposable penis has a point beyond the obvious curiosity value. It suggests intense competition and a high frequency of mating for this species. That means that simultaneous hermaphrodites may have to work as hard as non-hermaphrodite animals to mate successfully.

One narrative to explain simultaneous hermaphroditism is that it evolved against a backdrop of scanty opportunities for mating, either because not enough mates are available or because moving around and finding one isn’t easy. Clearly, this penis-disposing, sperm-scraping species that can copulate in a chain reaction of replaced penises doesn’t seem to fit that description. The authors suggest that this conceptualization should be revisited for C. reticulata.

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