You've no doubt heard that semen is nutritious, but it seems we're not the only species aware of nature's dirty little secret. Scientists have discovered that female southern bottletail squid readily — and consistently — consume the ejaculates and sperm of their mates.
What's more, they direct the nutrients specifically towards egg production and maintaining healthy bodies.
The females of many cephalopod species are able to store the spermatophores (protein capsules containing sperm) of their mates for later use. Female squid often stockpile spermatophores inside of a seminal receptacle near the exit of their reproductive tract, which is located inside of their mantle cavity. Some species also have an external storage facility in their buccal cavity, a fleshy pouch just below their mouth.
But the southern bottletail squid (Sepiadarium austrinum) is not like other squid.
"This species doesn't have a seminal receptacle in the mantle cavity or buccal cavity," says Ben Wegener, a biologist at Monash University in Australia. When a pair mates, the male will deposit his spermatophores into the female's buccal membrane. The spermatophores then evert to form a really tight spherical bulb called a spermatangia, which then attaches to the buccal cavity and releases sperm. The female fertilizes her eggs by passing them over the spermatangia in her buccal cavity before laying them onto a substrate.
But because these females lack seminal receptacles, they cannot store sperm released from the spermatangia. "Any sperm that's lost outside of the spermatangia is gone for good," Wegener tells io9.
Rather than using all of the sperm for fertilization, females often eat some of the spermatophores during copulation, by maneuvering their mouths in just the right way to scrape them off the buccal membrane. Wegener and his colleagues were able to recently discover this odd coital snacking because the squid is practically see-through (see video below).
This behavior led the team to wonder just how the squid were using the nutrients from the spermatophores. The best way to figure it out all, they thought, would be to track the path of radioactive sperm.
They began by placing a bunch of amphipods and Palaemonetes shrimp into a "radioactive bath of carbon 14," Wegener says. Then they mated male southern bottletail squid until they were depleted of sperm — that is, until they refused to mate with virgin females. The researchers fed the sperm-free males the radioactive crustaceans and isolated them for two weeks, allowing them to generate new, 14C-tinged spermatophores.
Next, the team mated the males with virgin females, which consumed some of the radioactive spermatophores during copulation, as expected (in fact, all of the females ate sperm). After a couple of weeks, they dissected the females and took samples of various tissues, including those from their eggs (both mature and immature), reproductive glands and mantle membranes. By measuring the levels of radioactivity in the different tissues, they discovered that the females were using the spermatophore nutrients to fuel their eggs and their bodies.
In other experiments, the team looked at how long the females could use the spermatophores in their buccal cavities — at least, those that they didn't eat — for egg fertilization. Though other cephalopod species can utilize stored spermatophores many months after copulation, S. austrinum females appear to have to use the sperm within 21 days of mating (some females continued to lay eggs after 3 weeks, but those eggs weren't fertilized).
The combination of short-term sperm storage and consistent spermatophore consumption likely has strong implications for the different sexual strategies males and females use, Wegener says. Females may actually be preferentially consuming the spermatophores of undesirable males and using the nutrients to develop healthier eggs, which are then fertilized by the sperm of more attractive males.
Males, on the other hand, are constantly trying to get around the reproductive cost of female speratophore consumption.
In a previous study, Wegener and his colleagues found that males prefer to mate with larger females. But they're not choosing larger females because they think they're reproductively better, they're choosing larger females because they know smaller females eat more spermatophores, Wegener explains. Moreover, males also appear to target females who are closest to being in an egg-laying condition. "Female spermatophore consumption has placed really unique selective pressures on the male," he says.
The scientists detailed their work in the journal Biology Letters.
Images via Julian Finn (Museum Victoria).