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Tigra Scientifica: Sexy squids

The Japanese pygmy squid, Idiosepius paradoxus, lives an innocuous existence tethered to the seagrass of the Japanese South Pacific Ocean.
At about the size of a child’s thumb, this delicate squid could be viewed as the less exciting dwarf to its more colorful and well-known cousin, the common cuttlefish. However, what I. paradoxus lacks in color and size, it makes up for in behavioral novelty, engaging in a rare mating practice called cryptic female choice (CFC).
CFC is the process by which females discard their mate’s sperm after copulation, allowing females to monitor the sperms’ quality, volume and fertilization success. For example, in I. paradoxus, once the sperm are deposited into the female’s seminal receptacle, she can dispose of them in two ways — she can either blow water into her seminal receptacle to dislodge the sperm, or she can consume them.
Although CFC has been documented in multiple species, its origin and effects are still dubious. Clemson evolutionary biologist Dr. Margaret Ptacek is hopeful that these squid could offer insight into the specific mechanism of CFC.
“While difficult to measure in many animal species, the squid system seems to offer a tractable way to separate female choice from male-male sperm competition,” Ptacek said. “A better understanding of how these post-mating processes influence fertilization can help us to better understand how female choice can still be a strong force of sexual selection.”
This is where a group of biologists led by Dr. Noriyosi Sato of Nagasaki University come in. Noriyosi and his team recently published a report in Evolution that explores how the sexual peculiarities of I. paradoxus influence sperm removal, fertilization success and sexual selection.
What they observed was confounding. Although the females removed more sperm from males with shorter copulation times, these removals did not necessarily eliminate the possibility of fertilization for those males. Instead, various fathers were represented in the offspring. This behavior may seem counterintuitive: why would the female remove a majority of a male’s sperm if only to allow that male to still fertilize their eggs?
Dr. Sato and his team propose that this behavior may provide some evolutionary benefit in what is called “genetic bet-hedging,” whereby the females collect sperm of many different males in order to birth a brood that is genetically diverse. This behavior is not only novel, but serves as a reminder of the importance that genetic diversity has in the survival and evolution of organisms. Dr. Norisoyi and his colleagues have shown that watching animals have sex is not only interesting, but imperative to the advancement of sexual selection research.

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