Mysterious Evolution of Shark Penis: Blame Sonic the Hedgehog?!

Here's something you probably didn't know. Sharks, skates, and rays - everyday Chondrichthyes - boast some very strange sexual organs. Like a pair of clasper-like penises, the male sex organs of a shark literally hook inside a female shark's cloaca in a unique reproductive strategy. Now, researchers think they have finally determined how these animals developed such strange members.

That's at least according to a study recently published in the journal Nature Communications, which details how unexpected regulation of the genetic circuit known as sonic hedgehog (SHH) by sex hormones has a strange impact on how limbs and the penis alike are formed.

As if talking about hook penises wasn't strange enough... yes, I did just mention everyone's favorite blue hedgehog with an affinity for "going fast" and collecting gold coins. While it's not exactly clear why this genetic circuit is named after Sega's iconic videogame character, what is clear is that Sonic isn't even alone. SHH is actually one of three major signaling pathways among most vertebrates - the others being desert hedgehog (DHH) and Indian hedgehog (IHH). But while those other two pathways seem most important among mammals, researchers are finding more evidence everyday that SHH is hugely important to a great many creatures, as it holds a key role in regulating the growth and formation of digits, limbs, and even helps organize the brain. Even after an animal reaches adulthood, Sonic is still playing his part, influencing the cell division of adult stem cells.

That latter duty has even been recently implicated in the development of one of the most difficult-to-treat kinds of bladder cancer, which you can read about here.

However, in the case of sharks and skates, it appears nearly solely responsible for male "clasper" development. (Scroll to read on...)

The Nature study details how, after observing little skate (Leucoraja erinacea) eggs in a controlled setting, researchers determined that claspers start to form in males during a late phase of pelvic fin development - almost as if these sex organs rise from the limbs.

That naturally turned their attention to SHH. They determined that the gene circuit and its related proteins (also called SHH) remained active for a month longer in males than females. Halting that activity early, stunted clasper growth entirely, and reactivating it in females causing them to develop male sex organs.

But what was naturally keeping SHH going longer in the first place? The researchers learned that the male sex hormone androgen was directly responsible for how long the network stayed active - an unexpected influence.

"At some point in their evolutionary history, chondrichthyans evolved a way for sex hormones to control the genes responsible for appendage development by extending the length of time they are activated, thus spurring the development of [unique] male sex organs," Emily DeMarco, who was not involved in the study, recently wrote for Science.

The researchers even go as far as to propose that this unexpected SHH influence could even explain how some of the earliest vertebrates known to man first developed their penises.

It may be a stretch, but all the same, I certainly won't ever look at Sonic the Hedgehog the same way again. I (or at least a lot of male fish) could owe big time.

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