The bizarre world of snail penises

For my final project, I was “lucky” enough to get to spend multiple hours examining periwinkle snails for signs of penises (really, it’s much harder than it sounds) so that I could determine the sex of individuals I used. In addition to developing a unique technique to elicit the snails to come out of their shells so that I could see if they had a penis behind their right tentacle, I also learned some interesting things about sexing snails. The presence of a penis is the common and generally agreed upon way to determine the sex of a snail, however, in my research for this project I found out that this is not a fool-proof method. Why? Because, just as humans exists in a variety of forms besides having a penis and not having a penis, snails too exhibit physical variation within each sex.


A periwinkle penis. Photo courtesy of Chris Harley. 

One reason penises are not always a reliable way to determine to sex of periwinkle snails is because after the mating season males tend to shed their penises (!). Amazingly, males will synchronously discard their organs, which may seem counter intuitive, but actually this may be an energy conserving strategy. Since the males can regrow this critical organ when the mating reason resumes, they do not have to waste energy keeping an organ around they aren’t using, and actually reabsorb some nutrients from these parts before the shedding occurs. For more on the evolutionary reasons behind penis-shedding, and for an interesting connection to our own anatomy check out this interesting post.

I also learned that another bizarre snail penis phenomenon occurs in response to human pollution. In Australia (and all over the world) female snails were found to grow penises on their head in the presence of tributyltin, or TBT, a chemical used to treat and preserve wood on boats. The way TBT works is that the chemical disrupts the hormone system of the snails (and oysters and clams too) causing penises to grow where they normally don’t. Luckily, since this chemical was banned in 2008, instances of female penis growth have declined.


Female snail with penis growth due to TBT (censored version). Photo from:

So, on this note I sum up my reporting from Experimental Invertebrate Biology and the final assignment of my undergraduate degree in Biology. I never expected the last thing I’d write would be about snail penises, but I’m sure it could be stranger… like this octopus that has a detachable penis.



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