How to Tell the Sex of a Slipper Snail .... And Why You'd Want to....

Quite a few people who ask about our research are surprised that snails can be males or females. They have probably never thought about invertebrates having genders. Some snails, like garden snails, are simultaneous hermaphrodites --- both males and females at the same time.  What's even more surprising to people is that some snails change sex.  In fact quite a lot of the research in our lab is focused on understanding when and why slipper snails change sex.

To conduct experiments on sex change, we need to know how to tell the difference between males and females, and how to identify individuals that are transitioning.

So, how can you tell the difference between a male and a female?  First of all... Not this way:

Mis-information about snail sex abounds on the internet!

Male snails have a penis
Like many males across the animal kingdom, male snails have a penis.  That is, most snails where reproduction occurs via copulation (as opposed to free spawning).  The penis is on the head - located behind the right tentacle.  In small slipper snails the penis can be as long as the male's body length. Imagine having something that size projecting from your head.

Here's a sketch from a publication from the 1940s by W.R. Coe, one of the first researchers to work on sex change in slipper snails.  In this picture the shell has been removed and you are looking at the dorsal view of the snail, with the guts positioned above the foot.

The sketch is not an exaggeration.  This photo from our lab shows a small male mating with a larger female.

In live snails, if you flip them onto their backs you can easily see the penis behind the tentacle as they move their heads around.  The penis is not retractable but they often curl it back behind the head, pointed into the mantle cavity. Flipping the snails onto their backs works well to tell the sex of other marine caenogastropod snails as well.  

Here's what it looks like in Crepidula incurva.

The blue arrow points to the penis.  You can see the eye spot at the base of the tentacle and the large white blob is the foot, with the propodium curling towards the head, looking for some substrate to grab onto.  It's hard to see that the penis is distinct from the neck in this photo. It is a lot easier to see when the animal is moving around in front of you.

Female snails have...
                                    .... lots of ducts and glands.  These structures are integrated into the mantle, on the anterior right side near the head.  The capsule gland and albumin gland play an important role in the packaging of eggs into capsules.  In Crepidula species the reproductive system ends in a female genital papilla (fgp) which extends into the mantle cavity.  This is the tube through which the eggs are extruded.  In Crucibulum and Calyptraea there is not fgp and it's more difficult to tell the sex of live snails.  

Here you can see the fgp of a Crepidula incurva. It is attached to the mantle cavity and extends into the mantle cavity.  

If you take female slipper shells out of the shell you will see something like this. The red arrows point to the capsule and albumin glands. In "B." the mantle is reflected and the gill is sticking out to the right.  In this case the animal does not have an fgp but if it did, it would be projecting out near the arrow on the right.

Transitioning
In slipper snails sex change happens gradually.  The snails always start life as males and change later to become females.  During the transition the males slowly lose the penis and the female glands and ducts slowly develop.  It is possible to find sexual intermediates that have both a penis and an fgp, and to find intermediates that lack both of these structures.  

You can read about some of our experiments with sex change in these publications:

Collin, R.,  M. McLellan, K. Gruber, and C. Bailey-Jourdain.  2005.  Effects of conspecific associations on size at sex change in three species of calyptraeid gastropods.  Marine Ecology Progress Series 293:89-97 [reprint]

Mérot, C. and R. Collin.  2012.  Effects of food availability on sex change in two species of Crepidula (Gastropoda: Calyptraeidae).  Marine Ecology Progress Series.  449:   173-181.   [abstract] 

Mérot, C. and R. Collin.  2012.  Effects of stress on sex change in Crepidula cf. marginalis (Gastropoda: Calyptraeidae).  Journal of Experimental Marine Biology and Ecology.   416-417:  68-71. [abstract]