The Banana Slug

The Banana slug (Ariolimax spp.) is a terrestrial slug located in the temperate forests of North America from San Diego up to Alaska. They get their common name from the striking yellow colouring that resembles a banana.  As with the majority of molluscs, the banana slugs are hermaphroditic, though what has captured a reputation of bizarre sexual behaviour is what is known as apophallation (Leonard et al., 2002). This is where a slug will amputate the penis of either the partner or its self by chewing it off (Leonard et al., 2007).

The penis of the banana slug is located at its head and when erect can reach a whopping 6 inches, which is almost the same size as its body. The female reproductive tract is located at the upper side of the body. Copulation occurs by intromission of the penis into the female reproductive tract. Both slugs will be in a position that resembles a yin yang, with head swinging and a chewing motion from the mouth.  This is when intromission occurs and the entire process including courtship can last over several hours (Leonard et al., 2007). After intromission it is believed that if the penis is not the right size, in particular too large for the opening, then it can become stuck in the reproductive tract. And of course, the only alternative is to chew it off.  Generally the recipient will chew the penis off its partner but there are reports of apophallation to be reciprocal if both gain intromission. Apophallation is a slow process and erosion of the penis is visible before amputation is complete. Once the penis is severed it is usually eaten by the partner. The amputated penis cannot regrow thus the individual will remain a female for the duration of its life (Leonard et al., 2002). 

Image of Banana Slug during copulation.

References:

 Leonard, J., Pearse, J.  & Harper, A. 2002, "Comparative reproductive biology of Ariolimax californicus and A. dolichophallus (Gastropoda; Stylommiatophora)", Invertebrate Reproduction & Development, vol. 41, no. 1, pp. 83-93.

Leonard, J.L., Westfall, J.A. & Pearse, J.S. 2007, "Phally polymorphism and reproductive biology in Ariolimax (Ariolimax) buttoni (Pilsbry and Vanatta, 1896) (Stylommatophora: Arionidae)", American Malacological Bulletin, vol. 23, no. 1, pp. 121-135.