Re: Does slug reproduction depend on photoperiod?

Hi Marty,

Thanks for the interesting questions! There are many species of terrestrial slugs, and I'm going to answer your questions using banana slugs as an example. A researcher named Dr. Janet Leonard studies banana slugs of the genus Ariolimax at the University of California, Santa Cruz. Here's what she has to say about reproduction in banana slugs.

Effect of photoperiod: Banana slugs generally lay more eggs during the fall and winter, when days are short. However, Dr. Leonard isn't sure if this increase in egg production is due to the shortened daylength per se, or to the lower temperatures that also occur at this time of year. She did say that slugs kept in the refrigerator at 8 degrees Celsius lay more eggs than slugs kept at cool room temperatures of ~15 degrees, so maybe temperature is the controlling factor. My guess is that if you want banana slugs to mate and lay eggs, you should treat them to a combination of short days and cool temperatures.

Egg size: Dr. Leonard says that for banana slugs, which are large slugs, egg size ranges from 0.09-0.6 grams. On the small end of the range, that's about as big as the diameter of a rice grain; the largest eggs can be the size of jelly bellies (those little jelly beans that come in many colors and flavors). You should definitely be able to see them with the naked eye. The eggs that Dr. Leonard showed me yesterday were the size of the salmon eggs that are sold as bait for fishing, but were a translucent white instead of bright pink. Here's a picture of eggs that were probably lain by a slug, from a garden site in England; there isn't a scale bar to indicate the size of the eggs, but they appear to be fairly large:

Other slug species may require different conditions to trigger breeding and egg-laying. The abstract of this scientific paper states that for the slug Limax valentianus, reproduction increased with long days and cool nights. In other words, it may be long OR short days that trigger reproduction, depending on the particular species you have.

I hope this information is helpful. Good luck with your slug breeding project!

Allison J. Gong
MAD Scientist