What Is That Smell?

Many nudibranchs give off aromas that smell like lemons, watermelons, vanilla, or flowers to us. Chemicals made by the animals produce these odors. Scientists think that the same chemicals that make nudibranchs poisonous may also be responsible for these aromas. Perhaps these odors we think of as delicious actually have a function similar to nudibranchs' bright colors and warn other animals to stay away.

Nudibranchs have eyes, but they probably can see only shades of light and darkness. They do not have ears or noses like ours. Instead, they have two long, fingerlike attachments, called rhinophores, which stick out of their heads and contain chemical sensors. The creatures use their rhinophores to detect chemical cues that tell them about their environment, similar to the way we use our abilities to see, hear, and smell. They can sense water currents and quality, the presence of other nudibranchs, or nearby food sources by their chemical makeup. Sea slugs leave slime trails, just as slugs and snails do on land. Scientists have shown that one nudibranch may follow another nudibranch's slime trail, either to find a mate, or (if it's another species), to eat it!

However, if one nudibranch has been stressed (perhaps, by a nearby enemy), a second nudibranch of the same species dropped on the trail of the first may immediately go in the opposite direction. Where are they getting the information to make these decisions? Scientists believe that perhaps nudibranchs can "read" chemical "messages" left in the slime trail.