SEA SLUGS: NOT WHAT YOU THINK!

When most of us think of slugs, we think of the ones that live in our backyard garden. They're gray, slimy and not very attractive. But in the underwater community of a coral reef, there is a dazzling rainbow of sea slugs, called nudibranchs. (In fact, there are so many of these spectacular sea beauties that we have given them their very own Cousteau Image Gallery!)

Not only are their brilliant colors and patterns different from what you expect, so are many other things about sea slugs. For example, the "horns" a nudibranch sports are actually rhinophores that it uses to taste and smell chemicals in the water. So, what else is not what it seems?

See the ruffled crown on the sea slug's back?  That's its gills, the breathing organ that most marine creatures have inside their body; the sea slug's proper name, nudibranch, means "bare gills."  Only very young nudibranchs have a shell; they cast it off as they grow older and never grow it back. In addition to gills, some sea slugs also wear parts of their digestive system on their back, in cerata that look like tentacles. The cerata are also handy for defense.  When these sea slugs munch on sea anemones or corals that have paralyzing stingers, the stingers pass harmlessly through the nudibranch's stomach and are stored in tiny sacs in the tip of the cerata.Then, if a predator gets too close, the cerata fire the stinging venom at it. If a few cerata are lost in the attack, they just grow back. The patterns and bright colors of sea slugs warn their predators of another defense.  Like skunks, sea slugs may protect themselves by secreting an acidic or strong-smelling substance that can make an enemy back off in a hurry. Nudibranchs have tiny teeth in a tiny mouth on the underside of their head. They also have specks of eyes under the skin not far from the rhinophores. The average nudibranch inches along on the sea floor on its flat "foot," searching for dinner. Some sea slugs swim by paddling with tiny hair-like cilia on their bottom side, and some can even be found hanging upside down--from the surface of the water! Sea slugs also like to climb on things, especially things to eat, like corals, hydroids and anemones.

Once or more each year, nudibranchs mate and lay millions of eggs in a flower-like swirl, attached to sand or rock, perhaps in a crevice.There the curling ribbon of eggs is rocked by currents as they develop. Egg-laying is often seasonal and sometimes depends on food supply. In a couple of weeks, nudibranch larvae emerge, born with a shell to protect them in their planktonic (drifting) stage of life. Finally, the larvae settle to the sea floor, lose their shell and become a new generation of sea slugs.