What do SEA SLUGS eat?

Dear Sheila,
Sea Slugs have a wide variety of diets, and most species are very specialised, eating only one or a few items of food.

Some, such as the Sea Hares are herbivores, eating sea weeds. Others such as the nudibranchs are specialised carnivores, each species feeding on a particular food. If you look around the Sea Slug Forum you will find some photos of different Sea Slugs and their food. A few examples of nudibranchs feeding are:
Glaucus atlanticus which feeds on the floating cnidarian Physalia,
Gymnodoris subflava, which feeds on other nudibranchs,
Favorinus japonica, which feeds on other Sea Slug eggs, and
Jorunna sp., which is one of many dorid nudibranchs that feed on particular sponges.

Another group of herbivores are the sacoglossans which feed by sucking the contents out of individual cells.

If you look at the yellow bar at the top of your message you will see a SEARCH button. If you type in 'food' or 'feeding', you will get a list of pages in the Forum which are about the food of different species.

Concerning your second question about the advantages of having or not having a backbone. One is not better than the other, they are different. Many invertebrates have an outer shell or exoskeleton which gives them support and protection and others like worms and slugs use their blood or other body fluids to give them a hydrostatic skeleton. I'm afraid the question is a bit too big to answer here. Just remember that invertebrates make up 99% of the animal Kingdom so it is obviously a very successful body plan.

Best wishes,
Bill Rudman.