The bottom photo shows clams which have become unfortunate victims of a moon snail. The tiny holes seen in each of the clam shells were drilled by the moon snail, using a special organ called a radula. Although most gastropods possess a protective outer shell, some of them do not. The sea slugs are an example of gastropods without a protective shell. Several species of sea slugs live in the IRL, but they are usually small and difficult to find, and are rarely present in high numbers.
Most species have very specialized diets - some eat only a single type of algae.