Opisthobranchs are gastropod mollusks commonly known as sea slugs. They are among the most beautiful and colorful marine animals inhabiting the Pacific Coast of Costa Rica. T. E. Thompson, an outstanding English malacologist, wrote in his book Biology of the Opisthobranchs: ...."opisthobranch gastropods are to the mollusks as orchids are to the angiosperms or butterflies to the arthropods."

Hypselodoris agassizii

Opisthobranchs are a relatively small group with only about 6,000 living species in the world. But they are well known to divers and nature enthusiasts because of their remarkable color patterns and surprising and extravagant forms that go beyond human imagination.