Nudibranchs, Opisthobranchs, Sea Slugs and more confusing nomenclature
Opisthobranchs have gained a great following in the last few years. Amateur collectors have always been numerous among shell, or prosobranch, collectors because the shells are easy to come by. Anyone, without special training or equipment, can walk down the beach and admire sea shells; however, most opisthobranchs are not quite as easy to find. You often have to go diving to find them. As such, the ever increasing number of recreational divers has exposed more and more people to the strange and colorful world of opisthobranchs and has accelerated the acruement of information on these fascinating creatures.
Not All Sea Slugs Are Nudibranchs
Sea slugs are part of the phylum Mollusca. They belong to the subclass Opistobranchia. Most of these animals do not have shells, but they are evolved from animals which had shells. Experts usually group the Opistobranchia into eight orders. The largest of these orders is Nudibranchia. This order contains more species than all the other orders of Opistobranchia combined. True Nudibranchs do not have shells. Now, if you look at the photograph of this sea slug, you can see that there is a slight discoloration underneath the orange tissue of the animal's mantle. The discoloration is an internal shell. So, the photograph is that of an opisthobranch but not that of a Nudibranch.
Sex
Virtually all opisthobranchs are simaltaneous hermaphrodites. This means that they have both female and male sex organs. While this may seem like a lot of fun, most opisthobranchs cannot self fertilize. However, this simplifies finding mates, since each individual of a given species can mate with any other individual of that species. Applysia californicus are known to make long mating chains on occasion.
In contrast to opisthobranchs, most typical marine snails, or prosobranchs, have separate sexes
Eating
Most opistobranchs feed with the same silverware that all other Molluscs except bivalves use: radula. A radula is a ribon of chitin with an array chitonous teeth. These are found in the buccal region (mouth cavity) just behind the mouth.
Radula can be very helpful in species identification. A scanning electron microscope reveals a wide variation in tooth morphology between species. The individual characteristics elucidated with the microscope can function similar to a fingerprint in helping identify these creatures. Unfortunately, this is not a sure fire method due to the occasional overlap in teeth number and shape within families and genera.
Bibliography
Barnes, Robert D, 1980, Invertebrate Zoology. (book)
Behrens, David W. 1991. Pacific Coast Nudibranchs. (book)
McDonald, Gary R and James W Nybakken. 1980. Guide to the Nudibranchs of California. (book)
Wells, Fred E and Clayton W Bryce. 1993. Sea Slugs of Western Australia. (book)
