Dotted Sea Slug from the Mediterranean

by B. N. Sullivan

Today we present the next example of 'Exotic Underwater Nudies' -- our series on nudibranchs (a.k.a. sea slugs) from around the world. This particular nudibranch lives primarily in the Mediterranean Sea. Its usual common name is the Dotted Sea Slug. Its scientific name is Peltodoris atromaculata -- but it used to be called Discodoris atromaculata. More on this below.

We have seen this species frequently along the coasts of both mainland Greece, in the Greek islands in the Aegean Sea, and Crete. We also have seen this species in the waters around Cyprus. It reportedly exists in many other areas of the Mediterranean region as well, including the Adriatic Sea.

Let's have a look at this nudibranch. Keep in mind that the images displayed here are macro photos. This nudibranch is actually about two inches (5 cm) long. You can click on any of the photos to enlarge them. (Note: The photos on this page all were taken near Cape Sounion, Greece).

The first two photos are of the same individual. The first shows the creature at rest. You can see how it got its common name, Dotted Sea Slug. By the way, the exact pattern and shape of the 'dots' on these nudibranchs is unique for each individual. They are similar enough to recognize that individuals belong to the same species, but they are not identical.

The second photo shows the underside of this creature. (Yes, I flipped it over on purpose so that I could photograph its underpinnings, but I then returned it to its original position before I swam away.) You can see the creature's 'foot' -- the muscular structure that it uses for locomotion -- and you can see how the mantle extends like a skirt to obscure the foot when the animal is in its normal position.

Like many nudibranchs, this species feeds on sponges. For a long time it was thought that it fed preferentially on only one sponge species, but then better and more refined methods of study revealed that it did indeed feed on at least a second sponge species, and it is possible that there may be others in its diet as well, at least occasionally.

The third photo shows the nudibranch on a sponge, Petrosia ficiformis. There is evidence that the nudie has been munching away at it for some time. These nudibranchs eat by scraping off the top layers of the sponge's tissues with their mouth parts. The scars on the sponge may eventually heal over, at least partially.

Now, about the name. Regular readers of The Right Blue know by now that we like to give the scientific names of the critters we write about or show in our photos, because common names often vary from one location to the next, and certainly from one language to the next, while the scientific name is standardized: it is the same for a given species across languages and locations. When we know and use a creature's scientific name, we can be sure that we are all talking about the same thing.

Sometimes, though, the scientific name can change, too. By this I do not mean to suggest that different scientific names are used in different locations haphazardly. Instead, creatures sometimes are re-named. That is the case with the Dotted Sea Slug.

When a new species is discovered, its anatomy is carefully examined in order to classify it. In the old days, this meant dissection in a laboratory so that its organs and other structures could be assessed for how similar or different they were to those of other, known, species. This is still done, of course, but more recently -- thanks to both the accumulation of knowledge and the development of more advanced methods and instruments -- much finer details about a creature's anatomy can be determined. In short, taxonomists (i.e., those whose job it is to categorize life forms) now can identify differences that were not visible before. One animal may 'look like' another at first glance, but when their cellular structures are examined and compared, for example, we may find significant differences that set one apart from the other in important ways.

Renaming a creature is not done on a whim. Rather, it happens when important new information about a creature becomes available some time after it was originally categorized and named. Discovery of sufficient anatomical differences may lead to re-assigning the creature to a different group or, in some cases, establishing an entirely new classification for it.

As best I can tell from what I have read, at the time this nudibranch was discovered and first described, in 1880, it was classified as a Dorid nudibranch, belonging to the genus Discodoris. It was given the scientific name Discodoris atromaculata. It kept that name for more than 100 years. Then, in the late 20th century, taxonomists using more refined methods began to notice anatomical features that seemed to distinguish this species from others in the genus Discodoris, and placed it instead in the genus Peltodoris. Slowly, the literature on this species is being amended to reflect the re-naming. Meanwhile, it is possible to search on either name and come up with information about the same critter.

Confused yet? Oh dear, I thought so. Don't worry -- there won't be a test!