Platydoris argo is a nudibranch (phylum Mollusca, class Gastropoda) belonging to the family Discodorididae, and within this to the genus Platydoris, consisting of a total of 25 species. Specifically Platydoris argo inhabits the waters of the Mediterranean Sea, and is also present in some locations of the Atlantic Ocean such as the Canary Islands or the Cape Verde Islands.
The body of Platydoris argo can reach considerable length for a nudibranch, the maximum described length being 10 cm. With a very flat and relatively wide body, the color adopted by the specimens of this species is reddish-brown with small white spots. The color of the specimens of this species makes them go completely unnoticed on the rocky bottoms where this species lives, if we do not pay special attention to look for them. The edge of the mantle is not completely smooth, but presents small undulations and indentations. On the cephalic part of the mantle, we find two lamellated rhinophores also reddish-brown, although slightly darker than the rest of the body. These rhinophores can be completely retracted if the specimen feels threatened in any way. At the opposite end of the mantle, we find the gill structure that surrounds the anal opening, formed by 6 retractile appendages, which also has the ability to retract completely inside its body when threatened. A very striking curiosity of this species, and very rarely observable is the ventral part of the specimen. Curiously, the color is not the reddish-brown of the dorsal part, but rather the belly adopts a yellow color with some brown spots and the narrow foot of the specimen is completely orange and is relatively narrow compared to the mantle.
Like the vast majority of nudibranchs, Platydoris argo is a hermaphrodite species that requires cross reproduction to generate offspring. It is common to observe specimens of this species, during the reproductive season (late spring and early summer), lining up one behind the other to exchange sperm and mutually fertilize each other's eggs. After cross-fertilization, the individuals deposit a yellow-orange spiral-shaped gelatinous ribbon with wavy upper edges, inside which are present the thousands of eggs deposited by each individual.Photos: