Blue Dragon

Pteraeolidia ianthina, commonly known as the blue dragon, is an aeolid nudibranch that belongs to the Facelinidae family. Alongside Glaucus atlanticus and Glaucus marginatus, it shares the same name. This species is widely distributed throughout the Indo-Pacific region. Recent studies have indicated that Pteraeolidia ianthina is, in fact, a complex of multiple species.

Among divers in Eastern 🇦🇺 Australia, Pteraeolidia ianthina is often referred to as the "blue dragon" due to its striking resemblance to a Chinese dragon. It is one of the most prevalent aeolid nudibranchs found in this region and has the ability to inflict a painful sting on humans.

The body color of Pteraeolidia ianthina is translucent tan, but it is the cerata, ranging from dark purple to lavender to golden brown, that give the nudibranch its distinct coloration. Green specimens are also not uncommon. The slug has an elongated body, measuring approximately 7 cm (2.8 in), adorned with numerous clusters of medium-large sized cerata. The rhinophores and cephalic tentacles are noticeably thick and feature two dark purple bands. The tips of the cerata contain nematocysts.

Inside the body of Pteraeolidia ianthina, algal zooxanthellae of the Symbiodinium genus, acquired from their food, continue to carry out photosynthesis, resulting in brown and green pigmentation. It is believed that both the Symbiodinium and nematocysts are derived from the sea slug's prey, which are coelenterates. These Symbiodinium reside within vacuoles in host cells that stem from the endoderm. Young specimens of Pteraeolidia ianthina are notably shorter, possess fewer cerata, and are often mistaken for other nudibranch species.

This sea slug primarily feeds on hydroids that harbor Symbiodinium, microscopic photosynthetic dinoflagellates. The Symbiodinium are "farmed" within the slug's digestive diverticula, where they photosynthesize sugars that sustain the slug without the need for additional feeding.