Glaucus marginatus is a species of small, floating, blue sea slug; a pelagic (open-ocean) aeolid nudibranch; a marine opisthobranch gastropod mollusc in the family Glaucidae.Valdés A. & Campillo O.A. (2004) Systematics of pelagic aeolid nudibranchs of the family Glaucidae (Mollusca, Gastropoda). Bulletin of Marine Science 75(3): 381–389. This species is closely related to Glaucus atlanticus, and is part of a species complex (Informal clade Marginatus) along with Glaucus bennettae, Glaucus thompsoni, and Glaucus mcfarlanei.Churchill C.K.C, Valdés A. & Ó Foighil D. (2014) Molecular and morphological systematics of neustonic nudibranchs (Mollusca : Gastropoda : Glaucidae : Glaucus), with descriptions of three new cryptic species. Invertebrate Systematics 28(2): 174-195. Like Glaucus atlanticus, it is commonly known as a blue dragon.
Description This nudibranch is dark blue, and in many ways it resembles a smaller version of Glaucus atlanticus. However, in this species the cerata are inserted in multiple series presenting an irregular arrangement (compared to single series in G. atlanticus). The first two groups of cerata are pedunculate with a long stalk. The last two groups are sessile and partially fused.
While G. atlanticus is up to long, G. marginata is shorter relatively, and its tail is shorter than its cousin. The species has a dark blue foot that is broad and well developed and cerata are dark blue with small light blue spots .
Distribution This species is pelagic, and can be found in the Pacific Ocean.
While they do not usually inhabit coastal regions, hundreds of the creatures were observed washing up on one of Sydney’s Northern Beaches, near Long Reef, in February 2021.
Habitat and behaviour These small nudibranches float upside down on the surface tension in temperate and tropical seas. They eat colonial cnidarians such as the Portuguese man o' war (a.k.a. bluebottle, or Physalia utriculus), blue buttons (Porpita porpita), and the by-the-wind sailor (Velella velella).
The nudibranch has been observed to exhibit aggregation patterns. This is in response to prey such as Por. Pacifica. This makes it so that it is easier for the nudibranch to outnumber and attack its prey.
References
Sources Bergh, L.S.R. (1860). Om Forekomsten af Neldefiim hos Mollusker. Vidensk. Meddel. Naturh. Foren. Kjöbenhavn, p. 309-331, pl. 8. Burn R. (2006) A checklist and bibliography of the Opisthobranchia (Mollusca: Gastropoda) of Victoria and the Bass Strait area, south-eastern Australia. Museum Victoria Science Reports 10:1–42. Australian Museum Online sections - sea slug forums
Huang, H. D., Chiu, W. C., Chang, M., & Lee, K. S. (2017). First record of a rare aggregation of the neustonic nudibranch Glaucus marginatus (Mollusca: Gastropoda: Glaucidae) from Kenting National Park, Southern Taiwan. Collection and Research, 30, 17-21.