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Glaucus marginatus
(Reinhardt & Bergh, 1860)
Glaucus marginatus
Maximum size:  about 5 mm.

Identification:  This broad, flattened aeolid is bright blue on the ventral surface (facing upward in the photo) and lighter blue on the dorsal surface. Its cerata extend laterally from three distinct pairs of lobes. In contrast to Glaucus atlanticus, the cerata are stacked in multiple, irregular rows on each lobe with the uppermost ones being shorter.

Natural history:  Glaucus marginatus is a rarely seen pelagic species. They are holoplanktonic, spending their entire lives drifting with the foot oriented toward the surface. They float partially by means of an air bubble that they have swallowed and stored in their gastric cavity (visible in above photo) and are able to move toward prey or mates by using their cerata to make slow swimming movements. They eat a variety of drifting prey including the siphonophore Physalia utriculus (Portuguese man-o-war). Like many other aeolids, they store the nematocysts from their prey in the tips of their cerata for protection against predators. Unlike in most aeolids, the sting can be felt by humans. The dark ventral coloration (which faces the sky) may help in concealing them from birds while the light dorsal coloration (facing down) may help in concealing them from fish. Like Glaucus atlanticus, it lays floating, linear egg strings.

Distribution:  Big Island, Maui and Niihau: cosmopolitan.

Taxonomic notes:  This species was described from the north Pacific (as Glaucilla marginata). (Note 1) It was first recorded from near-shore Hawaiian waters at Lehua Islet, Niihau by Cassidy Grattan on June 1, 2015.

Photo:  Cassidy Grattan: about 5 mm: Lehua Islet, Niihau; June 1, 2015.

Observations and comments:

Note 1:  Churchill, et. al., 2013 and Churchill, et. al., 2014 found Glaucus marginatus to be a cryptic species complex with three members in the north Pacific: G. marginatus, Glaucus thompsoni Churchill, Valdés & Ó Foighil, 2014 and Glaucus mcfarlanei Churchill, Valdés & Ó Foighil, 2014. Since the three species are superficially identical, it's possible that the illustrated animal could belong to one of the latter two rather than G. marginatus.
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