Glaucus marginatus (Reinhardt & Bergh, 1860)
|Maximum size: about 5 mm.
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.
Glaucus marginatus is a rarely seen pelagic species. They are holoplanktonic, spending
their entire lives drifting with the foot oriented toward the surface.
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
prey including the siphonophore Physalia
utriculus (Portuguese man-o-war). Like many other aeolids, they store the nematocysts
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: 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:
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.