Thanks to SFKelly for pointing me to nudibranches.
"Glaucilla marginata. This aeolid nudibranch, and its close relative Glaucus atlanticus, are beautifully adapted for their life of floating upside down in the sea, feeding on the 'bluebottle' or 'Portugese man o' war', Physalia. They store the most powerful of the bluebottle's stinging cells at the tips of their slender cerata (20mm)"
From the "seaslugforum: "The cerata of
Glaucus atlanticus (Fig. 5) are arranged in a single row in each arch while Glaucilla marginata (Fig. 6) they are in multiple rows." So this must be Glaucilla marginata?
From the wikipedia site: "Glaucus atlanticus is a nudibranch of the family Glaucidae, the only member of the genus Glaucus. It is 4cm long. The slug is distributed throughout the world's oceans in temperate and tropical waters. G. atlanticus preys on the Portuguese Man o' War, Physalia physalis, Velella velella, the Blue Button, Porpita porpita, and the violet snail, Janthina janthina.
With the aid of a gas-filled sac that is located in its stomach, G. atlanticus is able to stay afloat at the surface. Due to the location of the gas sac the blue sea slug actually floats upside down. The dorsal surface, actually the foot and underside, is either a blue or blue-white coloration. The true dorsal surface is completely silver-grey. This coloration is an example of counter-shading, which helps protect G. atlanticus from predators from both below and above.
G. atlanticus is able to feed on P. physalis due to its immunity to the venomous nematocysts. The blue sea slug will consume the entire organism. After consumption it will select the most venomous nematocysts and use them as a defense."
A kid was collecting plenty of those "fishes" at Honey moon bay, I shot them in his (green) bucket, they were about 10-20 mm large. They were not very reactive; folding in a ball seemed to be their most extreme reaction. Sea creatures were very blue there that day (those, blue bottles and some others).
Note: as many, I recently went to see "Varekai" by cirque du soleil...and was a bit frustrated by not being allowed to take pictures...but a guess: this (or similar) may have been a source of inspiration for the great costume designer Eiko Ishioka.
Un enfant ramassait plein de ces "poissons" a Honey moon bay, je les ai photographies dans son seau (vert), ils mesuraient environ 10-20 mm. Tres passifs, leur principale defense semblait etre de ce plier en boule. Ce jour la, les petites creatures de l'ocean etaient toutes bleues (ceux-ci, les blue bottles , et d'autres)
Another great link on the cousin Glaucus atlanticus