>Janthina is said to be purple because the snails feed on purplish
>by-the-wind sailors (Velella velella, harmless floating coelenterates
>related to the Portuguese man-o'-war).
Janthina also feed on Portuguese man-o'war and a round coelenerate called
Porpita. All are purple, as is the pelagic nudibranch Glaucus atlanticus,
which also eats all three of these floating jellyfish. I think the reason
for the purple color is cryptic in the open ocean. The larger of the
Janthina, Janthina janthina, is darker on the underside of the shell and
lighter on the upper. In life the animal spends its time upside down, with
the dark side up, presenting a color similar to the color of the ocean from
above (sea bird's eye view) and a color similar to the color of the sea
surface with sun shining through to a fish.
 
>What season of the year is the most likely for finding windrows of Janthina
>on the beach? My finds were both in winter.
 
Quite likely the best time for windrows of Janthina is winter, because of
winter storms.
 
By the way, Janthina also lay their eggs (which are purple) under bubble
rafts. Did we address the bubble rafts? To stay afloat, Janthina builds a
raft of bubbles by trapping air in a mucous envelope. Without the raft, the
animal would sink and die.
 
Peggy
 
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