They were all caused by the sun shining through the thin, upright sails of Velella velella:
It became quite fun, because you never knew what you were going to find next. The shadows were different shapes and sizes, depending on the outline of the sail and the angle of the sail relative to the sun. The experience reminded us of looking at sundials, and in this case the gnomon (the part of the sundial that casts the shadow) was Velella's sail.
Here are two of my favorite shadows of the night:
Although the sails were ~2 cm tall, many of the longest shadows were ~6 cm long.
Doesn't it make you wonder — if Velella is in the middle of the ocean, during very calm conditions, does it cast the same shadows on the surface of the sea?