Ideas:

1. The idea that shark might mean shadow is read also in this:

"In here was kept an image of a whale, shadow of the god Ta'aroa, made of the... breadfruit wood... It was charred black with sacred fire... which gave the image the appearance of black stone. With it, but uncharred, was another image made of the same kind of wood, representing the handsome-blue-shark of Ta'aroa." (Henry according to Barthel)

2. Among my handwritten notes (without source noted):

"The Rule of Six: ta ta ta ta taa - long vocal sound on the 5th.

Example: Ka unga te róngo, ki 'a Hina Mángo..."

In Polynesia Hina often seems to be used as a name for the Moon and this cryptic note of mine seems to imply that there is a shark in the moon.