As far as the bioluminescent dinoflagellates go, I have received the suggestion (or came up with; it was part of dialogue and I can't recall who thought up what) mermaids using recovered, corked bottles to contain water with dinoflagellates. They just have to shake the bottle to make it glow for a few seconds. (This is how the original torches/flashlights worked; batteries were so weak, initially, one could only turn them on for a brief flash of vision, hence the American name.) But the dinoflagellates probably would be shaken by the process of being carried while swimming, so the bottles would be best as tools kept in the colony for when the mermaids must see or hear something approaching.
On the subjects of seeing and hearing:
Seeing: How many of the Sark caves have permanently dark rooms, or are they small enough (or arranged in such a way) that there are openings to the outside all over?
Hearing: In the youTube theflatfishking's channel, I hear sound from the dives in the videos. Is that a good representation o the things one hears while diving, and how much of that was caused by air bubbles, which wouldn't work for my mermaids. (Except when they are breathing air, which they must do.) Any sounds you can her from fish?
Oh, and, on dinoflagellates: how often can their glow be seen in the C.I. waters? Large patches or small? Regularly located of a variety?
On the subjects of seeing and hearing:
Seeing: How many of the Sark caves have permanently dark rooms, or are they small enough (or arranged in such a way) that there are openings to the outside all over?
Hearing: In the youTube theflatfishking's channel, I hear sound from the dives in the videos. Is that a good representation o the things one hears while diving, and how much of that was caused by air bubbles, which wouldn't work for my mermaids. (Except when they are breathing air, which they must do.) Any sounds you can her from fish?
Oh, and, on dinoflagellates: how often can their glow be seen in the C.I. waters? Large patches or small? Regularly located of a variety?