These are Caribbean reef squid, or Sepioteuthis sepioidea. The actual size of these ones is about 2cm long and 1cm wide: they are juveniles. Growing at about 0.7mm each day, they reach maturity at around 6 months. Then they tend to go offshore.
Trying to look scary
Apparently these squid are generally calm and sociable. I suppose calmness is an anthropomorphic imagination derived from the fact that they don’t always skitter off when you approach them.
Colour changes are controlled by the nervous system directly affecting chromatophores. The patterns are not random, they seem to select certain disguises depending on whether they are being friendly, aggressive or even courting.
The young squid hide just below the surface of the water, beneath vegetation or, as we found them, under buoys.
This little fish was almost invisible until it left one bouy chain in preference for the next one.