Thanks Don,

Yeah sounds like a fast acting causative agent may have been at work. I have seen other inverts (not coral reef related) do that in near coastal and estuarine environments in response to hypoxic events and pollutants. Usually if there is time and available routes of escape the mobile nekton can scoot out of the affected area,which based on your account would be fairly easy for them to do. Also, if there was any dead floating nekton it would have been dispersed with the tide. [...]