I have spent more than three hundred nights in the water with these organisms. I have had thousands of them sitting on my skin, blinking, close enough to my eyes to touch.
I have never once seen one.
Everything I know about them from the water, I learned from their light. It is the only part of them any of us will ever see with our own eyes.
What they are
The species in our bays is Pyrodinium bahamense. Each one is a single cell, twenty to fifty microns across. You would need between five hundred and thirteen hundred of them lined up to cross one inch.
How they move
They are not drifting. Each cell has two whip-like tails called flagella, one wrapped around its middle and one trailing behind. Together they make the cell spin as it swims, on a corkscrew path, all night long, under everybody.
That matters. A drifting particle goes where the water takes it. A swimmer can hold its place. Part of why a bay keeps its density is that the cells are staying on purpose.
The name, corrected
You will read almost everywhere that "dinoflagellate" means fire and whirling. I repeated it myself for years. It is wrong.
Dinoflagellate is dinos, Greek for whirling, and flagellum, Latin for whip. Whirling whips. It describes how they swim.
The fire is in the other name. Pyrodinium is pyr, fire, and dinos, whirling. The whirling fire. It was named in 1906, in the Bahamas.
How one cell makes light
Two things are needed: luciferin, the molecule that gives off light, and luciferase, the enzyme that drives the reaction. Both sit inside tiny compartments called scintillons, along the inside of the cell wall.
Nothing is released into the water. Not one molecule. The whole reaction happens inside the cell, and what reaches your eyes is light passing out through the wall. That is why the bay does not burn itself out in a night.
What pulls the trigger
Something bends the cell. Your arm. A fish. A paddle. That sends an electrical signal across the membrane, acidity rises sharply inside the scintillon, and the acidity is the switch. The result is a photon of blue-green light at around four hundred and eighty nanometres.
It takes about a tenth of a second. That is why you see a spark and not a glow.
They keep their own hours
They run on an internal clock. Scientists have held them in total darkness around the clock, and they still did not flash during what should have been daytime.
The bay sleeps by day, even in the dark.
Two ways to eat
They are mixotrophs. By day they charge on sunlight, like a plant. When the sun fails them, they can take energy from the organic matter in the water, which the mangroves supply. Part of the research that established this for our species was done at La Parguera.
Why they flash
Not to frighten what is eating them. The flash lights up the attacker so that something bigger can find it.
It is not an alarm that scares the burglar. It is one that calls the police.
So when you move your arm and the water lights up, you are setting off several hundred thousand of those calls at once.
They can be spent
This part is my own observation, not published science. They charge by day and discharge only when the water moves. After several overcast days with boats stirring the bay every night, the whole bay reads dimmer. It is not the clouds that night. It is the accumulated draw of the days before.
The same organism, somewhere else
In much of the world this species is not a wonder. It causes harmful algal blooms, and in some regions it produces a toxin that builds up in shellfish and poisons the people who eat them.
The same creature. Put it in the wrong water and it is an emergency. Put it in ours and people cross oceans to see it.