ID guide › Dinoflagellates
Gambierdiscus sp.
Toxic. Gambierdiscus makes ciguatoxins, the cause of ciguatera. One aquarium professional dealing with a suspected outbreak reported feeling nauseated and dizzy near the tank, and so did the tank's owner. Ventilate, and read the safety notes before you start.
Gambierdiscus is closely related to Ostreopsis and has the same basic design: a flat disc that looks like a circle from above, instead of an oval. The cells are large, so if you see big round cells even at fairly low magnification, Gambierdiscus is worth considering. It has not been seen as often as Amphidinium and Ostreopsis in aquaria.
A clue to look for: the outline is almost, but not quite, a perfect circle. It looks like someone drew a circle and, when they came around to the end, the line did not quite meet up. That small step or mismatch is where the sulcus meets the cingulum. Remember that in these flat cells the cingulum runs around the rim.
I do not have a photo of Gambierdiscus of my own yet. If you have a confirmed one, I would love to see it. The small unidentified dino can be mistaken for it when you cannot judge the size.
What to do
Safety comes first. Ventilate, run fresh carbon, and read the safety notes before you try anything. I do not have enough data on Gambierdiscus in aquaria to say what works best, and aquarists report different results. See the treatment notes for the approaches I lean toward.
If you have a suspected Gambierdiscus outbreak, I would be very interested in good pictures and, if you can keep a small sample alive, a sample for further study. Please get in touch.