Dinoflagellates
(cf. Ostreopsis spp.)
I finally managed to find a few
cf. Ostreopsis spp. cells again and was able to take some videos with the new objectives and camera.
Ostreopsis dinoflagellates are feared for producing palytoxin analogues (ovatoxins, ostreocins, mascarenotoxins) (Chomérat et al., 2020 and Accoroni & Totti, 2016), and while it is certainly wise to treat every bloom as if it is toxic, the actual toxicity varies enormously between strains and environmental conditions (Ben-Gharbia et al., 2016; Gémin et al., 2020). That means even if we were capable of identifying the species, we still wouldn't know much about its toxin production (Chomérat et al., 2020).
If your tank inhabitants are not showing any signs of distress, you should still take protective measures for them and for yourself when you approach the bloom for manual removal. There is always a chance your tank inhabitants have learned to deal with those toxins, while humans most certainly have not (Accoroni & Totti, 2016).
The first video shows a little bit of the flagella on the top right of the cell in a wave motion. It also shows the typical movement of
Ostreopsis cells, which don't really move in straight lines and instead spin around their own axis or around a fixed point in the distance (Fukuyo, 1981; Begnaud, 2019).
(400× mag. DIC showing
cf. Ostreopsis sp. cell movement)
The movement is probably the best indicator if you don't have access to magnifications of 400× or higher. If the cells are shaped like pumpkin seeds or tears and move in a circling motion, chances are you are dealing with some
Ostreopsis cells in your tank (Steidinger & Meave del Castillo, 2018; Begnaud, 2019). Round cells with a similar movement are more likely
Coolia or one of the more rarely found genera (Verma et al., 2023; Begnaud, 2019), while the other common genera (
Prorocentrum and
Amphidinium) don't show that circling motion (Begnaud, 2019). In nature the differentiation between
Ostreopsis and other dinoflagellates would not be that easy, but in reef tanks planktonic species don't seem to survive for long, and even other benthic genera appear to be rare.
("side-view" showing edges of thecal plates, image sharpened in post)
Thecal plates are very hard to see on these cells, but they are a great way to tell
Ostreopsis apart from
Amphidinium, which has no thecal plates at all (Steidinger & Meave del Castillo, 2018). There is a little trick to make them easier to see. You can simply let the microscope slide sit for a while. Evaporation of the water raises the salinity, and together with the lack of water flow, changes in light intensity and changes in temperature this can cause cells to enter a dormant state in which they "shrink", which makes the theca visible. However, don't confuse the theca with a resting cyst like in the
Amphidinium post (page 1, post #3), which some
Ostreopsis species are also capable of forming (Accoroni & Totti, 2016).
(
cf. Ostreopsis shortly after entering an inactive state and "shrinking" so the surrounding translucent theca becomes more visible)
| Common Name | Dinos / Dinoflagellates / Ostreopsis |
| Classification | cf. Ostreopsis sp. |
| Verdict | Harmful (during blooms in large numbers) |
| Size | ≤ 195μm |
| Found | Substrate, Rockwork, Glass |
| Rarity | Obligatory (either rare or they take over) |
(630× mag. DIC showing
cf. Ostreopsis sp. cell)
Sources:
Dehydration, extreme heat, extreme cold, starvation, exposure to outer space, ... They are insane 
. Currently I'm looking into staining techniques, hoping to help with certain identification processes or simply improve visibility of things that might otherwise lack contrast.
