Actiniscus pentasterias, a marine dinoflagellate

René · Post by **René** » Sat Mar 02, 2013 4:14 pm

Not all silica thingies are diatoms: here is a very typical dinoflagellate with a silicified bit of armour around it's nucleus. It's quite rare, and the silica star is difficult to see in a living cell. But geologists came across fossilized stars like this for over 50 mln years. It took some time before it became known these thingies are still about today, and totally different of what they thought it would be...

This is Actiniscus pentasterias, a marine dinoflagellate from an offshore location. Diameter is roughly 40um. In the middle of the cell is a dino-nucleus visible, with typically condensed chromosomes; around the nucleus are 2 silicified stars located.

They are not always easy to spot, but when the cell starts moving, you can see the structure from another angle, and recognize the star like structure:

Schiller took this species up in his 1937 dinoflagellate monograph within the Rabenhorst-flora:

The question remains: what's the use of it?

Best wishes, René

Baley · Post by **Baley** » Sat Mar 02, 2013 9:05 pm

Haven't a clue as to use unless it might have to do with polarization (not optical) for cell division; but my question is how did Schiller get such remarkable detail in the 1930s of such a small specimen?

René · Post by **René** » Sun Mar 03, 2013 5:13 am

Hi Baley,

The refraction index of silica is practically similar to that of cytoplasm. That's the reason why it is so difficult to see the structure within a cell.

You do find loose skeletons freely in the water, when cells degrade. Details are then very well visible; geologists found them already for over 100 years by the time Schiller wrote his monograph.

Concerning the use of such a skeleton for an organism, Schiller mentions it has a very flexible membrane, in german:

So I think he implies that the skeleton gives protection against different osmotic pressures. However, we never see this species near-shore (where you might come across salinity changes), so I'm not convinced yet by this argument.

Best wishes, René

Wim van Egmond · Post by **Wim van Egmond** » Sun Mar 03, 2013 6:59 am

Very nice and interesting images, René!

Dinos are weird.

Wim

arturoag75 · Post by **arturoag75** » Sun Mar 03, 2013 10:05 am

Great job

BJ · Post by BJ » Sun Mar 03, 2013 11:21 am

René,

fantastic work !

thank you so much for posting,

best wishes,

Brian

Charles Krebs · Post by **Charles Krebs** » Sun Mar 03, 2013 3:50 pm

Hi René,

Superb details visible on such a small subject.
(And I appreciate your additional information).

BJ · Post by BJ » Mon Mar 04, 2013 7:56 am

Hello René,

something that you do not comment on is the bundle of rods anterior to the nucleus. The rods are a type of ejectile organelle termed a "docidosome" - I believe they are unique to Actiniscus. Schiller has also depicted them. The cells also have trichocysts.

As to the function of the pentasters, I like the idea that they may provide physical protection for the nucleus - perhaps, for example, when the cell is engulfying some food item which distorts the cell. However, some support for them giving protection from osmotic stress comes from the Actiniscus found in freshwater in the Canadian Arctic - apparently it is not clear whether these are the same or related species. In the freshwater environment the cells have more pentasters.

I have never knowingly seen live Actiniscus but I have been lucky enough to find some pentasters in marine sediments of Miocene age - took me a while to work out what they were!

thank you again for posting,

regards,

Brian

Pau · Post by **Pau** » Mon Mar 04, 2013 8:44 am

Most interesting organism, very nicely imaged.

René · Post by **René** » Mon Mar 04, 2013 11:55 am

Hi guys, thanks for the kind words. Especially Brian, for the extra information about freshwater relatives! Hope to see one of those, some time (hint!). Regarding that typical bundle of extrusion organelles (docidosome), I've seen something like it in other gymnodinoids, but I'm not sure it is exactly the same. Unfortunately I haven't got access to Hansen's electron microscopical study of Actiniscus, if you have a pdf, please send me a note!

Best wishes, René

BJ · Post by BJ » Mon Mar 04, 2013 1:38 pm

Hi René,

i do have a pdf of hansen´s paper - only 14 pages, but it is a massive 48mB file - I think the way to get this to you would be via dropbox. If you are registered with dropbox, send me a pm with your email address and then you can access it.

Alternatively, if you like, you could email me your address and I could post you a hard copy.

I am afraid that I will not be able to do either for a day or two as i will be in a technology-free zone.

regards,

Brian