by Roland Mortimer, Brazil
Once in a while I receive seawater samples from friends who travel around a lot more than I do. I received a seawater sample taken off the coast of Argentina last weekend and while searching for diatom types I came across several creatures which strangely enough I'd never seen in any of the samples of seawater from around Rio.
I decided to try to identify them and managed to do so from the book by Wynne and Bold "An Introduction to the Algae". I found only one type of Dinophysis and several types of Ornithocercus in the sample, Dinophysis meaning: Greek-dinein, to whorl and Greek-physis, nature. Ornithocercus also comes from the Greek-Ornitho, bird + Kerkos, tail, hence the apt description bird's tail.
Both types of algae make up part of the phytoplankton and are photosynthetic. The rib extensions are known as lists and are in the form of a wing in Dinophysis, whereas in Ornithocercus they may be very large and several may be present enclosed by a web-like membrane, or this may be absent or so small it's almost unnoticeable, just covering the ribs and not extending to the neighbouring ribs.
The hypotheca and epitheca (upper and lower portion of the cellulosic 'shell') are amply perforated just like diatoms and look beautiful under darkfield illumination. Being of cellulose they are difficult to mount as the cells 'disappear' when mounted in the synthetic resins replacing Canada balsam. I found they're easier to see if mounted dry and a ring of varnish placed around the mount then this is covered with a round coverglass. When dry and the cover lightly pressed into contact with the varnish, the cover is sealed with a ring of asphalt varnish. The cells can then be viewed dry in any illumination without halos from the water containing the cells.
These algae may be found in profusion in tropical/sub-tropical waters, although I've never seen them off Rio's coast; the specimens shown in Wynne and Bold are all from the Gulf of Mexico.
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