Desmid lookalikes
Although, because of their remarkable
morphological symmetry, desmid algae usually can be recognized in itself at
once, there are some representatives of other algal groups that resemble them
in a deceptive way.
Examples of such confusing non-desmids
particularly occur in the algal group of the xanthophytes. Xanthophytes are
characterized by other photosynthetic pigments than chlorophytes (to which the
desmids belong), but under the microscope there is usually no significant
difference in colour to be seen. Another essential difference between
xanthophycean desmid lookalikes and true desmids is in the way of reproduction.
Xanthophytes do not sexually reproduce by means of conjugation (no zygospores).
As, however, sexual reproduction stages in most desmid species are (extremely)
rare, this is not a workable discriminating characteristic either. In practice,
xanthophycean desmid lookalikes best can be recognized by their chloroplasts.
Whereas true desmids in principle have two large chloroplasts per cell, each of
them filling in a semicell, cells of xanthophytes contain (many) more than two,
relatively small, discoid chloroplasts, distributed all over the cell. As a
consequence, the midregion of a xanthophyte cell is coloured by pigments, in
contrast to a transparent midregion (isthmus) in true desmid cells.
Two commonly occurring xanthophycean
desmid lookalikes are Isthmochloron trispinatum and Pseudostaurastrum
lobulatum.
Isthmochloron trispinatum originally was described as a desmid, i.e. Arthrodesmus
trispinatus (West & West, 1902). Skuja (1948) transferred it to the
xanthophyte genus Isthmochloron.
Image © Henk Schulp
Cell of Isthmochloron trispinatum.
Notice that the isthmial region is filled in with pigment particles.
Cell dimensions ca 15 x 15 µm
Image © Koos Meesters
Another cell of Isthmochloron
trispinatum. When it is empty, it can easily be confused with a desmid cell,
e.g. of some small-sized Xanthidium species. |