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.

Cells of Pseudostaurastrum lobulatum (also known as Isthmochloron lobulatum) which might be confused with some biradiate Staurastrum (e.g., S. bibrachiatum). Scale bar = 10 µm.

References:

Peerapornpisal, Y., 2005. Freshwater Algae in Northern Thailand. — The Biodiversity and Training Program (BRT), Chiang Mai, 361 pp.

Skuja, H., 1948. Taxonomie des Phytoplanktons eniger Seen in Uppland, Schweden. — Symbolae Botanicae Upsaliensis 9 (3), 399 pp, 39 pls.