|Most red algae, like this
Batrachospermum, are complex,
multicellular seaweeds (though
this is a freshwater genus). From
Oltmanns, Morphologie und Biology
der Algen. 1922, Fig.469.
While green algae have varied growth forms, the red algae are mostly complex, multicellular seaweeds. Seaweeds, like their photosynthetic cousins on land, are organisms that set down "roots" in a suitable location, and stay put for the rest of their life. But being glued to one spot makes it even more important that they have a means to disperse their genetic information during their reproductive cycles. In algae, that's typically a 2-step process: spores that travel long distances to mingle with other populations, and sperm cells to travel relatively shorter distances to seek out a suitable egg (see "The truth about sex in plants"). For that they need some means of locomotion, and for single-celled organisms that usually means slender whip-like organelles called flagella (or cilia, which are smaller, but of the same structure).
They mystery of the red algae arises from the fact that neither their sperm cells nor their dispersal spores have flagella. Such cells are literally adrift in the ocean "without a paddle." Throughout this entire group there is no sign of flagella in any species or in any part of the life cycle. There is no sign even of the flagella-supporting structures (centrioles), usually found in all cells (not just cells with flagella) in other algae and land plants. It is as if the ancestors of red algae never had flagella at all. That hypothesis is however complicated by the apparent relationship of red and green algae and their common descent from the first photosynthtic eukaryotes.
|In the unicellular Chlamydomonas, all cells (except for the
dormant zygote) have two flagella. In multicellular green algae,
only the gametes and zoospores are so equipped.
From Haupt, Plant Morphology, 1953, Fig. 16.
So the mystery has several layers, including a dilemma: red algae have the older form of chloroplast, and presumably came first, with the green algae evolving from them. But green algae have the original mode of locomotion provided by flagella, and the flagella-less red algae must have evolved from them! The simple solution to this is that the first algae had red chloroplasts and flagella, and subsequently split into two groups. Red algae as we know them today have the original chloroplast but have lost their flagella. The green algae evolved a new type of chloroplast but retained flagella.
But now we have to ask "why did red algae lose their flagella?" How do their sperm cells find eggs without any means of self-directed movement? Textbooks are vague on this issue, generally implying that the loss of flagella must have been accidental. But it seems that the elimination of flagella and associated support structures, indeed all the genes involvd in producing flagella, was ruthlessly thorough and therefore must have happened for some adaptive "reason."