Fig 2A & B from Nozaki et al. 2020. Light microscopy of Eudorina compacta Nozaki sp. nov. originating from Lake Victoria. (A) Surface view of 32-celled vegetative colony showing compactly arranged cells. (B) Optical section of 32-celled vegetative colony showing a hollow structure.

Hisayoshi Nozaki and colleagues have described another new species of volvocine algae, a member of the genus Eudorina from Lake Victoria in Tanzania. Unlike most species in this genus, the cells of Eudorina compacta are tightly packed around the surface of the colony, which is ellipsoidal. They coexist in Lake Victoria with Colemanosphaera charkowiensis, another species that Dr. Nozaki and colleagues described in 2014.

What makes the new species unique is that tight packing of cells, since adult colonies of most Eudorina species have their cells separated by extracellular matrix:

Eudorina elegans. Image by Y. Tsukii from the Protist Information Server.

I think of this tight versus loose packing of cells as a function of the volume of extracellular matrix: for a given number of cells, more extracellular matrix means more space for the cells to spread out. The volume of extracellular matrix seems to be one of the most evolutionarily labile traits in the volvocine algae, increasing and decreasing all willy-nilly throughout the phylogeny:

Figure 3E from Herron & Michod 2008. Expanded volume of extracellular matrix.

Black branches here are species with a large volume of extracellular matrix, white without, and the pie charts are relative likelihoods of ancestral states at each node (that is, a 3/4 black pie chart means that ancestor is ~75% likely to have had a large volume of extracellular matrix). Eudorina compacta belongs in the part of the tree boxed in red, where the other species (Eudorina elegans and Eudorina minodii) have a large volume of extracellular matrix.

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