The cryptophytes provide an important living example of endosymbiosis in progress. Chloroplasts are thought to have originated by the engulfment of a cyanobacterium by an ancestral algal cell, allowing eukaryotes to acquire the ability of photosynthesis.
Cryptophytes gained photosynthesis by engulfing not a cyanobacterium, but a red algal cell. This cell-within-a-cell situation is called secondary endosymbiosis. The red algal nucleus has been reduced to a mere remnant, called the nucleomorph.
The cells have two flagellae, as well as organelles called ejectisomes. Ejectisomes contain ribbons of rolled up material which are ejected violently, resulting in spasm-like movements of the cell.
Small cells which swim swiftly by flagellar motion, but also make sudden spasms when ejectisomes are activated. Cells are greenish-brown, a browner hue compared to the green algae. Ejectisomes are barely visible as a U-shaped arrangement in the center of the cell.
Labeled: Ejectisomes (red box with arrow). The two flagellae, moving too fast to be visible, emerge from a groove (red arrow).
Much smaller than Cryptomonas. U-shaped, flattened cells with a pocket at the anterior end (the mouth of the “U”) from which two flagella arise. These flagella beat rapidly in swimming; the cell body also rotates about its axis when in motion. Ejectisomes are present near the anterior pocket, visible in these images as a small dark or greenish dot.
Common and abundant in some samples, but easily overlooked because they are close to colourless, small, and fast-moving.