This group is composed primarily of flagellates that occur in both marine and freshwater environments. The cells contain chlorophylls a and c 2 and phycobiliproteins that occur inside the thylakoids of the chloroplast. The cell body is asymmetric with clearly defined dorsi-ventral/right-left sides (Figs. 9.1, 9.6). The asymmetric cell shape results in a peculiar swaying motion during swimming. Most cryptophytes have a single lobed chloroplast with a central pyrenoid.

Cell Structure

There are two apically or laterally attached flagella at the base of a depression. Each flagellum is approximately the same length as the body of the cell (Figs. 9.1, 9.2, 9.6). The flagella are anchored in the cytoplasm by microtubular roots, striatedfiber roots, and a rhizostyle (Nam and Shin, 2016). Depending on the species, there are one or two rows of microtubular hairs attached to the flagellum. In Cryptomonas sp., the hairs on one flagellum are 2.5 μm long and in two rows whereas the hairs on the other flagellum are only 1μm long and arranged in a single row (Heath et al., 1970 ; Kugrens et al., 1987). Small organic scales 150 nm in diameter (Fig. 9.1) are common on the flagellar surface and sometimes on the cell body (Lee and Kugrens, 1986). The outer portion of the cell, or periplast (Gantt, 1971), is composed of the plasma membrane and a plate, or series of plates, directly under the plasma membrane, except under the furrow-gullet complex into which the contractile vacuoles empty (Figs. 9.1, 9.2) (Kugrens and Lee, 1987 ; Hoef- Emden, 2014). The number and shape of these plates are used to characterize genera taking into consideration that the haploid and diploid phases of a single genus can have different plates (Hoef- Emden and Melkonian, 2003). New periplast plates are added in an area adjacent to the vestibulum (Brett and Wetherbee, 1996). Sulfated fucose-rich polysaccharides can be excreted outside of the cell (Giroldo and Vieira, 2002).

The chloroplast most likely evolved from a symbiosis between an organism similar to the phagocytic cryptomonad Goniomonas (Figs. 9.5, 9.6) and a red alga (Kugrens and Lee, 1991 ; Liaud et al., 1997 ; McFadden et al., 1994).