PHYLUM CERCOZOA – Diversity of Life

EUKARYA> CHROMALVEOLATA> RHIZARIAE> CERCOZOA

Cercozoa (sur-ko-ZO-uh) is derived from two Greek roots that mean tail (kerkos -κέρκος); and animal (zoa -ζώο). The phylum is named for a common genus, Cercomonas, which is a motile cell with heterodynamic flagella. The trailing flagellum looks like a tail.

INTRODUCTION TO THE CERCOZOA

The cercozoans include a diverse collection of unicells that are testate , motile, radiolarian, and heliozoid (Figures 1-5). The group is defined strictly by molecular ssu-rRNA and actin (mainly Cavalier-Smith 2003, Nikolaev et al. 2004, and Howe et al. 2011) or polyubiquitin (mainly Bass et al. 2005) molecular phylogenies. Thus, there are no clear synapomorphies to define them. All can produce filopodia, but this is a synapomorphy of the Rhizariae, in general. By and large, groups of the cercomonads were orphan taxa, unaffiliated with other groups and defined as sisterless groups by Patterson (1999). Members of the cercomonads resemble carpediemonads and other excavates, testate amoebae, and heliozoans.

Howe et al. (2011) describe two unequal groups. Most of them are gliding cells with filose pseudopodia and are made up of 4 classes: Imbricatea (cells that have silicaceous scales -e.g. Euglypha, Figure 1), Sarcomonadea (gliding cells without scales or tests, e.g. Cercomonas, Figure 2), Thecofilosea (cells with thecae, see Figures 3-5), and Metromonadea (gliding cells with an external layer or surface coating). The other two groups are naked cells that may or may not be flagellated, but they are non-gliding. These are made up of 2 classes: Granofilosea (cells with granular filopodia, e.g. Clathurlina, Figure 6), and Chlorarachnea (cells with filopodia, often forming a net and many are photosynthetic, e.g. Chlorarachnion, Figure 7).

The phaeodarians, a group of the Thecofilosa, were once considered sarcodines (amoeboid animals) and affiliated with the radiolaria (a group of biomineralized heliozoan-like cells: polycyctines and acantharians). They have a test, which is a cytoskeleton that is mineralized with amorphous silica. They are rare members of the marine plankton and discovered only near the end of the 19th century during the voyages of discovery on the open ocean. The expedition of the HMS Challenger (1872-1876) was one such voyage, which was charged with studying the oceans of the world. To that end, it carried biologists, chemists, physicists, geologists, and archaeologists. During the four years of discovery, the ship circumnavigated the globe, took many soundings, temperature profiles, and bottom dredges. In the process, the expedition catalogued 4717 new species, many of them from the plankton. In fact, the unexpected diversity of the plankton in the ocean was one of the greatest discoveries made by the Challenger and her crew. Challengeron (see Figure 5) was one of the phaeodarians discovered during and named after the expedition. It was bilaterally-symmetrical, and, like other phaeodarians, had an internal capsule that surrounded the nucleus and a bilaterally-symmetrical outer capsule. The feeding axopods originated at one end of the internal capsule and emerged from the outer capsule at the bottom of the illustration in Figure 6. Ernst Haeckel, was a prolific writer, researcher, and artist, who popularized many of the organisms collected by the Challenger and other expeditions of the late 19th and early 20th centuries. Figure 6 is his illustration of an organism that he described and named.

The chlorarachniophytes are filose green amoebae that link together in a net-like web (see Figure 7), forming a plasmodium , a large confluent cellular mass with many nuclei (Hibberd and Norris 1984, Hibberd 1990a, McFadden et al. 1997, and Van Den Hoek 1995). They occur only in marine environments where they are associated with siphonous green algae. The individual cell bodies have about four bi-lobed chloroplasts each. In addition, each chloroplast has a degenerate nucleus, a nucleomorph, which is bound within membranes of the chloroplast (Hibberd and Norris 1984). This condition is considered to be the best evidence that some eukaryotes became photosynthetic by entering into a endosymbiotic relationship with another eukaryote (Bhattacharya and Melkonian (1995). Because the chloroplast stores photosynthate as paramylon and uses chlorophylls a and b , it is likely that the enslaved eukaryote was a euglenoid; however, Takahashi et al. (2007) provide evidence that the enslaved was a green alga and McFadden et al. (1997) show that the photosynthate is stored by the host and not the symbiont. Either way, the enslavement of a eukaryote is called secondary endosymbiosis, a condition that occurs also in the cryptomonads.

The filose amoebae have diverse types of life cycles (e.g. Ishida et al. 2000) can release uniflagellate swarmer cells, each of which has a single, posteriorly-directed flagellum that is inserted antapically and winds down the cell in a groove. Whether these also function as gametes is not known.


FIGURE 1. An SEM micrograph of the scaly test of a euglyphid amoeba from a soft water pond in central Pennsylvania. Image from Systematics Biodiversity Image Archive	FIGURE 2. A DIC micrograph of Cercomonas, a motile cell with heterodynamic flagella (text with tooltip) . The recurrent flagellum lies in a ventral groove. Image from: http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/cercomonas2_lpw.jpg	FIGURE 3. An illustration of Aulocantha. This taxon has both tangential needles and denticulate radial spines. Image from: http://microscope.mbl.edu/	FIGURE 4. An illustration of the mineralized cytoskeleton of Aulosphaera in the the peripheral network is joined in a triangular pattern. The radial spines emerge from the joints of the network. Image from: http://microscope.mbl.edu/


FIGURE 5. Challengeron, an illustration by Ernst Haekel (from his Kunstformen der Natur (Artforms in Nature). Haekel also described this particular species, C. wyvillei. Image from: Grell (1973)	FIGURE 6. An illustration of Clathurlina showing the stalked heliozoan structure with the filopodia emerging from openings in the organic capsule surrounding the cell. Image from: Grell (1973)	FIGURE 7. A DIC micrograph of Chlorarachnion that shows the characteristic filopodia and chloroplasts. Image from: http://microscope.mbl.edu/baypaul/microscope/images/t_imgAZ/

FIGURE 8. A cladogram based on Bass et al. (2005 and 2009) showing a simplified view of the relationships between the major taxa of the Cercozoa (those taxa highlighted in the shaded box). The taxa are from Cavalier-Smith (2003) and Howe et al. (2011).

SYSTEMATICS OF THE CERCOZOA

The earliest work (e.g. Kuhn et al. 2000) focused on the cercomonads, a group of amoeboflagellate taxa and much of the focus confirmed a relationship between the cercozoans and the foraminifera (Archibald and Keeling 2004; Archibald et al. 2003; and Longet et al. 2003). The “core cercozoa” has emerged as a coalescence of sisterless taxa from the last years of the 1990s to 2005 (Patterson 1999, Kuhn et al. 2000; Wylezich et al. 2002; Archibald et al. 2003; Longet et al. 2003, Nikolaev et al. 2003; Cavalier-Smith and Chao 2003; Archibald and Keeling 2004; Nikolaev et al. 2004; Bass et al. 2005; see Figure 8). The systematics of this phylum is still in flux (e.g. Cavalier-Smith and Chao 2003, Adl et al. 2005, Bass et al. 2009, Chantangsi and Leander 2010), but it has settled on the association of filopodial taxa that range from creeping motile cells to the large phaeodarians with silicaceous skeletons. We recognize that significant changes likely will occur in the systematics of this group and offer the taxonomy as a tentative relationship based on a modifications of Cavalier-Smith (2003) and Howe et al. (2011).

LITERATURE CITED

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By Jack R. Holt. Last revised: 03/04/2013

PHYLUM CERCOZOA (Cavalier-Smith 2002 em. Adl et al. 2005)

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