life11e_ch26

Rhizarians typically have long, thin pseudopods

The three primary groups of rhizarians—cercozoans, foraminiferans, and radiolarians—are unicellular and mostly aquatic. The rhizarians have contributed their shells to ocean sediments, some of which have become terrestrial features over the course of geological history.

CERCOZOANS The cercozoans are a diverse group with many forms and habitats. Some are aquatic; others live in soil. One group of cercozoans possesses chloroplasts derived from a green alga by secondary endosymbiosis, and those chloroplasts contain a trace of the alga’s nucleus.

FORAMINIFERANS Some foraminiferans secrete external shells of calcium carbonate (Figure 26.11). These shells have accumulated over time to produce much of the world’s limestone. Some foraminiferans live as plankton; others live on the seafloor. Living foraminiferans have been found 10,896 m down in the western Pacific’s Challenger Deep—the deepest point in the world’s oceans. At that depth, however, they cannot secrete normal shells because the surrounding water is too poor in calcium carbonate.

Figure 26.11 Building Blocks of Limestone Some foraminiferans secrete calcium carbonate to form shells. The shells of different species have distinctive shapes. Over millions of years, the shells of foraminiferans have accumulated to form limestone deposits.

In living planktonic foraminiferans, long, threadlike, branched pseudopods extend through numerous microscopic apertures in the shell and interconnect to create a sticky, reticulated net, which the foraminiferans use to catch smaller plankton. In some foraminiferan species, the pseudopods provide locomotion.

RADIOLARIANS Radiolarians are recognizable by their thin, stiff pseudopods, which are reinforced by microtubules (Figure 26.12A). These pseudopods greatly increase the surface area of the cell, and they help the cell stay afloat in its marine environment.

Figure 26.12 Radiolarians Exhibit Distinctive Pseudopods and Radial Symmetry (A) The radiolarians are distinguished by their thin, stiff pseudopods and by their radial symmetry. The pigmentation seen at the center of this radiolarian’s glassy endoskeleton is imparted by endosymbiotic dinoflagellates. (B) The endoskeleton secreted by a radiolarian.

Radiolarians also are immediately recognizable by their distinctive radial symmetry. Almost all radiolarian species secrete glassy endoskeletons (internal skeletons). The skeletons of the different species are as varied as snowflakes, and many have elaborate geometric designs (Figure 26.12B). A few radiolarians are among the largest of the unicellular eukaryotes, measuring several millimeters across.