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Prokaryotic Cell Structure

Prokaryotic cells are about 10 times smaller than eukaryotic cells. A typical E. coli cell is about 1 μm wide and 2 to 3μm long. Structurally, prokaryotes are very simple cells when compared with eukaryotic cells, and yet they are able to perform the necessary processes of life. Reproduction of prokaryotic cells is by binary fission—the simple division of one cell into two cells, after DNA replication and the formation of a separating membrane and cell wall. All bacteria are prokaryotes, as are the archaea. Embedded within the cytoplasm of prokaryotic cells are a chromosome, ribosomes, and other cytoplasmic particles (Fig. 1). Unlike eukaryotic cells, the cytoplasm of prokaryotic cells is not filled with internal membranes. The cytoplasm is surrounded by a cell membrane, a cell wall (usually), and sometimes a capsule or slime layer. These latter three structures make up the bacterial cell envelope. Depending on the particular species of bacterium, flagella, pili (description follows...

Complex Invertebrates on the Coral Reef

In the sea, as well as on land, invertebrates dominate the animal kingdom. Sponges and corals fill the reefscape with bright colors and endless body forms. Worms hide in every crevice, crack, and safe spot they can find. As abundant as these animals are, they only make up a small portion of the reef invertebrate population. Living alongside them are myriad invertebrates that are more complex and sophisticated. Many of these have well-developed body systems, some of which are very similar to those found in vertebrates. Simple invertebrates, such as sponges, satisfy all of their oxygen needs by absorbing the gas directly through their cells. The larger and more complex invertebrates do not have this luxury. Complex invertebrates need greater quantities of oxygen than can be provided by simple absorption. In addition, their bodies are often covered with waterproof external structures, further reducing the amount of contact they have with oxygen-rich water. These structural character...

Damselfish, Clown Fish, Cardinal Fish, and Squirrelfish

Another colorful and abundant reef fish is the damselfish, a delicate-looking animal with a small mouth. Damselfish nibble on the encrusting algae that grow on coral and rock. These red, yellow, orange, or blue fish grow to be about 6 inches (15 cm) long. Damselfish Like most damselfish, the Pacific gregory (Stegastes fasciolatus) is active during the day. Most of this fish’s time is spent “farming” a patch of alga that may cover up to 10.7 square feet (1 sq. m) of the reef. An excellent gardener, the little damselfish weeds out unwanted corals and algae so that its favored algae can flourish. To supplement plant growth, the fish fertilizes its garden with its own feces. Stegastes fasciolatus One of the brightest and best-known reef fish is the percula clown fish (Amphiprion ocellaris), one of several species of clown fishes and a member of the damselfish family. Reaching only 2.4 inches (6 cm), this small animal is striped in orange, white, and black. The percula ...

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