Skip to main content


Crabs are crustaceans, like shrimp and lobsters, but their bodies are more rounded. Crabs tuck their abdomens and tails up under their bellies. Most move by crawling along the ocean floor, although there are some species that swim. They have large front pincers that help them find and catch prey like clams, small fish, snails, and other crabs. Crabs do not stalk or pursue their meals; they usually wait for prey to swim by. They also scavenge dead and decaying organisms and look for food by sifting through sand or silt with their legs and antennae.

The spotted porcelain crab (Porcellana saya) is about 1 inch (2.5 cm) long. Its exoskeleton is orange red with white and violet spots that are ringed with red. Some species of spotted porcelain crabs are free living, but others live in symbiotic relationships with various types of hermit crabs and queen conchs (Strombus gigas).
Porcelain Crab

Hermit crabs are not classified as true crabs. True crabs have five pairs of legs, including the claws, and hermit crabs have only four pairs. Like other crustaceans, a hermit crab has a crusty exoskeleton over most of its body; however, this protective armor does not extend over its abdomen, which is soft and vulnerable to predators. For this reason, hermit crabs have to find suitable shells to cover their abdomens. Without this protection, they could become a meal for some other animal.

To forage, hermit crabs travel across the reef floor, dragging their homes with them. Their shells are heavy, so the crabs cannot move very fast. To avoid some of their predators, they usually look for food at night. If frightened, hermit crabs retreat into their shells and cover the entrances with their large right claw. Their smaller left claw is for eating. The largest species in this group is the red hermit crab (Petrochirus diogenes), a rusty-red animal that can grow to 10 inches (25 cm). The red hermit crab is an active predator, but can scavenge food when necessary.

On the reef, true crabs include the lesser sponge crab (Dromidia antillensis), the purse crab (Persephona punctata), and the green reef crab (Mithrax sculptus). The lesser sponge crab is only about 3 inches (7.5 cm) long with the last pair of legs bent over the back to hold a sponge. The purse crab is even smaller, at 2 inches (5 cm), with a white to gray shell that is marked with large red-brown spots. The extremely common tiny green reef crab is only 1 inch (2.5 cm) long and has hairy-looking legs.

Popular posts from this blog

Advantages and Disadvantages of an Exoskeleton

More than 80 percent of the animal species are equipped with a hard, outer covering called an exoskeleton. The functions of exoskeletons are similar to those of other types of skeletal systems. Like the internal skeletons (endoskeletons) of amphibians, reptiles, birds, and mammals, exoskeletons support the tissues and give shape to the bodies of invertebrates. Exoskeletons offer some other advantages. Serving as a suit of armor, they are excellent protection against predators. Also, because they completely cover an animal’s tissues, exoskeletons prevent them from drying out. In addition, exoskeletons serve as points of attachment for muscles, providing animals with more leverage and mechanical advantage than an endoskeleton can offer. That is why a tiny shrimp can cut a fish in half with its claw or lift an object 50 times heavier than its own body.
Despite all their good points, exoskeletons have some drawbacks. They are heavy, so the only animals that have been successful with them …

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

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)…