The most common type of arthropod on the reef is the crustacean. Crustaceans include shrimp, lobsters, and crabs. As shown in Figure below, the body segments of crustaceans are grouped into three specialized areas: head, thorax, and abdomen. Crustaceans have paired appendages attached to each segment that are used for walking, sensing, feeding, and defense. In a number of species, appendages form claws that are capable of exerting hundreds of pounds of pressure.

The biggest reef crustacean is the lobster. Even though the first pair of appendages are modified as claws in several lobster species, most reef lobsters are clawless. The appendages on the thorax of lobsters are adapted for walking along the reef floor and for swimming. Several lobster species make their homes on the reef. Two common ones are the spiny lobster (Panulirus argus) and the slipper lobster (Parribacus antarcticus).

The body of a crustacean is divided into three areas: head, thorax, and abdomen. The head and thorax are fused to form a cephalothorax.
The body of a crustacean is divided into
three areas: head, thorax, and abdomen. The head
and thorax are fused to form a cephalothorax.

The spiny lobster, as shown in the upper color insert on page C-5, is clawless, and its cylindrical body measures about 1 foot (30 cm) long. For protection the lobster is covered with numerous, formidable sharp spines. Body color varies with age and sex, ranging from bright yellow to reddish brown and blue. The legs display yellow stripes, and the abdomen is covered in yellow spots.

The spiny lobster has a fascinating and complex, six-stage life history that has received a lot of attention from marine biologists. Studies show that this animal develops in several areas of the sea, including the open ocean, shallow coastal water, and coral reefs. During its 30-year life, a spiny grows from a miniscule larva to an adult of more than 11 pounds (5 kg), taking seven to 10 years to reach sexual maturity.

Generally, the mature spiny lobster hides in reef crevices, holes, and caves during the day, emerging at dusk to look for food in the sand and sea grass. A spiny will wander several yards from its den, preying on a variety of organisms including crabs, small fish, sea urchins, algae, and seaweed. During nighttime forages, lobsters may gather in small social aggregates.

Before dawn, each lobster returns to the safety of its own, or a neighbor’s, den. These nighttime wanderers can find their way around in the dark because they are very sensitive to the Earth’s magnetic field and can use small variations in the field as landmarks. Their ability to find their dens is similar to the navigational senses of homing pigeons.

In the fall of each year, usually after a period of stormy weather, spiny lobsters congregate on the reef floor and begin a migration. They form a single file line called a queue, the head of one animal touching the tail of the animal in front of it, to march to deeper water. The lobsters travel day and night until they reach their destination. In the spring, they return to the reefs one by one. Scientists believe that these activities are related to water temperature and reproductive cycles.

Like the spiny, the slipper lobster is also clawless. The slipper lobster is drab or pale yellow and may grow to be 10 inches (25 cm) long. This carnivore also searches for food at night and will eat snails, oysters, clams, and the bodies of dead organisms. Slipper lobsters use their jaws to crack open prey. The bodies of these animals are flattened, a trait that helps them travel among the rocks unnoticed. Flat, shovel-like extensions on the head are the antennae.