Diversity Of Crustaceans

Crustaceans exist in a range of sizes, but most are small. For example, copepods (KOH-puh-PAHDZ), as shown in Figure 36-5a, are no larger than the comma in this sentence. At the other end of the size spectrum is the Japanese spider crab, shown in Figure 36-5b. With a leg span of 4 m (13 ft), this crab is the largest living arthropod.

Copepod Diversity

(a) Copepod, Cyclops sp.

(c) Goose barnacle, Lepas anatifera figure 36-5

(a) Copepod, Cyclops sp.

(b) Japanese spider crab, Macrocheira kaempferi

(c) Goose barnacle, Lepas anatifera

Aquatic Crustaceans

Copepods are extremely abundant in marine environments and may be the most abundant animals in the world. Copepods are an important part of the ocean's plankton, the collection of small organisms that drift or swim weakly near the surface of the water.

In freshwater environments, on the other hand, much of the plankton is composed of crustaceans known as water fleas, which are about the size of copepods. A common type of water flea is a member of the genus Daphnia.

Barnacles, such as the one shown in Figure 36-5c, are marine crustaceans that are sessile as adults. Free-swimming barnacle larvae attach themselves to rocks, piers, boats, sea turtles, whales, and just about any other surface. They then develop a shell composed of calcium carbonate plates that completely encloses the body in most species. Their swimming appendages develop into six pairs of long legs called cirri (SIR-IE) (singular, cirrus). The cirri extend through openings in the shell, sweeping small organisms and food particles from the water and directing them to the mouth.

Terrestrial Crustaceans

Sow bugs and pill bugs are terrestrial members of a group of crustaceans called isopods. Terrestrial isopods lack adaptations for conserving water, such as a waxy cuticle, and can lose water quickly through their exoskeleton. Therefore, they live only in moist environments, such as under leaves and rocks, in crevices around garden beds, and in the spaces between house foundations and sidewalks. In addition, pill bugs are capable of rolling into a ball when disturbed or threatened with desiccation, as shown in Figure 36-6. Sow bugs and pill bugs generally feed on decaying vegetation, but they may also eat garden bulbs, vegetables, and fruits that lie on or in the soil.

figure 36-5

Crustaceans include tiny species, such as the copepod (a), as well as giant species, such as the Japanese spider crab (b). Some aquatic species, such as barnacles (c), attach themselves to submerged marine surfaces.

figure 36-6

Pill bugs are isopod terrestrial crustaceans that are often found in moist environments with decaying vegetation. Pill bugs and sow bugs look similar, but only pill bugs roll up when disturbed.

Sow Bugs Look Alike


Cephalothorax (head and thorax) _L


Cephalothorax (head and thorax) _L

Marine Crustacean With Pairs Limbs

Antennules figure 36-7


Telson Uropod

Walking legs

Pill Bug Body Structure


Mandible / . Maxillipeds

figure 36-7

(a) Each of the 20 body segments of a crayfish has a pair of appendages, as seen in this ventral view. (b) Some of the appendages of the anterior cephalothorax are visible in this closer ventral view.

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