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Arthropods Chapter 7 pgs. 134-140 Arthropods: The Armored Achievers • Arthropods make up the largest phylum of animals. • The majority of marine Arthropods are Crustaceans. Characteristics of Arthropods • Their bodies are segmented and are bilaterally symmetrical. • They have jointed appendages such as legs and mouth parts. • They have an exoskeleton, which is a non-living external skeleton. Characteristics continued: • They shed their exoskeleton by molting. • Molting takes place when a new shell forms under the prior one. After the old one is discarded the new one hardens after the animal takes in water to expand itself. Characteristics of Crustaceans • There are approximately 68,000 known species of crustaceans. • Their chitinous skeleton is usually hardened by calcium carbonate. • Their appendages are specialized for swimming, crawling, attaching to other animals, mating, and feeding. Characteristics of Crustaceans continued: • Crustaceans have 2 pairs of antennae, which are usually involved in sensing the surroundings. Small Crustaceans • Small crustaceans are everywhere in the ocean. • There are many groups of small crustaceans such as: Copepods, barnacles, amphipods, isopods, krill, and shrimp and lobsters. Goose Barnacles pictured. Copepods • Copepods are extremely abundant and important in the plankton. • Their mouth parts filter out and capture food. • Keep from sinking by using their first pair of antennae to swim. • Many species are parasitic and are no bigger then a small bag of tissues. Barnacles • Are filter feeders that usually live on attached surfaces, such as whales and crabs. • Most barnacles look almost like mollusks because their bodies are enclosed by heavy calcareous plates. • They use cirri, which are actually legs, to sweep the water and move. Amphipods • Small crustaceans with a curved body, that is flattened sideways. • Most are only 2 cm big. • The head and tail typically curve downward and the appendages are specialized according to function. • They kind of act like parasites for whales and burrow into their skin. Isopods • Found in many of the same environments as amphipods. • They are easily identified because of their flattened back. • Fish lice are an example of Isopods. Krill • Are planktonic, shrimp-like crustaceans, up to 6 cm long. • Head is fused with some of the body segments to form a distinctive carapace that covers the anterior half of the body like armor. • Many krill are filter feeders that feed on plankton and diatoms. • Are extremely common in polar waters. Shrimp, Lobsters, and Crabs • The decapods are the largest group of crustaceans. • They are also the largest crustaceans in size. • Decapods feature five pairs of legs and three pairs of maxillipedes, which help sort out food and push it toward the mouth. • They have a cephalothorax and an abdomen. Shrimp and Lobsters • They have compressed bodies and elongated abdomens. • Shrimp are typically scavengers. • Many colorful shrimp particularly in the tropics live on the surface of other invertebrates or remove parasites from the skin of fish. Shrimp and Lobsters continued: • Lobsters are mostly nocturnal, and hide during the day in a rock or coral crevices. • They are scavengers and predators that crush mollusks and sea urchins. Hermit Crabs • Not true crabs, but are scavengers. • They hide their long soft abdomens in empty gastropod shells. • Some cover shells with sea anemones to protect themselves. Biology of Crustaceans • The diversity of forms among crustaceans is paralleled by equally diverse functional features. Feeding and Digestion • Filter feeding is very common in copepods and many small planktonic crustaceans. • Appendages are adapted for piercing and sucking in parasitic copepods and isopods. • Food passes through the stomach, which is 2 chambered in decapods, and connects to digestive glands that secrete digestive enzymes and absorb nutrients. • The nutrients are then distributed by an open circulatory system. Nervous System and Behavior • Crustaceans have a small, relatively simple brain, but well-developed sensory organs. • Most have compound eyes, which consist of a bundle of about 14,000 light-sensitive units grouped in a mosaic. • Crustaceans have a keen since of smell, which is that they are very sensitive to chemicals in the water. • Crustaceans are among the most behaviorally complex invertebrates. • They use a variety of signals to communicate with each other. Nervous System and Behavior continued: • Many of the signals used for communication involve special body postures or movements of the legs and antennae, which are often marked or colored to make the signals more conspicuous. • This type of communicating has helped crustaceans with settling arguments and in courtship. Reproduction and Life History • The sexes are separate in most crustaceans. • Males use specialized appendages to transfer sperm directly to the female. • Mating in decapods usually takes place immediately after the female molts, while the exoskeleton is still soft. • Females of many species can store sperm for long periods of time and use it to fertilize separate batches of eggs. Reproduction and Life History continued: • In decapods, females carry their eggs using pleopods, or swimmerets. Which are specialized appendages beneath the body. • Their larva is called nauplius. Other Marine Arthropods • Very few arthropods other then crustaceans are common in the ocean. Most belong to 2 small and entirely marine groups. A third group, huge and mostly terrestrial, includes a few shy invaders of the sea. Horseshoe Crabs • The horseshoe crabs are the only surviving members of the group (class Merostomata) that is widely represented in the fossil record. • The 5 living species of horseshoe crabs aren’t true crabs, but “living fossils.” Horseshoe Crabs continued: • They live on the soft bottoms in shallow water on the Atlantic and Gulf coasts of North America and Southeast Asia. • Their most distinctive feature is a horseshoe- shaped carapace that encloses a body with the 5 pairs of legs. Sea Spiders • Sea spiders are in the class Pycnogonida. • They only superficially, resemble true spiders. • They have 4 or more pairs of jointed legs that stretch from their small body. • A large proboscis with the mouth at the tip is used to feed on soft invertebrates such as sea anemones. • They are most common in cold waters. Insects • They only have 3 pairs of legs. • Most diverse group of animals but are rare in the sea. • Most marine insects live at the water’s edge, where they scavenge among seaweeds, barnacles, and rocks. • One marine insect that is found far from shore is the marine water strider (pictured).
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