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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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