Arthropoda II

Arthropoda II

Subphylum Crustacea 

Subphylum Crustacea

• Class Ostracoda

• Class Branchiopoda

• Class Thecostraca

• Class Malacostraca 

• There are more classes but these are the focus

General Characteristics

• Mostly marine, many freshwater, few terrestrial

• distinguished from other arthropods by 2 pairs of antennae

• a pair of mandibles, two pair of maxillae, pairs of appendages on all other body segments

• most with 16-20 segments; some with 60

  • head of 5 fused segments

  • thorax of 8 segments

  • abdomen of 6 segments

• ancestral condition is to have more segments

• derived condition is to have fewer with greater tagmatization

• major tagma include:

  • head

  • thorax

  • abdomen

• most crustaceans have a cephalothorax

• rostrum and telson are both unsegmented

• external features: appendages

  • some may have appendages on all segments; others not

  • biramous appendages separated into endopod and exopod

  • the diversity of appendages arrived from an ancestral appendage

• internal features:

  • hemocoel

    • major internal space

  • muscular system

    • striated muscles form a considerable part of the body

    • muscles arranges antagonistically

    • flexors draw appendages towards body

    • extensors move appendages away from the body

Homeostatic functions and Structures

• Respiratory system

  • small crustaceans: diffusion across thin areas of the cuticle

  • large crustaceans: gills (or lung-like organs)

• circulatory system:

  • open circulatory system

  • no separation of blood from interstitial fluid

  • heart is located dorsally

  • paired ostia

• excretory system

  • in adult crustaceans, takes the form of antennal glands (aka, green glands)

  • consists of end sac, labyrinth, tubule and bladder

  • mostly for osmoregulation

  • nitrogenous wastes lost via diffusion through gills

Feeding structures and Habits

• diverse across the range crustaceans 

• generally, mandibles and maxillae ingest food

• maxillipeds hold and crush food

• walking legs capture food

• gastric mill may be present to further digest ingested food

Nervous system and sensory structures

• brain a pair of supraesophageal ganglia

• well developed eyes (stalked or not; compound)

• body covered in tactile hairs

reproduction and life cycle

• most are dioecious

• most will brood their eggs

• indirect development is the norm

• larval stage is the nauplius

• larvae undergo metamorphosis

Diversity of Crustacea

Class Ostracoda

• 6,000 extant species (10,000 fossils)

• worldwide distribution

• important in aquatic food webs

• fusion of the trunk

• difficult to tell abdomen from thorax

• feeding and locomotion by head appendages

• may be bioluminescent

Class Branchiopoda 

• 10,000 species

• diverse morphologies

• flattened, leaf-like legs

• mostly a freshwater group

• important part pf zooplankton

• reproduction produces only females in summer

• as weather worsens, males are produced

• fertilized eggs can withstand harsh conditions

Class Thecostraca

• barnacles

• sessile and enclosed in a secreted shell

• head reduced, abdomen absent

• thoracic legs long

  • used for capture of food

• all are marine

• all are hermaphroditic

Class Malacostraca

• 20,000 species

• diverse in morphology

• ecologically and economically important

• Order Isopoda

  • successful invaders of land, but marine and freshwater forms remain

  • some are parasitic

  • dorsoventrally flattened

  • carapace lacking

  • immobile eyes

  • ability to roll into balls for protection

  • direct development except in parasitic form

• order Euphausiacea

  • “Krill”

  • 90 species but of immense importance

  • important to baleen whales and many fishes

  • many are bioluminescent

• order Decapoda

  • 18,000 species

  • contains the familiar crabs,lobsters, shrimp, etcs

  • 3 pairs of maxillipeds

  • 5 pairs of walking legs

  • microscopic to largest of the arthropods