Subphylum Crustacea - the crustaceans

(crabs, shrimp, etc)

Defining features of the crustaceans:

  • Two pairs of antennae
  • In contrast, Arachnids have no antennae, and insects have one pair.
  • First pair of mouthparts are mandibles for biting and chewing
  • In contrast, Arachnids have chelicerae
  • Most species have 2 additional pairs of mouthparts called maxillae
  • Underside of a crab, showing mandibles:

    Crustacean lifestyles and habitats:

  • Most are aquatic, including freshwater and marine habitats
  • Along with the annelids (which are especially important in the sediments), crustaceans are often among the most abundant and influential invertebrates in marine environments.
  • Some are terrestrial
  • Some terrestrial species live entire lives on land (e.g. terrestrial isopods - the pillbugs and sowbugs)
  • Many terrestrial species must return to water to breed, and have aquatic larvae (e.g. most terrestrial crabs)
  • The crustaceans include species that are:
  • predators (some are FIERCE predators!)
  • herbivores
  • scavengers
  • filter feeders
  • parasites
  • In other words, this phylum does it all!

  • Unlike many of the other animal groups, marine crustaceans DO NOT release eggs and sperm into water.
  • crustaceans have internal fertilization
  • mating occurs just after female molts, while exoskeleton still soft
  • crustaceans normally carry egg until they hatch, and release larvae into water.
  • most marine crustaceans have planktonic larval stages
  • some exceptions:
  • amphipods and isopods carry young until they are well-developed miniature versions of adults.

  • A bit of crustacean anatomy

    Diagram of major organ systems:

  • Circulatory (red)
  • Digestive (orange)
  • Ventral nerve cord (bright green)
  • Excretory (blue-green)
  • Reproductive (dark pink)


    Circulatory and respiratory systems:


    Heart (including small holes called ostia through which "blood" enters heart):



    Cross-section of body (e.g. of a crayfish) showing circulatory system (red), digestive system (orange), and gills (lavendar-purple):


    Note: water is moved through gill chamber by modified appendages on each side of head called gill bailers. Water is drawn through openings near lower rear of animal (just in front of tail) on each side of the body, and it exits gill chamber through openings on each side of the lower portion of the head, on either side of the mouth.


    Dissected crab (that had been preserved in formaldehyde, with pink latex injected into circulatory system), showing heart (pink thing in center), gills (sort of pink things on left and right), gastric mill (the object below the heart in this photo). Note: the head region of crab is facing bottom of picture.


    A sampling of some of the major crustacean groups:


    Class Branchiopoda - water fleas, brine shrimp, fairy shrimp, etc.

  • Cladocerans -"Water fleas" (e.g. Daphnia)
  • very common filter feeders
  • most species freshwater
  • "hop" through water propelled by antennae
  • most species parthenogenic (and all female) when conditions are good, males only present when conditions deteriorating.
  • sexual reproduction results in "resting eggs" resistant to drying and freezing.
  • Nice photos of Cladocerans can be found at:

    Cladocera web site: http:default.html


  • Brine Shrimp (popularly known as "sea monkeys") - very cool animals!
  • don't live in the sea
  • don't look like most of the photos found here ->
  • live in hypersaline environments (e.g. Great Salt Lake, Utah)
  • major form of animal life (just about ONLY animal) in Great Salt Lake.
  • "resting eggs" harvested in vast quantities
  • hatch in about 24 hours in saltwater, providing live larvae invaluable in aquaculture for feeding baby fish
  • brine shrimp egg harvesting and sales is a big business (e.g. see )
  • adult shrimp harvested and sold frozen as fish food
  • Here is a live adult female (these are lovely creatures when alive!):

  • Class Ostracoda - the ostracods, or seed shrimp

  • major group in freshwater and marine habitats
  • look like bivalve with shrimp body inside
  • antennae stick out at front, legs reach out as animal scurries about
  • If you are really inspired about ostracods, here is an Australian site with more info: Ostracods: http:ostbiol.html


    Class Copepoda - the copepods

  • This is one of the most diverse classes of crustaceans- over 7500 species!
  • Aquatic, both freshwater and marine
  • Extremely abundant
  • Many are planktonic and play major roles in marine ecosystems
  • Many are sort of teardrop shaped with large antennae that propell them in a jerky start and stop sort of motion (this sort of movement is very typical of many copepods)
  • predators, filter-feeders (often feeding on planktonic algae), etc.
  • Copepod showing water flow patterns that aid in food capture:


    Copepod with paired egg sacs:


    Photos of some freshwater copepods can be found here: Freshwater copepod photos: http:zoop-copep.htm


  • Some copepods are parasitic on fish. These look like crustaceans when larvae, but are worm-like as adults!
  • See: Lernea sp. (anchor worms)

    Class Cirripedia - the barnacles

  • bizarre, sessile, filterfeeding crustaceans
  • "A shrimp that stands on its head and kicks food into its mouth with its feet"

  • all species are marine
  • very common animals, attached to all sorts of things, including rocks, wood, clams, crabs, sea turtles, whales.....
  • have motile larvae that look like typical crustacean larvae
  • Diagrams of adult goose barnacles (most other barnacles don;t have this sort of muscular stalk):
  • Additional barnacle photos: Australian Barnacle gallery

  • Some barnacles are parasitic....and really bizzare parasites at that!
  • e.g., Sacculina, a parasite of crabs:
  • see Diagram of Sacculina
  • also see: Malin Werner-Sacculina researcher (includes photos of Sacculina larvae)
  • See Barnacles: Aquarium Frontiers Library if you would like more information on barnacles, including the really strange parasitic ones (optional reading).

  • Class Malacostraca - crabs, lobsters, shrimp, and others

  • almost 3/4 of all known crustaceans
  • includes the largest crustaceans
  • Here are a few of the major orders:

    Order Decapoda - the decapods

  • Decapod means "ten legged" (though often first pair are claws, not walking legs)
  • at least 8500 described species (1/3 of the known crustaceans)
  • marine, freshwater, land
  • very diverse lifestyles and diets (just about everything you can imagine)
  • this is the most familiar group of crustaceans, including most of the larger crustaceans, including:
  • Shrimp - adapted for swimming: Nice shrimp photos
  • Crabs - adapted for walking: Nice crab photos
  • tail is reduced and curled under body
  • Lobsters and crayfish - intermediate: a few photos
  • others
  • Order Amphipoda - the Amphiods

  • more than 5500 species
  • mostly small in size
  • marine, freshwater, a few on land (on beaches)
  • herbivores and scavengers mostly, some predators and parasites
  • COMMON animals
  • typically laterally compressed
  • A typical amphipod:

  • Freshwater amphipods like the one pictured above are quite small (average length usually about 1/2 inch or smaller, depending on species) but can be exceedingly abundant (dozens per square meter....or even hundreds per square meter sometimes!), and are important food for many animals (fish, birds, etc).
  • In Minnesota, amphipods living in marshes and other wetlands are particularly important for migrating Lesser and Greater Scaups - ducks with blue bills that are commonly refered to as "bluebills"
  • Bluebill populations are declining, and shrinking amphipod populations seem to be the cause.
  • An article on bluebills and their dependence on amphipods can be found here --->
  • A slide show showing these birds can be found here:

    Order Isopoda - the Isopods

  • more than 4000 species
  • mostly marine, but some very commons ones in freshwater and on land
  • the terrestrial isopods (pillbugs and sowbugs) are the only fully terrestrial crustaceans in most areas.
  • For more info see: Terrestrial isopods: http:isoinfo.htm
  • mostly scavengers, omnivores, but some predators and parasites
  • dorsoventrally flattened (a major thing that distinguishes them from the amphipods)
  • Diagrams of typical isopods (top and bottom):
  • Photos and info re: nasty fish predators/parasites that are isopods:
  • Parasitic isopod on fish: http:page.cgi?list=Damsels&filename=A7056.jpg
  • Article on isopods by Ron Shimek....parasitic/carnivorous isopods are discussed at end of article:
  • Also see photos of giant deep sea isopods:

    Are there other orders in this class?

  • YES...lots of others
  • too many to go into here!

  • A few final crustacean tidbits

    (sorry, no shrimp sauce to go with these tasty morsels of knowledge!)

    A few amazing crustaceans:

  • Japansese spider crab:
  • See also:

  • Mantis shrimp (stomotopods)- truly one of the pinnacles of arthropod evolution!
  • What are stomatopods?
  • What's Special about Stomatopods?
  • Stomatopods in Multimedia - some really cool film clips


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