Arthropoda

• highly successful

  • >million different species

    • 80% of all known extant species

  • occupy every known niche

  • highly abundant (10²¹ copepod crustaceans)

  • variable body forms

Arthropod Characteristics

• cuticle

• hemocoel

• segmentation

• highly developed nervous system

• compound eyes

• jointed legs (arthr + pod)

Cuticle

• made of chitin and protein

• laid down by epidermal cells

• multiple layers

• support and attach muscles

• variable thickness

  • hard over claws

  • soft outer joints

• waxy outer layer

  • waterproofing

  • temperature control

• hardened by calcium salts

  • crustacea

• hardness prevents expansion

• therefore, growth requires molting (“ecdysis”)

• swallow medium to increase internal pressure

• many arthropods go through metamorphosis

Hemocoel

• main body cavity is hemocoel

• open circulatory system important after ecdysis

  • distributes oxygen, food, hormones, phagocytes

Segmentation

• beginning with embryo, cells are subdivided into sharply defined populations of cells

• evolved many times (i.e. convergence)

• advantage?

  • enables efficient locomotion

  • specialization

Highly Developed Nervous System

• three-lobed brain

  • dorsal lobe: optical axons

  • middle lobe: antennae axons

  • ventral lobe: motor nerves

• double ventral nerve cord

  • connects to ganglia in each segment

Compound Eyes

• many (up to 4,000) light-receiving units (ommatidia)

• small external eyes

• cylindrical retinula cells

• light is focused on rhabdome

• pigments are depolarized

• stimulates neurons to signal brain

• sensitive to movement and UV

Jointed Legs

• how do joints help improve the functionality of an animal with a hardened exoskeleton?

  • flexibility

  • agility

Subphylum Trilobitomorpha

• extinct group of marine organisms that were well-preserved in the fossil record due to hard exoskeleton

• appeared in the early Cambrian and flourished for 270 million years before disappearing at the end of the Permian (~250 mya)

Arthropod Classification (Extant)

• subphylum Chelicerata (horseshoe crabs, sea spiders, arachnids)

  • 2 body regions

  • no antennae

  • chelicerae

    • grasping oral appendages

• subphylum Myriapoda (centipedes and millipedes)

• subphylum Crustacea (branchiopoda, copepoda, malacostraca, theostraca)

  • biramous appendages

  • 2 pairs of antennae

• subphylum Hexapoda (insects)

  • uniramous appendages

  • 3 body regions

  • 1 pair of antennae

  • 2 pairs of mouthparts

Class Merostomata: Horseshoe Crabs

• external reproduction

• exhibit “conditional mating strategies”

• dominant “attached males” fertilized 10% more eggs than others

• Some males don’t even try to attach, but try to sneak sperm in

• why?

  • “satellite males” fertilized more than they would otherwise

• eggs provide critical food source for red knots

Class Pycnogonida: Sea Spiders

• 1,300 species

• marine

• long-legged

• small body

• organs extend into legs

• each muscle = 1 cell

Class Arachnida (“Arachnids”)

• insectivorous predators and parasites

• often use poison to paralyze prey

• digestion is often external

Arachnids: Spiders

• males attract mates with gifts and dancing

Spider Webs

• silk is extremely strong

• made up of glycine and alanine

• glands are at the posterior end of the abdomen

• legs secrete oily substance to prevent sticking

• young spiders can build perfect webs

Arachnids: “Ballooning”

Arachnids: Ticks

• complex, ectoparasitic (external) life cycles

• common vector of disease (i.e. lyme disease)

Arachnids: Mites

• example: house dust mites

• feed on human skin flakes

Scorpions

• 1,700 species

• only 25 have venom capable of killing humans

Subphylum Myriapoda

Class Chilopoda (centipedes)

• 3,000 species

• dorsoventrally flattened

• laterally projecting legs

• one pair of legs per abdominal segment

• predatory carnivores

Class Diplopoda (millipedes)

• 10,000 species

• rounded bodies

• ventral legs

• two pairs of legs per segment

• feed on plants and detritus

Subphylum Crustacea

• predominately marine today

• >50,000 species known

• abundant

• 2 pairs on antennae

• many have hardened cuticle

• crustacea classification

  • Branchiopoda (water fleas, fairy shrimp, brine shrimp)

  • Maxillopzzoda (copepods, barnacles, fish lice)

  • Ostracoda (seed shrimp)

  • Malacostraca (decapods, isopods, amphipods)

• most crustaceans have nauplius

• either within egg or as early as larval stage

• very different from adults

• advantage?

  • minimized competition

Class Branchiopoda

• filter feeding shrimp

• 2-3 cm in length

• important food base

Class Maxillopoda

Copepods

• extremely common

• planktonic shrimp

Parasitic Copepods

• ectoparasites

  • sea lice

  • attach to gills of fish

  • modified appendages

Barnacles

• only sessile group of crustaceans

• fastened by head

• kick food into mouth with legs

Class Ostracoda (Seed Shrimp)

• 70,000 species (13,000 extant)

• typically around 1mm in size

• flattened and protected by bivalve-like shell

Class Malacostraca

• 23,000 species

• gills

• dorsal carapace (plate covering cephalothorax)

• hardened chitinous exoskeleton

Decapods

• shrimp, crabs, crayfish, lobsters

• 10 (“deca-”) thoracic walking appendages

  • 1st pair enlarged

    • chelipeds

    • largest crustaceans

• primitively small and marine

• large number of similar appendages

• body enlargement and appendage modification has evolved many times

Isopods

• “same legs”

• dorsoventrally

• pillbugs and woodlice

Amphipods

• laterally compressed

• important food-base

• freshwater

• shallow marine areas

Krill

• important food base in southern oceans