Phylum Arthropoda: Characteristics

Phylum Arthropoda

• Arthropoda means "jointed foot" in Greek
• Segmented, jointed, and hardened chitinous exoskeletons are produced by the epidermis
• Individual appendage joints have intrinsic musculature
• Motile cilia are absent in both adults and larval stages

Introduction and General Characteristics

• Phylum Arthropoda contains approximately 85% of known animal species, the arthropod body plan is by far the best represented
• Arthropods are abundant in the fossil record
• Insects, spiders, scorpions, pseudoscorpions, centipedes, crabs, lobsters, brine shrimp, copepods, and barnacles are arthropods
• Like annelids, arthropods are metameric, adding new segments from a budding zone at the rear during development
• In modern arthropods, metamerism is masked by segment fusion and body region modification for specialized functions
• Tagmatization, the specialization of segment groups, is prominent in arthropods
• Insecta and Crustacea, two major arthropod groups, possess a head, thorax, and abdomen as distinct tagmata
• Cilia are lacking, even in the larval stages

The Exoskeleton

• The exoskeleton is secreted by epidermal cells
• The outermost layer or epicuticle is generally waxy
• The epicuticle contains lipoprotein and lipid layers
• Water cannot pass through the cuticle, making it impossible for gas exchange
• Arthropods are resistant to dehydration because of water-proof cuticle
• Epicuticle is around 3% of the total exoskeletons thickness
• Procuticle comprises bulk of the exoskeleton, consisting primarily of polysaccharide chitin with various proteins
• Chitin is strong, nonallergenic, and biodegradable, hence commercial interest in its use is ongoing
• Chitin can be processed into fibers to make fabrics and surgical sutures
• Chitin can be produced to make biodegradable capsules for drug delivery
• Clear chitin film is an alternative substitute for plastic wrap
• Chitin and its derivatives readily bind to organic and inorganic compounds, are indigestible, and could reduce caloric and cholesterol uptake
• A good candidate for removal of toxic compounds from water is made up of chitin
• Calcium carbonate deposits in some procuticle layers of crustaceans contribute to hardening for it
• Tanning hardens the procuticle’s protein component
• Sclerotization, or tanning, creates cross-linkages between protein chains
• Hardening of exoskeleton done entirely via tanning among insects
• In bivalves, hinges and byssal threads form through sclerotization
• Procuticle is of varied thickness and not uniformly hardened, forming joints
• Jointed skeletons work much like vertebrate skeletons because of appropriate musculature
• Resilin, or "animal rubber", found in some joints, such as wings, stores and efficiently releases energy upon compression
• Development of a jointed, flexible exoskeleton is the essence of arthropod success because this grants lifestyle options such as flight

Hemocoel

• The coelom is too small to aid locomotion in heavily armored animals
• Hemocoel is the main body cavity, being part of the blood circulatory system
• Arachnids extend legs by increasing hemocoel blood pressure, but fluids/cavities do not typically impact arthropod movement

Molting

• Unlike mollusc shells, arthropod protective coverings grow throughout the body simultaneously
• The outer covering cannot be gradually added at a growing edge because the arthropod's outer covering is secreted over all regions of the body simultaneously
• Arthropods are literally encased, but for sensory hair/gland openings
• Major sections of the foregut and hindgut have a cuticle lining
• Molting process: arthropod sheds cuticle (including the gut lining), grows larger, and hardens a new cuticle
• The split from enzymatic secretions degrades and allows for its removal
• Potential body collapse is a greater issue in air during molting
• Soft-bodied crabs depend on increased hemocoel blood pressure until outer layers harden
• Terrestrial arthropods are smaller than aquatic ones, perhaps due to air being less supportive
• Potential vulnerability to predators increases between ecdysis and cuticle hardening, and arthropods generally seek protective shelter
• Tissue growth is continuous in arthropods as size increases although these increases in size are discontinuous
• The number of epidermal cells, for instance, increases continuously
• Epidermal tissue is folded or pleated until ecdysis
• Ecdysis/new exoskeleton formation are under neural and hormonal control
• Ecdysteroid hormones are produced from Y-organs (crustaceans) or prothoracic glands (insects) and stimulates molting
• Brain production of separate hormone triggers ecdysteroid creation among insects
• X-organ in crustacean eyestalks makes hormone inhibiting ecdysteroid production betwen molts
• Ecdysis is impossible until the X-organ does not produce this inhibitory hormone
• Surgically removing eyestalks results in sped up ecdysis
• Important arthropod roles also under neurohormonal control, like reproduction, fluid pressure, eye pigments, and chromatophore granules

Nerves and Muscles

• Unique arthropod nervous system: muscle fiber contraction strength relies on delivered nerve impulse rate to the fibers
• Vertebrates differ with muscle strength depending on fibers contracting/number of axons fired
• A single arthropod muscle fiber may receive signals from up to five neuron types
• Rapid vs. sustained contraction relies on the stimulation source
• Some neurons are inhibiting; their delivered potentials alter/change output
• Multiple functional/physiological muscle fiber types occur in arthropods
• Single muscle fiber type impacts contraction rate
• Control over movement depends on stimulated muscle fibers and neuron interaction on a single fiber
• A single arthropod neuron can stimulate many fibers
• The converse may be true: only a few neurons stimulate entire muscle
• Unique muscle physiology: entirely striated
• Other invertebrate groups possess mostly smooth muscle
• Arthropods could never be capable of flight without striated muscle

The Circulatory System

• Though closed blood vessels leave arthropod hearts, blood enters through perforations (ostia) in heart wall directly from hemocoel
• Arthropod circulatory systems are, therefore, open
• Oxygenated blood moves via sinuses for gas exchange
• Diagnostic feature: having heart with ostia

Arthropod Visual Systems

• Arthropods possess either ocelli or compound eyes
• Ocellus: small cup with light-sensitive surface backed by light-absorbing pigment
• Simple photoreceptors
• Photosensitive pigment is a protein with a vitamin A derivative, chemical change generates action potentials after light stimulations
• Do not form images usually
• Compound Eyes can form images
• Common among insects and crustaceans, and sometimes found with ocelli
• Large compound eyes were discovered on Anomalocaris fossils, a potential arthropod ancestor
• Compound eyes are evolved independently and operate similarly to those of insects and crustaceans in some polychaete annelids and bivalve molluscs
• For eyes to form images, light has to be focused, animals also need sophisticated nervous systems
• Human Eye vs Compound Eye
  • Human
    • Light enters via the lens and is focused back to the retina
    • Components are extracted by millions of cells
    • Impulses interpreted by brain
  • Compound
    • Multiple lenses with focus that cannot be changed, fewer cones
    • Image is extracted from dozens of ommatidium to produce "image"
• Compound Eyes- Made up of repeating units
  • Ommatidia
    • convex shape of eye broadens FoV
    • Simplest and most common kind of compound eye -Insects such as Bees, ants cockroaches such as true crabs (Infraorder Brachyura), myodocarpid ostracods, and isopods
  • Molecular evidence suggests that myodocarpid ostracods had an independent evolutionary origin of compound eyes
  • Retina image is broken up, so each ommatidium draws from only parts the complete image

Vision at the Ommatidium

• Composed of
  • The lens of the fixed focus eye has enough depth of field to capture most objects
  • Gelatinous crystalline cone helps with light refraction
  • Retinular cells are bodies containing photosensitive pigments
  • Cylindrical cells
    • Contain shielding pigments
    • Insulate each unit from surrounding elements
  • Neural Cartridge
    • Base of the ommatidium
    • Bundled nerves that collect data and send it to optic lobe
• Insect and crustacean eyes use polarized light for flight and mate recognition
• Can see ultraviolet patterns invisible to humans
• Some insects can see infrared at dawn and dusk
• The retinular cells are made up of rhabdomeres to hold light receptive pigments
• Rhabdomeres inside each unit
  • From microvillar outfoldings of retinular cell walls
  • Make up each ommatidium, the rhabdom
• "Closed" or "Fused" Rhabdoms
  • No space between light receptive units
  • All cells function as a single unit
  • In other words not a single structure but an entity built by microvilli working together
  • Tip captures intensity at a center, it then sends single down rest of unit to get to the neural cartridge

Open Rhabdoms (less common)

• Each receptive unit serves as a separate light channel, increasing photo capture for motion
• Seem in house flies and fruit flies
• Structural proteins can change closed structure to open

Superposition Eye

• Many elements combine image to single receptive source
• Common among night hunters, or those in light diluted areas
• To better function the pigment changes
  • Bright
    • Pigments get denser blocking light from adjacent ommatidium
    • Functions like a single element
  • Dark
    • Light comes in and increases signal of original
• Image clarity dependent on amount of light and receptor alignment

Arthropod Reproduction

• Sexual fertilization occurs in most species
• Fertilization is external or even indirect
• Gonochoristic- separate sexes
• Hermaphroditic- both sexes
• Parthenogenesis - development of unfertilized eggs occurs in certain insecta
• Many use pheromone production to communicate presence

Arthropoda Classifications

• Arthropoda has 8 classes and over 2,400 individual families
• Arachnids are over 550 of those families
• They base taxonomic distinctions on -Appendage form -Appendage Function -Origin of limbs -Distribution -Number
• 3 major classes are -Arachnida -Insecta -Crustacea
• Phylogenic connections among this group are murky, this lead to defining characteristics being excluded from text.

Subphylum Trilobitomorpha (= Trilobita)

• Defining feature: Has two Anterior or Posterior furrows which divide the body into 3 regions
• 4000 species no longer living now known from fossil records
  • Extinct 225mya
  • Common 500mya at time
• Located compound eyes and mouth on posterior part of organism
  • Mouth had labrum (chitinous lip)
  • Each posterior section held appendages biramous
• All appendages uniform, inner had no setea and was used for movement, outer had long filaments for swimming to dig

Subphylum Chelicerata

• Defining features include
  • Lack of antenna
  • Body of Prosoma and Opisthosoma
  • Appendages come in three types the Chelicerae
• First segment doesn't bear appendages, the second has clawed appendages for mouth grabbing