Dawn of the Arthropods: An Introduction to 85%.

Dawn of the Arthropods:An Introduction to 85%

The Arthropods

• First arthropods Early Cambrian 600 mya– All environments, diverse ecologies – Tiny mites + crustacea < 1 mm --- Japanese

spider crabs 3 m long – 85% of described animal taxa– 3 - 100 million species

Tardigrades and Onychophorans

Similar to Arthropods• chitinous cuticles

Phylum Tardigrada

• ~900 species• 50 – 1200 µm•Surface films of freshwater on terrestrial plants, mosses and lichens•abundant (3 million m-2)

Phylum Tardigrada

Defining characteristic: mouthparts: protrusible, oral stylet for piercing plant and animal tissues to obtain fluids

Phylum Tardigrada

• chitinous cuticle lining outer surface and gut• arthropod-like striated muscles• hemocoel• gas exchange across body surface

• NO RESPIRATORY STRUCTURES• lack cilia• 4 pairs of clawed appendages (non-jointed)

Phylum Tardigrada• nervous system similar to arthropods

• paired ventral nerve cord

• offspring develop as miniature adults • male deposits sperm into female seminal receptacle• cuticle never calcified• cryptobiosis – dehydrate with environmental extremes• gonochoristic•ANABIOSIS: reduced metabolism •CRYPTOBIOSIS: extreme anabiosis

video

Phylum Tardigrada

Phylum Onychophora“velvet worms”

Defining Characteristics:•2nd pair of appendages modified into jaws•3rd pair of appendages form oral papillae

Phylum Onychophora“velvet worms”

Defining Characteristics:•slime glands discharge adhesive through oral papillae•hydrostatic skeleton

Phylum Onychophora

• ~ 100 species • moist terrestrial habitat • thin, non-waxy cuticle• nocturnal

Phylum Onychophora

• carnivores, herbivores, omnivores• predators shoot proteinaceaous glue from oral papillae• some use glue as defense

video

Phylum Onychophora

• circular, longitudinal, and diagonal muscles• 1 pair feeding appendages• no jointed appendages• hydrostatic skeleton• pair of nephridia / segments

• Ocelli; no compound eyes, photonegative• Sperm morphology similar to oligochaetes and leeches•Mostly dioecious •Oviparous, viviparous, or ovoviviparous•Direct development

Phylum Onychophora

Phylum Arthropoda

• 6 classes• 80 orders

• 2400 families

• Loss of motile cilia

• Segmented, jointed, sclerotized chitinous exoskeleton

• Flexible joints, hemocoel, Sense organs, ecdysis, specialized segments (head, thorax, abdomen)

Phylum Arthropoda: Pechenik Classification

Subphyla:1)Trilobitomorpha (extinct)

1) Class Trilobita2)Chelicerata

1) Class Merostomata2) Class Arachnida3) Class Pycnogonida4) Class Eurypterida (extinct)

3)Mandibulata1) Class Myriapoda

1) Order Chilopoda/Diplopoda2) Class Hexapoda

1) Subclass Entognatha2) Subclass Insecta

3) Class Crustacea

Arthropoda Exoskeleton• water impermeable• procuticle – mostly chitin•EXOSKELETON LIMITS CONSTANT GROWTH

Arthropoda: Support and Locomotion• exoskeleton• Muscles connect body

segments + joints

• Exoskeleton across joints NOT absent, but thin and flexible;

Phylum Arthropoda

• metamerism• most taxa have fused and modified specialized segments

Molting

• exoskeleton covering except for sensory hairs, gland openings, spiracles• parts of gut also lined with cuticle• ecdysis – loss of exoskeleton

Molting

• new cuticle secreted first, but then hardened• period of vulnerability• under neural and hormonal control

Arthropoda Nervous System and Sense Organs

• Brain • Tactile receptors = bristles or setae

Arthropoda Nervous System and Sense Organs

• Brain • Tactile receptors = bristles or setae• Proprioception necessary with jointed appendages• Stretch receptors span joints and provide knowledge of limb

positions

Arthropoda Nervous System and Sense Organs

• Brain • Tactile receptors = bristles or setae• Proprioception necessary with jointed appendages• Stretch receptors span joints and provide knowledge of limb

positions• Three photoreceptors: simple ocelli, complex lensed ocelli,

compound eyes• Compound eyes: ommitidia• Ommitidium: nerve tracts and field of vision• Visual fields of neighboring ommitidia overlap

– movements detected by each– compound eyes suitable for detection of subtle movement

Arthropoda Nervous System and Sense Organs

• Brain • Tactile receptors = bristles or setae• Proprioception necessary with jointed appendages• Stretch receptors span joints and provide knowledge of limb

positions• Three photoreceptors: simple ocelli, complex lensed ocelli,

compound eyes• Compound eyes: ommitidia• Ommitidium: nerve tracts and field of vision• Visual fields of neighboring ommitidia overlap

– movements detected by each– compound eyes suitable for detection of subtle movement

Visual System

• ocelli or compound eyes• light sensitive surface with light-absorbing pigment

Circulation and Gas Exchange

• Gas exchange structures = invaginations in cuticle

• Inwardly branching trachea: hemocoel via spiracles

• gas exchange with hemolymph

Circulatory System

• heart with ostia = open circulatory system• hemolymph

Arthropoda Digestive System

• Foregut- ingestion, transport, storage, mechanical digestion

• Midgut- enzyme production, chemical digestion, absorption

• Hindgut- water absorption, preparation of fecal material

Arthropoda Excretion and Osmoregulation

• Nephridia and malpighian tubules• nephridial pores excrete concentrated nitrogenous waste

products• Urine from malpighian tubules to gut, concentrates urine

(reabsorb water)• Terrestrial arachnids, myriapods, and insects excrete

predominantly uric acid

Arthropoda Repro and Development

• Dioecious • Fertilization internal, parental care common

Class Trilobita

• 4000 described species• extinct• dorsoventrally flattened• 3 sections• biramous appendages

Crustacea

• Terrestrial and aquatic

• All depths in marine, brackish, and freshwater

• > 67,000 described species, likely 5-10x that number• Diverse form, size, and habitat• 5 classes, 34 orders

Crustacea

Subclass Malacostraca - Order Isopoda (pillbugs) - Order Amphipoda (scuds) - Order Euphausiacea (krill) - Order Stomatopoda - Order Decapoda (crabs, shrimp, lobsters, hermit crabs, crayfish)

Subclass Branchiopoda – brine (monkeys)Subclass Ostracoda - ostracodsSubclass Copepoda – copepodsSubclass Pentastomida – nasal parasites Subclass Cirripedia (maxilopoda) – barnacles

Characteristics

• Head = 5 segments, trunk divided into thorax and abdomen

• Carapace or cephalic shield• Appendages multi-articulate; either uniramous

or biramous• Mandibles are modified limbs that function as

jaws• Gas exchange by diffusion across specialized

surfaces

Characteristics

• Nephridia• Simple and compound eyes in at least one

stage– Compound eyes on stalk

• Gut with digestive cecae• Nauplius larvae, either mixed or direct

development• 2 pair antennae

Crustacean Bauplan

• Thorax– anterior segments fused = cephalon

• Maxillipeds: additional mouthparts• Number of segments in thorax varies • Thorax appendages = pereopods

– swimming, walking, gas exchange, feeding, defense– ultiarticulate and biramous

Basic Crustacean Bauplan

• Abdomen– Segments – Number of segments used in ID

• Appendages = pleopods– Biramous, flap-like– swimming

– Culminate in telson• Anus• caudal rami• w/uropods (last pair of abdominal appendages) forms tail

fan

Adult Crustacean

Crustacean Bauplan

• Nauplius Larvae– Single, median, simple eye– 3 pairs of sectioned, functional limbs

• Become antennules, antennae, and mandibles

Circulation

• Open circulatory system– Dorsal ostiate heart– Internal organs bathed in fluid– Simple heart and vessels in most

• Sessile species no heart; pumping vessels

• Blood– Variety of cell types

• Dissolved hemoglobin or hemocyanin• Explosive cells release a clotting agent at injury sites

Heart Shape

• Heart long and tubular; to postcephalic region• Or, globular, box shape, in thorax; association

with thoracic gills

Gas Exchange

• Aquatic– Small organisms = diffusion– Concealed gills for protection, prevent dessication– External gills

• Modified thoracic limbs• Gills are thin; maximize gas exchange• Most species beat gills to maintain flow

Gas Exchange

• Terrestrial– Cutaneous Respiration

• Membranes on legs of some species

– Gills• Concealed

– Pseudotrachea• Internal blind sacs to outside through small pores• Air in sacs, gas exchange with blood • Internal gills moist

Feeding Strategies

•Water currents•Hydraulic vacuum•Filter feeding•Feeding basket•Passive•Twirling antennae•Direct manipulation•Sand grazers or Sand lickers•Predators•Parasitism

Water Currents• Thoracic limbs for swimming and creating

suspension feeding currents• Water drawn into space • Particles trapped by setae• moved to food groove and toward head

Hydraulic vacuum

• Mouth appendages = paddles

• Water containing food drawn into interlimb space

• Food particles are not filtered, but captured in small parcels of water

• Individual algal cells are captured this way

Filter feeding

• Sessile crustaceans have feathery cirri to filter feed

• food up to one mm• = detritus, bacteria, algae and various

zooplankton• Some can coil cirrus around large prey in a

tentacle fashion

Filter feeding in slow water

• Extend pairs of cirri like a fan• Sweep rhythmically through water

Filter feeding in fast water

• Allow water to run through filter• video

Passive feeding

• Use cirri to passively strain• Burrow into sand with anterior facing upward• Extend cirri to capture bacteria, protists and

phytoplankton• Antennae brush food towards mouth

Twirl antennae

• Create spiraling currents that bring food toward mouth

• Food entangled in setae near base of mouth, brushed in

Direct manipulation

• Manipulation by mouthparts, pereopods and subchelate anterior legs

Sand grazers or Sand lickers

• Brush sand grains with setose mouthparts

• Select individual sand grain, rotate and tumble against mouthparts to remove organic material

Predator

• Grab prey with chelae pereopods• Tear, grind and shear with mouthparts

• Hunters or ambushers use raptorial subchelae to stab, club or smash prey

• Some hold prey in cage using endopods; others inject and suck out tissues

Snapping Shrimp

• Use large cheliped to snap close: produces loud popping sound and “shock” wave

• Pressure wave stuns prey, pull into burrow

• Foregut– Lined with cuticle that is continuous with exoskeleton, molted– Short pharynx-esophagus, stomach– Stomach = chambers for storage, grinding and sorting

Digestive system

HindgutHindgut Short, to anusShort, to anus

Midgut intestine Length varies with

shape size, diet

digestive ceca

Excretion and Osmoregulation

• Ammonia by nephridia and gills• nephridial excretory organs as antennal glands

(green glands) or maxillary glands

• Inner blind end is coelomic remnant of nephridium = sacculus

• Sacculus - Actively remove and secrete material from blood into excretory lumen

• metabolic waste removal and water and ion balance

Other osmoregulation

• Thin areas of cuticle– Gill surfaces

• terrestrial isopods: ammonia diffuses from the body as gas

Nervous System and Sense Organs

• CNS• Brain: three fused ganglia• Protocerebrum• Deutocerebrum• Tritocerebrum• Primitive nervous system = ladderlike

Nervous System and Sense Organs

• variety of sensory receptors• innervated setae or sensilla: contain

mechanoreceptors and chemoreceptors

• Propioceptors

• Animals in Class Malacostraca: statocysts

Nervous System and Sense Organs

• Two rhabdomeric photoreceptors:• Median simple eyes• Lateral compound eyes• Most possess both, either simultaneously or

during development

• Naupliar eye = primitive, secondarily lost

Nervous System and Sense Organs

• Lateral compound eyes• Lack visual acuity• Discern shapes, patterns and movement• Color vision in some• Lacking in many taxa

Nervous System and Sense Organs

• Underwater vision• Problems with angular distribution of light, lower intensity,

and narrow range of wavelengths than in air

• Solution: Mount eyes on stalks, increase information available to eyes. Increases field of view, and binocular range

Nervous System and Sense Organs

• Complex Endocrine and Neurosecretory Systems• Not well known

• Molting, chromatophore activity, and reproduction under hormonal and neurosecretory control

• Bioluminescence in several groups

Reproduction And Development

• Exploit virtually every life history scheme imaginable

• Usually dioecious• Hermaphroditism in remipedes,

cephalocarids, cirripedes, few decapods• Parthenogenesis common among

branchiopods and certain ostracods

Reproduction And Development

• Gonads paired structures in trunk• Pair of gonoducts from gonads to genital

pores on trunk segment• Male pair of penes, or single fused median

penis• Female include seminal receptacles• Most Crustacea Copulate in Pairs