- Body divided into segments, or somites, regionally fused into specialized groups by tagmosis (i.e., 5 segments form head)

- Each body segment has a pair of jointed appendages

- Cuticle forms well-developed exoskeleton, made up of plates called sclerites

- Growth by ecdysis (hormone-induced molting)- no cilia on larvae or adults

- Main body cavity = hemocoel, with open circulatory system; coelom reduced to region around gonads

- exoskeleton used as attachment point for muscles

Phylum Arthropoda >1,000,000 species!

Arthropod Phylogeny

5 Sub-phyla:

(1) Trilobites (extinct since Paleozoic)

(2) Chelicerates (spiders, horseshoe crabs, pycnogonids)

(3) Hexapoda (Insects)

(4) Crustacea (crabs, lobsters, shrimp)

(5) Myriapoda (centipedes, millipedes)

Arthropod Success 1: Exoskeleton

Modification of segmented body plan, by growth of a hard external covering = cuticle, or exoskeleton

protection, osmoregulation - a major pre-adaptation for the transition to life on land

Loss of circular muscles, rigid body coelom not needed for hydrostatic skeleton replaced w/ open circulatory system + hemocoel (like in molluscs, but independently evolved)

Growth must proceed through a series of molts, controlled by the hormone ecdysone (where clade Ecdysozoa gets name)

Arthropod Cuticle

hemocoel

Sensorychetae

cuticle

cuticle divided into outer layers for water retention, inner for rigidity

Arthropod Cuticle

Chitin: high mol. weight nitrogenous polysaccharide

hemocoel

Sensorycheta

gland cell pore

procuticle

epicuticle

epicuticle has water-retaining layers of oily lipoproteins, wax

procuticle of chitin, cross-linked proteins and CaCO3

Body WallEpidermis is a single layer of epithelial cells that secrete the cuticle in layers

Outer layer is epicuticle, with water-repellant hydrophobic layers - outermost layer: lipoproteins (fat + protein) - middle layer: waxy layer of fats, wax esters - inner layer: protein

Inner layer is the procuticle, of protein + chitin; hardened by:

(1) sclerotization = cross-linking of proteins into a 3D matrix (2) mineralization = depositing calcium carbonate in procuticle of crustaceans (i.e., crab shell)

Molting

Stages between molts are termed instars

- this is when actual tissue growth occurs, although there’s no size increase until after the molt

Cuticle is weakened enzymatically, then animal crawls out

After molting, animal sucks in air/water to inflate new cuticle, which then hardens

Molting

Arthropod Cross Section

Muscles anchor on inside of cuticle, connect into the jointed appendages on each segment

Heart arteries hemocoel collecting vessels

pericardium ostia heart

Open Circulatory System

Arthropod Success 2: Tagmosis

Success as a group is due largely to diversity of body form, possible because of specialization of segments, regions, + appendages

Tagmosis = segments are grouped together + specialized for particular functions greater efficiency

- head, thorax and abdomen are tagmata, regions specialized for performing different tasks

- arise from spatially restricted expression of Hox genes and other developmental regulatory genes

Segmentation + Tagmosis

Head Thorax Abdomen

Arthropod Success 3: Jointed Appendages

How do you move without cilia? Jointed appendages (limbs) + specialized muscles to move the limb pieces, or podites

- extrinsic muscles connect to body wall- intrinsic muscles are contained entirely inside the limb

Ancestral condition, found in crustaceans, is to have biramous limbs: each limb has 2 branches

Uniramous (1 branch) - insects

Biramous - crustaceans

SUB-PHYLA:

Trilobitomorpha: Trilobites (extinct)

Cheliceriformes: Chelicerates Pycnogonids

Crustacea: Crustaceans

Hexapoda: Insects

Myriapoda: Centipedes, Millipedes

Phylum Arthropoda

Most common group of fossil arthropods- once abundant in oceans- disappeared by Paleozoic (345 million yr ago)

SubPhylum Trilobitomorpha (Extinct)

Cephalon

Thorax

Pygidium

SUB-PHYLA:

Trilobitomorpha: Trilobites (extinct)

Cheliceriformes: Chelicerates (spiders, scorpions, and horseshoe crabs)

Pycnogonids

Crustacea: Crustaceans

Hexapoda: Insects

Myriapoda: Centipedes, Millipedes

Phylum Arthropoda

~ 65,000 spp.Cheliceriformes spiders, scorpions, and horseshoe crabs

- Body of 2 tagmata: cephalothorax + abdomen

- 1st pair of appendages = chelicerae (look like fangs)

- no antennae

- Gas exchange by book gills, book lungs or tracheae

- Separate sexes

Phylum Arthropoda SubPhylum Cheliceriformes

Class Pycnogonida – “Sea Spiders”

Class Chelicerata SubClass Merostomata

Order Eurypterida – extinct giant sea scorpions Order Xiphosura – Horseshoe crabs

SubClass Arachnida- scorpions, spiders, ticks, mites

SubPhylum Cheliceriformes

2 groups are marine

~1,000 spp.

- marine, intertidal to abyssal depths; worldwide distribution

- benthic, live on seaweeds or other invertebrates

- sucking proboscis on 1st head segment used to feed on soft- bodied invertebrates

- males: brood eggs on ovigers, special leg appendages

- females: hollow legs filled with eggs

Pycnogonida: Sea spiders

1st segment Ovigerous leg, used by males for brooding eggs

chelicera

Phylum Arthropoda SubPhylum Cheliceriformes

Class Pycnogonida – “Sea Spiders”

Class Chelicerata SubClass Merostomata

Order Eurypterida – extinct giant sea scorpions Order Xiphosura – Horseshoe crabs

SubClass Arachnida- scorpions, spiders, ticks, mites

SubPhylum Cheliceriformes

2 groups are marine

SubClass MerostomataOrder Xiphosura – Horseshoe crabs

5 living species (all others extinct)

- inhabit shallow marine waters

- burrow just under sand surface, prey on buried animals like bivalves

- small chelicerae

- limited distribution: Limulus polyphemus restricted to east coast of North America

- distinctive telson, or tail spine

Horseshoe crab: Limulus

prosoma

opisthosomabook gills

telson

anus

cheliceraepedipalp:1st walking leg

4 pairs ofwalking legs

- once very abundant on beaches in the Atlantic

- heavily harvested to make feed for farm animals

>60,000 spp.

- opisthosomal (abdominal) appendages absent, or modified as spinnerets for spinning silk proteins into webs

- no compound eyes

- gas exchange by tracheae or book lungs

…Scorpions, spiders, mites, ticks

SubClass Arachnida

SUB-PHYLA:

Trilobitomorpha: Trilobites (extinct)

Cheliceriformes: Chelicerates (spiders, scorpions, and horseshoe crabs)

Pycnogonids

Crustacea: Crustaceans (crab, lobster, shrimp)

Hexapoda: Insects

Myriapoda: Centipedes, Millipedes

Phylum Arthropoda

SubPhylum Crustacea- body of 3 tagmata: 5-segmented head, thorax, abdomen

- cephalic shield or carapace present to protected dorsum

- mandibles, modified limbs, act as jaws; 2 pairs of antennae

- biramous limbs (2 forks)

- “gills” actually legs modified for gas exchange

- excretion by nephridia (glands near antennae)

- simple ocelli + compound eyes (often stalked)

- nauplius larva, which molts and goes through several instars

~75,000 spp.

Crustacean Head

23

54

15-segmented head, each segment with its own appendages

thorax limbs, “borrowed” by the head

Crustacean Head

If 1st thoracic segment fuses with head, its appendages grow as maxillipeds

- next segment is thus 2nd thoracic

5 segments 1st antennae= antennules

2nd3rd

1st

thorax

head

External Anatomy of a Crayfish

Crustacean limbsBranches join at base, the protopod

Inner branch = endopod

Outer branch = exopodprotopod

endopod

exopod

body

Crustacean limbsBranches join at base, the protopod

- extensions on outer side = epipods- often flattened; function as gills, gill cleaners

- extensions towards the body = endites- often form a spiny,

grinding surface

Inner branch = endopod

Outer branch = exopod

endopodexopod

body

epipodendites

Arthropod limbs

Protopod

Epipod

Endopod

Exopod

Endites

Class Malacostraca - crabs, shrimps, lobsters

Class Maxillopoda - ostracods, barnacles, copepods

Class Branchiopoda - brine shrimp, tadpole shrimp

Class Remipedia - small cave-dwellers; basal lineage

Class Cephalocarida - small benthic detritivores; basal

SubPhylum Crustacea

Class Malacostraca

Pereopods, orwalking legs

Pleopods, orswimming legs

Thorax segmentsAbdomen

telson

Body of 19 segments: head, 5 thoraxthorax, 8

abdomen, 6 + telson (tail)

Class Malacostraca – 19 body segments (5-8-6 body)

3 major Super-orders:

Hoplocarida - stomatopods (mantis shrimps)

Eucarida - krill, crabs, shrimps, lobsters

Peracarida - isopods, amphipods, mysids

Class Maxillopoda - ostracods, barnacles, copepodsClass Branchiopoda - brine shrimp, tadpole shrimpClass Remipedia - small cave-dwellers; basal lineageClass Cephalocarida - small benthic detritivores; basal

SubPhylum Crustacea

seen in today’s lab

Vicious predators, snagging prey with raptorial limbs

Class Malacostraca (5-8-6 body) SuperOrder Hoplocarida Order Stomatopoda

= 2nd pereopod

Class Malacostraca (5-8-6 body) SuperOrder Eucarida

Order Euphausiacea (krill)

Krill form feeding swarms, especially at poles - primary food source for many whales

Class Malacostraca (5-8-6 body) SuperOrder Eucarida

Order Euphausiacea (krill)Order Decapoda (14,000 spp.)

InfraOrder: Caridea (shrimps) Astacidea (crayfish) Palinura (lobsters) Brachyura (true crabs)

Anomura (hermits, king crabs) Thalassinidea (ghost shrimp)

3 maxillipeds, leaving 5 pairs of pereopods (hence name) - at least one pair usually modified as chelae, or claws

Large carapace covering head, branchial chamber

decapod =“10 feet”

Order Decapoda, Infraorder: Palinura Brachyura (lobsters) (true crabs)

Crab abdomen reduced, folded under thorax

- 5th walking legs modified for swimming

Abdomen

Carapace

5th walking leg

Order DecapodaInfraorder Brachyura (true crabs)

Order Decapoda Infraorder Anomura (hermit + king crabs)

- 3 to 4 pairs of walking legs

- 5th leg reduced, often gill cleaner

sand crab

hermit

King crab

Both the 1st & 2ndantennae are betweenthe eyes in theanomurans

Order DecapodaInfraorder Caridea (shrimp)

Development in Crabs

In decapods, the nauplius stage occurs inside the egg

Pelagic stages are zoea, then megalops (transparent juvenile)

Crab zoea larva