Class Crustacea

"Crusted Forms" - hard flexible crust

Over 38,000 species

Nearly all are aquatic - "Insects of the Sea"

Microscopic to Largest Living Arthropods

• American Lobster - 60 cm, 22 kg
• Spider Crab - 3.6 m clawspan

Characteristics of Crustacea

1. External

a. Appendages

• 2 prs of antennae: 1st homologous to antennae of insects & myriapods, 2nd homologous to chelicerae
• 3rd pair of head appendages are mandibles
• Biramous - terminal segment with endopod and exopod

b. Tagmata

• Head and trunk (often divided into thorax and abdomen)
• A carapace may cover dorsal and lateral surfaces

c. Exoskeleton

• chitin may be calcified in larger species

2. Feeding and Digestion

a. Nearly all are suspension filter-feeders

b. Larger species predator/scavengers

c. Appendages specialized for cutting, tearing, collecting

d. Foregut enlarged into cardiac and pyloric stomach

e. Midgut bears two digestive glands (hepatopancreas)

• comprised of highly branched, blind canals
• secrete enzymes
• absorb nutrients
• store glycogen and fat

3. Gas Exchange, Circulation, and Excretion

a. Respiration

• All Crustaceans have gills
• May be within thoracic cavity or on appendages

b. Circulatory System

• Heart --> dorsal vessel --> sinuses --> gills -->pericardial sinus --> ostia in heart
• Respiratory proteins (hemocyanin) in blood of larger species

c. Excretion

• Excretory organs near antennae (antennal glands) or maxillae (maxillary glands)

4. Nervous and Sensory Structures

a. CNS is typical for arthropods:

• dorsal brain, paired ventral nerve cord, ganglia

b. Eyes of two types

• median simple eye (naupliar - from nauplius larvae)
• compound eyes - ommatidia (25 to 14,000)

c. Prioprioceptors and statocysts

• in soft tissues between segments
• single pair at base of 1st antennae

d. Sensory hairs for mechano- and chemoreception

Class Crustacea

• Subclass Malacostraca
  • Order Decapoda
  • Order Amphipoda
  • Order Isopoda
• Subclass Branchiopoda
• Subclass Ostracoda
• Subclass Copepoda
• Subclass Cirripedia

Subclass Malacostraca

• "Soft Shell"
• Over half (>22,000 spp.) of Crustacea
• Typically 14 segments: 8 thorax, 6 abdomen
• First 1-3 thoracic are forward - maxillipeds
• Abdominal appendages called pleopods
  • 1st pair often used in mating
  • 6th pair is turned backward for swimming (uropods)

Subclass Malacostraca: Order Decapoda ("Ten Legs")

• 10,000 species - crabs, shrimp, lobster, crayfish
• Commercially and ecologically important
• Mostly marine, crayfish common freshwater, some intertidal crabs are "semi-terrestrial"
• 3 prs maxillipeds, 5 prs legs (1st pr often chelate)
• Gills in branchial chamber enclosed by carapace

Subclass Malacostraca: Order Amphipoda ("Double Legs")

• 6000 species
• Mostly marine or freshwater, damp terrestrial
• Laterally compressed body, mostly small (< 2 cm)
• Abdomen is not clearly demarcated from thorax
• Large thoracic coxae on legs
• Appear "hunchback"

Subclass Malacostraca: Order Isopoda ("Same Legs")

• 4000 species
• Mostly marine benthic, some freshwater
• Wood lice are largest group of terrestrial crustaceans
• Most are < 2 cm; some quite large (> 40 cm)
• Dorso-ventrally flattened
• Head is "shield-shaped"
• Abdomen same width as thorax

Subclass Branchiopoda

• "Gill Feet"
• Fairy and brine shrimp (Order Anostraca)
• Water fleas (Order Cladocera)
• 800 species, almost all freshwater
• Especially common in ponds, small lakes
• Appendages are lobe-like with setae as gills
• Two terminal processes (cercopods)

Subclass Ostracoda

• "A shell", seed shrimp
• >5000 species, marine and freshwater
• Body enclosed in calcified bivalve shell
• Trunk segmentation absent

Subclass Copepoda

• "Oar-Footed"
• Largest Class of microscopic crustacea
• > 8,500 species, marine and freshwater
• May be very important in aquatic food webs
• Feed mostly on bacteria and phytoplankton
• Bulbous head, tapered posteriorly
• Trunk segmentation well-developed
• Paddle-like appendages
• A pair of caudal cerci are distinctive
• Compound eyes absent, median naupliar eye

Subclass Cirripedia

• Barnacles, Cirripedia is "feet with curls of hair"
• 900 species, exclusively marine intertidal
• Only crustacean to have evolved a sessile habit
• Stalked and unstalked forms
• First antennae modified into peduncle (stalk) or cement gland (unstalked) for attachment
• Plates are derived from a greatly modified carapace
• Chitinous shell is heavily calcified
• Trunk appendages modified into feeding cirri, which are exposed at high tide to trap food particles