Arthropods, the largest phylum on Earth, include diverse marine crustaceans like crabs, lobsters, and shrimp, characterized by flexible bodies, bilateral symmetry, and a chitinous exoskeleton. They exhibit specialized appendages for various functions and exhibit complex behaviors and reproductive strategies, with separate sexes and planktonic larvae. Other marine arthropods include horseshoe crabs and sea spiders, while insects, though rare in marine environments, can be found scavenging near the water's edge.

1. 6. Arthropods: Armored Achievers
Crabs, Lobster, Shrimp, Barnacles

2. Arthropods
• Largest phylum (3/4 of all species on earth)
• Insects – largest group
• Majority of marine arthropods are crustaceans
(subphlyum Crustacea)
• Flexible, segmented, bilateral symmetry
• Jointed appendages moved by sets of attached
muscles

3. Arthropods exhibit bilateral symmetry and a chitinous
exoskeleton. Provides support, protection, and increased
surface area for muscle attachment.
To grow they must molt the exoskeleton and absorb water to expand
before the new exoskeleton hardens.

4. Arthropods: Subphylum Crustacea
• Most marine with gills for gas exchange
• Appendages specialized for swimming, crawling,
attaching to other animals, mating, and feeding
• Two pairs of antennae involved in sensing
surrounding.

5. Arthropods: Subphylum Crustacea
• Small Crustaceans
– Copepods (cope-a-pod)
• Planktonic, use mouthparts to filter feed, some may
swim, many are parasitic
– Barnacles
• Filter feeders that usually live attached to surfaces,
even living organisms
• Cirri (sear-I) (feathery legs) sweep water for food
• Crustacean larvae that swim and attach before
metamorphosing into adults

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7. Arthropods: Subphylum Crustacea
• Small Crustaceans
– Amphipods
• Curved, flattened bodies (sideways)
• Beach hopers, common in shore debris, seaweed,
burrowing in whales, or planktonic
– Isopods
• Parasitic fish lice that are dorsoventrally flattened
• Marine pill bug

8. Amphipods and Isopods
Sea louse: a marine pill bug
Orchestoidea, a beach hopper
are often found on marine
mammals.

9. Arthropods: Subphylum Crustacea
• Small Crustaceans
– Euphausiids (yoo-fa-ze-id) (Krill)
• Planktonic, shrimp-like, filter feeders
• Common in polar waters in giant schools
• Most exclusive food source for whales, penguins and
fish

10. Arthropods: Subphylum Crustacea
• Shrimps, Lobsters, and Crabs
– Decapods (10 legs)
• Largest in size, great commercial importance
• 5 pairs of legs w/ first pair being claws used for
feeding and defense
• Well developed carapace encloses cephalothorax
• Rest of body called abdomen

11. Arthropods: Subphylum Crustacea
• Shrimps and Lobsters
– Laterally compressed
– Shrimp - scavengers feeding on detritus
• Some may remove parasites from skin of fish
– Lobsters – Marine scavengers and predators that crush
molluscs and sea urchins
– Hermit crabs – not true crabs that hide soft body in
empty shells

13. Arthropods: Subphylum Crustacea
• Crabs
– Abdomen small and tucked
under large cephalothorax
• V shaped abdomen = male
• U shaped abdomen = female
– Highly mobile and walk
sideways

14. Arthropods: Subphylum Crustacea
• Crabs
– Scavengers/predators
– Some have specialized diets of seaweeds,
organic matter, or coral mucus
– Live along rocky shores or sandy beaches
– Land crabs live most of life on land but may
return to ocean to release eggs

15. Biology of Crustaceans
• Feeding and Digestion
– Filter feeding common among small crustaceans
• Bristles on some appendages used to gather food
• Other appendages move food from bristles to mouth
• Some may use appendages to pierce or suck
(parasitic)
• Bristles sift, chitinous teeth in stomach grinds

16. Biology of Crustaceans
• Feeding and Digestion
• Decapods have 2 chambered stomach connected to
digestive gland that secretes enzymes and absorbs
nutrients (extracellular)
• Intestine ends in an anus
• Open circulatory system distributes nutrients

17. Biology of Crustaceans
• Nervous System and Behavior
• Small, simple brains but well-developed sensory
organs
• Compound eyes
• Keen sense of smell (chemical sensitivity)
• Have statocysts for balance
• Most behaviorally complex of all invertebrates
• Have special body posture and movement of legs and
antennae
– Helps settle disputes between neighbors and courtship

18. Biology of Crustaceans
• Reproduction and Life History
– Separate sexes in most crustaceans
– Males use specialized appendages to transfer sperm
directly to female
– Decapods - takes place after molting and females can
store sperm to use on different batch of eggs
– Most have planktonic larvae type and number of larval
stages vary widely

20. Other Marine Arthropods
• Horseshoe Crabs (class Merostomata)
– Only surviving members
– Widely represented by fossil records
– 5 living species and not true crabs
– Live on soft bottoms of shallow waters on Atlantic and
Gulf coasts of North America and Southeast Asia
– Emerge on beaches to reproduce

21. Other Marine Arthropods
• Sea Spiders (class Pycnogonida) Pic – no – ga- ni- da
– Superficially resemble spiders
– Four or more pairs of legs
– Large proboscis with mouth at tip used to feed on soft
invertebrates such as sea anemones and hydrozoans
– More common in cold water but do not occur throughout
oceans

23. Other Marine Arthropods
• Insects (class Insecta)
– 3 pairs of legs as adults
– Rare in the sea
– Live at waters edge scavenging for seaweeds, barnacles,
and rocks
– Inhabit decaying seaweed that accumulates at high tide