Arthropods- Crustaceans

Numerical importance of arthropods

The biggest diversity in the invertebrates

Crabs and Carcinization

• Largest assemblage of species in animal kingdom

• Protostomes

• Much in common with annelids

•  segmentation

•  nervous system

•  early development

•  appendage in each segment (primitive)

• Alaskan king crabs

• Blue crabs

• Stone crabs

• Rock crabs

• Spiny lobster

• Lobster

• Shrimp

Fisheries- Species

Fisheries

• High commercial value

• Highest bycatch of any fishery

Exoskeleton

• Chitinous cuticle or _____ covers whole body

  • Divided into separate plates for movement

  • Has given rise to other important morphological adaptations

    • Hemocoel

    • Jointed appendages

  • No growth (must molt to grow)

• Many arthropods molt throughout life, though further apart as get older

Molting- Stages

• Proecdysis – energy reserves and blood calcium rises

  • comes from hepatopancreas or resorbed from exoskeleton

• Ecdysis – exoskeleton splits as animal takes on water due to salt accumulation and animal extracts itself

  • Water may equal ½ premolt body weight

• Postecdysis – cuticle secreted and calcified around water-swollen body

  • Excess water excreted

• Intermolt – growth of soft tissues, energy reserves increase

• Hydrostatic skeleton (hemocel) during molt.

Molting Mechanics

• Ecdysis in crustaceans

  • Y-organ produces ecdysone (steroid hormone)

  • X-organ produces molt inhibiting hormone

    • MIH inhibits production of ecdysone

  • Eyestalk ablation

ecdysone (steroid hormone)

Y-organ produces…

molt inhibiting hormone

X-organ produces…

of ecdysone

MIH inhibits production…

Limitations to body size (at least on land)

• Large animals need very heavy exoskeleton

• But when molting, if too big, new skeleton collapses before hardens

  • This keeps arthropods generally small

  • Large arthropods aquatic only due to buoyancy

Filter feeding, Deposit Feeding, and Predation

What are the 3 forms of nutrition of the Crustaceans?

Nutrition (Filter feeding)

• likely the primitive mode of nutrition

• Many modes of filter feeding in crustaceans

  • Virtually every appendage has been modified in some group for filter feeding

  • Large variation (different appendages) suggest has evolved numerous times

Nutrition (Predators)

• Predators (annelids, bivalves, cannibals), scavengers, detrivores, herbivores

• Morphology reflects feeding habits

• Food captured with chelipeds

  • Passed to third maxilleped

  • Held by mandibles and bitten into chunks by maxillipeds

Open circulatory System

• =hemolymph=interstitial fluid

• Blood bathes organs

• Moves by heart pumping AND body movement

• Pigment is hemocyanin

• Blood contains amebocytes for phagocytosis and clotting

  • Explosive cells disintegrate, releasing substance that converts fibrinogen to fibrin, causing clotting

  • Very fast!

Why if they lose an appendage they don’t die of blood loss?

Vision

• Compound eyes on end of stalk

  • Variable number of ommatidia

    • 10-15,000 (14,000 in lobsters)

  • Crabs and shrimp can discriminate form and size of objects

  • May discriminate color

  • Polarized light in some

Crabs Reproduction

• Most are Dioecious (separate sexes)

• Often ritualized interactions between sexes

• Often (but not always) mate after female molts while carapace is soft

• Female carries eggs on abdomen while they mature

Nauplius larva

• 3 pairs of appendages (2 antennae and mandibles)

• Use to swim

• No segmentation

• Naupliar eye

Zoea larva (often multiple zoeal stages)

• Only in large species (crabs, shrimp, etc.)

• Additional swimming appendages

Carcinization

• Light body, laterally compressed, slight legs, small or no chelapeds

• Heavier body, larger claws

• Built for lateral walking

Reasons for Success of Carcinization

• Found in all marine habitats

• Size

• Defenses

  • Chelipeds

  • Decoration (camouflage)

  • Commensals (pea crab)

  • Swimming

    • Portunids

 

HERMIT CRABS

An effort to reduce demands of exoskeleton formation

KRILL

•  Filter feeders

•  Molt rapidly (few seconds) when startled

•  Extensive vertical migrations

•  Important in food webs (blue whales eat 4 tons/day)

AMPHIPODS

• Laterally flattened

• Benthic and pelagic

• Mostly scavengers

ISOPODS

• Dorsoventrally flattened body

• Scavengers, omnivores and herbivores, deposit feeders

• Many parasitic forms

• Wood boring (Limnoria spp.) – cellulase excreted from hepatopancreas

• One of most abundant deep-sea fauna`

• Major portion of gray whale diet in Arctic

• Leave characteristic scrape marks on benthos

• Lack carapace

COPEPODS

• Often most abundant marine zooplankton

• Important link in food web

• Excellent swimmers