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Zoology 375 > Phylum Mollusca > Flashcards

Phylum Mollusca Flashcards

1
Q

Molluscs

A 
• 2nd largest phylum (after Arthropoda)
• 50,000-120,000 species
Lophotrochozoan (Trochophore larvae)
Protostome 
Bilaterian (Triploblastic)
Apomorphies:
-Dorsal epithelium - mantle secretes shell or "spicules"
-Cuticular band of teeth - radula (except for bivalves)
-Ventral body well muscles - foot (for locomotion or clinging)
-Head
-Visceral mass
-1-way gut
-Coelum reduced 
-Body cavity mostly homocoel
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2
Q

Radula

A

Ribbon with chitin & protein
2 rows of teeth
Rasps food particles into mouth

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3
Q

Trochophore larvae

A

Common to most annelids & molluscs & some other protostome groups
Complete gut
Band of cilia encircles body - for swimming & Collecting food apical tuft of flagellae
Usually planktonic

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4
Q

Visceral Mass

A

Major body parts between head and foot

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5
Q

Odontophore

A

Supports radula

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6
Q

Buccal mass

A

Ontophore + radula complex

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7
Q

Shell

A 
Secreted by mantle
3 layers
-periostracum
-prismatic
-nacreous (mother of pearl)
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8
Q

Mantle

A 
Secretes shell
3 lobes
-muscular
-secretory
-sensory
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9
Q

Hemocoel

A

Contains circulatory fluid

Persistent blastocoel

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10
Q

Mantle cavity

A

Open to exterior
Contains gills
Becomes “lung” in pulmonate gastropods
Exit site for excretory, digestive and reproductive systems

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11
Q

Ctenidia

A 
Gills
Gas exchange (with cilia)
Food collection (some species) 
Brood chamber!
Not present in all species
work by countercurrent exchange
“blood” (hemolymph) & water flow in opposite directions
- water moved by ctenidial cilia
- maintains maximum gradient for oxygen
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12
Q

Osphradium

A

Chemoreceptive / tactile receptor

Usually near ctenidia (tests water quality)

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13
Q

Gas exchange

A
  • ctenidia
  • diffusion across body wall
  • secondary “gills” from body wall
  • “lungs” in terrestrial molluscs (=vascularized air sac modified from mantle cavity)
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14
Q

Circulation

A

hemocoel = circulatory space, open
Cephalopods with closed system (movement)
Network of vessels & heart (not in scaphopods)
Blood = hemolymph (carries gases & nutrients etc.)
- gas carried primarily by hemocyanin (copper-based, not as efficient as hemoglobin (less stamina))

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15
Q

Nervous system

A

Nervous system:
From simple (ganglia absent (chitons)) to complex: ganglia large & abundant
Cephalopods - with true brain & nervous system comparatively larger than that of fishes)
- paired ganglia
- form ring around esophagus

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16
Q

Giant Fibres and Stellate Ganglion

A
  • specialized for rapid impulse conduction
  • used to coordinate jet propulsion
  • large & easy to study process of nerve impulse transmission
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17
Q

Excretory system

A
  • urine produced by kidneys (usually 2)
  • kidneys collect from pericardium (around heart)
  • drains into mantle cavity at renal pore (nephridiopore)
  • urine as ammonia
  • selective resorption of filtrates
  • secretion of toxins & wastes into urine
  • osmoregulation by kidneys too
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18
Q

Reproduction

A
  • most species gonochoristic
  • some gastropods parthenogenetic
  • “virgin birth” - female produces clones
  • some spp. protandric hermaphrodites • sex changes from male to female with age
  • external fertilization common in most taxa
  • Gastropoda with both internal & external
  • Cephalopoda with only internal fertilization
  • some simultaneous hermaphrodites • pulmonate gastropods
  • reciprocal copulation (unlike turbellarians)
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19
Q

Indirect Development

A

– trochophore
– veliger (trochophore modified with velum)
• Planktonic

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20
Q

Glochidium

A
  • nonswimming
  • ectoparasite on fish
  • freshwater bivalves
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21
Q

Direct development

A 
(no larval stage )
– Pulmonate gastropods (terrestrial snails & slugs)
– Cephalopods
• Eggs develop in gelatinous mass
• Protected by mother - parental care
-Mini adults
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22
Q

More characteristics of Molluscs

A
  • primarily marine
  • some bivalve & gastropods in freshwater
  • only some gastropods are terrestrial
  • filter feeders, detritivores, predators, herbivores, ecto- & endoparasites
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23
Q

Classes

A

7
1. Aplacophora - wormlike, spicules, no shell 2. Polyplacophora - chitons
3. Monoplacophora - “living fossils”
7 Classes:
4. Gastropoda - snails, slugs, etc. (~80% of spp) 5. Cephalopoda - squid, octopuses, nautiluses 6. Bivalvia - clams, mussles, oysters, etc.
7. Scaphopoda - tusk shells

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24
Q

Class Aplacophora

A 
Apomorphies:
• Vermiform (wormlike)
• Foot as narrow keel? – Or gone?
• small (<5 mm, some 30cm)
• deep ocean
• calcareous spines
• ~320 species
• evolved before the shell
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25
Q

Class Polyplacophora

A 
“Chitons”
• shell of 7 or 8 plates (apomorphy)
• ~ 800 species
• lateral mantle = girdle
• most in intertidal zone
• cling tightly to rocks
• ganglia lacking or few & small
External & internal segmentation in Polyplacophora
26
Q

Class Monoplacophora

A

Evidence for internal segmentation in Monoplacophora
Apomorphies
• 8 pairs of foot retractor muscles
• 3-6 pairs of ctenidia
• 6-7 pairs of kidneys
• “living fossils” - discovered alive in 1952 • ~ 20 species
• deep sea - 2,000 m
• single unhinged shell
• limpet-like
• “segmented” internally
• possible ancestor of remaining molluscs

27
Q

Class Gastropoda

A

• Snails, slugs, limpets… huge diversity
• 51-80% of Mollusc species
• ~40,000 species (200,000 total?)
• Marine, freshwater & terrestrial
• Only molluscs on land
• Diverse habitats:
Rivers, lakes, trees, deserts, mountains, marine intertidal, plankton, deep sea
Apomorphies:
• Torsion
• Operculum - not present in all species – Not a good apomorphy…but a good diagnostic

28
Q

Operculum

A
  • proteinaceous shield on foot
    Can withdraw head & foot into shell & close operculum for protection
  • may have been important in invasion of terrestrial systems
29
Q

Torsion

A

Visceral mass and nervous system twist 90°-180° during embryonic development
Happens quickly during embryonic development Not related to the spiraling of the shell!
Brings mantle cavity over head
Benefits mixed at best:
Allows head to withdraw into mantle cavity
-Ctenidia & osphradium in cleaner water
-But anus & renal pore also near head
Some species show detorsion (reversal)
-Contaminate water with waste (symmetrical flow undesirable)
Typical: visceral mass protected by coiled shell atop the foot

30
Q

Shell of Gastropods

A

Coiled (Probably for efficient packing of visceral mass as gastropod grows larger)
Most species shells coil dextrally
Many paired organs reduced on right side
Few species coil counterclockwise (sinistral)

31
Q

Columella

A

Central axis

32
Q

Columellar muscle

A

-Extends from foot to the columella

Very important muscle - involved in many body movements

33
Q

Dextral Shell

A

Clockwise = “right handed”
Aperture on right with columella vertical
Many paired organs reduced on right side

34
Q

Sinistral

A

Counterclockwise = “left handed”

Aperture on left with columella vertical

35
Q

Gastropod movement

A
  • small species
    cilia on ventral surface of foot
  • Most species use pedal waves of muscle contraction (contraction starts in anterior, squeezing the foot forward; contraction moves posteriorly in a wave)
36
Q

Subclasses

A

“Prosobranchs”
Opisthobranchia
Pulmonata

37
Q

Subclass “Prosobranchs”

A 
• Most gastropod species
• Mostly marine
• Few terrestrial & freshwater species
• Diverse: predators, herbivores
detritivores, parasites, sessile & free-living
• Rare: symbioses with bacteria
38
Q

Siphon

A
  • Entry into mantle cavity

– Water flows in to osphradium & gills

39
Q

Limpets

A

No coil to shell

40
Q

Abalone

A

Perforated shell for water flow

41
Q

Slipper snails (Crepidula fornicata)

A

Form stacks, oldest are female, on bottom

42
Q

Planktonic Heteropods

A

Some shell-less

43
Q

Subclass Opisthobranchia

A

Apomorphy: Mantle cavity lateral or posterior
due to detorsion, or lost • < ~2,000 species
• Sea slugs, sea hares, sea butterflies etc. • Most are marine
Reductions:
• Shell - reduced- lost or internalized
• Operculum - lost or reduced
• Torsion limited
• Mantle cavity - reduced / lost
• Ctenidia - reduced / lost
(But larvae typically show unreduced state)

44
Q

Nudibranchs

A

Replaced ctenidia with cerata
Replaced osphradium with rhinophores
Usurpation of cnidae & defensive toxins

45
Q

Subclass Pulmonata

A

Apomorphy: Mantle cavity highly vascularized to
form a lung
• ~17,000 species (4x no. of mammal species)
• Few marine species (intertidal)
• Terrestrial snails & slugs
Torsion 90° - cavity opens to the right

46
Q

Pneumostome

A

Opening to mantle cavity (lung)

47
Q

Pulmonata reproduction

A
  • some simultaneous hermaphrodites
  • reciprocal copulation (unlike turbellarians)
  • direct development (no larvae)
48
Q

Class Bivalvia

A

• Headless (virtually)
• Eyeless - not really
• No radula / odontophore complex
• Foot typically without sole
Sedentary
• Some swim, some dig
• Marine & freshwater (10-15%)
• Most species are suspension / filter feeders
- use ctenidial cilia
• Gills used for gas exchange & feeding (in most spp)
• Often dominate biomass in shallow waters
• Important ecological indicators - invasive
species & endangered species
• Commercially important - food & pearls

49
Q

Water flow in Bivalves

A

– Enters - incurrent siphon
– Exits - excurrent siphon
– Tubular extensions of mantle tissue
– Allow animal to remain buried

50
Q

Giant clam - Tridacne gigas

A

Largest bivalve species
Length over 4’ (1.2 m)
Weight over 500 lbs (227 kg)
Mantle contains endosymbiotic photosynthetic algae

51
Q

Shipworms

A

(not worms!)
Lamellibranch bivalves that eat & burrow into wood
Analogous to termites -
symbiosis with bacteria that can digest cellulose

52
Q

Class Cephalopoda

A 
Apomorphies:
• Shell (if present) divided by septa 
• Closed circulatory system
• Foot modified as arms & siphon
• Ganglia fused to form a large brain encased in a cartilaginous “cranium”
53
Q

Cephalopod characteristics

A 
• Fast moving, active predators
– Jet propulsion of water out a siphon
• Complex behaviors - problem solving
• 100% marine
• With ctenidia & radula
• ~ 600 species (10,000 extinct species)
• Only Nautilus species have external shell
54
Q

Subclass Nautiloidea

A

Only 6 species left “evolutionary relict”
Relatives of extinct ammonites
Probably driven to extinction by fish
Animal only in last chamber of shell

55
Q

Siphuncle

A

A calcareous tube containing living tissue

Controls gas content of each chamber

56
Q

Cephalopod locomotion

A 
Buoyancy
– Gas control
– Internal shell of cuttlefish & squid 
• Cuttlebone, pen, Spirula
– Heavy cations in water replaced by low-weight ammonium ions (chemical bouancy chamber) to maintain neutral buoyancy:“ammoniacal” squids
57
Q

Coleoid defense

A 
Bioluminescence
Photophores - light organs
– To startle predators
– To attract mates
– To eliminate silhouette (crypsis)
Ink from ink sac
– Discharged from anus at will
– Might have a mild narcotic effect – To startle & confuse predators
58
Q

Coleoid morphology

A

Squid have 8 arms and 2 longer tentacles
With suckers & taste receptors
Sensory reception: excellent eyes taste, touch (poor hearing)

59
Q

Coleoid eyes

A

Image-forming
Very similar to mammal eyes
Classic example of convergent evolution
Like mammals - with cornea, lens, iris, diaphragm & retina

60
Q

Coleoid reproduction

A
  • most leave egg cases
  • direct development
  • some exceptions now known
61
Q

Architeuthis

A
  • Giant squid
    Hundreds of dead specimens found on beaches Never seen alive (by a scientist)
    A favorite prey item of bull sperm whales
62
Q

Mesonychoteuthis

A
  • Colossal Squid”

Zoology 375 (7 decks)

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