Phylum Mollusca Flashcards
Molluscs
• 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
Radula
Ribbon with chitin & protein
2 rows of teeth
Rasps food particles into mouth
Trochophore larvae
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
Visceral Mass
Major body parts between head and foot
Odontophore
Supports radula
Buccal mass
Ontophore + radula complex
Shell
Secreted by mantle 3 layers -periostracum -prismatic -nacreous (mother of pearl)
Mantle
Secretes shell 3 lobes -muscular -secretory -sensory
Hemocoel
Contains circulatory fluid
Persistent blastocoel
Mantle cavity
Open to exterior
Contains gills
Becomes “lung” in pulmonate gastropods
Exit site for excretory, digestive and reproductive systems
Ctenidia
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
Osphradium
Chemoreceptive / tactile receptor
Usually near ctenidia (tests water quality)
Gas exchange
- ctenidia
- diffusion across body wall
- secondary “gills” from body wall
- “lungs” in terrestrial molluscs (=vascularized air sac modified from mantle cavity)
Circulation
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))
Nervous system
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
Giant Fibres and Stellate Ganglion
- specialized for rapid impulse conduction
- used to coordinate jet propulsion
- large & easy to study process of nerve impulse transmission
Excretory system
- 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
Reproduction
- 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)
Indirect Development
– trochophore
– veliger (trochophore modified with velum)
• Planktonic
Glochidium
- nonswimming
- ectoparasite on fish
- freshwater bivalves
Direct development
(no larval stage ) – Pulmonate gastropods (terrestrial snails & slugs) – Cephalopods • Eggs develop in gelatinous mass • Protected by mother - parental care -Mini adults
More characteristics of Molluscs
- primarily marine
- some bivalve & gastropods in freshwater
- only some gastropods are terrestrial
- filter feeders, detritivores, predators, herbivores, ecto- & endoparasites
Classes
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
Class Aplacophora
Apomorphies: • Vermiform (wormlike) • Foot as narrow keel? – Or gone? • small (<5 mm, some 30cm) • deep ocean • calcareous spines • ~320 species • evolved before the shell
Class Polyplacophora
“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
Class Monoplacophora
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
Class Gastropoda
• 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
Operculum
- proteinaceous shield on foot
Can withdraw head & foot into shell & close operculum for protection - may have been important in invasion of terrestrial systems
Torsion
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
Shell of Gastropods
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)
Columella
Central axis
Columellar muscle
-Extends from foot to the columella
Very important muscle - involved in many body movements
Dextral Shell
Clockwise = “right handed”
Aperture on right with columella vertical
Many paired organs reduced on right side
Sinistral
Counterclockwise = “left handed”
Aperture on left with columella vertical
Gastropod movement
- 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)
Subclasses
“Prosobranchs”
Opisthobranchia
Pulmonata
Subclass “Prosobranchs”
• Most gastropod species • Mostly marine • Few terrestrial & freshwater species • Diverse: predators, herbivores detritivores, parasites, sessile & free-living • Rare: symbioses with bacteria
Siphon
- Entry into mantle cavity
– Water flows in to osphradium & gills
Limpets
No coil to shell
Abalone
Perforated shell for water flow
Slipper snails (Crepidula fornicata)
Form stacks, oldest are female, on bottom
Planktonic Heteropods
Some shell-less
Subclass Opisthobranchia
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)
Nudibranchs
Replaced ctenidia with cerata
Replaced osphradium with rhinophores
Usurpation of cnidae & defensive toxins
Subclass Pulmonata
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
Pneumostome
Opening to mantle cavity (lung)
Pulmonata reproduction
- some simultaneous hermaphrodites
- reciprocal copulation (unlike turbellarians)
- direct development (no larvae)
Class Bivalvia
• 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
Water flow in Bivalves
– Enters - incurrent siphon
– Exits - excurrent siphon
– Tubular extensions of mantle tissue
– Allow animal to remain buried
Giant clam - Tridacne gigas
Largest bivalve species
Length over 4’ (1.2 m)
Weight over 500 lbs (227 kg)
Mantle contains endosymbiotic photosynthetic algae
Shipworms
(not worms!)
Lamellibranch bivalves that eat & burrow into wood
Analogous to termites -
symbiosis with bacteria that can digest cellulose
Class Cephalopoda
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”
Cephalopod characteristics
• 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
Subclass Nautiloidea
Only 6 species left “evolutionary relict”
Relatives of extinct ammonites
Probably driven to extinction by fish
Animal only in last chamber of shell
Siphuncle
A calcareous tube containing living tissue
Controls gas content of each chamber
Cephalopod locomotion
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
Coleoid defense
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
Coleoid morphology
Squid have 8 arms and 2 longer tentacles
With suckers & taste receptors
Sensory reception: excellent eyes taste, touch (poor hearing)
Coleoid eyes
Image-forming
Very similar to mammal eyes
Classic example of convergent evolution
Like mammals - with cornea, lens, iris, diaphragm & retina
Coleoid reproduction
- most leave egg cases
- direct development
- some exceptions now known
Architeuthis
- Giant squid
Hundreds of dead specimens found on beaches Never seen alive (by a scientist)
A favorite prey item of bull sperm whales
Mesonychoteuthis
- Colossal Squid”
