Download Outline 20: Mollusca

Invertebrate Zoology Lecture 20, June 4, 2008
Phylum Mollusca (Introduction & Cephalopoda)
I.
II.
Major Classes of Phylum Mollusca (see PowerPoint)
Evolutionary relationships
A.
Classic protostome, with all features as described for Phylum Annelida.
1.
Closely aligned with Annelida and Arthropoda in “traditional” phylogeny
2.
Somewhat more distant from Arthropoda according in molecular phylogeny.
III. Bauplan basics: sketch of hypothetical ancestral mollusc (HAM) to review overall
organization. I am not assuming this is truly the ancestor, but useful for showing
key features.
A.
Shell
B.
Mantle
1.
Functions
2.
May have associated sensory structures (such as?)
C.
Mantle cavity: external fold of mantle, contains
1.
Ctenidia
2.
Anus
a)
What keeps it from pooping on its ctenidia?
3.
Renal/genital pore also opens into mantle cavity
D.
Foot: wide, flat and muscular
1.
Note epipodial tentacles in some
E.
Head
1.
cephalic tentacles
2.
radula
F.
Visceral mass: enclosed by mantle (not the same as the mantle cavity!) Contains
body organs.
IV. Digestive system
A.
Complete
B.
salivary glands may have toxins, digestive enzymes
V. Respiration/circulation
A.
Gas exchange: ctenidium (aquatic) or mantle (terrestrial and some aquatic forms)
1.
Close-up view of ctenidium showing structure as well as hemolymph and
water flow  countercurrent exchange
B.
Circulation [Fig. 20.34]
1.
Open circulatory system except in Class Cephalopoda
2.
Heart includes two atria and a muscular ventricle
a)
Circulation pattern? Path of oxygenated hemolymph? Deoxygenated
hemolymph
VI. Osmoregulation/excretion (briefly)
A.
Metanephridia (=”kidneys”) associated with the pericardial sac/coelom
VII. Nervous system
A.
Variable depending upon body plan
B.
“Primitive” form (i.e. Aplacophora, Polyplacophora)
1.
Head-associated ganglia
2.
Four main nerve cords (two visceral = dorsal; two pedal = ventral)
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3.
Cross-connections between cords is reminiscent of Platyhelminthes
C.
Gastropoda et al.
1.
Head-associated ganglia
2.
Two main nerve cords
3.
Ganglia in strategic locations
D.
Sensory structures vary among groups; will get into as we cover specific organisms
VIII. Reproduction
A.
Some gonochoristic, many hermaphroditic
B.
Gonads associated with reduced coelom
1.
Ancestral condition: simple spawning of gametes via genital pores into…?
2.
Many molluscs have more elaborate structures for internal fertilization
IX. Focus: Class Cephalopoda
A.
Diversity of modern cephalopods (slides): Nautilus, Cuttlefish, Squid, Octopus
B.
Evolution of cephalopod features (focus on form and function)
1.
Ancestral cephalopod (~500 mya): differences from HAM. Beginnings of a
mobile, predatory lifestyle!
a)
Shell adaptations
 Original adaptive value of chambered shell?
 Siphuncle: What is it and how does it work?
b)
Elongation of body; sometimes coiled over the head (different
directionality of coiling that in Gastropoda)
c)
Development of anterior epipodial tentacles  “arms”
 Adaptive value at this point? Two hypotheses
d)
Development of the beak. Function?
 Maintains the radula as well
e)
Development of lateral extensions of foot
 Functions?
 Further evolution into funnel/siphon  jet propulsion

Describe jet propulsion!

Other function of one-way movement of water through
mantle cavity?
2.
Three key lines branch off this ancestral line
a)
Nautiloids, including five living species. NOTE
 Epipodial “arms” derived from foot. No suckers.

Also has specialized tentacles for mating that are likely
derived from the cephalic tentacles.
 Siphon/funnel derived from foot
 Chambered external shell with siphuncle
 Relatively simple eye, similar to that found in Gastropoda.
 NOTE: Viscera are in visceral mass, not mantle cavity
b)
Ammonoids
 Somewhat similar to Nautiloids

“Suture patterns” more elaborate
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
Known from fossils only; extinct at end of Cretaceous
Coleoidea
 Branched off somewhat later
 Elongate, torpedo-shaped bodies
 Shell became lighter and internal; covered with mantle tissue

Belemnoidea maintained a relatively substantial shell

Other branch: trend toward reduced shell
Focus: extant Coleoidea and their predatory adaptations
1.
Already discussed some of their predatory features above
a)
Streamlined shape
b)
Reduction of shell weight
c)
Funnel/siphon  jet propulsion
2.
Further development of arms
a)
Became more muscular, fewer in number.
b)
Suckers evolved; better able to grasp prey via suction and hooks.
c)
Used also to crush the exoskeletons of crustacean prey.
3.
Further development of cephalic tentacles in squid and cuttlefish
a)
Extend explosively via hydrostatic pressure
b)
Used for rapid prey capture as well as in mating.
4.
Eyes: At some point a complex, vertebrate-like eye developed
a)
Classic example of convergent evolution
b)
Giant squids have the largest eyes of any animal (50 cm diameter!)
c)
Special adaptations…
5.
Nervous system
a)
Cerebral ganglia are consolidated into a highly effective brain
b)
Giant axons penetrate mantle for rapid response
c)
How intelligent are they?
6.
Further adaptations of the mantle
a)
Increased musculature for more effective jet propulsion
b)
Chromatophores (slides):
 Pigment filled “cell” with muscles radiating outward that allow
for shape change and change in pigment distribution
 Function: color change for camouflage and communication
 Also, octopuses & cuttlefish have even more muscular control
to create textural changes
 Regulation: visual input
c)
Photophores: bioluminescent! Function?
d)
Fins: steering (squid, cuttlefish)
7.
Ink sac
a)
Location and function?
b)
Other fun facts…
Life cycle: Loligo opalescens (the market squid)
1.
Mating, Egg laying, Development, Hatching
2.
Direct or indirect development?
c)
C.
D.
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