Mollusca
Diversity and Classifcation
90,000 species
8 classes
Caudofoveata
Solenogastres
Polyplacophora
Monoplacophora
Gastropoda (VERY diverse, can live in freshwater or on land)
Bivalvia (2nd most diverse, can live in freshwater)
Scaphopoda
Cephalopoda
Biology and Importance
Molluscs are astonishingly diverse
Humans have exploited them in many ways
shells were used as money
shells and pearls are used in jewelry and decoration
we eat clams, oysters, scallops, mussels, snails, abalone, squids, and octopi
molluscs are critical for maintaining unpolluted riverine, coastal, and oceanic habitats
range from fairly simple organisms to some of the most complex invertebrates
microscopic to 20m in length
shells of some giant clams reach 1.5 m to length and weigh more than 250 kg
can be herbivorous grazers, predaceous carnivores, filter feeds, detritus feeders, and parasites
molluscs originated in the sea
only bivalves and gastropods moved into freshwater habitats
only slugs and snails (gastropods) invaded the land
Form and Function
body bilaterally symmetrical
unsegmented
often with definite head
triploblastic
eucoelomate: coelom is limited to a space around the heart
Body-plan has a head-foot portion and a visceral mass portion
head foot contains the feeding, cephalic sensory, and locomotor organs
visceral mass contains the digestive, circulatory, respiratory and reproductive organs
Head-foot
more active region of a mollusc
within the mouth is a structure unique to molluscs - the radula
radula: tongue like rasping organ
present in all except bivalves and solenogastres
may have 250,00 “teeth”
new teeth continually produced
supported on cartilage-like rod (odontophore)
works like a conveyor belt to deliver food to digestive tract
posterior to the mouth is the chief locomotor organ - the foot
foot:
ventral body wall specialized as a muscular foot
foot is present in all molluscs
used for locomotion, digging, attachment (byssal threads)
Visceral mass:
Dorsal body wall forms a pair of folds called the mantle
Mantle:
made of epidermis
may have sense oragns and glands
glands secrete mucus and shell (some)
mantle may create mantel cavity
Mantle cavity, is modified into gills or lungs, and secretes the shell
Mantle cavity
cavity houses respiratory organs
for some, mantle cavity may offer protection
Shell:
secreted by mantle
much variability in form and structure
made of calcium carbonate
3 layers
periostracum (outer): thick in freshwater; thin or absent in marine
prismatic (middle): very dense
nacreous (inner): glossy and secreted throughout life
Homeostatic functions and structures
Respiration
most molluscs have folded, cillated gills
used for feeding in bivalves
bivalves may have extended siphon
some molluscs respire through skin
many terrestrial snail lack gills (pulmonates)
mantle is modified into sac-like lung for breathing air
pneumostome present
Circulation
most have an open circulatory system
blood is not entirely contained within vessels
adequate for slow moving animals
cephalopods have closed circulation
heart is simple and has few vessels
2 auricles and 1 ventricle
blood has both oxygen and carrying cells and white blood cells
Feeding and Digestion:
Detritus feeders, burrowers, grazers, carnivores, filter feeders, etc
radula aids in feeding
most digestive organs are embedded in visceral mass
complete digestive system:
foregut
foregut: receives and prepares food
buccal cavity
mouth
radula
salivary glands
esophagus
midgut
midgut: storing region, crushing region
stomach
digestive glands
hindgut
hindgut: absorption of nutrients, and formation of feces (usually largest and longest)
intestine
Excretion
1 pair of metanephridia
term kidney is used, but different from vertebrate kidney
some excretion through body walls and gills
Nervous system
central nervous system - ring of ganglia in head area
pair ganglia extend to other parts of the body
nervous system is simple compared to vertebrates
cephalopods rival some mammals in reasoning and learning
Reproduction and Development
most are dioecious
some gastropods ( snails and slugs) are monoecious
marine forms produce larval stage known as trochophore
for some, a secondary larval form (veliger) will form
cephalopods have direct development
Class Polyplacophora
General Aspects (Biology/Morphology)
found generally on rocky surfaces in intertidal regions
move very little over lifetime
clings to rocks with broad foot
capable of rolling up like armadillo for protection
at low tide, can press margin of mantle against substrate, (lessens water loss)
small, 2-5 cm
dorsoventrally flattened
convex dorsal surface
7 to 8 plates on dorsal side (mantle may cover)
head reduced
mantle cavity extended along sides of foot
most diverse class of molluscs
variety of body forms and names: snails, limpets, slugs, whelks, conchs, periwinkles, sea slugs, sea hares, sea butterflies
generally slow moving or sedentary with heavy shells
Shells:
always univalve (single piece)
may be coiled or uncoiled
top is apex and contains the oldest and smallest whorl
whorls become larger as they spiral down the columella (central axis)
open is aperture and may be covered by operculum
Class Gastropoda
Diversity
Major groups of gastropods:
Prosobranchs: marine snails, and some freshwater and terrestrial groups
Opisthobranchs: sea slugs, sea hares, sea butterflies, bubble shells
Pulmonates: land snails and freshwater snails (only one that isn’t marine, breathes through a rudimentary lung)
General Aspects (biology/morphology)
common bivalves include valms, oysters, mussels, and scallops
marine and freshwater (majority marine)
majority are benthic filter feeders
adult shell sizes are bivalves vary from millimeters to over a meter long
lack head, radula, and cephalization (concentration of sensory structures)
shells always in two parts
hinge ligament at dorsal side
drawn together by adductor muscles
umbo is oldest part of shell
Forms and function
Feeding and digestion
most will feed by creating cillary currents that draw water and suspended food particles over gills (filter feeding)
as water is drawn in, mucus is secreted to entangle food particles
mucus and food slide down glls to good groove at base of gills
Reproduction:
sexes usually separate
for many, fertilization is external, and gametes are shed into water
freshwater forms have internal fertilizations
many produce glochidia larvae
live parasitically on gills of fish
some discharge glochidia into water
others attract fish and cast glochidia directly at fish gills
Class Cephalopoda
General Aspects (Biology/Morphology)
Common names: squids, octopus, nautiluses, devilfish, cuttlefish, and extinct ammonites
all are marine
found at great depths and shallow intertidal zones
sfdhs (??)
foot is merged with head
forms funnel for expelling water and a crown of tentacles
may be small (2-3 cm) or large (19m).
ancestrally, shell was present and straight
derived forms have curved, coiled, reduced, or absent shells
shell coiled in nautiloids and extinct ammonoids
although heavy, made by gas filled chambers
chambers walled-off from one to another and only last is inhabited
shell internal and curved in cuttlefish
squids reduced to the pen (glassy)
absent in octopuses
Form and Function
Nervous system:
more elaborate than other molluscs
largest brain of any invertebrate
senses are well developed
eyes are single-lens, large, and complex
no color vision; but superb underwater resolution
Communication:
through chromatophores (cells with pigments)
cell shape can be changed, thus the amount of pigment shown can also be changed
allows for rapid change in appearance