Transcript Lecture 08, molluscs 1, simplified - Cal State LA

Nephrozoan animals are
divided into 2 major lineages
that are distinguishable by
features of their embryos:
Protostomes
Deuterostomes
Protostomes
Platyhelminthes
(flatworms)
In 2 most
speciose groups,
coelom has been
greatly reduced
Annelid
worms
Molluscs
Nematodes
Arthropods
Replaced by:
- open circulatory system
- exoskeleton for muscle
attachment
Phylum Mollusca
~100,000
species
Soft body divided into 3 parts:
a) foot, for locomotion
b) visceral mass = internal organs
c) mantle, a flap of tissue that secretes the shell
- Open circulatory system spouts blood through a body cavity
called hemocoel (replaces coelom, not needed due to shell)
- Ctenidia = flaps used as gills, for respiration (getting oxygen)
- Radula = toothed ribbon used to scrape food
- Develop through a larval stage called a trochophore, like
annelid worms, to a unique veliger larva
Phylum Mollusca
Class Aplacophora - Small + wormlike, primitive (no shell)
Class Monoplacophora - Deep sea, cap-like shell; segmented?
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, cuttlefish, nautilus
Class Scaphopoda - Tusk shells
Molluscan Body Plan
Ventricle
Coelom
Mantle
Mantle
cavity
Visceral nerve Pedal nerve
Mantle cavity encloses gills; receives waste, gametes;
water may be pumped through it by cilia, or muscle action
Shells
Mantle tissue expresses proteins that cause calcium carbonate
to crystallize out of the ocean water as calcite or aragonite,
different crystalline forms
- form layers of minerals + conchin, a cross-linked protein
- inner layer of pearly nacre in primitive groups
Shell = physical defense against predators
Also acts as a type of exoskeleton, to which
muscles can attach and work off of
- molluscs no longer need a coelom
as as internal skeleton; coelom is
very reduced, to a sac around heart
Shells vary among classes
Chitons: 8 separate plates
Bivalves (clams): 2 shells, hinged
at dorsal umbo
Gastropods (snails): 1 twisted shell
with an operculum (trapdoor)
Cephalopods:
nautilus 1 shell

internal shell
octopus
no shell

squid
Gastropod Shell
each turn
= 1 whorl
trapdoor =
operculum
Made of:
aperture
most spiral
to the right
(clockwise)
(1) outer organic layer
(2) chalky calcium carbonate layer
(3) inner pearly nacreous layer (missing in most
higher gastropods)
Spire = all the old whorls
added up together
Columella
Body whorl = most recent
whorl, where most of the
body is located
Shell Diversity
Siphonal opening
Look for spines or ridges, which prevent predatory crabs from
getting their claws around shell to crack it open
Radula
Unique molluscan characteristic: ribbon of chitonous teeth
- projects from bucchal cavity in throat
- anchored over odontophore structure
- moved by protractor (out) and retractor (in) muscles
- tips of teeth may be hardened by iron deposits
May add up to 5 rows of teeth per day
Can be used for herbivory, scraping algae, diatoms off rocks
Can be used for predation (rasping sponges, tissue from other
animals, carrion); may be highly modified (poison-injecting
teeth of cone snails)
Radula
Marginal
teeth
Median tooth
Vetigastropods:
- primitive condition, w/
many marginal teeth
Caenogastropods:
- advanced condition, w/
no marginal teeth
- these sweep particles
into central furrow, where
they’re caught on median teeth
- median teeth used for
tearing, punching holes
- protractors extend the
radula, odontophore
- retractors withdraw,
scraping surface with
teeth as radula pulled
back inside its sac
Digestive system
Molluscs have complete guts:
(1) buccal cavity (mouth) containing radula
(2) esophagus links foregut to stomach
- salivary glands to lubricate radula
- gizzard to help grind up vegetation in some herbivores
(3) stomach with finger-like extensions called digestive ceca or
diverticula that weave into many regions of the body
(4) intestine leading to anus, which opens into mantle cavity
- also used in
filter feeding
Ctenidia
Ctenidia (gills)
grow from
wall of mantle
cavity
O2-poor blood enters
through afferent vessel
O2-rich blood leaves
through efferent vessel
Ciliary beating moves water past incoming O2-poor blood
Veliger larva
Eggs develop into a trochophore, like annelids
Trochophore may develop in the plankton (abalone, limpets) or
within a benthic egg capsule (all others) produced by mom
Next stage is veliger larva, unique to molluscs
Mantle
Eye
Velum
cilia
Statocyst
Propodium
Operculum
Velum
shell
eyespots
Ciliary currents used for:
-- swimming
-- catching phytoplankton
cells for food
Bands of cilia
Food
groove
Phylum Mollusca
Class Aplacophora - Small + wormlike, primitive (no shell)
Class Monoplacophora - Deep sea, cap-like shell; segmented?
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, cuttlefish, nautilus
Class Scaphopoda - Tusk shells
Class Aplacophora
Class Monoplacophora
No foot, head, tentacles
Rodlike spicules, instead of shell
Split off early, before shell
evolved... or lost it later?
Single shell, “segmented”
arrangement of organs?
Phylum Mollusca
Class Aplacophora - Small + wormlike, primitive (no shell)
Class Monoplacophora - Deep sea, cap-like shell; segmented?
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, cuttlefish, nautilus
Class Scaphopoda - Tusk shells
Class Polyplacophora: Chitons
1,000 spp.
- Marine, mostly intertidal
- No eyes, tentacles, or crystalline style in gut
- Paired ctenidia in mantle cavity
- may coat radular teeth with iron deposits for strength
Synapomorphies:
- 8 valves (dorsal shell plates) with articulamentum layer
- Mantle forms thick girdle: may secrete calcareous spicules
Chiton external anatomy
spicules imbedded
in girdle
tail valve, showing inner
articulamentum layer
(outer layer)
Chiton internal anatomy
Water flow through mantle
cavity is controlled cilia
on ctenidia
Phylum Mollusca
Class Aplacophora - Small + wormlike, primitive (no shell)
Class Monoplacophora - Deep sea, cap-like shell; segmented?
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, cuttlefish, nautilus
Class Scaphopoda - Tusk shells
Old Systematics
Class Gastropoda - Snails + slugs
Subclass Prosobranchia (marine snails)
Archaeogastropods – limpets + abalone
Mesogastropods – snails
Neogastropods – smarter snails
Subclass Opisthobranchia – sea slugs
Subclass Pulmonata – terrestrial snails + slugs
Modern gastropod phylogeny
Orthogastropoda
opisthobranchs
(sea slugs)
pulmonates (land snails + slugs)
Gastropoda: Snails + slugs
~ 70,000 spp.
Asymmetrical body usually inside a coiled shell; undergo
torsion during development, to fit body into spiral shell
Successfully colonized dry land
1) muscular foot for movement
2) radula for herbivory on plants: ready food supply
In heterobranchs, ctenidia replaced with other gill structures
- sea slugs have special posterior gills
- pulmonates have lungs for breathing air
Head with eyes, 1-2 tentacles, complex radula
Very complex reproductive systems
Torsion
Developmental process that allows body to fit into spiral shell
with both anus and head at the opening
In late veliger stage, visceral mass rotates 180o over the foot,
counter-clockwise
Proceeds in 2 steps:
- first a 90o twist, done by foot retractor muscle
- second 90o twist by differential tissue growth on the 2 sides
Combined with spiral shells, led to reduction or loss of organs
on right side of adult body (ctenidium, atrium, nephridium)
- sole exception is the gonad, which remains on the right
(we’ll see why in a bit)
Torsion
PRE-TORSION
POST-TORSION
mantle cavity, anus go from posterior to anterior
Torsion
Molluscs have typical protostome paired, ventral nerve chords
Nerve chords end up twisting around in a figure-8
Gastropod Reproduction
Basal gastropods are dioecious (have separate sexes)
In derived gastropods, the right nephridium (kidney) was lost
during evolution of torsion, along with most right-side organs
- its plumbing and pore to the outside was taken over by
the gonad, however
- gonad doesn’t share ducts and pore with the excretory
system, as is typical for many protostomes (e.g., annelids)
- allowed complex reproductive systems to evolve, including
egg capsules for embryos
Derived heterobranch gastropods evolved hermaphroditism
Vetigastropoda
Abalone, top snails, keyhole limpets
- Basal gastropods
- Shell w/ nacre layer on inside
- Radula for herbivory, w/ numerous
teeth in rows
- Mantle cavity lacking a siphon
- Most organs are paired
- Separate sexes; most are broadcast spawners
Wavy Top Shell,
Astrea undosa
Vetigastropods
Giant Keyhole Limpet,
Megathura crenulata
Tegula sp.
Abalone (Haliotis)
excurrent
tremata
epipodial
tentacles
cephalic
tentacles
red abalone
pink abalone
green abalone
black abalone
Broadcast spawning in abalone
Males and females spawn gametes
into the surrounding sea water
Sperm navigate owards,
then bind to egg
In more derived (advanced) gastropods, fertilization is internal
Abalone larval settlement
Abalone zygotes pass through a swimming trochophore stage,
then become veliger larvae, and finally settle + metamorphose
on encrusting coralline red algae – which gives the shells
of red abalone their reddish coloration