Download Lecture 10, molluscs 3 - Bivalvia, Cephalapoda

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 Bivalvia
~20,000 species
Shell of 2 valves, left + right; hinged by ligament on dorsal side
- held together by adductor muscles
Byssal threads (protein) anchor some to hard substrata
Reduced head, no radula; most are filter-feeders
No eyes on head, but can have
numerous eyes on body
1 large pair of ctenidia used in
filter feeding + gas exchange
Large mantle cavity, often with
mantle edges fused into large
siphons for water intake
Class Bivalvia – clams, mussels
~20,000 species
Shell with 2 valves, held together by powerful
adductor muscles (the meat of a scallop)
Shell
Labial palp,
scrapes food
off gills and
into mouth
foot
Ctenidia = gills,
used also in
filter feeding
Class Bivalvia – clams, mussels
Shell with 2 valves, held together by powerful
adductor muscles (the meat of a scallop)
Shell
Posterior
adductor
muscle
Anterior
adductor
muscle
foot
Foot used for digging
into bottom
Bivalve shell: Muscle scars
hinge ligament
pedal
retractor
posterior
adductor
muscle
umbo
anterior
adductor muscle
pallial line
serrations
Muscle scars can be identified inside a bivalve shell
Muscles & ligaments
control the opening and
closing of the bivalve shell
When muscles relax, inner ligament
pushes out
shell opens
When muscles contract, inner
ligament is compressed
shell
closes tightly
a relaxed clam opens its shell;
staying shut takes energy
Self-test : know the major anatomical features
of a bivalve
Bivalves filter-feed with their gills
to mouth
Cilia (tiny hairs) beat to
generate water currents
(like sponge choanocytes)
Trap tiny particles, which are
then carried to the mouth
Use their ctenidia (gills) for both filter feeding & respiration
- clams burrow into mud/sand; use siphon to draw in water,
pass it over their gills for both oxygen and food
Food particles are swept into food groove at base of W
- scraped off, sorted by labial palps
- passed to mouth
- unsuitable particles rejected as pseudofeces
food groove
Foot is a thin, grooved appendage used to pour the
fast-hardening liquid prottein that forms byssal threads
Byssus
- foot is held firmly against a hard surface (rock, other shells)
- gland secretes a liquid protein that drips down foot groove
- protein hardens into tough thread, sticks out of gland
- foot releases from surface, then re-planted and repeat process
Anatomy of the Mussel Mytilus
mantle
Byssal gland
Byssal threads
posterior
adductor
muscle
ctenidium
foot
palp
anterior adductor muscle is greatly reduced
ctenidia are thin flaps that fill most of the shell
Anatomy of the Mussel Mytilus
Mussels: Dominant Spatial Competitors
use fast-hardening
protein to form
byssal threads
that glue them
to rock surface
Mussels cover rocks in the intertidal zone
out-compete other organisms for space, unless their #’s
are limited by predators
Space Invaders
The ability of bivalves to occupy space, reproduce in huge
numbers, and efficiently filter feed has made them particularly
disruptive invasive species in many ecosystems
- devastatingly harmful, both ecologically and economically
Dreissena (zebra mussel)
– spread from Europe to
Great Lakes, down Mississippi
to 20 states
– economic loss of >$5 billion
so far
Space Invaders
The ability of bivalves to occupy space, reproduce in huge
numbers, and efficiently filter feed has made them particularly
disruptive invasive species in many ecosystems
- devastatingly harmful, both ecologically and economically
Dreissena
– spread from Europe to Great Lakes to 20 states
(zebra mussel) – economic cost: >$5 billion to date
Potamocorbula – alien clams filter San Fran Bay so thoroughly,
eliminated the spring phytoplankton bloom
Musculista – Asian Date mussel, advancing across the globe
The giant clam genus Tridacna includes the largest bivalves
- may be 4 feet long, >400 pounds, 100 years old
Inhabit nutrient-poor tropics:
not much to filter feed on
Mantle tissue is filled with
zooxanthellae symbionts,
same as hard corals
- release fixed carbon
to fuel clam growth
- now endangered
from over-fishing
Unionids: Endangered freshwater mussels
Freshwater mussels evolved “vampire larvae” called glochidia
that clamp onto fish gills and drain nutrients from their host
- larvae don’t get washed downstream,
have plentiful food source
Females show amazing adaptations to
lure their host fish close enough to
spray larvae into the fish’s gills
glochidia
- most Unionids have one fish species that acts as a host
- many Unionids are now endangered as a result
of human activity that disrupts freshwater ecosystems
Freshwater mussels (Unionids)
edge of mussel’s mantle
female
mussel
actual prey fish
Freshwater mussels mimic small fish, insects with parts of their
body to lure larger, predatory fish  spray vampire-larvae
into big fish’s gills, where the larvae drink its blood!
Glochidia larvae
(220 mm)
...Glochidia are released into the
fish’s face, where they clamp
on to the thin gill tissue
Other species
produce clumps
of eggs called
ovisacs that mimic
insect larvae
- these mimic
blackfly larvae
clinging to rocks
When a fish bites the ovisac, it ruptures, releasing
glochidia into the fish’s gills
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 Cephalopoda
SubClass Nautiloidea (chambered shell)
Subclass Coleoidea
shell reduced
to internal
remnant
Order Teuthoida - squids
Order Sepioida - cuttlefish
complete loss of shell
Order Octopoda - octopuses
Class Cephalopoda
~900 species
Nautilus, squid, octopus
Most intelligent invertebrates, complex eyes
- Only molluscs with closed circulatory system: hunt by
zooming backwards by high-speed jet propulsion
- flex mantle muscles, forcing water out of siphon
- Defense without a shell: inking, color + texture change
- Foot divided into prehensile tentacles with flexible suckers
nautilus
(external shell)
squid
(thin, internal shell)
octopus
(no shell)
Chambered Nautilus
siphuncle
Nautilus lives at >600 meters deep; comes to surface at night
to hunt, catching prey with its tentacles
- pumps gas into sealed chambers through tube called
the siphuncle, to adjust buoyancy (floaty-ness)
Order Teuthoida (squids)
- elongated body with lateral (side) fins
- 8 short arms + 2 long tentacles
Order Teuthoida (squids)
contractile
arms
suckers
siphon
ventral
arms
fin
club of
tentacle
Order Teuthoida (squids)
Squid
Fast-moving mid-water predators
shoot out 2 extra-long tentacles to snag
fish, using their suckers
internal
shell
giant squid = world’s
biggest invertebrate
Giant squid,
Architeuthis
Glow-in-the-dark squid
Bioluminescence results from lightproducing bacterial symbionts
- Vibrio bacteria live in a shuttered organ
the squid can selectively open to emit
desired amount of light
- bacteria glow whenever they are at a
threshold density in the light organ
Euprymna scolopes
Glow-in-the-dark squid
Bioluminescence results from lightproducing bacterial symbionts
Mid-water squid use light for countershading to hide from predators below
 nocturnal squid cast a shadow when
there is moonlight, which predators
see and attack from beneath
Euprymna scolopes
 by opening shutters on light organ,
squid releases same amount of light
from ventral side as is hitting its dorsal surface
 no shadow visible to predators beneath them; essentially,
become invisible
Glow-in-the-dark squid
Quorum sensing is how bacteria tell when there’s enough of
them present to start glowing (or doing other things)
- each cell releases a chemical signal
- when enough signal builds up, all cells turn on genes
required for bioluminescence
- studies on this process led to breakthrough understanding
of how bacteria cooperate during human infections:
they wait until quorum sensing indicatesEuprymna
sufficient scolopes
#’s
to turn on pathogenicity genes and mount an attack
- led to a whole new line of research, looking for antibiotics
that disrupts bacterial communication
Loligo, the reef squid
Order Sepioda (cuttlefish)
Eye
Arms
Fin
Tentacle
most precise ability to
mimic background color
of any animal
chromatophores - colored pigment sacs expand or contract
to change color
iridocytes - light reflectors in skin
Order Sepioda (cuttlefish)
Order Octopoda
- body short & round
- no fins
- no internal shell
- 8 similar arms
- mostly benthic
advanced brain with
many distinct lobes
ability to learn, even by
observing other octopuses perform a task
~200 spp.
Octopus
- 8 arms
- no shell
- best vision
- smartest invertebrate
~200 spp.
buccal
mass
ctenidium
Cross-section
of arm
siphon
Octopus’ Compound Eye
Epidermis
Retina
Iris
Cornea
most advanced compound
eye of any invertebrate
image-forming, much like
our camera eyes
sensory cells
pigment cells