Download Phylum Mollusca

Phylum Mollusca
Cephalopods, Gastropods, Bivalves
and their Relatives
Characteristics of the Group
• Most numerous phylum in the ocean by # of
species (200,000)
• Soft bodied with a calcium carbonate shell
• Bilaterally symmetrical but not always
• Gas exchange through paired gills
• Ubiquitous in marine environments
• Unsegmented
• Have a head, foot, and body in some arrangement
Unique Structures
• Mantle – thin layer of
tissue that secretes the shell.
• Foot – ventral, muscular
organ that is used in
locomotion (sometimes
modified into tentacles)
• Radula – ribbon of small
teeth made of chitin that are
used in feeding
Mollusc Body Plan
Intestine
Shell
Gonad
Stomach
Heart
Digestive Gland
Mantle
Gill
Foot
Nervous System
Radula
Gastropods
• Means “stomach
footed”
• Most numerous and
varied group of
molluscs
• Include snails,
limpets, abalones, and
nudibranchs
Characteristics of Gastropods
• “Coiled mass of vital organs surrounded by
a dorsal shell”
• Have a ventral foot
• With their radula they inhabit most all
feeding niches – carnivores, detritivores,
herbivores
• Shells can be very elaborate, may be
internal, or could be missing entirely.
Gastropod Shells
Parts of a Gastropod Shell
Nudibranchs
• Sea slugs
• Name
means
“naked gill”
• Have lost
the shell
entirely
• Usually brightly colored
• Can keep and incorporate noxious chemicals and
undischarged nematocysts from prey for defense.
Bivalves
• Have a two valved hinged
shell.
• Body is laterally
compressed.
• No head or radula
• Gills are used to obtain
oxygen and filter food
• Mantle surrounds body.
• Strong muscles are used
to close the valves of the
shell
Clams
• Burrow in sand and
mud
• Use siphons to move
water in and out –
permits eating and
respiring while
buried.
• Some contain
iridescent chemicals
in their tissues
Mussels & Oysters
• Instead of burrowing,
they secrete byssal
threads that attach them
to rocks and other
surfaces.
• Oysters cement
themselves to the
substrate.
• Some bivalves like
shipworms bore into
wood. The wood is
digested by bacteria in the
gut of the organism.
Oyster Anatomy
Cephalopods
• Means “headfooted”
• About 650 species
• Body such that
head is between
“feet” and body.
• Includes octopuses,
squid, cuttlefishes,
and the chambered
Nautilus
Giant Pacific Octopus
Cephalopod Characteristics
•
•
•
•
Agile swimmers
Complex nervous system
Shell is reduced or lost entirely
Foot is modified into tentacles usually
equipped with suckers
• Eyes are well developed
• Move by forcing water out of the siphon, a
flexible, funnel-shaped tube on the side of
the head.
Octopuses
•
•
•
•
Eight arms and no shell
Bottom dwellers
5cm – 9m
Predators on crabs,
lobster, shrimp, small
fish, and other molluscs
• Have a hard “beak”
• Some like the blue-ring
secrete a highly toxic
venom in their bite
• Can emit a dark ink to
confuse attackers
Blue ringed octopus
Squid
• Elongate body
• Mantle contains two
triangular fins.
• Eights arms + two
retractable tentacles
• Shell reduced to a
chitinous pen inside the
mantle
• Few cm to 12 m
Cuttlefishes
• Resemble squid but
with a flattened body
and a rippling fin
surrounding the
mantle.
• Have a calcified
internal shell or
“cuttlebone.”
Cuttlefish vs. Squid
The Chambered Nautilus
• Have a smooth coiled shell
up to 25 cm in diameter.
• Chambers in the shell are
filled with gas to provide
buoyancy
• Has 60-90 short, suckerless
tentacles for capturing fish
• Living fossil
Other Molluscs
• Chitons (Polyplacophora)
• Tusk shells (Scaphopoda)
• Monoplacophorans
Molluscan Digestion
• Separate mouth and anus
• Have salivary and digestive glands
• Use their radula to scrape, drill, cut, or capture
prey.
• Digestion is extracellular and intracellular.
• Bivalves have a crystalline style in their gut that
secretes enzymes.
• Some can even retain ingested chloroplasts that
continue to photosynthesize and produce food
for the mollusc
Mollusc Circulation
• Open circulatory system
– Muscular heart pumps blood to all tissues.
• Cephalopods – closed circulatory system
– Blood contained in vessels
– More efficient – meets the demands of large
organs such as the brain.
Mollusc Nervous System
• Gastropods and bivalves have ganglia.
• Cephalopods have a large fused brain that
is divided into regions that control
particular functions and behaviors.
• Octopus and cuttlefish can learn.
• Sophisticated color changes are believed to
be coordinated by the nervous system.
Mollusc Reproduction
• Some have external fertilization (tusk
shells, bivalves, chitons, & some
gastropods)
• Others have internal fertilization - a
spermatophore, or sperm packet is
deposited into the female via a modified
arm (cephalopods) or a penis (gastropods).
Mollusc Reproduction continued…
• Some create a trocophore larvae which
develops into a veliger (a planktonic larva
with a tiny shell)
• Cephalopods lay eggs and the young
develop from the yolk. Female octopus
usually die after laying and guarding eggs
– most likely due to starvation.