PHYLUM MOLLUSCA

Cephalopods, Gastropods, Bivalves

and their Relatives

Characteristics of the GroupCharacteristics of the Group

• Most numerous phylum in the ocean by number of species (200,000)

• Soft bodied with a calcium carbonate shell• Bilaterally symmetrical but not always• Most have a head region with eyes and other sensory organs

Other Characteristics of Other Characteristics of MolluscsMolluscs• Unsegmented• Molluscs are coelomates, although the coelom is reduced• Separate mouth and anus; have salivary and digestive

glands that release digestive enzymes• Gas exchange through paired gills• Ubiquitous in marine environments; can be found from

intertidal zones to the deepest oceans• Have a head, foot, and body in some arrangement

http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php

Unique StructuresUnique 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 (tough carbohydrate) that are used in feeding

Mollusc Body PlanMollusc Body Plan

Gill

Mantle

Heart

Nervous System

Gonad

ShellStomach

Intestine

Digestive Gland

Foot

Radula

Types of MolluscsTypes of Molluscs• Gastropods –

• Snails, limpets, abalones, nudibranchs, whelks

• Bivalves – • Clams, mussels, oysters

• Cephalopods – • Octopus, squid, cuttlefish

Mulluscs: The Survival Game

“The ability to change in the ever-changing world is perhaps the greatest secret in the survival game.”•Watch the video: http://vimeo.com/37325960•Identify the evolutionary advances in the three types of molluscs.

GastropodsGastropods• Means “stomach footed”• Most numerous and varied group of molluscs • (~75, 000)

• The only group to invade terrestrial niches

• Include snails, limpets, abalones, and nudibranchs

Characteristics of GastropodsCharacteristics of Gastropods• “Coiled mass of vital organs surrounded by a dorsal shell”• Have a ventral foot typically used for crawling; may be

modified for burrowing, leaping, swimming, or clamping• 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 (nudibranchs).

Parts of a Gastropod ShellParts of a Gastropod Shell

Gastropod ShellsGastropod ShellsLeft-handed or right-handed?

NudibranchsNudibranchs(http://www.youtube.com/watch?v=nHVoV0MVwSc)

• 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.

BivalvesBivalves

• 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• Many bivalves use their foot for burrowing

ClamsClams

• 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

Clam AnatomyClam Anatomy

Mussels & OystersMussels & Oysters

• Instead of burrowing, they secrete byssal threads that attach them to rocks and other surfaces.

• Oysters cement their left shell to a hard substrate; sometimes other oysters creating oyster reefs

• Pearl oysters - thin layer of calcium carbonate coat irritating particles or parasites lodged between the mantle and the inner shell

Oyster Anatomy

Other BivalvesOther Bivalves• Some bivalve bore into coral, rock, or wood. • The shipworm bore into wood and have symbiotic bacteria

in their gut to digest the wood.

CephalopodsCephalopods

• Means “head-footed”• About 650 species• Body such that head is

between “feet” and body.• Includes octopuses, squid,

cuttlefishes, and the chambered Nautilus

Giant Pacific Octopus

Cephalopod CharacteristicsCephalopod 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• Body protected by thick muscular mantle• Move by forcing water out of the siphon, a flexible, funnel-

shaped tube on the side of the head.

OctopusesOctopuses

• 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

SquidSquid

• 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

CuttlefishesCuttlefishes

• Resemble squid but with a flattened body and a rippling fin surrounding the mantle.

• Have a calcified internal shell or “cuttlebone.”

The Chambered NautilusThe 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

Molluscan DigestionMolluscan Digestion

• Separate mouth and anus• Have salivary and digestive glands• Most use their radula to scrape, drill, cut, or capture prey.

• Bivalves – radula is absent and food particles are ingested and filtered by cilia on the gills; have a crystalline style in their gut that secretes enzymes.

Molluscan CirculationMolluscan 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.

Heart

Molluscan Nervous SystemMolluscan Nervous System• Gastropods and bivalves have ganglia – cluster of nerve

cells• Cephalopods - large fused brain that is divided into

regions that control particular functions and behaviors.• Octopus and cuttlefish can learn (http://www.youtube.com/watch?

v=9kuAiuXezIU) .• Sophisticated color changes are believed to be coordinated by the

nervous system. (http://www.youtube.com/watch?v=2x-8v1mxpR0, http://www.youtube.com/watch?v=fR7Dqf0vzzQ )

Mollusc ReproductionMollusc Reproduction

• Most have separate sexes, but some are hermaphrodites

• 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).

• Cephalopods lay eggs and the young develop from the yolk. Female octopus usually die after laying and guarding eggs – most likely due to starvation.

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.