Phylum MolluskaPhylum Molluska

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3 Main Classes of MollusksGASTROPODA : includes limpets, snails, slugs

and whelks

BIVALVIA: includes clams, oysters, muscles, scallops

and shipworms

CEPHALOPODA: includes octopi, squid, cuttlefish, nautilus and argonauts

Above: Scallop showing off its eyes!

Left: Cuttlefish and squid

Above: Limpet

All forms of Mollusks have a body composed of at All forms of Mollusks have a body composed of at least three distinct parts:least three distinct parts:

1.1. Visceral MassVisceral Mass- - A soft bodied portion that A soft bodied portion that

contains the internal organscontains the internal organs

2.2. FootFoot- - A strong, muscular portion A strong, muscular portion

used for locomotion (adapted used for locomotion (adapted as tentacles in cephalopods)as tentacles in cephalopods)

3.3. MantleMantle- A membranous, sometimes A membranous, sometimes

muscular covering that muscular covering that envelops but does not envelops but does not completely enclose the completely enclose the visceral massvisceral mass

- Can secrete a shellCan secrete a shell

Illustration of the general features of all mollusks

Identifying Characteristics of Mollusks• Have a Have a coelomcoelom which allows for which allows for specialization of organ specialization of organ

systemssystems• Most are Most are bilaterally symmetricalbilaterally symmetrical• All have a All have a visceral mass, foot and mantlevisceral mass, foot and mantle (see (see

previous slide)previous slide)• Possess a Possess a one-way one-way digestive systemdigestive system with specialized with specialized

organs and have feeding organs and have feeding mouthpartsmouthparts• Cephalopods have a closed Cephalopods have a closed circulatory systemcirculatory system, all , all

other classes possess other classes possess openopen circulatory systems circulatory systems• Many have highly developed Many have highly developed cephalizationcephalization with with sensory sensory

organsorgans• Many have calcium carbonate Many have calcium carbonate external shellsexternal shells• Respiratory systemRespiratory system includes the use of includes the use of gillsgills to extract to extract

OO22 and get rid of waste and get rid of waste• Many have a Many have a radularadula which is a which is a chitinous tonguechitinous tongue used for used for

feedingfeeding• Sexual ReproductionSexual Reproduction (have separate sexes: (have separate sexes: dioeciousdioecious))

The Gastropods : “belly” “foot”The Gastropods : “belly” “foot”• Usually are Usually are univalvesunivalves

(one shell)(one shell)• Large, Large, muscular muscular footfoot• Well definedWell defined headhead• MusclesMuscles move move in a in a

rippling motion for rippling motion for movementmovement

• Most possess Most possess gillsgills but but some have some have lungslungs

• Have a Have a radularadula to help to help obtain foodobtain food

• Most snails have a lid-like Most snails have a lid-like part called an part called an operculum operculum on the back of the foot on the back of the foot so so they can draw their they can draw their bodies into their shell and bodies into their shell and close off the openingclose off the openingUnderside of a Limpet

Gastropods in Love….Gastropods in Love….

.. and a lonely garden slug….

The Bivalves : “two” “shell”The Bivalves : “two” “shell”• Possess Possess 2 shells that 2 shells that

can be tightly closed can be tightly closed with strong muscleswith strong muscles

• Strong, Strong, muscular foot muscular foot and use it for and use it for locomotionlocomotion

• No No head or teethhead or teeth• Obtain food and oxygen Obtain food and oxygen

by by using a muscular using a muscular siphon that brings siphon that brings water to the gills water to the gills which which filter out Ofilter out O22 and plant and plant cell materialcell material

• Often Often attach attach themselves to surfacesthemselves to surfaces

• Possess Possess an open an open circulatory systemcirculatory systemGIANT CLAM

Above left: Elephant trunk clams, named for their large foot

Above right: an opened oyster commonly enjoyed raw or cooked as a delicacy

Lower right: Scallop showing off its many primitive eyes

The Cephalopods: “head” “foot”The Cephalopods: “head” “foot”• Bilaterally Bilaterally symmetricalsymmetrical• Prominent Prominent head, arms head, arms

and tentaclesand tentacles• Have Have developed braindeveloped brain, ,

“intelligent “ creatures“intelligent “ creatures• Arms that take in food Arms that take in food

surround surround hard, strong, hard, strong, beak-like jawsbeak-like jaws that tear that tear the prey and secrete a the prey and secrete a poison to paralyze itpoison to paralyze it

• Closed Closed circulatory system circulatory system with with hemocyanin hemocyanin (like (like hemoglobin)hemoglobin)

• Move Move by jet propulsionby jet propulsion• Can change color with Can change color with

the use the use of of chromatophoreschromatophores

• Possess Possess advanced visionadvanced vision• Secrete Secrete an ink cloud an ink cloud

when in dangerwhen in danger

SquidSquid

Caribbean Reef Squid Tentacles of a giant squid

Calamari: a deep fried squid dish Squid eye and gills

OctopusOctopus

Octopus beak Octopus on the beach

Swimming Octopus Octopus arm in detail

Octopi are extremely intelligent creatures Octopi are extremely intelligent creatures and have been known to learn to escape and have been known to learn to escape from their aquariums when in captivity from their aquariums when in captivity through very small cracks at the top.through very small cracks at the top.

Ecological Roles of MollusksEcological Roles of Mollusks• Mollusks are part of Mollusks are part of the the

aquatic food chainaquatic food chain (see (see picture to left)picture to left)

• Many mollusks are Many mollusks are keystone species: if their keystone species: if their numbers decrease the numbers decrease the decline of other marine decline of other marine species will followspecies will follow

• Filter feeders help to Filter feeders help to purify the waterpurify the water

• Many bivalves are Many bivalves are reef reef forming and provide an forming and provide an ecosystem for other ecosystem for other aquatic speciesaquatic species

• Reef builders Reef builders stabilize stabilize banks and bottom banks and bottom sedimentssediments

Starfish eating (digesting) a mussel

Squid FactSquid Fact

The largest squid ever caught was a 10 m long Colossal Squid off the coast of New Zealand and

weighed in at 495 kg. Colossal Squid are larger than the giant squid found off the Mexican coast and are

thought to grow in size to a whopping 13 meters!