Phylum Mollusca

Introduction

•Mollusca is the Latin for “soft”•Includes snails, squids, nautilus, octopus, clams, etc.•2nd largest phylum after Phylum Arthropods (Animal Kingdom)

Phylum Mollusca; the most malleable body plan in the animal kingdom

Sizes range from almost microscopic to giant squid ( Architeuthis). It can reach a length up to 18 m (normal size)

•Herbivorous, only eat plants•Predaceous Carnivores •Filter Feeder•Detritus Feeders •Parasites

•Have adapted to a wide variety of habitats• Terrestrial, marine, benthic, and

accomplished swimmers.•Molluscs evolved in the sea and most molluscs are still marine.

• Some gastropods and bivalves inhabit freshwater.

• A few gastropods (slugs & snails) are terrestrial

Edible mussels (Mytilus edulis)

•Humans use molluscs in a variety of ways:

• As food – mussels, clams, oysters, abalone, calamari (squid), octopus, escargot (snails), etc.

•Pearls – formed in oysters and clams.•Shiny inner layer of some shells used to make buttons.

“If a grain of sand, parasite or other foreign particle becomes trapped between the mantle and the shells’ inner surface, a pearl may form over a period of years. Natural pearl formation is a fairly rare event; perhaps only one oyster in 1,000 is likely to harbor a valuable pearl naturally”

• A few are pests or introduced nuisances: – Shipworms – burrow through wood, including docks &

ships.– Terrestrial snails and slugs damage garden plants.– Molluscs serve as an

intermediate host for many parasites.– Zebra mussels – accidentally introduced into the Great Lakes and reeking havoc with the ecosystem.

• Shipworms can be destructive to wharves & ships.• The valves have tiny teeth that act as wood rasps and allow

these bivalves to burrow through wood.• They feed on wood particles with the help of symbiotic bacteria

that produce cellulase and fix nitrogen.

Mollusc Phylogeny

• ~50-80K extant species

• ~40K extinct species

• Fossil records from precambrian period of proterozoic eon (>570my BP)

The first molluscs probably arose during Precambrian times.

Diverse molluscs found in the early Cambrian.

It is likely that molluscs split off from the line that led to annelids after coelom formation, but before segmentation appeared.

o“Hypothetical Ancestral Mollusc” oProbably lacked a shell or crawling foot.oProbably small (about 1 mm).oLikely was a worm-like organism with a ventral gliding surface.oProbably possessed a dorsal mantle, a chitinous cuticle and calcareous scales.

1. Dorsal Epithelium forming a mantle which secretes calcareous spicules or one or more shells

2. Cuticular band of teeth(radula) in the esophagus used for feeding (not present-lost? in bivalves)

3. Ventral body wall muscles develop into a locomotory or clinging foot

Defining Characteristics

• Most but not all mollusc have shells consisting primarily of calcium carbonate set in a protein matrix.

• The Molluscan body plan includes– Head-foot– Visceral Mass -containing digestive, circulatory,

respiratory and reproductive organs.

Molluscan Body Plan

An example of mollusc illustrating their common body plan

• Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods)

The Head-Foot Region

The Head-Foot Region

• The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves).– Ribbon-like

membrane with rows of tiny teeth.

The Head-Foot Region

• The foot of a mollusc may be adapted for locomotion, attachment, or both.

• Pelagic forms may have a foot modified into wing-like parapodia.

Muscular foot

Mantle and Mantle Cavity

• The space between the mantle and the visceral mass is called the mantle cavity. – The respiratory organs (gills or lungs) are

generally housed here.

When present, the calcareous shell is secreted by the mantle and is lined by it. It has 3 layers:

• Periostracum – outer organic layer helps to protect inner layers from boring organisms.

• Prismatic layer – densely packed prisms of calcium carbonate.

• Nacreous layer – iridescent lining secreted continuously by the mantle – surrounds foreign objects to form pearls in some.

Pearl is formed between the mantle and the shell.

Internal Structure and Functions

• Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses.

• Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.

• Most molluscs are dioecious, some are hermaphroditic.

• The life cycle of many molluscs includes a free swimming, ciliated larval stage called a trochophore.– Similar to annelid

larvae.

The trochophore larval stage is followed by a free-swimming veliger larva in most species

Major classesPolyplacophora – the chitonsAplacophora- MonoplacophoraGastropoda- snails and slugsBivalvia- clams, mussel and oystersScaphopoda- tusk shellsCephalopoda- octopus and squids

Mollusc Taxonomy

Class Polyplacophora

Greek: many plate bearingDefining characteristics: Shell forms as a series of 7 to 8 separate plates

Tonicella lineata

•Use radula to scrape algae off rocks•Live on rocky intertidal zones•Gills are suspended from roof of mantle cavity.•Water flows from anterior to posterior ; linear digestive tract

• Pair of osphradia serves as sense organ.• Light sensitive esthetes form eyes in some species – pierce plates.• Blood pumped by a three-chambered heart.

– Travels through aorta and sinuses to gills.• Pair of metanephridia carries wastes from pericardial cavity to exterior.• Sexes are separate.• Trochophore larvae metamorphose into juveniles without a veliger stage.

Class Aplacophora

•Quite small– usually only a few millimeters•Entirely marine•No true shell •The body is unsegmented and bears numerous calcareous spines or scales embedded in an outer cuticle

Greek: no shell bearingDefining Characteristic: Cylindrical, vermiform body with the foot forming a narrow keel

Aplacophora

Two subclasses: Neomeniomorpha or SolenogastresChaetodermomorpha or Caudofoveata

Solenogastres Caudofoveata

CaudofoveataAplacophora

•Caudofoveata are small (1-30 mm), mainly deep sea molluscs. They are worm-like, lacking shells or distinct muscular feet; they instead have scales and calcareous spines called sclerites, for movement.•live by burrowing through soft sediment and feed by lying vertically in the sediment with just the mouthparts exposed and taking in passing organic detritus.

SolenogastresAplacophora

Subclass Solenogaster•do not have true ctenidia, although their gill-like structures resemble them•during development many solenogastres are covered by a spiny scleritome comprising spines or scale-like plates; this has been likened to the halwaxiid scleritome

Aplacophora

Solenogasters feed on cnidaria and ctenophores, either sucking their bodily fluids or eating their tissue.

Solenogastres

Class Monoplacophora

Greek : one shell bearingDefining characteristics: 1. 3 to 6 pairs of ctenidia, 6 to 7 pairs of nephridia2. Multiple (usually 8) pairs of foot (pedal) retractor

muscles

-Was known only from fossils until 1952-Entirely marine and all collected from depth at least 2000 m-A single unhinged cap shaped shell is present-The largest species is about 37 mm

Class Gastropoda

Greek: stomach footDefining Characteristics:1. Visceral mass and nervous system become

twisted 90-180 ⁰ (exhibiting torsion) during embryonic development

2. Proteinaceous shield on the foot (operculum) to which columellar muscles attaches

______________________________________Gastropoda is the largest of the molluscan

classes.– 40,000 to 75,000 living species.– Include snails, slugs, sea hares, sea

slugs, sea butterflies.– Marine, freshwater, terrestrial.

• Benthic or pelagic

Gastropoda

• Gastropods show bilateral symmetry, but due to a twisting process called torsion that occurs during the veliger larval stage, the visceral mass is asymmetrical.

Gastropoda

Torsion is an anticlockwise twisting of most of the body (the visceral mass) through 180⁰ during early development

Gastropoda

• The shell of a gastropod is always one piece – univalve – and may be coiled or uncoiled.– The apex contains the

oldest and smallest whorl.

– Shells may coil to the right or left – this is genetically controlled.

Gastropoda

• Coiling is not the same as torsion.

• Early gastropods had a planospiral shell where each whorl lies outside the others.– Bulky

• Conispiral shells have each whorl to the side of the preceding one. – Unbalanced

• Shell shifts over for better weight distribution.

• herbivores and feed by scraping algae off hard surfaces using the radula.

• scavengers of dead organisms, again tearing off pieces with radular teeth.

• carnivores and have a radula modified into a drill to bore through the shells of other molluscs. They use chemicals to soften the shell.

Feeding Habits Gastropoda

• Snails in the genus Conus feed on fish, worms, and molluscs.– Highly modified radula used for prey capture.– They secrete a toxin that paralyzes their prey.

• Some are painful, even lethal, to humans.

Feeding Habits Gastropoda

• Flamingo tongue snails feed on gorgonians.

• Mantle is brightly colored and envelops the shell.

Generalized AnatomyGastropoda

• Pulmonates lack gills.– Have a highly vascular area in

mantle that serves as lung.– Lung opens to outside by

small opening, the pneumostome.

– Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon.

In Terrestrial gastropods: The mantle cavity serves as a

“lung”

GastropodaInternal Form and Functions

• Most have a single nephridium and well-developed circulatory and nervous systems.

• Sense organs include eyes, statocysts, tactile organs, and chemoreceptors.

• Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea.

• Sensory osphradium at base of the incurrent siphon may be chemosensory or mechanoreceptive.

GastropodaInternal Form and Functions

• Respiration in many performed by ctenidia in mantle cavity.

• Derived prosobranchs lost one gill and half of remaining gill.– Resulting attachment to

wall of mantle cavity provided respiratory efficiency.

GastropodaInternal Form and Functions

Pneumostome• Slugs lack

shells• Mantle

thickened• Pneumostome

– Air intake into mantle cavity

Gastropoda

• Monoecious and dioecious species.• Young may emerge as veliger larvae or pass

this stage inside the egg.• Some species, including most freshwater

snails, are ovoviviparous.

Subclass Prosobranchia Gastropoda

Greek: Anterior GillDefining Characteristic:1. Mantle Cavity generally anterior due to torsion

•Prosobranchia includes most marine snails and some freshwater and terrestrial gastropods.•Largest of three subclasses; mostly marine•Generally free-living and mobile; some have evolved sessile or even parasitic lifestyles•Warm water cone snails (Conus sp.) are carnivorous produce potent venoms•Most primitive of gastropods; two other subclasses evolved from prosobranch like ancestors

Subclass Opisthobranchia GastropodaGreek: Posterior GillDefining Characteristic:1. Mantle cavity lateral or posterior due to detorsion, or lost

Opisthobranchia includes sea slugs, sea hares, sea

butterflies, and canoe shellsSea hare Aplysia has large anterior

tentacles and a vestigial shell.

Subclass Opisthobranchia Gastropoda

– Most are marine, shallow-water.– Partial to complete detorsion -

anus and gill(s) are displaced to right side.

– Two pairs of tentacles, one pair modified to increase chemo-absorption.

– Shell is reduced or absent.– Monoecious

Compared to Prosobranchia:1. A trend toward reduction or loss of the

shell2. Reduction or loss of the operculum3. Limited torsion during embryogenesis4. Reduction or loss of the mantle cavity5. Reduction or loss of the ctenidia

Subclass Opisthobranchia Gastropoda

Locomotion is generally made by cilia and pedal waves along the ventral surface of the foot

Some opisthobranchs, ex. sea hares, can swim in short spurts by flapping lateral folds called parapodia

Subclass Pulmonata Gastropoda

Latin : LungDefining Characteristic:1. Mantle cavity highly vascularized and other modified to form a lung

Compare to other subclasses, only few of these species are marine and those few species occur only in intertidal and in estuaries

Subclass Pulmonata Gastropoda

•Pulmonata includes land and most freshwater snails and slugs.

• Ancestral ctenidia have been lost and the vascularized mantle wall is now a lung.

• Air fills lung by contraction of mantle floor.

• Anus and nephridiopore open near the pneumostome.

• Waste is forcibly expelled.• Monoecious• Aquatic species have one pair of tentacles.• Landforms have two pair of tentacles and

the posterior pair has eyes.

Class BivalviaLatin: Two valved [Greek: hatched foot]PelecypodaDefining Characteristics:1. Two valved shell2. Body flattened laterally

-Includes clams, scallops, mussels , shipworms and oysterMostly sessile filter feeders.No head or radula.

Two major subclasses:•Protobranchia•Lamellibranchia

-and one very small subclass;The Septibranchia

• Bivalves are laterally (right-left) compressed and their two shells are held together by a hinge ligament on the dorsal surface.

• The Umbo is the oldest part of the shell, growth occurs in concentric rings around it.

• Part of the mantle is modified to form incurrent and excurrent siphons.– Used to pump water

through the organism for gas exchange and filter feeding.

– Sometimes used for jet propulsion.

• Shipworms can be destructive to wharves & ships.• The valves have tiny teeth that act as wood rasps and

allow these bivalves to burrow through wood.• They feed on wood particles with the help of symbiotic

bacteria that produce cellulase and fix nitrogen.

• Bivalves move around by extending the muscular foot between the shells.

• Scallops and file shells swim by clapping their shells together to create jet propulsion.

• Like other molluscs, bivalves have a coelom and an open circulatory system.

• The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange.

• Scallops have a row of small blue eyes along the mantle edge. Each eye has a cornea, lens, retina, and pigmented layer.

Suspended organic matter enters incurrent siphon.Gland cells on gills and labial palps secrete mucus to entangle particles.Food in mucous masses slides to food grooves at lower edge of gills.Cilia and grooves on the labial palps direct the mucous mass into mouth.Some bivalves feed on deposits in sand.

• Bivalves usually have separate sexes.

• Zygotes develop into trochophore, veliger, and spat (tiny bivalve) stages.

Glochidia – Unique Larval Stage of Freshwater Bivalves

In freshwater clams, fertilized eggs develop into glochidium larvae which is a specialized veliger.

Glochidia live as parasites on fish and then drop off to complete their development.

Subclass Protobranchia

Greek: first gillDefining Characteristics:1. Gills small, functioning primarily as gas

exchange surfaces2. Food collecting by long , thin, muscular

extensions of tissue surrounding the mouth (palp proboscides)

Subclass Protobranchia

-Entirely marine, and all species live in soft substrate- feed on sediments taken in and the organic fraction is digested (deposit feeding)-Palp boscides, long, thin muscular extensions of the tissue surrounding the mouth is responsible for food collection , not in the gills-Much more common in deep water

Subclass Lamellibranchia

Greek: plate gillDefining Characteristics:1. Gills modified to collect suspended food particles, in

addition to serving as gas exchange surfaces2. Secretion of proteinaceous attachment material

(usually in the form of threads) by a specialized gland (the byssus gland) in the foot

Most bivalves are lamellibranchs. Majority are marine, and some belong to Unionidae, freshwater bivalve species.

Commercially important for many years as food (e.g., Oysters and scallops)

Widely used to assess environmental pollution

Subclass SeptibranchiaGreek: Fence gillDefining Characteristic:Gills highly modified to form a muscular septum, which pumps water through the mantle cavity for respiration and feeding

Small groups of carnivorous bivalves that feed on zooplanktons and on pieces of decomposing animal tissueAll species are marine and found in very deep waterThe septibranch ctenidium is highly modified, lacking filaments and forming a muscular septum

Subclass Septibranchia

Septibranchs feed as organic vacuum (suck small crustaceans and annelids).The stomach is lined with hardened chitin, to grind up ingested food.

Class ScaphopodaGreek: Spade footDefining Characteristics:1. Tusk-shaped, conical shell, open at both

ends2. Development of anterior, threadlike,

adhesive feeding tentales

Young group, first appearing in the fossil records in the middle Ordovician (450 years ago)300-400 species,Entirely marine, lives sedentary lives in sand or mud substratePossess no ctenidia

The scaphopod shell is never spirally wound, but rather grows linearly as hollow, curved table; hence known as “tooth shell” and “tusk shell”.

-capture small food particles, including Foraminiferans ,from the surrounding sediment and water using specialized, thin tentacles known as captacula.

Rear end, through which water is driven

into the pallial cavity by ciliary action

Burrowing foot pulling the animal into the ground

Scaphopoda shells (many of them from Vancouver Is, British Columbia) were the shells used to make the North American Indian trade money "Wampum“. Scaphopod shells have been used as decoration, jewellery and money by many peoples.

Scaphopods have separate sexes, and external fertilisation. They have a single gonad occupying much of the posterior part of the body, and shed their gametes into the water through the nephridium.Once fertilised, the eggs hatch into a free-living trochophore larva, which develops into a veliger larva that more closely resembles the adult, but lacks the extreme elongation of the adult body.

Class CephalopodaGreek: Head footDefining Characteristic:1. Shell divided by septa, with

chambers connected by the siphuncle: a vascularized strand of tissue contained within a tube of calcium carbonate (shell reduced or lost in many species)

2. Closed circulatory system3. Foot modified to form flexible arms

and siphon4. Ganglia fused to form a large brain

encased in a cartilaginous cranium

Cephalopoda

• Cephalopods include octopuses, squid, nautiluses and cuttlefish.

• Marine carnivores with beak-like jaws surrounded by tentacles of their modified foot.– Modified foot is a funnel for

expelling water from the mantle cavity.

Cephalopoda

Cephalopods are the supreme testament to the impressive plasticity of the basic molluscan body plan.Ctenidia and a radula are present in all cephalopod speciesThe head and associated sensory organs are extremely well-developed.Nautilus locomotes by jet propulsion, expelling water from the mantle cavity through flexible, hollow tube called the siphon or funnelCephalopods swim by expelling water from the mantle cavity through a ventral funnel.They can aim the funnel to control the direction they are swimming.

Cephalopoda

• Cephalopod fossils go back to Cambrian (570 mya) times.• The earliest had straight cone-shaped shells.• Later examples had coiled shells similar to Nautilus.• Ammonoids were a very successful group, some had quite

elaborate shells.

Anatomy of Nautilus

Cephalopoda• Shells of Nautilus and early

nautiloid and ammonoid cephalopods were made buoyant by a series of gas chambers.

• Nautilus shells differ from gastropod because they are divided into chambers. The animal lives in the last chamber. A cord of living tissue extends through each chamber.

Cephalopoda

Cuttlefishes have a small curved shell, completely enclosed by the mantle.

Cephalopoda

The squid shell is also internal, but it is little more than a thin, stiff, proteinaceous sheet, called the pen

FUN FACT: The largest cephalopod Mesonychoteuthis hamiltoni, (Fig. 17) called the colossal squid, is longer than a city bus, while the smallest cephalopod, Idiosepius notoides, the pygmy squid, could fit on your fingernail.

CephalopodaGeneral Anatomy Cephalopod

Anatomy

All cephalopods have arms, but not all cephalopods have tentacles .Octopuses, cuttlefish, and squid have eight non-retractable arms, but only cuttlefish and squid (Sepioidea and Teuthoidea) have tentacles (two each). Arms usually have cirri (fleshy papillae/palps), often suckers, and sometimes hooks (modified suckers) along their undersides. Tentacles are longer than arms, are retractable, and usually have a blade-shaped or flattened tip, called a club, which is covered in suckers

Anatomy of an Octopus

Cephalopoda

•Cephalopods have a closed circulatory system.•Nervous and sensory systems are more elaborate in cephalopods than in other molluscs.•The brain is the largest of any invertebrate.•Most cephalopods’ skin contains several layers of tiny colored cells called chromatophores, which overlay reflective cells called iridocytes- responsible for coloration of skin

CephalopodaCommunication

Visual signals allow cephalopods to communicate.

Movement of body and armsColor changes effected by chromatophores

Most cephalopods have an ink sac that secretes sepia, a dark fluid containing the pigment melanin.

When a predator tries to attack, the animal ejects the ink into the water where it hangs between the animal and the predator screening a quick escape

Color & Morphology Changes for Camouflage

Cephalopods have an amazing ability to change color very rapidly. They accomplish this feat using numerous pigment-filled bags, called chromatophores.

Chromatophores are found in the skin, and expand and contract to reveal or conceal small dots of color (left). Additionally, an iridescent dermal tissue can also be manipulated by some cephalopods to aid in camouflage, courtship rituals, or accompany color changes.

Cephalopoda

Cephalopods have two eyes

Nautilus spp. Eyes are simple and

function as on the pinhole camera

principle; no lens While other

cephalopods eyes are incredibly similar to

mammals

Differing eye morphologies in cephalopods. From left, a squid (Loligo), octopus, cuttlefish, and Nautilus. Note the hyponome below the octopus eye — this is a muscular tube, that when contracted, expels water in a jet, propelling the octopus backwards. The hyponome can be aimed in various directions, giving the octopus finer control over its escape route.

Octopuses are the only cephalopods with a completely protected "closed" cornea. This structure is probably the most sophisticated eye of all invertebrates and is as complex as the vertebrate eye.

CephalopodaReproduction

• Sexes are separate in cephalopods.• Juveniles hatch directly from eggs – no free-

swimming larvae.• One arm of male is modified as an intromittent

organ, the hectocotylus.– Removes a spermatophore from mantle cavity

and inserts it into female.

CephalopodaReproduction

•Most females then lay large yolky eggs in clusters on the ocean floor or on any other hard substrate. •Eggs develop by dividing unequally instead of in the spiral pattern of other molluscs. It is thought this is a derived mode of development.•After a period of development within the egg, juveniles hatch out directly without the swimming larval stage common to many other molluscs. Most males and females die shortly after spawning.

Direct Development in Cephalopods

The brain. Finally, one of the most intriguing aspects of cephalopods is their intelligence. With a centralized brain, the largest of all invertebrates, and highly developed eyes and other sense organs, they are able to remember and learn by example or through trial and error.

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