Published on July 8, 2012
Introduction•Mollusca is the Latinfor “soft”•Includes snails, squids,nautilus, octopus,clams, etc.•2nd largest phylumafter PhylumArthropods (AnimalKingdom)
Phylum Mollusca; the most malleable body plan in the animal kingdom
•Have adapted to a wide variety of habitats •Terrestrial, marine, benthic, and accomplished swimmers.•Molluscs evolved in the sea and most molluscsare still marine. •Some gastropods and bivalves inhabit freshwater. •A few gastropods (slugs & snails) are terrestrial
Edible mussels (Mytilus edulis)•Humans use molluscs in a varietyof ways: •As food – mussels, clams, oysters, abalon e, calamari (squid), octopus, escargot (snails), etc.•Pearls – formed in oysters andclams.•Shiny inner layer of some shellsused to make buttons.
“If a grain of sand, parasite or other foreign particlebecomes trapped between the mantle and the shells’ innersurface, a pearl may form over a period of years. Natural pearlformation is a fairly rare event; perhaps only one oyster in 1,000 islikely to harbor a valuable pearl naturally”
Mollusc Phylogeny • ~50-80K extant species • ~40K extinct species • Fossil records from precambrian period of proterozoic eon (>570my BP)
The first molluscs probably aroseduring Precambrian times. Diverse molluscs found in the early Cambrian. It is likely that molluscs split off fromthe line that led to annelids after coelomformation, but before segmentationappeared.
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 shells2. 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
• 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.
An example of mollusc illustrating their common body plan
The Head-Foot Region• 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 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, attachme nt, or both.• Pelagic forms may have a foot modified into wing-like parapodia.
Mantle and Mantle Cavity
When present, the calcareous shell is secreted by the mantleand 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.
• 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
Mollusc TaxonomyMajor classes Polyplacophora – the chitons Aplacophora- Monoplacophora Gastropoda- snails and slugs Bivalvia- clams, mussel and oysters Scaphopoda- tusk shells Cephalopoda- octopus and squids
Class PolyplacophoraGreek: many plate bearingDefining characteristics: Shell forms as a series of 7to 8 separate plates •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 Tonicella lineata
• 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 cuticleGreek: no shell bearingDefining Characteristic:Cylindrical, vermiform bodywith the foot forming anarrow keel
Two subclasses: Neomeniomorpha or Solenogastres Chaetodermomorpha or CaudofoveataSolenogastres Caudofoveata
•Caudofoveata are small (1-30mm), mainly deep sea molluscs.They are worm-like, lackingshells or distinct muscular feet;they instead have scales andcalcareous spinescalled sclerites, for movement.•live by burrowing through softsediment and feed by lyingvertically in the sediment withjust the mouthparts exposedand taking in passing organicdetritus.
Subclass Solenogaster•do not have true ctenidia, although their gill-like structures resemble them•during development many solenogastres are covered by a spiny scleritomecomprising spines or scale-like plates; this has been likened to the halwaxiidscleritome
Solenogasters feed oncnidaria andctenophores, either suckingtheir bodily fluids or eatingtheir tissue.
Class MonoplacophoraGreek : 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 GastropodaGreek: stomach footDefining Characteristics:1. Visceral mass and nervous system become twisted 90-180 ⁰ (exhibiting torsion) during embryonic development2. 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
• Gastropods show bilateral symmetry, but due to a twisting process called torsion that occurs during the veliger larval stage, the visceral mass is asymmetrical.
Torsion is an anticlockwise twisting of most of the body (the visceral mass) through 180⁰ during early development
• 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.
• 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.
• 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.
• Flamingo tongue snails feed on gorgonians.• Mantle is brightly colored and envelops the shell.
• 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”
• 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.
• 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.
• Slugs lackPneumostome shells • Mantle thickened • Pneumostome – Air intake into mantle cavity
• 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.
Greek: Anterior GillDefining Characteristic:1. Mantle Cavity generally anteriordue 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
Greek: Posterior GillDefining Characteristic:1. Mantle cavity lateral or posterior due to detorsion,or lost Opisthobranchia includes sea slugs, sea hares, sea butterflies, and canoe shells Sea hare Aplysia has large anterior tentacles and a vestigial shell.
– 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 shell 2. Reduction or loss of the operculum 3. Limited torsion during embryogenesis 4. Reduction or loss of the mantle cavity 5. Reduction or loss of the ctenidia
Locomotion is generally made by ciliaand pedal waves along the ventralsurface of the foot Some opisthobranchs, ex. sea hares,can swim in short spurts by flappinglateral folds called parapodia
Latin : LungDefining Characteristic:1. Mantle cavity highly vascularized and othermodified 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
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, vel iger, and spat (tiny bivalve) stages.
In freshwater clams,fertilized eggs developinto glochidium larvaewhich is a specializedveliger. Glochidia live as parasites on fish and then drop off to complete their development. Glochidia – Unique Larval Stage of Freshwater Bivalves
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)
-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 tissuesurrounding the mouth is responsible for food collection , not inthe gills-Much more common in deep water
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
Greek: Fence gill Defining Characteristic: Gills highly modified to form a muscular septum, which pumps water through the mantle cavity for respiration and feedingSmall groups of carnivorous bivalves that feed onzooplanktons and on pieces of decomposing animaltissueAll species are marine and found in very deep waterThe septibranch ctenidium is highly modified, lackingfilaments and forming a muscular septum
Septibranchs feed as organic vacuum (suck small crustaceansand annelids).The stomach is lined with hardened chitin, to grind upingested 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 substrate Possess no ctenidia
The scaphopod shell isnever spirally wound, but rathergrows linearly as hollow, curvedtable; hence known as “toothshell” and “tusk shell”.
-capture small food particles, including Foraminiferans ,from thesurrounding sediment and water using specialized, thin tentacles knownas captacula.
Rear end, through which water is driven into the pallial cavity by ciliary actionBurrowing foot pulling the animal into the ground
Scaphopoda shells (many of them fromVancouver Is, British Columbia) were theshells used to make the North AmericanIndian trade money "Wampum“. Scaphopodshells have been used as decoration, jewelleryand money by many peoples.
Scaphopods have separate sexes, and externalfertilisation. They have a single gonad occupyingmuch of the posterior part of the body, and shedtheir gametes into the water through thenephridium.Once fertilised, the eggs hatch into a free-living trochophore larva, which develops intoa veliger larva that more closely resembles theadult, but lacks the extreme elongation of theadult body.
Class Cephalopoda Greek: Head foot Defining 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 system 3. Foot modified to form flexible arms and siphon 4. Ganglia fused to form a large brain encased in a cartilaginous cranium
• 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.
Cephalopods are the supreme testament to theimpressive plasticity of the basic molluscan body plan.Ctenidia and a radula are present in all cephalopodspeciesThe head and associated sensory organs are extremelywell-developed.Nautilus locomotes by jet propulsion, expelling waterfrom the mantle cavity through flexible, hollow tubecalled the siphon or funnelCephalopods swim by expelling water from the mantlecavity through a ventral funnel.They can aim the funnel to control the direction theyare swimming.
• 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
• Shells of Nautilus and early nautiloid and ammonoid cephalopods were made buoyant by a series of gas • Nautilus shells differ chambers. from gastropod because they are divided into chambers. The animal lives in the last chamber. A cord of living tissue extends through each chamber.
Cuttlefishes have a small curved shell, completely enclosed by the mantle.
The squid shell is also internal, but it is little more than a thin, stiff, proteinaceous sheet, called the pen
FUN FACT: The largestcephalopod Mesonychoteuthishamiltoni, (Fig. 17) called thecolossal squid, is longer than a citybus, while the smallestcephalopod, Idiosepius notoides, thepygmy squid, could fit on yourfingernail.
General Anatomy Cephalopod Anatomy
All cephalopods have arms, but not allcephalopods have tentacles .Octopuses, cuttlefish, and squid have eightnon-retractable arms, but only cuttlefish andsquid (Sepioidea and Teuthoidea) havetentacles (two each).Arms usually have cirri (fleshypapillae/palps), often suckers, and sometimeshooks (modified suckers) along theirundersides. Tentacles are longer than arms, areretractable, and usually have a blade-shapedor flattened tip, called a club, which is coveredin suckers
Anatomy of an Octopus
•Cephalopods have a closedcirculatory system.•Nervous and sensory systems aremore elaborate in cephalopodsthan in other molluscs.•The brain is the largest of anyinvertebrate.•Most cephalopods’ skin containsseveral layers of tiny colored cellscalled chromatophores, whichoverlay reflective cells callediridocytes- responsible forcoloration of skin
CommunicationVisual signals allowcephalopods to communicate. Movement of body and arms Color 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
Cephalopods have an amazing ability to changecolor very rapidly. They accomplish this featusing numerous pigment-filled bags, calledchromatophores. 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.
Cephalopods have two eyes Nautilus spp. Eyes are simple and function as on the pinhole camera principle; no lens While othercephalopods eyes are incredibly similar to mammals
Octopuses are the only cephalopods with a completely protected"closed" cornea. This structure is probably the most sophisticated eye of allinvertebrates and is as complex as the vertebrate eye.Differing eye morphologies in cephalopods. From left, a squid(Loligo), octopus, cuttlefish, and Nautilus. Note the hyponome below theoctopus eye — this is a muscular tube, that when contracted, expels water in ajet, propelling the octopus backwards. The hyponome can be aimed in variousdirections, giving the octopus finer control over its escape route.
Reproduction• 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.
•Most females then lay large yolky eggs in clusters on theocean floor or on any other hard substrate.•Eggs develop by dividing unequally instead of in the spiralpattern of other molluscs. It is thought this is a derived modeof development.•After a period of development within the egg, juvenileshatch out directly without the swimming larval stage commonto many other molluscs. Most males and females die shortlyafter spawning.
The brain. Finally, one of the most intriguing aspectsof cephalopods is their intelligence. With acentralized brain, the largest of allinvertebrates, and highly developed eyes and othersense organs, they are able to remember and learnby example or through trial and error.
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