Molluscs

Development of the coelom

• In the precambrian era, the most complex animals in the seas were acoelomate

• With time, some animals developed a more elaborate anatomy, a space called a coelom– It is lined with mesoderm, and organs within it are

suspended by mesenteries– The coelom is an efficient hydrostatic skeleton, with

circular and longitudinal muscles acting against each other• Development of the coelom was a major step in the

evolution of the larger and more complicated phyla, beginning with the molluscs

Characteristics of the Phylum Mollusca

• A very large phylum with 90,000 living species and 70,000 extinct fossil species– Many more molluscs have not been formally

named yet• While many have a shell, the body is soft• This phylum includes chitons, tooth shells,

snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squid, octopuses and nautiluses

Representative Molluscs

Mollusc Characteristics

• Some molluscs such as Tridactna (the giant clam) may weight (counting the shell) as much as 900 pounds.

• Squid may be over 50 feet long; many molluscs, however are about 2 inches

• Shell collecting has been a popular hobby for many people for hundreds of years– Native Americans in the Gulf of Mexico region used

seashells as money in trade with neighboring tribes

Classes of Molluscs

• Gastropoda - snails with shells and slugs• Bivalvia - clams, oysters• Polyplacophora – chitins• Cephalopoda – squid, nautiluses, octopuses• Monoplacophora• Scaphopoda• Caudofoveata• Solenogastres

Relationships among Molluscs

Ecological Relationships

• Molluscs are found from the tropics to the polar seas• Most live in the sea as bottom feeders, burrowers,

borers, grazers, carnivores, predators and filter feeders• Fossil evidence indicates that molluscs evolved in the

sea and most have remained marine• Some bivalves and gastropods have moved to estuaries

(brackish) and freshwater• Only snails have been successful living on land, they are

most common where the habitat is moist and sheltered, with calcium available in the soil for their shells

Economic Importance

• Oysters “on the half-shell” are a way of life in Florida (and other states as well)

• Digging clams at low tide in coastal states provides much needed income

• Culturing of oysters for pearls is probably a billion dollar industry; in Japan in the 1960’s, you could pay $1 for an oyster and were guaranteed a pearl

• - There are freshwater pearls too!!• Teredo, the burrowing worm (but really a mollusc), causes

untold damage to wooden docks, as well as wooden boats• Snails and slugs are unwelcome guests in the garden

Mollusc Body Plan• The head-foot portion contains the feeding,

sensory, and locomotor organs• The visceral mass portion contains digestive,

circulatory, respiratory, and reproductive organs• The protective mantle is formed by two folds of skin• The space between the mantle and body wall is the

mantle cavity• Within the mantle cavity are the gills (or lungs, in

the case of pulmonate snails)• In most molluscs, the mantle secretes a protective

shell over the visceral mass

Anatomy of a generalized Mollusc

The head-foot complex

• Most molluscs have a well-developed head bearing the mouth and some sensory organs

• Photosensory receptors range from simple eyes to complex eyes such as in the squid

• Tentacles may be present• Posterior to the mouth is the chief locomotor

organ, the foot

The Radula (Fig. 10.4)• The radula is found only in the molluscs; it is found in all

except bivalves, some gastropods and solenogasters• It is a protruding, rasping (like a wood file), tongue-like

organ• On the radula are rows of tiny teeth, up to 250,000,

pointed backward• By a filing action, the radula rasps off fine particles of

food material from surfaces, but can also drill a hole through hard shells

• The radula moves the particles of food to the digestive tract

• Movements of the radula are supported by complex muscles and cartilages (called the odontophore)

The molluscan foot

• The foot is usually ventrally located• It can be used to attach to the bottom of the

ocean or lake (called the substratum) or can also be used in location (clam vs squid or the tortoise and the hare)

• Modifications of the foot include the attachment disc of limpets, and the siphon jet of squid)

• The foot often secretes mucus which aids in adhesion or in helping some molluscs glide on cilia

The Visceral Mass

Mantle and Mantle Cavity• A mantle is a sheath of skin on each side of the body, it secretes

the shell when present• The mantle cavity contains the gills or lungs that develop from

the mantle• The exposed surface of the mantle also serves for gaseous

exchange• Digestive, excretory, and reproductive products are emptied into

the mantle cavity• In aquatic molluscs, a continuous flow of water brings in oxygen

and food, and flushes out wastes• Cephalopods (squid) use the head and mantle cavity to create jet

propulsion

The Shell

• When present, the shell is secreted by the mantle and is lined by it

• The outer layer of the mantle is called the periostracum, and is composed of conchiolin (a tough membrane made of protein)

• The middle of the shell is the prismatic layer made of calcium carbonate

• The periostracum protects the shells of freshwater molluscs from acid caused by decaying leaves

• The shell appears during the larval period and grows throughout life

The shell and mantle of a bivlave

Internal Structure and Function

• The open circulatory system includes a pumping heart, blood vessels and blood sinuses– The open circulatory system is the most common

type in molluscs• Most cephalopods have a closed system with a

heart, vessels, and capillaries• The digestive system is quite complicated and

specialized for different feeding habits

Internal Structure and Function

• Most molluscs have a pair of kidneys called metanephridia

• The kidney ducts also discharge sperm and eggs• The nervous system has pairs of ganglia with various

specialized sense organs• Most molluscs are dioecious (separate sexes) but some

gastropods are hermaphroditic• The egg hatches into a free-swimming trochophore

larvae• In many gastropods and bivalves, there is an

intermediate stage, the veliger, wich occurs next

Trochophore Larva

Class Monoplacophora

• Thought to be extinct, a living specimen was found in 1952; a dozen species are now known

• Small molluscs with rounded shells resembling limpets

• Some indication of segmentation (or metamerism) suggesting the ancestor was also segmented

• Neopilina, however, only shows pseudometamerism

Class Monoplacophora

Class Polyplacophora (chitons)

• Flattened mollscs about 2 inches long, with 8 overlapping dorsal plates

• Most prefer rocky intertidal surfaces, and survive by scraping algae from the rocks with their radula

• The mantle extends around the chiton margin, forming a girdle

• Gills are suspended from the roof of the mantle cavity and grooves form a closed chamber so water flows from anterior to posterior

• The plates have photosensitive structures to perceive light• Osphradia serve as a sense organ to sample water in the

mantle cavity• Sexes are separate; a trochopore larva is present

Anatomy of a Chiton

Scaphopoda (the tooth shells)

• Live in the marine environment• Slender body covered with a mantle; the shell is open

at both ends• Foot protrudes from the larger end to burrown into md• The foot and ciliary action move respiratory water

through the mantle cavity• Gills are absent and gaseous exchange occur via the

mantle• Detritus and protozoa are caught on cilia on the foot or

the mucous-covered knobs of the tentacles

Dentalium, a scaphopods

Class Gastropoda

• The most diverse of the molluscs; contains 70,000 living and 15,000 extinct species

• Includes snails, slugs, limpets, whelks,conches, periwinkles, sea slugs, sea hares, sea butterflies

• From marine forms with ancestral features to air-breathing terrestrial snails and slugs

• Gastropod shells are one-piece univalves, coiled or uncoiled

Torsion in the gastropods

• All living gastropods have descended from ancestors which were both coiled and torted, whether they show those features today or not

• Torsion is a twisting process that begins with the veliger larva, changing it from a bilateral to an asymmetrical form

• The mantle cavity moves from the rear position (shown in the picture of the ancestral mollusc) and comes to rest over the head, facing towards incoming water

• When torsion is completed, the gills, anus, and reproductive glands

Torsion in the gastropods• It is not known what the original advantages were to

ancestral gastropods in adopting torsion• It may have some advantages and disadvantages– An advantage may be that the gills face fresh incoming

water with high oxygen content– The disadvantage is the waste products from the anus

come out just above the head where sensory organs are located

– However, the mantle cavity is designed in such a way that wastes are quickly dispersed into the water

– Also, the sperm and eggs are also sent out into the water in the same way

Torsion and coiling in the gastropods

Feeding habits of gastropods

• Having a radula results in much variation in how snails feed

• Many are herbivorous; some graze on plankton• Some scavenge on decaying flesh, others are active

carnivores using the radula to ear apart prey• Oyster drills and moon snails drill holes in the

shells of bivalves and insert their proboscis to such out tissues and juice

• Species of Conus can deliver a lethal stink to capture prey

Other gastropod structures

• Land snails that are pulmonates have a highly vascular area in the mantle that acts as a lung

• Aquatic pulmonates come to the surface to expel a gas bubble and then inhale by forming a siphon with their mantle

• The sense organs of gastropods include eyes, statocysts, tactile organs and chemoreceptors

• Gastropods include both monecious and dioecious species

Class bivalvia

• This class includes mussels, clams, scallops, oysters and shipworms (Teredo)

• They range from 1 mm to hundreds of pounds• Most are sedentary suspension feeders

dependent on ciliary currents to bring in small particles of food

• They lack a head, radula, or any other kind of cephalization

• Most are marine; some live in freshwater streams, ponds and lakes

Shell morphology• The two shells or valves are held together by a very strong

hinge ligament• The valves are drawn together by strong adductor muscles• The umbo is the bulge, and is the oldest part of the shell with

growth occurring outward in rings

Body and Mantle

• A visceral mass is suspended from the dorsal midline, with gills hanging down on each side

• The foot is attached anteroventrally• The posterior edges of the mantle form

excurrent and incurrent openings– These “siphons” can be very long in clams that

burrow in sand or mud• Bivalves have a three-chambered heart that

pumps blood to the gills, mantle, and kidney

Feeding mechanism of freshwater clams

Anatomy of a freshwater clam

Feeding and digestion

• Suspended organic matter enter the incurrent siphon• Gland cells on the gills secrete mucous which entangles

the food particles• The food particles then slide to food grooves at the

bottom of the gill• Cilia and grooves on the labial palps direct the mucous

mass into the mouth• In the stomach, the mucus and food particles are kept

mixed by the crystalline style which produces enzymes• The stomach sorts food particles and directs them to the

digestive gland for intracellular digestion

Locomotion in the bivalves• The slender foot is extended out between the

valves• Blood is pumped into the foot; it swells and

anchors the bivalve in the mud• Shortening the foot then pulls the clam forward• Locomotion in scallops is accomplished by

clapping their valves to create a jet propulsion effect, the mantle guides the stream of water

• Some mussels don’t move once they complete the larval stage; mussels attach by byssal threads

Reproduction and development• Sexes are usually separate, oysters undergo

protandry where they start out as males and then change sex and become females

• Fertilization of eggs by sperm is external• Embryos develop as trochophore larva, some

then develop into veligers, and end up as spat larval stages before settling down to the bottom and metamorphosing into adults

• The larvae of freshwater clams develop into glochidia which attach to the gills of fish where they live for a short time as parasites

The glochidia of freshwater clams

• The glochidia eventually leave the gills of the fish and settle to the bottom; this hitchhiking helps to increase the distribution of the species

• Freshwater clams are highly sensitive to pollution and thus their survival is in question

Class Cephalopoda

• This class includes squids, octopuses, nautiluses, devilfish and cuttlefish

• All are marine predators• The giant squid is the largest invertebrate

known• The eye of the giant squid (Architeuthis”) is 10

inches in diameter and very sophisticated in structure

Class Cephalopoda

• Cephalopods feed on fish, mollusks, crustaceans, and worms

• The foot is in the head region and is modified for expelling water from the mantle cavity

• Octopuses and cuttlefish secrete venom from the salivary glands

• The beaklike jaws grasp prey; the radula tears off pieces of flesh

The shell of the cephalopod• The shell of the cuttlefish and squid are

enclosed internally within the mantle cavity• The octopus has completely loss the shell• The Nautilus in ancient times, had a shell which

was straight but overtime, has become coiled– The shell of the Nautilus fills with gas which causes

it to stay upright; they can swim– Because the Nautilus may be found swimming as

deep as 400 meters, the atmospheric pressure on the shell may be as high as 600 pounds per square inch

The chamber Nautilus

Body and Mantle of the Cephalopod

• Octopods have 8 arms with suckers; squid have 10 arms, 8 arms with suckers and pair of long retractile tentacles

• Tentacles grasp food causing it to adhere with secretions, not suckers

Body and Mantle of the Cephalopod

• Water enters the mantle cavity through the neck region– Contraction of the mantle edges about the neck expels

water through the funnel– Water currents provide oxygen to the gills, they also

create water propulsion, and carry waste and reproductive products away from the body

• The cephalopod brain is the largest of an invertebrate • Sense organs are well developed, the eyes are

complex, complete with cornea, lens and retina

The eye of the cuttlefish

Special features of cephalopods

• Cephalopods can change color by expanding and contracting chromatophores in the skin

• This process is controlled by the nervous system

• Many of the color changes are associated with alarm signals or mating

• Some deep-sea squids are bioluminescent• Many cephalopods can eject ink which forms a

smokescreen that may confuse an enemy when released

Reproduction

• Sexes are separate• Males display and court females• Fertilization is internal and are attached to

stones or other objects to develop• Some of the octopii provide parental care to

their eggs until they hatch