Molluscs

Introduction Introduction. One of the largest animal phyla

after the arthropods. 50,000‑100,000 living species, 35,000 extinct.

Mollusca means soft body. Range from fairly simple to some of the most

complex and intelligent inverts., and in size from microscopic to giant squid 18m long, including tentacles. They may weigh 1000 lbs. Shells of some of the giant clams, Tridacna gigas which inhabit Indo‑Pacific coral reefs, reach 1.5m and weigh more than 500 lbs. = extremes. 80% are < 5cm in max. shell size. In cludes some of most sluggish and most active inverts. Very diverse feeding habits ‑ herbivorous grazers, predaceous carnivores, filter feeders, detritus feeders and parasites.

Includes Chitons, tooth shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses and nautiluses.

Molluscan classesPolyplacophor

a (Chiton)Gastropods(snails)

ourworld.compuserve.com/homepages/ BMLSS/molluscs.htm

www.sonic.net/~jmp/ solomon01.shtml

Gastropod (nudibranch)

Cephalopods

Chambered Nautilus

www.manandmollusc.net/.../ beginners_intro_printable.html

Octopus

Bivalves

Zebra Mussels

www.zoo.cam.ac.uk/zoostaff/ aldridge/dreissena.html www.fossilbeach.net/

fossilshelldirectory.html

Major Divisions Protostomes Deuterostomes determinate spiral cleavage indeterminate radial

cleavage

mesoderm from a single cell mesoderm from endoderm

coelom is a schizocoel coelom from pouches

mouth from blastopore mouth from new opening, anus from or near blastopore

Lophotrochozoans & Ecdyso- zoans

Transition from flatworm to Mollusc

Habitats. Range from tropics to polar seas, at

altitudes > 7000 m, in ponds, lakes and streams, on mud flats, in pounding surf, and in open ocean from the surface to the abyssal depths.

Most live in the sea and represent a variety of life styles including bottom feeders, burrowers, borers and pelagic forms.

Origin Fossil evidence shows originated in sea

and most have remained there. Much of their evolution occurred along the shores, where food was abundant and variety of habitats available.

Only bivalves and gastropods moved on to brackish and f.w.

Only snails are terrestrial (bivalves are filter feeders so can't leave water) but snails are limited in range by need for humidity, shelter and presence of calcium in soil.

Economic importance Food. Pearls, natural and cultured. Most from marine

oyster, Meleagrina, found around eastern Asia. Mother of pearl. Mississippi and Missouri river

basins supply most for U.S., but supply depleted and trying to raise the bivalves artificially.

Destructive. Burrowing shipworms are bivalves which destroy wooden ships and wharves.

Snails and slugs frequently damage garden and other vegetation.

Snails are intermed. hosts for parasites. Boring snail rivals sea star in destroying oysters.

Characteristics

Distinguished by presence of a muscular foot, a calcareous shell secreted by a fold of the dorsal body wall called the mantle, and a toothed feeding organ, the radula.

Characteristics Earliest mollusc, the hypothetical

aggregate mollusc, (archetype?) is thought to have had:

a flat creeping foot, a dorsal shield‑shaped shell secreted

by the mantle, ctenidia (gills) housed in a mantle

cavity, and an anterior head with sensory

tentacles and a radula.

Characteristics Head bears terminal mouth, eyes, tentacles

and other sensory organs. Missing in bivalves

Primitively, possess gills housed within mantle cavity created by the overhanging mantle and shell.

Gills composed of flattened filaments projecting to either side of a supporting axis. Ea. filament w/ lateral cilia for ventilating current and frontal cilia to remove particles. Ancestral m. prob. had several prs. of such gills located posteriorly in mantle cavity.

Characteristics

Radula = belt of teeth stretched over a cartilage base, the odontophore.

Functions as scraper in feeding, but secondarily modified for other modes of feeding in many (e.g. as boring tool to cut through shells of relatives)

Radula SystemRadula System

RadulaRadularetractorretractor

EsophagusEsophagus Radula sacRadula sac

OdontophoreOdontophore

RadulaRadula

Salivary glandSalivary gland

Buccal cavityBuccal cavity

MouthMouthOdontophoreOdontophoreprotractorprotractor

RadulaRadulaprotractorprotractor

OdontophoreOdontophoreretractorretractor

Characteristics-digestion Primitively, the stomach is adapted for processing fine

particles of food (esp. algae) scraped from hard surfaces by radula.

A rotating mucous mass in style sac ( = posterior end of stomach) acts as windlass to pull in a mucous string loaded with food from the esophagus.

Particles are separated over a sorting region, and fine particles are sent up the ducts of the surrounding digestive gland, where intracellular digestion occurs.

Usually find some extracellular digestion within stomach (advanced).

Intestine is for formation of feces. Out through anus which opens at posterior margin of mantle cavity and wastes swept away by exhalent current.

Formation of fecal pellets important. Decreases chances of fouling gills with wastes

Characteristics-circulatory

Open blood vascular system (completely closed in cephalopods).

Blood drains from gills into one or more pairs of auricles ‑‑> central ventricle ‑‑> pumped out through aorta to blood sinuses (=spaces surrounding organs) ‑‑> back to gills.

http://biodidac.bio.uottawa.ca/thumbnails/catquery.htm?Kingdom=Animalia&Phylum=Mollusca&category=diagcl

Characteristics-excretion

Coelom located in mid‑dorsal region of body. Surrounds heart (pericardial) and a portion of intestine (perivisceral).

Cavity drained by excretory organs, metanephridia, which empty into mantle cavity.

Excretory systems

Movie

Characteristics-excretion Pericardial coelom receives waste from two sources.

Heart wall delivers a filtrate from the blood with the waste that has been collected during the circulation of the blood.

Glands in pericardium secrete waste into coelom. Pericardial fluid then passes thru ciliated opening

(=nephrostome) into kidney. Further secretion + some selective reabsorption by

kidney tubule wall. Final urine into mantle cavity. (Note that with development of a coelom go from

protonephridia which are blind sacs with only one opening, to metanephridia with two openings, one of which is associated with the coelom and the other = a passageway to outside. All kidneys are elaboration of this plan.)

Characteristics-nervous system

Nervous system. Ground plan = nerve ring around esophagus from which one pair of nerve cords go to the foot and another pair go to the viscera and mantle.

Characteristics-sensory Primitively find a pair of statocysts in

the foot. Commonest sense organs are: pair of tentacles with an eye at end of each. one or two osphradia (= patches of sensory

epithelium near gills) which monitor the incurrent water as chemoreceptors and to determine the amt. of sediment in the current passing through the mantle cavity.

Characteristics-reproduction

Primitively, molluscs are dioecious, with a pair of gonads in the visceral mass adjacent to the coelom.

Maturation occurs in coelomic cavity and metanephridia function as gonoducts. In such primitive molluscs, fertilization is external and development is planktonic.

Spiral cleavage results in trochophore larva. A later larva, called a veliger, is typical of many groups and the trochophore may be suppressed.

Mollusc larvae

www.willapabay.org/~bcfarms/ veliger2.html

http://departments.colgate.edu/biology/People/DMcHugh.html

Gastropoda Largest and most successful class. About 35,000

living species and 15,000 extinct. Very diverse. Include snails, limpets, slugs, whelks, conchs,

periwinkles, sea slugs, sea hares, sea butterflies, and others.

Range from some of the most primitive of marine molluscs to the highly evolved terrestrial air‑breathing snails and slugs.

Size range from microscopic to giant marine forms e.g. Pleuroploca gigantea, a snail with a shell up to 60 cm (2 ft.) long and the sea hare Aplysia, wh can get up to 1 m long in some species. Most are between 1 and 8 cm long. Range of habitats large.

In sea; littoral zones (region between tide marks), very deep, some pelagic. Brackish water, f.w., terrestrial, but limited by mineral content of soil, temp. extremes, dryness and acidity. Widespread nevertheless; great altitudes and even polar regions. All kinds of habitats; small pools or large lakes and oceans, woodlands, pastures, under rocks, in mosses, on cliffs, in trees, underground and on the bodies of other animals. Do everything except fly. Usually sluggish (no pun intended), sedentary ans. because have heavy shell and slow loc. organs. Shells are chief defense, but also protected by coloration and secretive habits.

Torsion A 180o twist that moves the mantle cavity, which

was originally posterior, to the front of the body. Twists the visceral organs and nervous system

in the process. Anus and mantle cavity open anteriorly

above the mouth and head Gill, kidney and other organs, originally on the

left side, are shifted to the right side while organs orig. on the right side move to the left side.

Digestive tract is bent and nerve cords are twisted into a figure 8.

http://nighthawk.tricity.wsu.edu/museum/ArcherdShellCollection/Illustrations/TorsionStages.html

Gastropod shell Coiled shell is developed for hydrodynamic

reasons. It is easiest shell to carry around. Weight and bulk of the main body whorl, the

largest whorl of the shell pressed on right side of mantle cavity and interfered with the organs on that side.

Accordingly, the gill, auricle of the heart and kidney of the right side have been lost in all except primitive living gastropods = bilateral asymmetry.

Important because solved one of the major problems of torsion. Now anus is located at right edge of mantle cavity where exhalant water current removes waste.

Detorsion

In some gastropods, e.g. sea hare, and slugs bilateral symmetry is restored by detorsion. This is in ansimals without shells or with very reduced ones.

Gastropod Mantle Cavity Water generally enters mantle cavity on

the left side of the head, passes over left gill (since right gill is usually missing) and exits on right side of head.

System modified in shelless marine species. They have secondary gills in back or projections of dorsal body wall for respiration.

In terrestrial snails, mantle cavity is modified into lung.

Gastropod features

Radula important. Use it for scraping algae or tearing prey.

Some are filter feeders. See text for digestion,

reproduction and sense organs.

Bivalves Mussels, clams, scallops, oysters

and shipworms. Range in size from tiny seed shells

1-2 mm to giant South Pacific clams >1m.

Bivalves Most are sedentary filter feeders that

depend on ciliary currents produced by the gills to bring in food materials.

Unlike the gastropods, they have no head, no radula and very little cephalization.

Most are marine, but many in brackish water and in streams, ponds and lakes.

Bivalves Shell is bivalved. See in lab. Note

mantle in lab. Best way to get an idea of general form and function of body plan of bivalves is to study them in lab.

Bivalves Note direction of water currents through

mantle cavity and over gills. Does the water enter from the anterior end or

posterior end? Where does it exit? What is the adaptive value of such a system (i.e. why do you think the water current runs this way)?

What is/are the function(s) of the gills? What is meant by "a ciliary filter feeder"? What is the function of mucous in the feeding of bivalves? What has happened to the radula? Why? What are some of the diverse life styles of bivalves?

How do they manage to burrow? What keeps burrowers from clogging their gills?

Structure and function of bibalve gill

Gill function

 Cephalopods Most advanced molluscs - in fact,

in many ways, they are most advanced invertebrates.

Include squids, octopuses, nautiluses, and cuttlefish.

All are marine and all are active predators.

http://www.multimania.com/martingerard/img0212.htm

http://www.multimania.com/martingerard/img0211.htm

Cephalopods Name means "head foot". Modified foot is

concentrated in head region. Foot edges are drawn out into arms and tentacles bearing sucking discs for seizing prey; also, part of the foot is modified to form a funnel for expelling water from the mantle cavity

Range in size from 2 or 3 cm to giant squid which is largest invert. known. Once were "rulers of the sea", but beaten out by fish.

Incredibly complex and complicated animals. Learn about basic structure in lab.

Squid locomotion

Movie Live Nautilus

Behavior. Much research has been done on the behavior of cephalopods especially octopus.

Its nervous system is incredibly complex for an invertebrate.

They are capable of learning complex tasks and of adapting behavior to situations.

Complicated signaling system using chromatophores.

Polyplacophora (the Chitons)

See material on this group in your text.

Be sure to include their features in your molluscan chart.

Cambrian environment and evolutionary trends

The following material on the evolution of mollusks in the late Precambrian and early Cambrian are distilled from the ideas of Dr. Robert B. Willey.

Late Pre-Cambrian: approx. 600,000,000 years before present

First evidence of multicellular animals (soft-bodied cnidarians and other marine inverts., burrowing wormlike animals).

Multicellular algae and fungi. Average temp. of ocean = around

0oC. Beginning of steady increase in temp.

Cambrian

Unlike Pangaea, Cambrian continental masses were probably similar to today, but more tropical (no obvious glacial deposits found.)

Fewer mountains - mostly worn down or wearing down from previous mountain building periods.

Cambrian Climate tropical to cold temperate, but

mostly centered on the hot equator, probably few if any cold arctic areas.

Some areas of cold and hot deserts, some constant rain areas and many seasonal tropical monsoon belts.

All areas subject to periodic or constant heavy wind erosion (no plants to protect).

Cambrian Sea level 400-600 ft. higher than present on flat centers

of continents as well as on the continental slopes (less water tied up in polar ice caps).

All areas subject to heavy deposition from land erosion because no plants to hold down land

Numerous oceanic currents tended to keep mineral sediments in suspension long after monsoons were over.

More of water surface than now was suitable for luxuriant plankton growth.

During quiet intermonsoon much of continental shelf covered with dead plankton sifting down from surface water = rich food source

Much of same areas subject to periodic burial several meters deep as thick mud and land flowed from the river systems.

Cambrian: 570-500 m.y.b.p. Land Mostly bare of any vegetation

except algae Severe unretarded erosion by wind

and water widely separated continental areas

affording great surface area for continental slopes below sea level and epicontinental seas.

Later, approx. 280 m.y.b.p., get one continental mass = Pangaea.

Preadaptations necessary to exploit the periodic deposition areas

Planktonic larva to allow growth free of muck until reached ---

Size capable of burrowing up through the heavy burial when metamorphosing to a benthic form (Trochophore first, then veliger, then metamorphosis into benthonic adult when conditions are suitable for settling)

Preadaptations Most food lay on surface mud and was most

available during quiet intermonsoons. So, there was size increase pressure in

order to have strength to maintain position near surface and to have food storage capacity enough to weather the lack of food during the monsoons. (Remember, there was little life on land or in fresh water, therefore new deposits were not nutritious)

 Preadaptations Most early Cambrian forms very small, but

probably capable of burrowing up through a few inches of suddenly deposited mud. Increase in size very rapid - only partly explained as response to predation.

Could also be response to exploitation of more dangerous parts of the monsoon belts.

Burrowing adaptations and swimming adaptations to maintain surface position during mud flow

Univalve shell and foot and torsion

Archetype was slow moving with a well developed head and mantle posterior. Wide foot. Conical shell offered protection from equally slow moving predators. Osphradium in rear or lateral, gill typical (probably several pairs), radula = scraping organ.

All points to an animal not adapted to mud or ooze, but rather a rock dweller grazing on encrusting life. Chitons similar, used foot to cling to outer rocks and managed to hang on by flattening or migrated to quieter abysses. But other niches were available to be exploited. How to take advantage of rich food in mud and plankton?

Univalve shell and foot and torsion

But other niches were available to be exploited. How to take advantage of rich food in mud and plankton?

Univalve shell and foot and torsion

Feed on surface mud. Hydrodynamics of bubble- like shell tend to keep it floating above deposits.

Foot provides ideal "mud shoe". Torsion in gastropods probably evolved

as way to put the incurrent opening at front of animal where sediments would be less stirred up by animal's movements.

Probably true gastropods appeared within 50 million years of beginning of Cambrian.

Univalve shell and foot and torsion

Cephalopods (appear approx. 100 million years later) descended from forms which stayed afloat - just over the benthos.

Also had univalve dorsal shell, just became more conical.

Tended to long planktonic stage. Adult could easily escape inundation by remaining afloat.

Became major planktonic predator. Finally gave up shell in favor of active life.

Bivalve shell and foot Bivalves closed up their shell and

slowed down metabolism during deposition, burrowed upward with foot until siphons could extend to surface.

Evidence for Many Continents

Very large no. of species and regional differences in fossil faunas from Cambrian through Silurian (425-405 m.y.b.p.).

Indicates many barriers to interbreeding and competition between faunas.

Evidence for Warm Climate

No glacial deposits

Evidence for Periodic Deposition

Each layer relatively thick. Perhaps = annual deposits.

Evidence for Turbulence Moon almost certainly 20,000

miles closer to earth than now. Tides alone would keep

epicontinental seas stirred up and spring and fall tides would be equivalent to tidal waves.

Evidence for Severe Deposition

Great thickness of shale, limestone and sandstone deposits.

Much greater than next 200 million years.

Hypothetical Aggregate Mollusc

http://online.sfsu.edu/~biol240/Labs/lab_18molluscs/Pages/architype.html