Transcript Mollusca

Deanne L. de Asis
Jessica Consad
• is one of the most diverse groups of animals on the planet. It includes
familiar organisms like snails, octopuses, squid, scallops, oysters and chitons..
• is the largest marine phylum, comprising about 23% of all named marine
organisms.
• Numerous mollusks also live in freshwater and terrestrial habits.
• also includes some lesser known groups like the monoplacophorans, a group
once thought to be extinct for million of years until one was found in 1952 in
the deep ocean of the coast of Costa Rica.
• they are a clade of organisms that all have soft bodies which typically have a
“head” and a “foot” region. Often their bodies are covered by a hard
exoskeleton, as in the shells of snails and clams or the plates of chitons.
• they have been important to humans throughout history as a source of food,
jewelry, tools and even pets.
• their shells are considered quite beautiful and valuable.
• they can also be nuisance, such as the garden snail; and they make up a major
component of fouling communities both on ducks and on the bulls of ships.
• they also have a very long and rich fossil record (more than 550 mya), making
them one of the most common types of organism used by paleontologists to
study the history of life.
• Molluscs and mollusks are words derived from the French mollusque,
originated from the Latin molluscus, from mollis which means soft.
• Malacology is the scientific study of molluscs.
The most general characteristics of molluscs is that they are unsegmented and
bilaterally symmetrical.
The following characteristics are present in modern molluscs:
• the dorsal part of the body wall is a mantle (pallium) which secretes
calcareous spicules, plates or shells.
• the anus and genitals open into the mantle cavity.
• there are two pairs of main nerve cords.
Fig. 1 Dorsal view of a chiton with a
spike bearing mantle.
• In 2001, Haszprunar estimated about 93,000 named
species which include 23% of all named marine
organisms.
• In 2009, Chapman estimated the number of described
living species at 85,000.
• Molluscs have more varied forms than any other animal
phylum. The majority of the species still live in the oceans,
from the seashores to the abyssal zone, but some form a
significant part of the freshwater fauna and the terrestrial
ecosystems. They are extremely diverse in tropical and
temperate regions but can be found at all latitudes.
• Freshwater and terrestrial molluscs appear exceptionally
vulnerable to extinction.
• Estimates of the number of non-marine molluscs vary
widely. However, in 2004 the IUCN (International Union
for the Conservation of Nature and Natural Resources)
Red List of threatened Species included nearly 2,000
endangered non-marine molluscs.
Fig. 2 About 80% of all
known mollusc species
are gastropods (snails
and slugs), including
the cowry(a sea snail)
pictured here.
• In terrestrial communities, gastropods can achieve
reasonably high diversity and abundance: as many
as 60-70 species may coexist in a single habitat and
abundance in lead litter can exceed more than 500
individuals in four liters or litter,
• Marine molluscs occur on a large variety of
substrates including rocky shores, coral reefs, mud
flats and sandy beaches.
• Gastropods and Chitons are characteristic of these
hard substrates and Bivalves are commonly
associated with softer substrates where they
burrow into the sediment.
• Tridacna Gigas are the largest living bivalve. It lives
on coral reefs and many bivalves (e.g., mussels and
oysters) attach themselves to hard substrates.
• Some microscopic gastropods live interstitially
between sand grains.
Fig 4 Tridacna Gigas
Fig. 3
Many marine molluscs
emerge from their eggs as
planktonic trocophore larvae,
however
Sinistral Pond Snails
(Physella sp) emerge from their
eggs as young snails the whitish,
jellybean-shaped organisms are
ostracodes (crustacenans).
• Large concentrations of gastropods and bivalves are found in
hydrothermal vents in deep sea. For example, the fauna of
Paleozoic hydrothermal vent communities includes the
molluscan groups Bivalvia, Monoplacophora and Gastropoda
as well as the outgroups Brachiopoda and Annelida.
• Based on current understanding of relationships the earliest
molluscs grazed on encrusting animals and detritus.
Herbivorous grazers are relatively rare and are limited to
some polyplacophorans and a few gastropod groups.
• Most
chaetodermomorph
aplacophorans,
monoplocophorans and scaphopods feed on protists and/or
bacteria while neomemiomorph aplacophorans graze on
cnidarians.
• Cephalopods are mainly active predators as are some
gastropods, while a few chitons and septibranch bivalves
capture microcrustaceans.
• Most bivalves are either suspension or deposit feeders that
indiscriminately take in particles, typically assimilating
bacteria, protists and diatoms.
Fig 5 The freshwater Sinistral
Pond Snail (Physella sp.)
scrapes algae from the glass
with its radula.
Fig. 6 Chiton Olivaceous:
West Indian Chiton
Scientific Classification
Kingdom:
Superphylum:
Phylum :
Animalia
Lophotrochozoa
Mollusca (Linnaeus, 1758)
Classes
Aplacophora
Helcionelloida
Bivalvia
Monoplacophora
Caudofoveata
Polyplacophora
Cephalopoda
Rostroconchia
Gastropoda
Scaphopoda
• is an informal group of smalll, deep-water,
exclusively
benthic, shell-less marine
mollusks found in all oceans of the world.
• They are cylindrical and worm-like, and
most very small, being no longer than 5 cm
(2 in), some species, however, can reach a
length of 3ocm(12 in)
• They mainly burrow into the substrate in
water regions deeper than 20m (66 ft).
They are typically either carnivores or
detritivores.
• This class was once classified as sea
cucumbers in the echinoderms. In 1987,
they were officially recognized as molluscs
and given their own class. This class is
polyphletic , and consists of two clades:
the Solenogastres and Caudofoveata,
• The approximately 7, 500 living species of
bivalves include each common animals as
clams, oysters, scallops and mussels.
• Derive their name from the two parts, or valves,
into which the shell is divided. One or two large,
well-developed adductor muscles are used to
close shell swiftly and tightly in times of danger
• Abundant in both salt and fresh water , most
adult bivalves are sedentary, herbivorous filter
feeders, using currents set up by cilia on their
gills to bring in food particles, usually
microscopic algae.
• The bivalves have sensory cells for
discrimination of touch, chemical changes, and
light. The scallop has quite complex eyes; a
single individual may have a hundred or more
eyes located among the tentacles on the fringe
of the mantle.
Most bivalves poses large gills for the
purposes of respiration and filtering out
of small food partiles
Fig.7 Interior view of right valve
showing the muscel scars.
Fig. 8 Bivalve with left valve and rmantle
removed.
Because of this diet and feeding,
they lack radula. They poses one or
two pairs of gills (ctenida) or
branchia for respiration.
Fig. 9 Section through the viscual mass showing
the internal organs.
•Also known as Chaetodermomorpha .
• Often combined with Solenogastres and termed
Aplacophora. However, such grouping is not
monophyletic; molecular data suggests that the
Caudofoveata are a sister group to the
cephalopods
• They 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.
• They 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.
• During sexual reproduction, the female
produces eggs which are fertilized and brooded,
and then the larvae swim freely.
Fig. 10 Anterior 1.4 mm of a
fixed and stained specimen
of Falcidens sp.
Fig. 11 Molluschi Aculiferi
(Caudofoveata) molto
piccoli
•Greek word “kephalópoda” means “head-feet”.
• Characterized by bilateral body symmetry, a prominent
head, and a modification of the mollusc foot, a
muscular hydrostat, into the form arms or tentacles.
• became dominant around Ordovician period.
• The class contains two extant subclasses
• Coleoidea – mollusk shell has been internalized or
is absent; includes the octopus, squid and
cuttlefish
• Nautiloidea – the shell remains; this subclass
includes the nautilus.
•Two important extinct taxa are
•Ammonoidea – the ammonites
•Belemnoidea – the belemnites.
• Fishing industry name this class as inkfish, referring to
many cephalopods’ ability to squirk ink,
• Teuthology – branch malacology. It is the sudy of
cephalopods.
Fig. 12 Cuttlefish (sepia)
seizing a shrimp with the
use of its tentacles
Fig. 15
Nautilus:
the only
shelled
cephalop
od
Fig. 13 Dorsal
view of a squid
(loligo)
in
swimming
position.
The
tentacles
and
arms are held
together
and
functions as a
rudder.
Fig.14
Sagital
section
of
nautilus.
Most of cephalopods are active and predatory
swimmers pressing jaws and radula. They poses eyes
as complex as those of humans, and a greater
capacity of learning than any other invertebrates.
•a class of animals that are more commonly known as snails and slugs.
• contains a vast total of named species, second only to the insects in overall
number.
• The fossil history of this class goes all the way back to Late Cambrian.
• There are 611 families of gastropods, of which 202 families are extinct, being
found only in the fossil record.
• they are previously known as univalves and sometimes spelled Gasteropoda.
• They are a major part of Mollusca and are the most highly diversified class in
the phylum, with 60,000 to 80,000 living snail and slug species.
• Representatives live in gardens, woodland, deserts and mountains; small
ditches, great rivers and lakes; estuaries, mudflats, rocky intertidal, the sandy
subtidal, in the abyssal depths of the oceans including the hydrothermal
vents, and numerous other ecological niches, including parasitic one.
• Gastropods without shell, with only a very reduced or internal shell are
known as slugs.
• Marine shelled species of gastropod include edible species such as abalone,
conches, periwinkles, whelks, and numerous other sea snails with coiled
seashells.
Fig. 16 Gastropod anatomy
Fig. 15 Air-breathing
land gastropod Helix
pomatia, the Roman
snail
Fig. 17 Sea slug
• Prosobranchia – they have a spiral shaped shell, well
developed head that poses tentacles, radula, and a large flat
foot for motion.
The primitive members are herbivores (rasp seaweeds
and micro algae).
The advanced forms are predators (poses a long
proboscis and cylindrical siphon).
• Opisthobranchia (sea slugs, sea hares, nudibranches etc.)
have forsaken their gills and shells. It has been speculated
that their ancestors were sand-burrowers, for whom these
would have been a hindrance.
• an extinct group of ancient
molluscs.
• the oldest known conchiferan
molluscs, that is, they had a
mineralised shell.
• They were mistaken for
Monoplacophorans.
• They were untorted and they
had coiled, cone-shaped shell.
• Modern reconstructions depict
them as resembling snails.
• They possessed a “snorkel”-like
opening which was most likely
used for breathing.
Latouchella costata
Yochelcionella, a member of
the small shelly Fauna of the
Tommotian.
• Means “bearing one plate”
• a class of mollusks with a cap-like shell, living on the bottom of deep sea
• have known as a recent class since 1952; previously they were known from
the fossil record.
• Geographically widespread component of the benthos. Most are known
from deep water (1800-6500 m), although several species are found
shallower waters ranging up to 200 m.
• They graze on microscopic organisms in mud or bottom detritus
• They have a single, flat, rounded bilateral shell that is often thin and
fragile; ranges in size from 3-30 mm
• The heart is divided into two equal halves, each with its own auricle,
ventricle and aorta.
• The nervous system has small ganglia around the esophagus from which
two pairs of main nerve cords run through the body; one pair supplying
the foot, and the other the visceral organs.
• There are two pairs of gonads, which release gametes into the water
through one of the pairs of nephridia. Sexes are separate and fertilization
is external.
Ventral view of the (fossil)
shell
of
Tryblidium
reticulatum
Lindström,
1880. There are visible
muscular attachment scars.
Head region is on the upper
part of the drawing. The
shell region is up to 43mm.
Drawing of the shell of
Pilina unguis. Head
region is on the left.
Dorsal view of the
shell of Tryblidium
reticulatum.
• Chitons are small to large, primitive marine molluscs in the class
Polyplacophora.
• Amphineura – 900 to 1000 extant species of chitons in the class which was
formerly known.
• Commonly known as sea cradles or “coat-of-mail shells”.
• They are also sometimes referred to more formally as loricates,
polyplacophorans, and rarely as polyplacophores.
• They live worldwide, in cold water and in the tropics. Most of them inhabit
intertidal or subtidal zones and do not extend beyond the photic zone.
• They live in hard surfaces, such as ion or under rocks or in rock crevices.
• They are exclusively and fully marine. A contrast to the valves which were
able to adapt to brackish water and freshwater, and the gastropods which
were able to make successful transitions to freshwater and terrestrial
environments.
• They are easily recognizable because of their shells that are split into eight
dorsal plates that cover the centre of their bodies.
Vental view of a chiton
with a spike bearing
mantle.
Dorsal view of a
chiton with a
spike nearing
mantle.
Chiton Olivaceous: West
Indian Ocean
Respiration occurs through 6-80 pairs of gills in a
groove around the foot. Chitons are herbivores
that have strongly toothed radulae. They are
chiefly found in shallow coastal waters.
• class of extinct molluscs dating from the early Cambrian to the
late Permian.
• Initially thought as bivalves.
• They have a single shell in their larval stage, the adult typically
has a single, pseudo-bivalved shell enclosing the mantle and
muscular foot.
• Rostroconchs probably lived in a sedentary semi-faunal
lifestyle.
• Heraultipegma is the earliest, very primitive, rostroconch
genus dating from the Late Terreneuvian. True Rostroconchs
appeared during the Ordovician,
• Ribeiroia had a single hinge in which all shell layers covered the
dorsal region resulting in a very rigid shell. It’s an early primitive
rostroconchs
• Conocardium – the outer shell layers do not cross the entire
margin, suggesting independent steps towards the bivalve
flexible hinge. It is amore advanced Rostroconch.
• Some evidences that conocardoid rostroconchs were the
predecessors to the Scaphopoda.
• also known as tusk shells. A class of shelled marine molluscs.
• Scientific name is Scaphopoda, meaning “shovel=-footed”.
• Shells of species range from about 15cm to about 5 mm in
length.
• They live on soft substrates offshore.
• Molecular data suggests that the scaphalopods are a sister
group to the cephalopods, although higher-level molluscan
phylogeny remains somewhat unresolved.
• They live in the bottom sediments where they feed on
detritus, foramanifera and microscopic animals.
• Well-known in the fossil record, with claims that they
appeared in the Mid-Ordovician, although the first wellidentified fossils date to the Mississippian.
• Most closely related to extinct molluscan class
Rostroconchia.
• Dentalium hexagonum, a scaphopod mollusc. The shells were
strung and used by the natives of the Pacific Northwest as
shell money.
•The shell and mantle are slenderly tubular, slightly curved
(shaped like an elephant tusk) and open at both ends.
•conical foot protrudes from the larger ventral end of the
shell and is used for burrowing.
•Delicate ciliated contractile tentacles are found around the
mouth to capture food.
•The large mantle cavity serves for respiration.
A
fossil
Dentalium
shell
from the Pliocene
of Cyprus
• The aragonitic shells of scaphopods are conical and
curved in a planispiral way, and usually whitish in color.
• the shell resembles a miniature elephant’s tusk.
• The shells are hollow and open at both ends; the
opening at the larger end is the main or anterior
aperture of the shell. The smaller opening is known as
the apical aperture.
• Some are minute, most are 4-6 cm (1.6 -2.4 in)long;
some species reach 15cm (5.9 in) in length.
Body Parts and Functions
Characteristics
Digestion and Excretion
Nervous System and Sensory Capability
Respiration and Circulation
Locomotion
There are three distinct body zones:
•head-foot - contains both the sensory and
motor organs
•visceral mass - contains the welldeveloped organs of digestion, excretion,
and reproduction
•Mantle- a specialized tissue formed from
folds of the dorsal body wall, that hangs
over and enfolds the visceral mass and that
secretes the shell.
• The mantle cavity, a space between the
mantle and the visceral mass, houses the gills;
the digestive, excretory, and reproductive
systems discharge into it.
• Radula - a toothed tongue, composed
primarily of chitin. It serves both to scrape off
algae and other food materials and to convey
them backward to the digestive tract. In some
species, it is also used in combat.
The digestive tract is complete and ciliated,
with a mouth, anus and complex stomach. The pattern
of the stomach varies according to the mollusks diet.
Food is taken up by cells lining the digestive
glands arising from the stomach, and then is passed into
the blood. Undigested materials are compressed and
packaged, then discharged through the anus into the
mantle cavity and are carried away from the animals in
the water currents. This packaging of wastes in solid
form prevents fouling of the water passing over the
gills.
Excretory functions are carried out by a pair of
nephridia, tubular structures that collect fluids from the
coelom and exchange salts and other substances with
body tissues as the fluid passes along the tubules for
excretion. The nephridia empty into the mantle cavity.
• Mollusks have a relatively complex nervous
system, which varies from species to species
reaching the height of complexity at the
octopus.
• The octopus is thought to be among the most
intelligent of all invertebrates, with a mental
capacity likened to that of a domestic cat.
• Sensory ability in some mollusks (notably the
cephalopods) is considerable, with a variety of
organ systems, as well as large, complex eyes.
• The eyes of the giant squid are the largest in
the animal kingdom, approaching the size of
dinner plates.
It has recently been
demonstrated that squid can successfully locate
and capture transparent prey in the water by
means of a specialized polarization vision.
• Mollusks
(excluding
cephalopods)have
an open circulatory system, the blood does not
circulate entirely within vessels but is collected
from the gills, pumped through the heart, and
released directly into spaces in the tissues from
which it returns to the gills and then to the heart.
• Hemocoel ("blood cavity"). In mollusks, the
hemocoel has largely replaced the coelom, which is
reduced to a small area around the heart and to the
cavities of the organs of reproduction and
excretion.
• Cephalopods have vigorous activities that
require the cells to be supplied with large quantities
of oxygen and food molecules. They have a closed
circulatory system of continuous vessels and
accessory hearts that propel blood into the gills.
• Herbivorous forms are commonly gliders, moving on waves of muscular
contraction.
• Carnivorous forms have achieved more advanced forms of locomotion.
• Cephalopods swim actively by a type of jet propulsion, in which water is
rapidly expelled from the mantle cavity via the siphon.
• The Cuttlefish and the Sea Hares rely upon undulating lateral fins for
highly maneuverable locomotion.
• In the bivalves the foot has developed into a tool for burrowing, which can
be remarkably rapid for example in the common Razor Shells.
•The three major classes range from:
largely sedentary or sessile filter-feeding animals, such as clams and
oyster (class Bivalvia),
through aquatic and terrestrial snails and slugs (class Gastropoda)
to the predatory cuttlefish, squids, and octopuses (class Cephalopoda).
1. Body is short and partially or wholy enclosed by a fleshy outgrowth of the
body wall called the mantle. Between the mantle and the visceral mass is a
mantle cavity containing components of several systems (secondarily lost in a
few
groups)
2. A shell (if present) is secreted by the mantle and consists of one, two or eight
parts. the head and the ventral muscular foot are closely allied (the foot
being variously modified for burrowing, crawling, swimming, or food capture).
3. The digestive canals are complete and intricate with ciliary canals for the
sorting of particles. The mouth with a rudula bearing traverse rows of minute
chitinous teeth to rasp food , except in Bivalvia. The anus opening in the
mantle cavity. A large digestive gland and often salivary glands are present.
4. The circulatory system is open, except in Cephalopoda and usually
includes a dorsal heart with one or two atria and one ventricle. An anterior aorta
and other vessels and many blood spaces (hemocoels) exist in the tissues.
5. Respiration occurs via one to many uniquely structured ctenidia (gills) in the
mantle cavity (secondarily lost in some), by the mantle cavity, or by the mantle.
6. Excretion by kidneys (nephridia), one or two or six pairs, or only a single one.
They usually connect to the pericardial cavity and they exit in the mantle cavity. The
coelom is reduced to the cavities of the nephridia, gonads and pericardium.
7. The nervous system is typically a circumesophageal nerve ring with multiple
pairs of ganglia and two pairs of nerve cords (one pair innervating the foot and
another the visceral mass).
8. The sexes are usually separate(some are monoecious, a few are protandric).
Gonads add up to four, two or one, all with ducts. Fertilization occurs externally or
internally. Egg cleavage determinate, spiral, unequal and total (meroblastic
in Cephalopoda). Trochophores and veliger larvae form, or a parasitic stage
occurs(Unionidae), or the development is direct (Plumonata, Cephalopoda).
9. Unsegmented (except Monoplasophora). Symmetry bilateral or asymmetrical.
•The Mollusca include some of the oldest metazoans
known.
•Late Precambrian rocks of southern Australia and
the White Sea region in northern Russia contain bilaterally
symmetrical, benthic animals with a univalved shell
(Kimberella),resembles those of molluscs.
• The earliest unequivocal molluscs are helcionelloid
molluscs that date from Late Vendian rocks.
•In the Early Cambrian the Coeloscleritophora are also
present (gastropods, bivalves, monoplacophorans, and
rostroconchs)
• Cephalopods are first found in the Middle Cambrian.
• Polyplacophorans in the Late Cambrian.
• Scaphopoda in the Middle Ordovician.
• Late Vendian-Early Cambrian taxa bear little
resemblance to the Cambrian-Ordovician taxa (most of
which remain extant today).
On the left is Inoceramus sp., a
bivalve from the Cretaceous of
Alameda County, CA. At right
is
Turritella
andersoni,
a
gastropod from the Eocene of
Ventura County, CA.
• The phylogeny (evolutionary "family tree") is a controversial subject.
• Molluscs are generally regarded members of the Lophotrochozoa , a
group defined by having trochophore, larvae and, in the case of
living Lophophorata, a feeding structure called a lophophore.
• The other members of the Lophotrochozoa are the annelid worms and
seven marine phyla.
• The molluscan shell appears to have originated from a mucus coating,
which eventually stiffened into a cuticle.
• An analysis in 2009 that used both morphological and molecular
phylogenetics comparisons concluded that the molluscs are not
monophyletic.
• A 2010 analysis similarly concluded that the molluscs are
not monophyletic, this time suggesting that solenogastres are more
closely related to the non-molluscan taxa used as an outgroup than to other
molluscs.
• Bivalves such as clams and mussels, have been an important food
source since at least the advent of anatomically modern humans—and
this has often resulted in over-fishing.
• bivalves and some gastropods whose shells are lined
with nacre are valuable. The best natural pearls are produced by pearl
oysters Pinctada margaritifera and Pinctada mertensi, which live
in the tropical and sub-tropical waters of the Pacific Ocean. Natural
pearls form when a small foreign object gets stuck between
the mantle and shell.
• Mollusc shells were used as a kind of money in several pre-industrial
societies. When used for commercial transactions they functioned as
commodity money.
STINGS AND BITES
• When handled alive, a few species of molluscs can sting or bite and with
some species, this can present a serious risk to the human handling the animal.
• All species of cone snails are venomous and can sting when handled
(carnivorous gastropods that feed on marine invertebrates ). Their venom is
based on a huge array of toxins, some fast-acting and others slower but
deadlier—they can afford to do this because their toxins require less time and
energy to be produced compared with those of snakes or spiders
PESTS
•Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is
"second only to malaria as the most devastating parasitic disease in tropical
countries. The parasite itself is not a mollusc, but all the species have
freshwater snails as intermediate hosts.
• snails and slugs, can be serious crop pests and when introduced into new
environments can unbalance local ecosystems.