• Another
clam of the same species had been verified at 220 years old, and a third may
have lived 374 years. But this most recent clam was the oldest yet.
• "Its
death is an unfortunate aspect of this work, but we hope to derive lots of
information from it," postdoctoral scientist Al Wanamaker told London's
Guardian newspaper. "For our work, it's a bonus, but it wasn't good for
this particular animal."
MOLLOSKS
•110,000 species in 7 classes. MOLLUSK EVOLUTION The
mollusks contain animals that are mostly crawlers or completely sedentary.
Theirs slowness results from having no legs and using a single foot. The only
group of mollusks that became active swimmers were the class of Cephalopods
which include octopi, squids, and the chambered nautilus.
Animal Kingdom
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•Their evolution
illustrates ways which modifications of the body plan can open up new ecological
options and close others as well as give rise to new kinds of animals.
•The chambered nautilus has come down to us almost unchanged
from the Cambium period, 500million years ago
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•It has its connection to other mollusks by having a large
roomy shell but it is distinctive that it consists of a series of chambers with
partitions between them, each being vacated one after another as the animal
grows but, the chambers are filled with gas (and can be controlled with part of
the animals tissue), and this allows the animal to achieve the specific gravity
of the surroundings...
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•These shelled cephalopods dominated the class during most of
its evolution until about 65 million years ago when they were replaced by the
soft bodied cephalopods...squid, cuttlefish, octopi.
•During their reign, the shelled cephalopods were the most
abundant, successful and varied creatures in the ocean (1).
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•After this period they
shared their dominion with fish and other animals and then their relative
numbers dwindled.
•The three major classes of Mollusks became recognizable at the
end of the Cambium period, and since then, have been following independent
paths of evolution.
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•Gastropod's have deviated little from the original body plan.
Because none became active predators, that can hunt, grasp and devour their
prey. Their muscular and nervous equipment for motility was deeply committed
from the start to carry them about by crawling over a surface that nothing like
limbs, fins, paddles, tentacles or jet propulsion ever evolved.
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•The bivalves became standardize early on as stationary filter
feeders that sift small particles of food through their gills and their nervous
and muscle system are committed to opening and closing their shells and digging
themselves deeper into the sand.
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•The diversity of mollusks encompasses food, dyes, pests,
pathogens, parasites, and pearls. Their variety is reflected in the range of
body forms and ways of life. Mollusks include the coat-of-mail shells or
chitin, marine and freshwater snails, shell-less sea slugs, tusk shells, clams,
mussels, octopuses, squids, cuttlefishes, and nautiluses.
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•While some mollusks can swim, most are attached or live
creeping along the bottom.
•The body is typically divided into a head (lost in bivalves)
muscular foot, and visceral hump containing the body organs.
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•There are no paired or
jointed appendages or legs.
•Two notable features are the mantle and a toothed tongue
called the radula (usually made of chitin).
•Mollusks have a gut with mouth and anus, a blood system,
nervous system,reproductive system, and an excretory system with kidneys.
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•Gills are present in
aquatic species which are used to extract oxygen from the water and in some, to
strain out organisms and detritus
from the water or bottom mud. The particles are then conveyed to the mouth by
tracts of cilia
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•Lack of an internal skeleton have kept mollusks small with
exceptions of the Giant Squid which can reach 60'. Some giant clams can reach
4.5'in shell length. Many measure less than 1cm and one is only 1mm when full
grown
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•The mantle...a fold of
skin, the mantle, forms ;
•forms a pocket housing
the gills
•a chemical sensory organ
•mucus secreting gland
•anus
•excretory pore
•sometimes the reproductive opening
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•The cells of the mantle are thickened at the edge of the
mantle skirt, secreting the shell and slime, acids and ink for defense, mucus
for protection and for cohesion of food particles is secreted by the gills and
mucus glands. Products of the mantle can be defensive, acting to deter
predators.
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•The purple gland in the mantle of the sea hare expels a purple
secretion when the animal is disturbed.
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•The Shell
Mollusks usually hatch from the egg complete with a tiny shell that is often
retained at the apex of the adult shell. The shell provides protection from
damage and predators and on the shore or land, it prevents the loss of body
fluids. New growth occurs at the shell lip in Gastropods and along the ventral
margin in bivalves.
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•Shell is secreted by glandular cells. Its mostly composed of
calcium carbonate and can show great variation in shape size thickness,
sculpture, surface texture and shine. Marine examples are often thick and heavy
while land specimens are light. The nautilus is the exception because it has a
light brittle spiral shell with thin walls which contain gas which can affect
buoyancy.
MOLLOSKS
•Many shells are sculpted into ribs, lines, beading, knobs, or
spines which are much admired by shell collectors and used in the
identification of species. Some have reduced or no shell at all, while some
have internal shells.
MOLLOSKS
•Class Polyplacophora (Amphenura) Chitons or coat of mail shells have an
oval shell consisting of eight plates bounded by a girdle. The plates of the
shell are well articulated, chitins can roll up in a ball when disturbed, and
these articulations are also an advantage when moving over uneven rocks. They
are usually restricted to rocky shores.
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•Class Gastropoda (stomach-foot) Gastropods are the largest class of
mollusks (90,000 species) and have only one shell. These include most of the
sea shells...limpets, cowries, cone shells, top shells, winkles, abalone,
oyster drills, nudibranchs/sea slugs .
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•The mouth of the gastropod is usually protected by a lid or
operculum which is secreted by glands on the upper side of the back of the foot
and is the last part of the animal to be withdrawn and acts as a trap door. Its
present in larva as well though the limpets loose it later.
MOLLOSKS
•Class Bivalvia ..bivalves (formerly Pelecypoda). Clams, mussels, scallops, oysters,
(bivalves)...from tiny fingernail clams to a 500 lb. Tridacna of the Pacific Reefs. Pelecypoda means
hatchet foot which reflects the laterally compressed body which is modified for
filter-feeding, the loss of the head and developed complex sheet of gill
derived tissues for screening microorganisms out of water currents.
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•The shell is closed by adductor muscles passing from one valve
to another. Where these attach to the shell, scars are formed on the inside of
the shell. These scars are important for identifying and classifying bivalves.
Most have two scars (muscles) but oysters and scallops have one.
MOLLOSKS
•Bivalves have a large pair of gills which fill the mantle
cavity which fill a dual role of respiration and feeding.
•Not all bivalves burrow, mussels secrete byssal threads to
attach to rocks and oysters cement their left shell to a hard surface. Scallops
live unattached and can swim for short distances by ejecting water from the
mantle cavity and clapping the valves.
MOLLOSKS
• Some even bore into coral, rock and wood...shipworm. Sexes
are separate although some oysters may alter sex in their life. Fertilization
is external either in the sea or mantle cavity.
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•Class Cephalopoda. Octopuses, squids, cuttlefish, nautiluses... Cephalopods..
differ from the rest of the mollusks in their appearance and their
specialization's for life as active carnivores. They include many pelagic
forms, swimmers in the open seas and bottom dwelling octopus and cuttlefish.
With the exception of nautiluses, these groups, most of which possessed shells
are extinct.
MOLLOSKS
•Most have internal
skeletons.
•They are good swimmers
catching moving fish, and have evolved various buoyancy mechanisms, very
•responsive to stimuli
•All but nautilus have an ink sac opening off the rectum which
contains ink for confusing the enemy.
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•The body color can change
in response to stimuli and change by pigment cells called chromatophores.
•Have a very well developed eye that focuses by moving its
position rather than change shape of the lens
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•Class Scaphopoda.....Tusk shells These are a small group of mollusks that
are entirely marine and live buried in sand or mud of fairly deep waters. Only
their empty shells are to be found on the beach. The tube of the shell is lined
by a mantle, no gills, mantle absorbs oxygen which has a few ridges with cells
bearing tiny hair-like cilia that help create a current .
MOLLOSKS
•Class Monoplacophora These are a group of what was thought extinct, primitive
Paleozoic (270-600 million yr.) mollusks, includes a living mollusk, radically
different from other mollusks in that is internally segmented. This
segmentation violates one of the basic criteria in
which characterized.
mollusks are
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•characterized. This living specimens, Neopilina galatheae,
was dredged up from off Costa Rica from 3.5 km below the surface in 1952. At
present, 12 species are known. Most live at great depths and all are marine.
Monoplacophorans are small and have a single, caplike shell, giving them a
limpet-like appearance.
MOLLOSKS
•Mollusca Life History Most sexes are separate but there are some species that are
hermaphrodites. Many mollusks have a trochophore larva like the polychaetes
(close affinities between them). The larva develop into a veliger larva (with a
tiny shell) Cephalopods lack a larva and the young develop in large yolk filled
eggs.
MOLLOSKS
•Ecology They colonized land, fresh and salt water, and most all marine
habitats, rocky, coral sandy, muddy, boulder, shingle, transition zones,
mangrove swamps, estuaries. The veliger larva of most marine mollusks float
passively in the upper waters of the sea forming plankton.
MOLLOSKS
•Mollusks can be eaten by other mollusks, starfish, bottom
living fish like rays, , whales eat squid, sea birds probe mud, humans use them
for food, fishing bait, currency, dyes, pearls. Some are pests....shipworms,
slugs, snails oyster drills.
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