Phylum Mollusca
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. 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. 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...
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). 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.
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.
The pelecypods 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.
DARWIN
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. 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.
- 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.
- 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
.
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.
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
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. The purple gland in the mantle of the sea hare expels a purple secretion when the animal is disturbed.
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. 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. 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.
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.
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 . 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.
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. 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. 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. 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.
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.
- 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.
- 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.
(left is what nautilus sees) 
Giant
Squid...Mote Marine Lab
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 . Sexes are separate and fertilization occurs externally at sea.
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 mollusks are characterized. This living
specimens, Neopilina galatheae,
was dredged up from off
- It has a single dome-shaped shell
- no evidence of twisting of the internal organs (like the Gastropods)
- 5 pr. of retractor muscles
- 5 pr. of gills
- six pr of nephridia
- a well developed radula.
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. Female octopus protect their eggs until they hatch...then she dies.
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. 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.
Check out this site... TheCephalopodPage
The
Mussel
1. What is a byssus
2. Where, geographically (range) are mussels found?
3. What organisms feed on mussels?
4 What can cause poisoning in mussels?
5. What is the nutrition value of mussels?
6. What is the main drawback of raising mussels in
coastal waters?