What are Mollusks?

What are Mollusks? • The name Mollusca (from the Latin mollis meaning soft), was first used by the French zoologist Cuvier in 1798 to describe squids. • All mollusks have a muscular foot (used for moving) and a mantle (an outgrowth that covers the animal). • Most mollusks have an external calcium carbonate shell that is produced by the mantle. • All body systems are present. • Many also have a radula (a unique organ that is mostly composed of a hard material called chitin) in the mouth that allows the animal to scrape food from surfaces by sliding back and forth. Like a rasping tongue. • Mollusks have a coelom (a body cavity), but the coelom is made from cell masses, making all species in this phylum protosomes. All organs are suspended in this coelom, between the outer covering and the digestive tube of the animal. However, animals in this phylum are unique to coelomates in that they lack body segmentation. Taxonomy

Body Systems of Mollusks • Respiratory- diffusion still occurs (through mantle) but specialized gills, lungs present (derived from mantle). • Circulatory- pumping heart, vessels and sinuses; open system in all but cephalopods; • Hemocyanin- oxygen combining pigment or “blood” that contains copper and is bluish when combined with oxygen. • Digestive- complete and complex; mouth to anus. • Excretory- 2 nephridia (kidneys) empty into mantle cavity. • Nervous- ganglia and connecting nerve cord; specialized eye in cephalopods. • Reproductive- most are dioecious (unisexual), but some gastropods are monoecious (heterosexual). (Go Back)

Phylum: Molluska • The 7 major classes belonging to Mulluska. • Aplacophora - Mollusks without shells • Bivalvia – Clams and their relatives • Cephalopoda – Squids and their relatives • Gastropoda - Snails and their relatives • Monoplascophora - Mollusks with one plate • Polyplacophora – Chitons • Schapoda - Mollusks with a tubular shell (Go Back)

Class: Aplacophora(belongs to Phylum: Molluska) • This class includes two subclasses called Caudofoveata and Solenogastres. • Both classes contain relatively few species. The Caudofoveata are aberrant molluscs which lack shells. They normally burrow in soft sediments and are quite common in the deep sea. The class Solenogastres is also very small. These animals also lack shells and generally live epibenthically (on the surface of the substratum). • They usually resemble worms, and are found in deep water, often more than 3,000 m. There are only about 300 species in this class, and besides lacking shells, they are all marine and generally very small. • NONE OF OUR SAMPLES BELONG TO APLACOPHORA. (Go Back)

Chaetoderma canadense(an Aplascophoran) (Go Back)

Class: Bivalvia(belongs to Phylum: Molluska) • The bivalves are bilaterally symmetrical (can be divided along their length into two mirrored - right and left halves) with extensive mantle lobes which secrete a single shell composed of two valves. The two valves of the shell are hinged dorsally where they are held together by a new structure, the ligament, and completely enclose the rest of the body. The valves are closed by two large muscles. Bivalve respiration and locamotion reflect the morphology described. • Class Bivalvia includes all of the bivalves. These are marine animals with a hinged shell divided into two halves. The hinge is head together by a ligament and one or two adductor muscles. Most live in sand or mud, and use their foot for digging and anchoring to surfaces. These bivalves use the hinges to take in food, and are also able to jet some distance away by closing the hinged shell and squirting the water taken in out of the mantle cavity. Bivalves lack a head and have a spacious mantle cavity as well. Some examples include clams, oysters, scallops, and mussels. • The bivalves are mainly marine, but a few species are found in freshwater habitats, although none have invaded the land. Many species of bivalve are of commercial importance. • NEXT

Respiration in Bivalves • The mantle cavity of bivalves is greatly expanded and now surrounds the rest of the body. Inside the mantle cavity are the large leaf-like ctenidia (feathery or comb-like structures), each of which consists of a dorsal (the back of an organ or body) vascular axis from which hang an inner and outer gill plate, or demibranch, made up of numerous parallel gill filaments. • Each plate is not single, but is folded back on itself in a V-shape and thus has an ascending and a descending lamella. The two plates, when seen in cross section have a W-shape. • Usually the lamellae are held together by junctions of different types; interlamellar junctions join the lamellae together and interfilamentar junctions join the adjacent filaments together. The more primitive bivalves, e.g. the mussel family, possess filibranch gills in which the interfilamentar junctions are only patches of interlocking cilia. (Go Back)

Shape of lamellar plates (Go Back)

Bivalve Locomotion • Most bivalves are sedentary and many are adapted for burrowing into soft sediments using the laterally compressed foot. A few are capable of boring into wood, and some are even able to bore into rock. • Although most bivalves are sedentary, some, especially members of the scallop family, are able to swim actively over short distances, usually to escape potential predators. • The scallops swim by rapidly ejecting water from the mantle cavity by rapid clapping together (adduction) of the shell valves, which forces water out on either side of the hinge. Scallops usually swim in short bursts, but may cover several metres at a time. (Go Back)

Orders belonging to Class: Bivalvia • Major orders belonging to Mollusca/Bivalvia. • Arcoida • Myoida • Mytiloida • Pterioida • Unionoida • Veneroida (Go Back)

Order: Arcoida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Arcoida • Arcidae (Ark shells) • Glycymerididae (Bittersweet clams) (Go Back)

Family: Arcidae(belongs to Order: Arcoida) • These shells almost always have straight hinge lines and strong umbones. Most are ovate. • Major Genera and species • Anadara • brasiliana (Incongruous Ark) • transversa (Transverse Ark) • Noetia • ponderosa (Ponderous Ark) (Go Back)

Family: Glycymerididae(belongs to Order: Arcoida) • These shells have V-shaped ligaments and many teeth. They are usually round or broadly ovate. • Major Genera and species • Glycymeris • pectinata (Comb Bittersweet) (Go Back)

Order: Myoida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Myoida • Pholadidae (Piddocks) (Go Back)

Family: Pholadidae(belongs to Order: Myoida) • Major Genera and species • Cyrtopleura • costada (Angel Wing) (Go Back)

Order: Mytiloida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Mytiloida • Mytilidae (Mussels) • Pinnidae (Pen shells) (Go Back)

Family: Mytilidae(belongs to Order: Mytiloida) • Major Genera and species • Geukensia • demissa (Atlantic Ribbed Mussel) • Ischadium • recurvum (Hooked Mussel) • Modiolus • americanus (Tulip Mussel) (Go Back)

Family: Pinnidae(belongs to Order: Mytiloida) • Major Genera and species • Altrina • rigida (Stiff Pen Shell) (Go Back)

Order: Pterioida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Pterioida • Anomiidae (Jingle shells) • Ostreidae (Oysters) • Pectinidae (Scallops) • Plicatulidae (Kitten’s Paws) (Go Back)

Family: Anomiidae(belongs to Order: Pterioida) • Major Genera and species • Anomia • simplex (Common Jingle Shell) (Go Back)

Family: Ostreidae(belongs to Order: Pterioida) • Major Genera and species • Crassostrea • virginica (Eastern Oyster) • Ostrea • equestris (Crested Oyster) (Go Back)

Family: Pectinidae(belongs to Order: Pterioida) • Major Genera and species • Argopecten • gibbus (Calico Scallop) (Go Back)

Family: Plicatulidae(belongs to Order: Pterioida) • Major Genera and species • Plicatula • gibbosa (Kitten’s Paw) (Go Back)

Order: Unionoida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Unionoida • NONE OF OUR SAMPLES BELONG TO Unionoida. (Go Back)

Order: Veneroida(belongs to Class: Bivalvia) • Major families belonging to Molluska/Bivalvia/Veneroida • Cardiidae (Cockles) • Carditidae (Carditas) • Cultellidae (Razor clams) • Donacidae (Donaxes) • Lucinidae (Lucines) • Mactridae (Surf clams) • Petricolidae (Petricolas) • Semelidae (Semeles) • Solecurtidae (Razor clams) • Solenidae (Razor clams) • Tellinidae (Tellins) • Veneridae (Venus clams) (Go Back)

Family: Cardiidae(belongs to Order: Veneroida) • Major Genera and species • Dinocardium • robustum (Giant Atlantic Cockle) • Trachycardium • egmontianum (Prickly Cockle) • muricatum (Yellow Cockle) (Go Back)

Family: Carditidae(belongs to Order: Veneroida) • Major Genera and species • Carditamera • floridana (Broad-ribbed Cardita) (Go Back)

Family: Cultellidae (belongs to Order: Veneroida) • Major Genera and species • Ensis • directus (Common Razor Clam) (Go Back)

Family: Donacidae (belongs to Order: Veneroida) • Major Genera and species • Donax • variabilis (Florida Coquina) (Go Back)

Family: Lucinidae (belongs to Order: Veneroida) • Major Genera and species • Divaricella • quadrisulcata (Crosshatched Lucine) • Parvilucina • multilineata (Multi-lined Lucine) • Phacoides • pectinatus (Thick Lucine) • Pseudomiltha • floridana (Florida Lucine) (Go Back)

Family: Mactridae (belongs to Order: Veneroida) • Major Genera and species • Mactra • fragilis (Fragile Atlantic Mactra) • Mulinia • lateralis (Dwarf Surf Clam) • Raeta • plicatella (Channeled Duck Clam) (Go Back)

Family: Petricolidae (belongs to Order: Veneroida) • Major Genera and species • Petricola • pholadiformis (False Angel Wing) (Go Back)

Family: Semelidae (belongs to Order: Veneroida) • Major Genera and species • Cumingia • tellinoides (Tellinlike Cumingia) (Go Back)

Family: Solecurtidae (belongs to Order: Veneroida) • Major Genera and species • Tagelus • plebeius (Stout Tagelus) (Go Back)

Family: Solenidae (belongs to Order: Veneroida) • Major Genera and species • Solen • viridis (Green Jackknife Clam) (Go Back)

Family: Tellinidae (belongs to Order: Veneroida) • Major Genera and species • Tellina • alternata (Alternate Tellin) • iris (Iris Tellin) • sybaritica (Dall’s Dwarf Tellin) • tampaensis (Tampa Tellin) (Go Back)

Family: Veneridae (belongs to Order: Veneroida) • Major Genera and species • Chione • cancellata (Cross-barred Venus) • Dosinia • discus (Disk Dosinia) • Macrocallista • numbosa (Sunray Venus) • Merceneria • campechiensis (Southern Quahog) (Go Back)

Class: Cephalopoda(belongs to Phylum: Molluska) • This class contains the squids, octopuses and cuttlefish. • Orientation of the body differs from most mollusks in that the ventral region (belly-side) is now anterior (up front) and the visceral mass is now the functional posterior (in back) end of the animal. This change in orientation has implications for sensory function and cephalopod locomotion. • Also differs from other mollusks in the alteration of much of the original foot into a series of large prehensile tentacles or arms at the anterior end which now surround the head. • Most either have an internal shell, like a squid. Some lack a shell, like an octopus. All cephalopods are predators, and use jaws and radula to crush and rip prey. Like bivalves, they are able to move by taking water into the mantle cavity and shooting it back out. • They have large brains enclosed in cartilaginous brain cases. Because of a developed brain, good eye sight, and good sense organs, they are considered to be one of the most advanced invertebrates. • NONE OF OUR SAMPLES BELONG TO CEPHALOPODA. (Go Back)

Octopus(a Cephalopod) (Go Back)

Eye of an Octopus (Go Back)

Cephalopod Locomotion • Most cephalopods, except the more sedentary octopus, are active animals, which swim by means of jet propulsion, during which water is rapidly forced out of the mantle cavity through the siphon by the contraction of the muscles in the mantle wall. • Squids, besides being the fastest cephalopods, are also the fastest swimmers of all aquatic invertebrates. Their long tapered bodies are ideally streamlined, and the lateral fins provide stability. The 'flying squids' are particularly interesting. These have highly developed fin vanes and may shoot out of the water and glide for some distance. (Go Back)

Cephalopod Tentacles • Squids and cuttlefish have ten appendages comprised of eight arms and two long tentacles possessing numerous cup-shaped, rimmed suckers. The tentacles are used to capture prey, which is held by the tentacles and their suckers as it is pulled towards the mouth. • The mouth lies at the base of the tentacles and possesses a parrot-like beak. • The octopuses have eight long tentacles (sometimes called arms) each with a row of suckers on the inner surface. (Go Back)

Class: Gastropoda(belongs to Phylum: Molluska) • Gastropods are the most successful group of molluscs not only in terms of the number of species, but also in the wide range of habitat in which they may be found. Marine species have become adapted to living on all types of substratum and some have even adopted a pelagic (living in open water either at the surface or at intermediate depths) existence. Others have successfully invaded all types of freshwater habitat as well as the land. • A univalve, generally spirally coiled shell is present in the majority of gastropods. The most significant change from the hypothetical ancestral mollusc that the gastropods have undergone during their phylogeny is the process of torsion. The third important feature of gastropods is the greater degree of head development or cephalization. • Class Gastropoda is the largest group of mollusks, with between 40,000 and 75,000 species. These are also the only mollusks that live on land. Gastropods, like Class: Schapoda, lack gills, and use the mantle as a makeshift lung for extracting oxygen and breathing. Most species have coiled shells, and this class has evolved tentacles and eyes. This class includes marine animals like the sea slug and terrestrial animals like the slug and snail. • NEXT

Gastropod Shell Coiling • One characteristic feature of the majority of gastropods is that the ancestral conical shell has now become coiled. This process was a separate evolutionary event and was not connected with torsion, indeed there is fossil evidence that it may have preceded it. In the most primitive gastropods, shell coiling was planospiral, whereas in the more advanced forms coiling is asymmetrical. • In this type of shell, the shell axis has now become shifted slightly to improve weight distribution and in modern forms it is carried obliquely to the long axis of the body, and its weight is now borne by the widest part of the foot, near the middle of the body. • The symmetrical shell and the angle at which it is carried has caused some occlusion of the mantle cavity on the right side, which in turn has resulted in the reduction or loss of the right ctenidium and the associated auricle. The shape of a gastropod's shell depends greatly on its habitat and its mode of life. (Go Back)

Gastropod Torsion • The most significant change from the hypothetical ancestral mollusc that the gastropods have undergone during their phylogeny is the process of torsion. During torsion, most of the body behind the head, including the mantle, mantle cavity and visceral mass are twisted anti-clockwise through 180 degrees. • Not all living gastropods are torted, however. Some groups have undergone detorsion during which the mantle cavity has moved to within 90 degrees of its original position, occasionally even 120 degrees in some species, and has resulted in some rearrangement of the internal anatomy. (GO BACK)

Orders belonging toClass: Gastropoda • 5 major orders belonging to Molluska/Gastropoda • Archaeogastropoda • Caenogastropoda • Heterogastropoda • Nudibranchia • Sigmurethra (Go Back)

Order: Archaeogastropoda (belongs to Class: Gastropoda) • Major families belonging to Molluska/Gastropoda/Archaeogastropoda • Turbinidae (Turban shells) (Go Back)

Family: Turbinidae (belongs to Order: Archaeogastropoda) • These shells are turban- or top-shaped, or occasionally flattened. They are thick, sculptured or smooth, with a rounded aperture that is pearly within. • Major Genera and species • Turbo • castanea (Chestnut Turban) (Go Back)

Order: Caenogastropoda (belongs to Class: Gastropoda) • Major families belonging to Molluska/Gastropoda/Caenogastropoda • Conidae (Cone shells) • Crepidulidae (Cup-and-saucer shells or slipper shells) • Fasciolariidae (Spindle shells) • Ficidae (Fig shells) • Melongenidae (Crown conch shells) • Muricidae (Murex shells) • Nassariidae (Dog whelk or Basket whelk shells) • Naticidae (Moon shells) • Olividae (Olive shells) • Terebridae (Auger shells) (Go Back)

Family: Conidae (belongs to Order: Caenogastropoda) • These shells are cone-shaped (narrow at bottom, wide at shoulder) with a low spire. • Major Genera and species • Conus • floridanus (Florida Cone) (Go Back)

What are Mollusks?