Chapter 12

Characteristics Of The Mollusca Phylum Being soft bodied is one of the characteristics of the mollusca phylum that makes them unique and wonderful creatures. The characteristics of the mollusca phylum in their own complex world are common in all classes of the molluscan phylum. When you think of mollusca you think of snails, shells, oysters, clams, mussels, squid, and octopus, and many other shelled and non-shelled animals crawling and swimming in the land and seas. With ages the characteristics of the mollusca phylum had evolved with them and how these amazing creatures survived and become the second largest in the animal kingdom is amazing.

The amazing characteristics of the mollusca phylum had been preserved from generations to generations it was noted that Arthropodas, one of the Molluscan are the most successful of the animal phyla in terms of numbers of species of about 110,000. These great molluscan species comprise the vast majority of habitats in both aquatic and terrestrial location. These vast diverse species spans from the arctic seas to the tropics and from rivers, streams, valleys to the forest and mountainsides and where ever they are the poses the same characteristics of the mollusca phylum that is distinct from the rest. Wherever they are characteristics of the mollusca phylum that are adaptable to the current or new location is admirable. The can easily adapt to live in deserts and they can become preys or predators to other animals and they know how to defend themselves from the invaders.

The uniqueness of the characteristics of the mollusca phylum is impressive and because of their basic body type these animals had evolved into a variety of different forms adapted to specific environments and their hard shell is amazingly protecting them and their shelter as well. In the vast seas the characteristics of the mollusca phylum that have no shells such as squids and octopuses are amazing. They can swim fast and can defend themselves against their attackers through their powerful muscles, tentacles, and the technique they used to hide from the enemy using the ink in their body. Many people love to see these amazing creatures and because of these uniqueness and characteristics of the mollusca phylum they appear in a wide range of sizes, colors, forms. They are considered to be one of the most very successful group in the animal kingdom. They had increased their population and they have more than 100,000 species since their existence began.

Class Gastropoda gastropods, slugs, and snails

The Class Gastropoda includes the snails and slugs. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell. Gastropods have a muscular foot which is used for "creeping" locomotion in most species. In some, it is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue (ganglion).

Gastropods are dioecious, and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other. Marine species have veliger larvae. Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products. Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.

Class Bivalvia bivalves and clams

Also known as pelecypods, the approximately 15,000 species of this taxon are found in marine and freshwater habitats throughout the world. A bivalve is characterized by possessing two shells secreted by a mantle that extends in a sheet on either side of the body. The oldest part of the shell, the umbo , can be recognized as a large hump on the anterior end of the dorsal side of each shell. The two shells are joined at the dorsal end by a region called the ligament. The ligament is comprised of the tensilium and resilium . Together they open the shells at rest. A bivalve closes its shells by contracting its powerful adductor muscles. Commonly there are two, an anterior and a posterior one, but in some taxa (such as scallops) there is only a single, central one.

The body is laterally compressed. The only external structures are the labial palps ; in some groups, there are sensory tentacles and photoreceptors at the edge of the mantle . Bivalves also possess two ctenida (in most cases) and a muscular foot. The edges of the mantle are fused in some taxa and prolonged to form tube-like siphons . One siphon carries water to the mantle cavity (the inhalent siphon) and one from it (the exhalent siphon); in many taxa they are fused but the water streams remain separate. A bivalve uses its muscular foot either to attach itself to a substrate or to burrow. Scallops propel themselves through the water by jet propulsion: rapid closing of the valves squirts water out of the mantle cavity, and the animal "swims" in the opposite direction.

Most bivalves are filter feeders, but some are scavengers or even predators. The four main feeding types of bivalves are defined by gill structure. In protobranchs, the ctenida are used only for respiration and food is caught by the labial palps. In filibranchs and lamellibranchs, the ctenida trap the food particles in their mucous coating and transfer the food to the labial palps via ciliary action. These two groups differ in that the branches of the ctenida are connected only by ciliary junctions in filibranchs whereas lamellibranchs have tissue connecting the branches of the ctenida. A septibranch bivalve has a septum across its mantle cavity, which functions to pump in food. Thus, Bivalvia is the only molluscan class characterized by the absence of a radula. Most marine bivalves go through a trochophore stage before turning into a free-swimming veliger larva. This type of larva looks like a miniature bivalve with a row of cilia along the edge of the mantle. Freshwater species lack these stages. Instead, some go through a larval stage known as the glochidium . Rather than being free-swimming, a glochidium attaches to fish or other objects that will not be swept downstream. Glochidia can be serious pests of freshwater fish.

The evolutionary relationships of bivalve lineages are not currently well understood. There appear to be many parallel lines of evolution that obscure relationships between taxa. However, there is some consensus that Protobranchia (Palaeotaxodonta), Pteriomorpha, Heterodonata, and Anomalodesmata are legitimate taxonomic groups. Pteriomorphans, characterized by reduced or absent siphons, usually have filibranch ctenida. Heterodontans are typified by possessing large siphons and have lamellibranch gill structure. Lastly, anomalodesmatans are the septibranch bivalves. Many bivalves (such as clams or oysters) are used as food in places all over the world. Pearl oysters are used for commercial production of pearls. Bivalves can also cause economic damage. The glochidia larvae of some freshwater mussels can be serious parasites of fish, and some marine bivalves bore through wood, causing damage to wooden ships, pilings, and other wood structures.

Bivalves dislodged from beneath a single rock in a coral lagoon in the South Pacific. Included are pen shells, pearl oysters and a young Tridacna . Note the bivalve with its inhalant (ventral) and exhalant (dorsal) siphons extended. Included is an anterior end view of a bivalve with its valves widely gaping, showing the mantle lining each valve, the foot, the two demibranchs on each side of the foot, and the labial palps.

Class Cephalopoda octopuses and squids

There are approximately 650 to 700 extant species of cephalopods in two subclasses and five orders. Cephalopods are strictly marine and are found in all of the world's oceans. Cephalopoda is the most morphologically and behaviorally complex class in phylum Mollusca. Cephalopoda means "head foot" and this group has the most complex brain of any invertebrate. Cephalopods are characterized by a completely merged head and foot, with a ring of arms and/or tentacles surrounding the head. The arms, tentacles, and funnel are all derivatives of the foot. Members of the order Nautiloidea have more than 90 tentacles; members of orders Sepioidea and Teuthoidea have eight arms and two tentacles; and members of orders Ocotopoidea and Vampyromorpha have eight arms. The mantle surrounds the visceral sac and possesses strong muscles required for contraction of the cavity and respiration. An opening in the mantle cavity serves as an inhalant aperture, whereas the funnel serves as the exhalent aperture. All cephalopods have one pair of unciliated ctenidia within the mantle cavity, with the exception of Nautilus, which has two pairs of ctenidia. The movement of water over the ctenidia is controlled by muscular contractions of the funnel or mantle wall. An external shell is possessed only by the Nautiloidea. Sepioids and teuthiods have reduced inner shells, while ocotopoids and vampyromorphans lack shells altogether. A cephalopod is also characterized by a horny beak secreted by the walls of the buccal cavity, and a radula within the buccal cavity.

All cephalopods are carnivorous. The strong beak is at the entrance to the buccal cavity , on the floor of which lies the radula. There are two pairs of salivary glands , one of which may be poisonous. The digestive tract consists of three parts: esophagus , which may contain a crop; stomach , which mashes food; and caecum , where most digestion and absorption occur. The posterior portion of the caecum contains a diverticulum that serves as an ink gland, producing a suspension of melanin that can be expelled through the mantle cavity. Locomotion in cephalopods is accomplished mainly by jet propulsion. To close its mantle completely, a squid fits two cartilaginous ridges on the mantle wall into two cartilaginous grooves on the opposite funnel wall; contraction of circular muscles around the mantle cavity then forces water out the funnel. The funnel can be aimed, allowing the animal to change its direction. Locomotion in other cephalopods can be accomplished by other means. Octopoids can use their arms to "walk," and sepioids and teuthoids possess lateral fins that can propel the animal.

Cephalopods are gonochoric. A female typically possesses a single oviduct. A male produces spermatophores that it transfers to the female's genital pore by means of a specialized arm or tentacle. In some species, the specialized arm tip may be pinched off and left in the female's mantle cavity: this is known as the hectocotylus arm. Mating in some cephalopods includes courtship rituals that may consist of color changes, body movements, or combinations of both. Cephalopods exhibit spiral cleavage and are protostomous, but they have no larval stage: their development is direct. Octopods typically tend their eggs until hatching. Most cephalopods are semelparous. With the exception of Nautilus, cephalopods contain pigment-rich cells in the epidermis surrounded by cells containing contractile fibers. These cells, called chromatophores, are responsible for the ability of the cephalopods to change color and patterns accurately and rapidly in response to danger or emotion. Chromatophores may also be under hormonal control. When the contractile fibers are stimulated, they contract and expose a larger amount of color.

Cephalopods possess well-developed nervous systems and complex sensory organs. The ganglia are large and close to each other, forming a large brain. Certain upper lobes within the brain serve as controls for memory and learning. Cephalopods also possess ganglia elsewhere within the mantle cavity linked to the brain by giant axons that are involved with muscular contraction. The eyes in Nautilus are primitive, but in other cephalopods are highly developed and resemble vertebrate eyes with a cornea, lens, retina, and iris. These eyes are capable of forming images and distinguishing colors. Cephalopods are of considerable economic importance to humans. Many species of squid and octopus are eaten. Nautilus shells are often used decoratively, and the internal shell of a cuttlefish, or cuttle bone, is sold in the pet trade as a calcium source for birds. Giant cephalopods such as squid and octopuses are also a great source of sea-monster folklore.

Polyplacophorans include about 600 extant species. Entirely marine, they inhabit hard bottoms and rocky coasts in all of the world's oceans. Although commonly intertidal, living chitons have been dredged from waters as deep as 7000 m. Of the two subclasses, Paleoloricata and Neoloricata, only the latter is represented by living examples. The shell plate of a neoloricatan extends beneath the adjacent plate. Lacking this feature, the paleoloricatates are considered less derived. The polyplacophoran nervous system and configuration of the heart and gonads resemble those of members of the molluscan class Aplacophora; also like an aplacophoran, a chiton has calcareous spicules in the mantle. Polyplacophorans are well suited for life in the fringe of the ocean where there are the combined dangers of beating waves and exposure to the atmosphere. A chiton is flattened and elongated, and can adhere tightly to hard substrata with its strong foot. It usually attaches by a combination of muscular contraction and adhesive secretions, but may attach more firmly by creating a seal against the surface with its girdle and then raising its inner margin to generate negative pressure.

Adults of modern species range from 8 mm to 33 cm in length. A chiton has eight overlapping shell plates , and can, if dislodged, roll into a rough ball with its plates on the exterior. The shell has three layers, and, at least in young individuals, small tubes called aesthetes that bear photoreceptors pierce the plates. The thickened mantle edge is termed a girdle; it extends onto the dorsal side of the plates, completely covering them in Cryptochiton stelleri. The girdle is reinforced with calcareous spicules. The head is reduced, and lacks eyes and tentacles. A chiton's subradular chemosensory organ can be extended out of the mouth to sense the substrate. Most chitons feed by rasping algae and other encrusted food off of the rocks on which they crawl. One genus is predatory, trapping small invertebrates under the fringe of the mantle, and then eating the captured prey. In some chitons, the radula has teeth tipped with magnetite, which hardens them. The digestive tract consists of a mouth, buccal cavity , esophagus, stomach, two-section intestine, and anus. Digestion is extracellular. Polyplacophorans have six to 88 pairs of bipectinate ctenidia , the number of which is typically not species specific, increasing as the individual grows. The ctenidia are arranged in the mantle folds that extend from front to rear along each side of the animal's foot.

The sexes are separate. Fertilization is external in seawater or in the female's mantle cavity; there is no copulation. Chitons disperse in the plankton as trochophore larvae , then settle directly as juveniles. The class Polyplacophora extends back to the Late Cambrian Period with the early genus Matthevia. Some fossil species had only seven plates. Examples of the order Paleoloricata are found from the Late Cambrian through the Late Cretaceous. The order Neoloricata extends from the present back to the Mississippian Period. The fossil record of chitons is difficult to assess given the rarity of articulated specimens. Class Polyplacophora is not of significant economic importance to humans in contemporary society, although indigenous people of the Pacific coast of North America are reported to have eaten Cryptochiton stelleri .

There are approximately 900 species of Scaphopoda, commonly called tusk shells. Scaphopods are found in marine regions around the world. Scaphopods are all marine species whose habitat ranges from shallow sub-littoral areas up to waters that are 4570 m deep. Most scaphopods are found in waters greater than 6 m. Scaphopods burrow in sediments ranging from muds to medium-coarse gravel. The two orders of this group may have slightly differing burrowing behaviors. Individuals in the Gadilida may burrow up to 30 cm in captivity. Many species in the Dentaliida burrow with the concave side just below the substrate. Scaphopods have a fossil record dating back to the early Devonian. Scaphopoda means "shovel foot". Scaphopods are most closely related to bivalves and share four synapomorphies: 1) reduced head, 2) decentralized nervous system, 3) mantle cavity is expanded to surround the body and 4) foot modified into a spate form. Within the Scaphopoda, two subtaxa are recognized, the orders Dentaliida (Gray, 1847) and Gadilida (Stoliczka, 1862). The Gadilida has a constriction on the anterior aperature and the central tooth of the radula is square or higher than wide. The Dentaliida have a conical foot, the shells are often ribbed, they may be small to large and the central tooth of the radula is wider than high. While the distinction of the orders has been clear, the taxonomy of genera among families has been debated .

Scaphopod shells usually have four layers, and these are used for identification. The shell is curved, tubular, and shaped like an elephant tusk. Most average 3 to 6 cm long, but can range from 4 mm to 15 cm. Fossils show specimens 30 cm long. The scaphopod shell is open at both ends. The wider end of the shell where the head and foot extends out is the anterior end . The posterior is the narrow end of the shell which usually is at or below the substrate. The shell surrounds a large mantle cavity, and wraps around the viscera to form a tube. The mantle cavity goes along ventral side to a smaller opening at the other end. No ctenidia are present, and gas exchange is through the mantle surface. Cilia an currents move water thorugh posterior aperature. Occasional muscular contractions expell water from the posterior end of the shell. The head is a short, conical projection (probosicis) with a mouth . Lobes on each side of the head have threadlike tentacles, called captacula, which are used to capture food.

After fertilization, the egg develops into a free-swimming trocophore larvae, then a bilaterally symmetrical veliger. The veliger usually metamorphoses in 5-6 days. At this point it becomes benthic. Scaphopods are gonochoristic or dioecious. Eggs are released singly through the right nephridium . Sperm is also released through the nephridium. Eggs are planktonic and fertilization is external. Scaphopods burrow by projecting their foot into the substrate and contracting pedal retractor muscles to pull the animal downward. Extension of the foot may help with water intake, and the scaphopods probably use foot movements to expel wastes from the posterior opening. The scapopod captacula may have tactile receptors, but this is unknown. Scaphopods have lost eyes, tentacles and osphridia found in other molluscs. The buccal cavity has a sub-radular (below the radula) organ which may be chemoreceptive. Scaphopods are fed on by fish and crabs. Their burrowing behavior is thought to keep them from predators.

Living representatives of this Class were not discovered until 1952, although Paleozoic fossil monoplacophorans had been known for some time. At present, 11 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. A number of their organs (nephridia, heart, etc.) are repeated serially, making them resemble metamerically-arranged species such as annelids and arthropods. Whether this resemblance indicates a close relationship between monoplacophorans and phyla exhibiting true metamerism is an open question.

Neopilina galatheae , top (dorsal) view. Neopilina is peculiar because of the replication of various of its organs and organ systems, reminiscent of metameric animals. The class Monoplacophora is well known as fossils, and until 1952 all of its members were believed to have been extinct since the Devonian period, about 350 million years ago. [This shell, relatively thin, was damaged while being dredged.]

Neopilina galatheae , bottom (ventral) view. The oval, flat foot is characteristic of the classes Monoplacophora, Polyplacophora and Gastropoda . The foot is bordered on each (left and right) sides by five gills.

Neopilina galatheae , side (lateral) view. The anterior end, denoted by the slightly coiled apex, is on the right.

There are approximately 320 described species in Aplacophora. However, it is likely there are many other species that have not been described. Aplacophorans are found throughout the oceans over the world Exclusively marine, aplacophorans mainly burrow into the substrate in water more than 20 m deep, and may reach densities up to 4-5 per square meter. Aplacophorans in the sub-class Chaetodermomorpha are limited by a minimum salinity of 28-30%. vermiform body ,reduced foot, reduced posterior mantle cavity, gonads empty to pericardial cavity with U-shaped gametoducts which exit to mantle cavity, no nephridia Aplacophorans are small, cylindrical, worm-like, and usually less than 5 cm long, but can range from 1 mm to 30 cm. Like other mollusks, it has no outer shell, but the epidermis secretes calcareous spicules or scales which are embedded in dorsal mantle. These spicules give the aplacophorans a sheen. Chaetoderms have a scaly appearance. All aplacophorans have a simple mantle cavity. The radula is not ribbon-like as in other mollusks, but is an expansion of the foregut epithelium. The teeth of the radula may be in simple plates in transverse rows, up to 50 rows with 24 teeth per row.

Development involves metamorphosis from a trocophore larva. Aplacophorans may be monoecious or dioecious with single or paired gonads. All discharge gametes through gonopericardial ducts into the pericardial (heart) chamber. Gametes then pass through gametoducts to the mantle cavity where they are then released outside the body. Animals in the Chaetodermomorpha have external fertilization while those in Neomeniomorpha are internally fertilized, and sometimes even brooded. Little is known about aplacophoran lifespans. Aplacophorans move via cilia through or on substrate.

Gadila aberrans occidental tuskshell Antalis entale

Chapter 12

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Chapter 12