Phylum Mollusca Lecture PDF

Phylum Mollusca  Mollusks are an extremely diverse and abundant Phylum. Most mollusks are marine. Some groups live in fresh water and land. The phylum includes animals with an external shell, for example snails and oysters, as well as mainly soft-bodied forms, for example slugs a...

Phylum Mollusca  Mollusks are an extremely diverse and abundant Phylum. Most mollusks are marine. Some groups live in fresh water and land. The phylum includes animals with an external shell, for example snails and oysters, as well as mainly soft-bodied forms, for example slugs and squids. Mollusks are first known from the early Cambrian. They utilize two/three part chitin, calcite and aragonite shell. Mollusks are divided into three main groups: 1. Gastropoda 2. Bivalvia 3. Cephalopoda Gastropoda Gastropods are the most diverse group of mollusks, living in terrestrial and all aquatic environments. The bivalve shell shape is constrained strongly by function so that mode of life can often be interpreted from the shell morphology. Cephalopods are the most morphologically complex mollusks. As active predators, occupying the same ecological niche as fish, cephalopods may be considered to be the most sophisticated invertebrate group. Cephalopoda Bivalvia  Basic morphology Most mollusks have an elongate, unsegmented body with a distinct head. The internal organs are held between a muscular foot and a calcareous shell secreted by an underlying tissue known as the mantle. The mantle tends to overhang the body forming a chamber at the posterior, the mantle cavity. This cavity contains the gills. The mouth opens anteriorly, at the other end of the mollusk. Ancestral mollusk internal morphology Sensory organs, such as eyes and tentacles, are concentrated in the head. Shell morphology is extremely diverse. Shells may be coiled or straight, chambered or undivided, singular or two-valved. Primarily, shells provide protection but they also may be used in burrowing or boring, or to enable buoyancy. The evolutionary development of some mollusks, such as octopus and squid, has tended to result in the loss of the shell.  Class Gastropoda Gastropods may have a calcareous shell or be entirely soft bodied. They have a well-developed head and sensory organs and an expanded muscular foot. In terrestrial gastropods the gills are lost and the mantle cavity is modified into an air-filled “lung”. Each complete coil is called a whorl. Dextral shell Gastropods are coiled and can be planispiral and therefore may have a superficial resemblance to ammonites. Posterior region Anterior region has muscular foot has head with tentacles  Torsion: Class Gastropoda Embryonic gastropods begin their development with their anus and gills at the rear of the body. As they develop, the entire visceral mass (a cluster of internal organs in mollusks that contains the digestive, excretory, reproductive, nervous, and respiratory systems), is forced to twist around into a U-bend. There is probably no simple, single answer to the puzzle of gastropod torsion. It probably brought mantle cavity in the front of the animal, so the snail can easily ventilate its gills, wash away wastes, and taste the oncoming water. Some scientists also suggest that torsion allowed the gastropod to retract into its mantle cavity for protection. In addition to these puzzles is the fact that some gastropods secondarily lose their torsion. Detorted gastropods are most common among the unshelled opisthobranchs.  Class Gastropoda Sublass Opisthobranchia Sublass Pulmonata Sublass Prosobranchia  Class Gastropoda Evolutionary history Cambrian gastropods were typically marine, herbivorous grazers with low, coiled shells. Cambrian gastropods By the Carboniferous, forms with a siphonal notch were common, indicating the presence of a siphon and therefore an infaunal mode of life. Palaeozoic gastropods generally occupied shallow water environments and were greatly affected by the end-Permian extinction. During the Mesozoic, prosobranchs diversified and deep burrowing, long-siphoned prosobranchs originated in the Cretaceous. These forms dominate the gastropod fauna today. Carnivorous gastropods were important predators in the Cenozoic. The Cenozoic also saw the radiation of gastropods into freshwater Early Paleozoic gastropods environments and the appearance of planktic opisthobranchs. Radiation of the air-breathing pulmonates into the terrestrial environment began in the Jurassic. Late Paleozoic gastropods Pleistocene gastropods Cretaceous gastropods  Class Gastropoda Planorbis (Jurassic–Recent) Belonging to the subclass Prosobranchiata, this freshwater gastropod has an almost planispiral shell (diameter approximately 1 cm). Although the morphology varies within the genera, most species have smooth shells. Living in a range of freshwater environments, Planorbis feeds on algae and plants. Planorbis Turritella (Cretaceous-Recent) It belongs to the subclass Prosobranchiata. The multiwhorled shell with a high pointed spire has a simple, circular aperture and spiral ribbing on the external surface. There is no siphonal canal. The height is approximately 5 cm. The mode of life is benthonic, infaunal, filter feeding. Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms suspended in water, typically by having the water pass over or through a specialized Turritella filtering organ that sieves out and/or traps solids.  Class Gastropoda Hygromia (Eocene-Recent) Buccinum (Pliocene-Recent) Hygromia is a terrestrial gastropod, the It has a moderately high-spired shell with an oval bristly snail, belonging to the subclass aperture and short siphonal canal. Pulmonata. Typically the shell has an external, ribbed sculpture. It has a modified, untwisted, mantle cavity The height is approximately 8 cm. that functions as an air-breathing lung. Living in sea water up to a depth of 200 m, Buccinum The shell is generally thin, smooth, and lives semi-infaunally with its siphon extended above conispiral. the sediment drawing clean water into the mantle Its height is approximately 5 mm. cavity. Buccinum is carnivorous. Hygromia is found in a range of terrestrial habitats and is most common in humid Hygromia environments with calcium-rich soils. Patella (Eocene-Recent) Patella is characterized by an uncoiled, conical, cap-like shell. Pronounced ribs that radiate from the Buccinum apex strengthen the shell. The height of the cone is approximately 3 cm. Patella lives in the intertidal zone and clings to rocks using its foot. Patella  Class Bivalvia Bivalves are laterally compressed mollusks enclosed within a pair of hinged shells or valves. The valves are closed by the adductor muscles. The shell is opened by relaxing these muscles and water currents are drawn into the cavity. In the majority of bivalves the gill is modified for filter feeding, although the earliest bivalves may have been deposit feeders (an organism that moves along the surface within soft sediments to ingest organic matter found in the sediment). Some bivalves retain this feeding strategy. Most forms are capable of limited movement and the foot can protrude into the sediment to enable them to burrow. Most bivalves are marine but there are some freshwater forms. As the shell forms of different species are adapted to the nature of the substrate, bivalves are very useful in paleoenvironmental reconstruction. Bivalve shells are multilayered. Two phases comprise the shell – an organic matrix and a crystalline calcareous component, in the form of aragonite or calcite (CaCO3). Internal morphology Class Bivalvia Beak: Tiny pointy origin of the shell. Umbo: “Humpiest” part of the shell of which beak is a part.  Class Bivalvia Heterodont dentition Pachydont dentition Types of dentition: No hinge teeth Schizodont dentition Dysodont dentition Desmodont dentition Taxodont dentition Isodont dentition  Class Bivalvia Bivalve ecology As shell shape is constrained by function, the mode of life of bivalves can be interpreted from the shell morphology. The major life habits of bivalves are: (i) burrowing in soft substrates; (ii) boring and cavity dwelling; (iii) attached (cemented or by byssus threads); (iv) unattached recumbant; and (v) intermittent swimming. Infaunal bivalves Bivalves that burrow in soft substrates tend to be equivalved and have a distinct pallial sinus. Burrowing is achieved by the foot, which penetrates the sediment and swells. The muscles in the foot then contract, drawing the shell down through the sediment.  Class Bivalvia Boring bivalves Bivalves that bore into hard substrates typically have elongate, thin shells that are resistant to abrasion. The shell edges are used to penetrate the material aided by acid secretions from the mantle. Epifaunal and swimming bivalves Epifaunal bivalves exploit three living strategies: (i) attachment to the substrate by byssus threads (a group of thread-like structures made of the protein collagen); (ii) cementation to hard surfaces; and (iii) recumbent, free lying on the sediment surface stabilized by the shell morphology. Byssally attached bivalves secrete threads of collagen that adhere to the substrate. Cemented bivalves produce a calcareous fluid at the mantle margin that crystallizes, fixing the bivalve firmly to the substrate. Recumbent, unattached bivalves have extremely asymmetric valves. Bivalves that are able to swim do so only intermittently. The shells of swimming valves tend to be thin, to reduce weight, although some forms have pronounced radial ribs.  Class Bivalvia Evolutionary history The earliest bivalves are known from Lower Cambrian rocks. These primitive forms are extremely small and may be descended from an unusual group of primitive mollusks called rostroconchs. Strangely, no bivalves are recorded from the Middle and Upper Cambrian. A major period of diversification occurred in the early Ordovician. Groups arose with taxodont, dysodont, and heterodont hinges and a range of feeding strategies. Deposit feeders, byssally attached bivalves, and burrowers colonized marginal and nearshore environments. After this rapid radiation the group stabilized and bivalves were not a particularly diverse or Cambrian bivalve abundant group during the Palaeozoic. Non-marine bivalves appeared in the Devonian and were abundant in the Carboniferous, particularly in deltaic environments. Ordovician bivalves Devonian freshwater bivalve  Class Bivalvia Evolutionary history During the early Mesozoic, bivalves underwent a second radiation. The presence of a muscular foot and the development of siphons allowed them to successfully exploit an infaunal mode of life. By colonizing new infaunal environments, bivalves expanded into the intertidal zone, burrowed deeper into the sediment, and developed mechanisms for boring into hard substrates. Diversification of epifaunal bivalves also occurred in the Mesozoic. The most important of these groups were the rudist bivalves. They colonized carbonate shelves, adopting conical forms similar to corals or encrusting or lying on hard substrates. This group was short lived, originating in late Jurassic times and becoming extinct at the end of the Cretaceous. Most bivalves survived the end-Cretaceous mass extinction. Bivalves have been ubiquitous in most shallow marine environments throughout the Cenozoic and they continue to be abundant in most present-day marine environments. Rudist bivalves Triassic bivalves  Class Bivalvia Radiolites (Cretaceous) Known as rudist bivalves, such Teredo (Eocene–Recent) highly modified forms of Teredo is a highly specialized bivalve able to bore into wood. cementing bivalves were The cylindrical shell is extremely reduced. common in the Cretaceous. Sharp sculpture on the external surface is used for tunneling Radiolites has two strikingly into the substrate. different valves. The lower valve The animal is essentially worm-like and lives encased in the is conical with thick walls and burrow, growing to fill the excavated space. Teredo the upper valve is reduced to a small, flat lid. Ensis (Eocene-Recent) Such coral-like rudists tended to Characterized by an extremely elongated, thin, featureless shell grow in groups, feeding in calm, (approximately 12 cm in length) with both posterior and clear waters above the sea bed. anterior gapes. Ensis lives infaunally in muds and sands in the intertidal zone. During feeding, the anterior part of the shell is close to the sediment–water interface. During low tide, the animal burrows actively down into deeper sediments using the muscular foot. Ensis Mya (Oligocene–Recent) This bivalve has an elongate, smooth shell with a posterior gape. The shell is approximately 3 cm from umbo to shell edge. Dentition is absent and there is a deep pallial sinus. Radiolites Modern species are infaunal, living in soft sediment in burrows 30 cm deep. Mya  Class Cephalopoda All cephalopods are marine. The body of cephalopods is elongated so that the mantle cavity is anterior and the visceral mass is at the posterior end of the animal. Living cephalopods swim using jet propulsion. Water is drawn into and expelled from the mantle cavity through the hyponome, a modified part of the foot. Cephalopods are divided into three subclasses: Nautiloidea: Nautiloids have an external, chambered shell with simple sutures between its chambers. Ammonoidea: Ammonoids also have an external shell, always coiled, with variable and more complicated sutures. Coleoidea: Coleoids have an internal and reduced shell. In some coleoids the shell is absent. Nautiloids (external shell) Coleoidea (internal shell) Ammonoids (external shell)  Class Cephalopoda Nautiloids (Subclass Nautiloidea) The animal occupies the final shell chamber, the body chamber. The head, sensory functions, and hyponome are situated near the chamber opening and the visceral mass is at the rear. The animal is connected to the rest of the shell by the siphuncle, a tube extending from the body chamber to the protoconch (the first part of the shell to be formed; new chambers are added onto this as the animal grows). Septal necks are where the septa are pierced to allow the tube of soft tissue (siphuncle) through. The septum is the wall of the chamber. A suture is a line along which the septum of the shell fuse. The umbilicus is the diameter of the depression between the inside margins of the last coil. The most recent line of growth, which traces the edge of the aperture opening, is called the peristome. All of the chambered sections of the shell are collectively called the phragmocone. Support structures for the siphuncle are called septal foramen.  Class Cephalopoda Buoyancy in Nautilus Nautilus is the only living cephalopod that retains an external, coiled Nautilus has an adjustable buoyancy mechanism that gives it shell. Living in cool waters in the southwest Pacific, at depths neutral buoyancy at different depths of the water column. between 150 and 300 m, Nautilus relies on buoyancy control to The shell chambers contain gas and sea water, the adjust its position in the water column. proportions of which can be changed. Nautilus is an opportunistic feeder (sustains itself from a number of Initially the chambers contain sea water. The siphuncle different food sources), grasping mainly crustaceans and small fish removes ions from solution in the sea water, drawing water with its tentacles. from the chambers into the mantle cavity. Gas bubbles then Nautilus is a poor swimmer. Sea water is drawn into the mantle diffuse into the space, making the animal more buoyant and cavity and expelled through the hyponome. As water is ejected a able to float higher in the water column. By pumping ions into force is exerted on the shell, causing it to lurch forwards. When the the chambers, Nautilus brings water back into the chambers, mantle cavity is emptied, the shell swings backwards generating a making the animal less buoyant so that it sinks to lower see-sawing motion. Consequently, Nautilus swims only for short depths. distances and relies on buoyancy control to maintain its position in the water column during feeding. It rests during the day on the sea floor. Nautiloids include not only the living genus Nautilus, but a wide variety of primitive forms going back as far as the Cambrian. The primitive forms tend to be straight-shelled (orthoconic) or slightly curved (cyrtoconic). Othoceras sp.  Class Cephalopoda Ammonoids (Subclass Ammonoidea) Most ammonoids had a chambered, planispirally coiled shell (the shell coils in a single horizontal plane with the diameter increasing away from the axis of coiling/centre) with complex sutures. The shell can be divided into three parts: (i) the body chamber, where the animal lived (ii) the phragmocone, the chambered part of the shell (each chamber represents part of a previous body chamber) (iii) the protoconch, the first chamber to form As with nautiloids, the chambers were connected by a tube called the siphuncle, although in ammonoids the siphuncle usually ran along the outer, ventral margin of the shell rather than through the center of the chambers. The buoyancy mechanism of ammonoids was therefore similar to that of nautiloids.  Class Cephalopoda Coiling There are 3 forms of coiling mode: Involute coiling: The inner coils are almost completely hidden by more recent coils. This type of coiling is demonstrated by Nautilus. Evolute coiling: Describes a coiling growth opposite to involute, in which the inner coils are easily seen, giving a wider umbilicus. This type of coiling is common in Ammonoids. Convolute coiling: Newly formed whorls partially overgrow prior whorls in ammonoid species. Involute coiling Convolute coiling Evolute coiling  Class Cephalopoda Ammonoid suture Sutures are the lines marking the junction between the chamber wall and the ammonoid shell. The suture pattern is an important feature used in ammonoid classification. Sutures are described in terms of saddles and lobes. Saddles are curved sections of line that “point” towards the body chamber. Lobes are the converse – curves that are directed away from the body chamber. Palaeozoic ammonoids generally had simple, straight suture lines, whereas most Mesozoic ammonoids are characterized by complex sutures.  Class Cephalopoda Ammonitic suture Ceratitic suture Goniatitic suture All ammonites are ammonoids, but not all ammonoids are ammonites. Ammonites are a specific group within the larger classification of ammonoids.  Class Cephalopoda Sexual dimorphism Mature ammonite shells collected from the same horizon can often be divided, on the basis of size, into two distinct morphological groups. The smaller ammonites are referred to as microconchs and the larger type as macroconchs. Microconchs may also have a modified aperture with lateral extensions, the lappets. The function of the lappets is unknown but it may be linked to sexual reproduction. Although microconchs and macroconchs may simply be closely related species, new characters appear in both groups simultaneously suggesting that they are males and females of the same species, though which is which is unknown.  Class Cephalopoda Heteromorphs Some ammonite groups developed peculiar, “heteromorph” shell forms, particularly in the late Cretaceous. Originally these forms were considered non-functional, evolutionary dead-ends. However, physical modeling has shown that they were stable and well adapted for floating within the water column. Class Cephalopoda  Class Cephalopoda Difference between ammonoids and nautiloids If the internal shell/fossil can be observed then this method is useful. The shape of the septa curvature differs between Nautiloids and Ammonoids. A Nautiloid shell will usually be smooth on the exterior with no ornament or ribbing. In contrast, an Ammonoids shell can have ornament and ribbing on its exterior. The siphuncle of a Nautiloid will always connect the body chamber to the original chamber through the centre of all previous chambers. In Ammonoids, the siphuncle also interconnects previous chambers but it has the tendency to run closer to the outer (ventral) margin rather than the centre. Coleoids (Subclass Coleoidea) Class Cephalopoda The subclass Coleoidea includes all living cephalopods except Nautilus. The shell in coleoids is internal and reduced or even absent. Cuttlefish have an internal shell with a buoyancy function. Squids are streamlined swimmers with an internal cartilaginous rod. Octopuses are benthic, shell-less coleoids with “webbed” arms that allow the animal to drift with the currents. Belemnites make up the majority of the fossil coleoids. Characterized by an internal skeleton with a robust, bullet-shaped, calcite counterweight, belemnites are abundant in Jurassic and Cretaceous rocks. Belemnites Belemnites had an internal skeleton unlike that of any living coleoid. It can be divided into three parts: (i) the robust anterior counterweight – the rostrum or guard; (ii) the buoyancy mechanism – the phragmocone, a chambered conical section with a siphuncle; and (iii) the pro-ostracum – the support for an open body chamber.  Class Cephalopoda Evolutionary history The first cephalopods were straight-shelled nautiloids. They appeared in late Cambrian times and underwent a rapid diversification in the Ordovician when they gave rise to the coiled forms that existed throughout the Palaeozoic and Mesozoic. Although never as abundant or diverse as Straight-shelled nautiloids ammonoids, they survive through to the present day. Ammonoids evolved from straight-shelled ancestors in the early Devonian. Their evolutionary history has been marked by a sequence of radiations followed by extinction. Peaking in diversity during the Jurassic, they declined through the remainder of the Mesozoic, becoming extinct at the end of the Cretaceous. The history of coleoids is less well known due to their reduced, internal shell. The first true coleoids are recorded from Carboniferous rocks. Belemnites became abundant in the Jurassic and Cretaceous. Squid and cuttlefish are known from the Jurassic, diversifying in the Cenozoic after the end- Cretaceous extinction event.  Class Cephalopoda Kosmoceras (Middle Jurassic) Dactylioceras (Lower Jurassic) This ammonite has a compressed shell This ammonite is typically serpenticone and the external with pronounced ribs that bifurcate surface is ribbed (the shell diameter is approximately 6.5 towards the venter. cm). The suture pattern is ammonitic. Sexual dimorphism is known. In the microconch, the smaller dimorph, the aperture is compressed and the lappets Dactylioceras are developed. The shell diameter of the macroconch is approximately 5 cm. Amaltheus (Lower Jurassic) Amaltheus has a compressed, oxycone shell sculpted with curved, sinuous ribs. A “pie crust” keel is developed along the venter. The suture pattern is ammonitic. The shell diameter is approximately 8 cm. Kosmoceras Amaltheus  Class Cephalopoda Scaphites (Cretaceous) Scaphites is a heteromorphic ammonite typical of the Cretaceous period. This partially uncoiled ammonite has a body chamber in the form of a hook with a slightly constricted aperture that faces upwards. The external surface of the shell is ribbed. Scaphites Neohibolites (Lower Cretaceous) This belemnite has a small, spindle-shaped guard (approximately 4 cm in length) with a long ventral groove in the area around the alveolus. The soft-body morphology of belemnites is known from exceptionally preserved individuals associated with fossil lagerstätten. Such specimens have long hooked tentacles and ink sacs. Neohibolites