Phylum Mollusca Lecture and Self-Assessment Questions PDF

CHAPTER 9 parts: the head-foot and the visceral mass. PHYLUM MOLLUSCA Mollusks have a mantle that secretes a Learning Objectives: calcareous shell and covers the visceral mass. 1. Identify features common to...

CHAPTER 9 parts: the head-foot and the visceral mass. PHYLUM MOLLUSCA Mollusks have a mantle that secretes a Learning Objectives: calcareous shell and covers the visceral mass. 1. Identify features common to all molluscs. The mantle cavity functions in excretion, gas 2. Describe various types of molluscs. exchange, elimination of digestive wastes, INTRODUCTION and the release of reproductive products. They exhibit bilateral symmetry, meaning their PHYLUM MOLLUSCA body can be divided into mirror-image halves. Mollusks typically have trochophore larvae, undergo spiral cleavage during development, and form a schizocoelous coelom. The coelom is reduced to cavities surrounding the heart, nephridia, and gonads. Most mollusks have an open circulatory system, except for the class Cephalopoda, which has a closed circulatory system. Additionally, a radula is usually present and is used for scraping food. Mollusks exhibit an impressive range of size and body form, making them one of the most diverse groups in the animal kingdom. They include the largest of all invertebrates, with about 100,000 species described and an additional 70,000 species known only from fossils. As the most malleable creatures in the animal kingdom, they vary significantly in size and form. For instance, the giant squid (Architeuthis) can reach an astonishing CLASSIFICATION OF PHYLUM MOLLUSCA length of 18 meters, while the smallest mollusks, like the garden slug, can be less than 1 centimeter long. Other well-known examples of mollusks include clams, snails, and octopuses. General Characteristics of phylum mollusca Phylum Mollusca is characterized by several distinct features. The body is divided into two 1|Pa ge BODY STRUCTURE OF MOLLUSCS overlays a fleshy, tongue-like structure The body structure of molluscs is divided into supported by a cartilaginous odontophore, two main regions: the head-foot and the allowing it to protrude from the mouth. visceral mass. The head, which is the anterior Muscles attached to the radula move it back part of the head-foot, contains the mouth and and forth over the odontophore, scraping food various nervous and sensory structures that and passing it posteriorly to the digestive are essential for feeding and perception. The tract. foot is an elongate structure used for attachment and locomotion, allowing the mollusk to move and secure itself to surfaces. Positioned dorsal to the head-foot, the visceral mass houses the internal organs responsible for digestion, circulation, reproduction, and excretion. The mantle of a mollusk attaches to the visceral mass, enfolds most of the body, and often secretes a protective shell. The shell is composed of three distinct layers. The outermost layer, called the periostracum, is made up of proteins and is secreted by mantle cells at the outer margin. The middle and thickest layer, known as the prismatic layer, consists of calcium carbonate mixed with organic materials. The innermost layer, the nacreous layer, is formed from thin sheets of calcium carbonate interspersed with organic matter and is secreted by cells along CLASS GASTROPODA the entire epithelial border of the mantle. Class Gastropoda (gas-trop’o-dah), derived from the Greek words’ "gaster" meaning gut The mantle cavity, located between the and "podos" meaning foot, includes snails, mantle and the foot, opens to the outside and limpets, and slugs. It is the largest and most functions in gas exchange, excretion, varied class within the phylum Mollusca, elimination of digestive wastes, and the boasting approximately 60,000 living species. release of reproductive products. Most TORSION: molluscs have a radula, a rasping structure in Torsion is a significant developmental the mouth that consists of a chitinous belt and modification in gastropods, involving a 180° rows of posteriorly curved teeth. The radula 2|Pa ge counterclockwise twisting of the visceral LOCOMOTION mass, mantle, and mantle cavity. This process Almost all gastropods possess a flattened foot positions the gills, anus, and openings from that is often ciliated and covered with gland the excretory and reproductive systems just cells, enabling them to creep across the behind the head and nerve cords, twisting the substrate. The smallest gastropods use cilia to digestive tract into a U shape. The adaptive propel themselves over a mucous trail, while significance of torsion includes protection, larger gastropods employ waves of muscular clean water intake, and enhanced sensory contraction to move across surfaces. In some perception. gastropods, the foot is modified for specific purposes: it is used for clinging in species such as abalones and limpets, and for swimming in sea butterflies and sea hares. SHELL COILING: The earliest gastropods had shells that were FEEDING AND DIGESTION coiled in one plane. However, modern snails Gastropods feed by scraping algae or other have secondarily returned to this shell form. In small, attached organisms from their modern snails, their shells are asymmetrically substrate using their radula. Other gastropods coiled. This asymmetrical pattern leaves less are herbivores, feeding on large plants, room on one side of the visceral mass for scavengers, parasites, or predators. The certain organs, indicating that organs which anterior portion of the digestive tract is are now single were probably paired modified into an extensible proboscis that ancestrally. contains the radula. Additionally, the digestive tract of most molluscs is ciliated. Food is trapped in mucous strings and incorporated into a mucoid mass called the protostyle, which extends to the stomach and is rotated by cilia. OTHER MAINTENANCE FUNCTIONS In gastropods, gas exchange involves the mantle cavity, with a rolled extension of the 3|Pa ge mantle called the siphon serving as an Gastropod Diversity inhalant tube. They have an open circulatory Gastropod diversity is organized into several system, where blood bathes cells directly in subclasses. tissue spaces called sinuses, and this blood Subclass Prosobranchia contains around also acts as a hydraulic skeleton, supporting 20,000 species, mostly marine, but some body structures. The nervous system is are freshwater or terrestrial. The majority of primitive, with six ganglia located in the head- these species are herbivores or deposit foot and visceral mass. Their sensory feeders, though some are carnivorous, structures include well-developed eyes, including heteropods. statocysts for maintaining balance, and Subclass Opisthobranchia includes fewer osphradia, which function as chemoreceptors. than 2,000 marine species, such as sea The excretory system in primitive gastropods hares and sea slugs. In this subclass, there had two nephridia, but modern species often is often a reduction or loss of the shell, possess only one. mantle cavity, and gills, with pteropods being an example. REPRODUCTION AND DEVELOPMENT Subclass Pulmonata contains about 17,000 Gastropods exhibit diverse reproductive and species that are predominantly freshwater developmental strategies. Some species are or terrestrial. Most species in this subclass dioecious, such as many marine snails, with are herbivores with a long radula for gonads located in the spiral of the visceral scraping plant material. In Pulmonata, the mass. These species discharge gametes into mantle cavity acts as a lung, and air or the sea for external fertilization. Other water enters through the pneumostome. gastropods are monoecious, including many hermaphroditic snails, and internal cross- CLASS BIVALVIA fertilization is common among them. In these Class Bivalvia (bi’val”ve-ah), derived from the species, the evolution of a penis from the body Latin words "bis" meaning twice and "valva" wall and glandular portions of the female meaning leaf, includes clams, oysters, reproductive tract aids in secreting mucus, mussels, and scallops. These mollusks have a protective jelly, or capsules around fertilized sheet-like mantle and a shell consisting of two eggs. Some monoecious snails are protandric, valves. Many bivalves are edible, and some meaning the testes develop first and later even form pearls. They are primarily filter transform into ovaries. Gastropod feeders, using their gills to strain food particles development often begins with a trochophore from the water. larva, which undergoes spiral cleavage and develops into a free-swimming veliger larva equipped with a foot, eyes, tentacles, and shell. 4|Pa ge Shell and associated structures The shell of bivalves is composed of two convex halves, known as valves. These valves are connected by a proteinaceous hinge and feature a series of tongue-and-groove modifications known as teeth. The umbo, the oldest part of the shell, is a swollen area near the shell’s anterior margin. Bivalves possess adductor muscles that allow them to close their shells effectively, aiding in defense against predators like sea stars. The mantle of Gas Exchange, Filter Feeding, and Digestion a bivalve secretes nacre, a combination of Bivalves exhibit several adaptations for gas aragonite and conchiolin, around irritants exchange, filter feeding, and digestion. such as sand grains or parasites to form Notably, they have lost their head and radula. pearls. High-quality pearls are produced by Their gills are composed of folded sheets Pacific oysters, specifically Pinctada known as lamellae, and cilia on the gills move margaritifera and Pinctada mertensi. water into the mantle cavity through an incurrent opening. As filter feeders, bivalves filter small particles of food from the water. These trapped particles are then moved by cilia to the food grooves, with the suprabranchial chamber ensuring efficient transport and processing of food. Labial palps sort the filtered food, while cilia carry small particles into the mouth and reject larger particles as pseudofeces. In the digestive tract, food is entangled in mucoid strings and moved to the stomach. The crystalline style rotates in the stomach, releasing enzymes for carbohydrate and fat digestion. 5|Pa ge  Other Maintenance Functions Bivalves possess an open circulatory system where blood flows from the heart to tissue sinuses, nephridia, gills, and back to the heart via a separate aorta. The nephridia and excretion system includes two nephridia located below the pericardial cavity. For locomotion and burrowing, blood is pumped into the foot, extending it into the substrate. Foot muscles then swell the foot into an anchor, and retractor muscles pull the body and shell into the substrate. The nervous system is composed of three pairs of interconnected ganglia associated with the esophagus, foot, and posterior adductor muscle. Reproduction and Development Most bivalves are dioecious, meaning they have separate sexes, while some are monoecious (hermaphroditic) and may be protandric, changing sex during their life. The gonads are located in the visceral mass, and they exhibit external fertilization. Fertilization occurs in the mantle cavity by sperm brought in with inhalant water. Development proceeds 6|Pa ge through trochophore and veliger larval stages. through cementation. Boring bivalves settle Freshwater bivalves in the family Unionidae onto the substrate as larvae, and the anterior brood their young to a modified veliger stage margin of their valves mechanically abrades called a glochidium, which is parasitic on the substrate. fishes. CLASS CEPHALOPODA Class Cephalopoda (sef’ah-lop”o-dah), derived from the Greek words "kephale" meaning head and "podos" meaning foot, includes octopuses, squid, cuttlefish, and nautiluses. Cephalopods are the most complex mollusks and, in many ways, the most complex invertebrates. The foot of cephalopods has evolved into a circle of tentacles or arms. These fascinating creatures Bivalve Diversity use a unique method of locomotion called jet Bivalves are found in nearly all aquatic propulsion, and their bodies are enclosed by a habitats, either completely or partially buried highly muscular mantle. in sand or attached to solid substrates. Their mantle margins are often fused to form siphons. Some surface-dwelling bivalves attach to substrates using byssal threads or 7|Pa ge Shell Ancestral cephalopods likely had conical shells shaped like a cone. Today, the nautilus is the only living cephalopod with an external shell, which is divided by septa into chambers. The siphuncle, a tissue cord, perforates the septa, absorbing fluids by osmosis and replacing them with metabolic gases. Locomotion Cuttlefish possess a unique internal shell Cephalopods depend on their ability to move called a cuttlebone, which is composed of thin quickly using a jet-propulsion system. This layers, creating small gas-filled spaces for system involves expelling water forcefully buoyancy. In squid, the shell is reduced to an through a funnel-like structure called the internal, chitinous structure called the pen. siphon, propelling them swiftly through the Additionally, squid have cartilaginous plates in water. The mantle of cephalopods contains the mantle wall, neck, and head that support radial and circular muscles that work together the mantle and protect the brain. to create the jet propulsion. Radial muscles contract to expand the mantle cavity, allowing 8|Pa ge water to fill the cavity, and circular muscles aiding in balance and orientation. Olfaction, contract to expel the water forcefully. This mediated by osphradia, allows cephalopods mechanism enables cephalopods to execute to detect chemical signals in their rapid and agile movements, essential for environment. Additionally, cephalopods have escaping predators and capturing prey. specialized cells called chromatophores that Additionally, many cephalopods can control enable them to change color and patterns. the direction of their movement by adjusting the angle of the siphon, allowing them to navigate with precision in their aquatic environments. FEEDING AND DIGESTION Cephalopods locate their prey using their excellent vision and tentacles equipped with adhesive cups. In squid, these cups possess small hooks for better grip. They have powerful, beak-like jaws for tearing food, and a radula, a tongue-like organ with tiny teeth that rasps food, forcing it into the mouth cavity. Cuttlefish and nautiluses feed on small invertebrates on the ocean floor, while octopuses are nocturnal hunters that feed on snails, fishes, and crustaceans. Squid primarily feed on fishes and shrimps. Cephalopods use peristalsis, coordinated muscular waves, to move food through their digestive system. OTHER MAINTENANCE FUNCTIONS Cephalopods have a closed circulatory system, where blood is confined to vessels throughout its circuit around the body. They Cephalopods possess an ink gland that possess nephridia, excretory organs closely releases a brown or black fluid containing associated with blood vessels, and have large melanin and other chemicals when brains that support their predatory habits and threatened. This ink cloud can obscure the dexterity. Octopuses, cuttlefish, and squid predator's view, providing the cephalopod have complex eyes structurally similar to with a crucial opportunity to escape. vertebrate eyes. Statocysts, located near the brain, respond to gravity and acceleration, 9|Pa ge REPRODUCTION AND DEVELOPMENT Cephalopods exhibit dioecious reproduction, with gonads located in the dorsal portion of the visceral mass. The male reproductive tract includes testes and structures for packaging sperm into specialized packets called spermatophores. The tentacle of male cephalopods, known as the hectocotylus, is specially modified for the transfer of spermatophores. In some species, such as Loligo and Sepia, the hectocotylus has rows of smaller suckers capable of picking up Cephalopods take care of their developing spermatophores. The female reproductive eggs; for example, octopuses use their arms to tract produces large, yolky eggs and is clean the eggs of debris and squirt water to equipped with glands that secrete gel-like maintain a suitable environment for cases around the eggs, which often harden embryonic development. Cephalopods when exposed to seawater. develop within the confines of the egg membranes, and when they hatch, they are miniature adults. Unlike some other animals, cephalopod young are not cared for after hatching and must fend for themselves. During copulation, male and female cephalopods intertwine their tentacles. The male removes spermatophores from his mantle cavity and inserts his hectocotylus into the female's mantle cavity, depositing a spermatophore near the opening to the oviduct. Spermatophores have an ejaculatory mechanism to release sperm for fertilization. Eggs are fertilized as they exit the oviduct and are deposited either individually or in string- like masses, often attaching to a substrate, CLASS POLYPLACOPHORA such as the ceiling of an octopus's den. Class Polyplacophora (pol”e-pla-kof’o-rah), derived from the Greek words "polys" meaning 10 | P a g e many, "plak" meaning plate, and "phoros" gills in the mantle cavity on each side of the meaning to bear, includes chitons. These foot. Cilia on the gills create water currents mollusks are commonly found in shallow that carry water through the mantle cavity. marine waters on hard substrates. While The digestive, excretory, and reproductive chitons have a fishy flavor, they are tough to tracts open near the exhalant area of the chew and challenging to collect. They possess mantle cavity. a reduced head, a flattened foot, and a shell divided into eight articulating dorsal valves. Chitons feed on attached algae, using a Additionally, chitons have a muscular mantle chemoreceptor called the subradular organ, that extends beyond the shell and foot, and located near the mouth, to detect food. The their mantle cavity is restricted to the space radula rasps food from the substrate, and between the margin of the mantle and the digestion and absorption occur in the foot. stomach, with waste products moving into the intestine. The nervous system of chitons is ladder-like, consisting of four anteroposterior nerve cords and numerous transverse nerves. A nerve ring encircles the esophagus. Sensory structures include osphradia, tactile receptors on the mantle margin, chemoreceptors near the mouth, and statocysts in the foot. Additionally, some chitons may have photoreceptors on the surface of their shells. Chitons crawl over substrates in a manner Chitons are dioecious, meaning they have similar to gastropods, attaching themselves distinct male and female individuals. to the substrate. They have a linear series of Reproduction involves external fertilization, 11 | P a g e resulting in a swimming trochophore larva. The trochophore eventually settles and metamorphoses into an adult without passing through a veliger stage. CLASS MONOPLACOPHORA Class Monoplacophora (mon”o-pla-kof’o- CLASS SCAPHOPODA rah), derived from the Greek words "monos" Class Scaphopoda, commonly known as tooth meaning one, "plak" meaning plate, and shells or tusk shells, includes marine burrowing "phoros" meaning to bear, includes mollusks animals. Their most distinctive characteristic with an undivided, arched shell, a broad and is a conical shell that is open at both ends. The flat foot, a radula for feeding, and serially head and foot extend from the wider end of repeated pairs of gills and foot-retractor the shell, while the rest of the body, including muscles. These mollusks are dioecious. This the mantle, is greatly elongated and extends group was known only from fossils until 1952, the length of the shell. Scaphopods use their when a limpet-like monoplacophoran named radula and tentacles for feeding, primarily Neopilina was dredged up from a depth of preying on foraminiferans. Functional gills are 3,520 meters off the Pacific coast of Costa absent in scaphopods; instead, gas exchange Rica. Since then, approximately 25 species occurs across mantle folds, allowing them to have been described. obtain oxygen and expel carbon dioxide. These animals are dioecious and reproduce by producing trochophore and veliger larvae. 12 | P a g e  CLASS SOLENOGASTRES CLASS CAUDOFOVEATA Class Solenogastres (Gk., solen = channel + Class Caudofoveata (kaw”do-fo’ve-a”ta), gaster = gut) is characterized by the absence derived from the Latin words "cauda" meaning of a shell. These mollusks have cylindrical tail and "fovea" meaning depression, includes bodies and crawl on their ventral foot, which is wormlike mollusks that live in vertical burrows modified into a pedal groove for locomotion. on the deep-sea floor. These mollusks range in They possess calcareous spicules and lack size from 2 mm to 14 cm and are dioecious. true gills, but have gill-like structures that They feed on foraminiferans using a radula serve a similar function for respiration. and lack a shell, but have scale-like spicules Solenogastres are carnivorous and often feed on their body wall and possess a muscular on cnidarian polyps, dwelling on the surface of foot. The use of nephridia for excretion corals and other marine substrates. They are suggests that this group may resemble the monoecious, meaning each individual ancestral mollusk. possesses both male and female reproductive organs. Reference: https://bio.libretexts.org/Courses/Lumen_Lear ning/Fundamentals_of_Biology_I_%28Lumen %29/14%3A_Module_11- _Invertebrates/14.04%3A_Phylum_Mollusca?fo rm=MG0AV3 Ponder, W. F., & Lindberg, D. R. (2008). Mollusca: Physiology, Evolution, and Systematics. University of California Press. 13 | P a g e  SELF-ASSESSMENT QUESTION: SAQ 10: Compare the development of bivalve SAQ 1: Describe the general body structure of and cephalopod larvae. What are the key mollusks. How is the body divided, and what are differences in their life cycles? the functions of the mantle and mantle cavity? SAQ 11: Describe the key features of Class SAQ 2: What are the key differences between Polyplacophora. How does the shell structure of torsion and shell coiling in gastropods, and chitons differ from that of other mollusks, and what are the adaptive significance of these what is the function of their eight dorsal valves? features? SAQ 12: Explain the feeding mechanism in SAQ 3: Explain the function of the radula in chitons. What role does the radula and the gastropods. How does it contribute to their subradular organ play in their feeding feeding behavior? behavior? SAQ 13: How does the reproductive system of SAQ 4: Discuss the differences in reproduction Class Scaphopoda differ from that of Class and development among gastropods. How do Polyplacophora? What type of larvae do both dioecious and monoecious species differ in groups produce? their reproductive strategies? SAQ 14: Discuss the unique characteristics of SAQ 5: What are the main subclasses within Class Solenogastres. How does their lack of a Gastropoda, and how are they distinguished by shell impact their anatomy and behavior? habitat and morphology? SAQ 15: Describe the habitat and feeding SAQ 6: Describe the shell structure and strategy of Class Caudofoveata. What is the associated features of bivalves. What are the significance of their scale-like spicules and lack functions of the adductor muscles and the of a shell? umbo? SAQ 7: Explain the process of filter feeding in PROVIDE YOUR REFERENCES: bivalves. How do the gills and labial palps contribute to this feeding strategy? SAQ 8: Describe the method of locomotion in cephalopods. How does jet propulsion work, and why is it important for cephalopod survival? SAQ 9: What is the function of the hectocotylus in cephalopods, and how does it facilitate reproduction? 14 | P a g e