Molluscs Bonneville High School Vertebrate Zoology CHAPTER 16

Unit Objectives Objective 1: Describe the form and function of the Molluscs Objective 2: Differentiate between the classes of Molluscs (Caudofoveata, Solenogastres, Monplacophora, Polyplacophora) Objective 3: Differentiate between the classes of Molluscs: (Scaphopoda, Gastropoda, Bivalvia, Cephalopoda) Objective 4: Describe the Internal form and function of gastropods, bivalves, and cephalopods. Objective 5: Describe the form and function of the Annelida Objective 6: Differentiate between the classes of Annelida (Polychaeta, Oligochaeta, Hirudinea, ) Objective 7: Describe the Internal form and function of the following classes: Polychaeta, Oligochaeta, Hirudinea. Objective 8: Describe the life cycle and reproductive strategies for Molluscs and Annelids

Fluted Giant Clam

Characteristics Phylum Mollusca Over 90,000 living species and 70,000 fossil species Soft body and protostomes Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses

Marine Snail Nudibranch Chiton Pacific Giant Clam Octopus

Characteristics Characteristics: Herbivorous grazers, predaceous carnivores, filter feeders, and parasites Most are marine, but some are terrestrial or freshwater aquatic

Characteristics Evolution Fossil evidence Some bivalves and gastropods Indicates molluscs evolved in the sea Most have remained marine Some bivalves and gastropods Moved to brackish and freshwater Snails (gastropods) successfully invaded land Limited to moist, sheltered habitats with calcium in the soil Cephalopods Evolved to become relatively intelligent

Economics Characteristics Many are used as food Culturing of pearls is an important industry Snails and slugs are garden pests

Mollusc Body Plan: Head-Foot and Visceral Mass Portions Form and Function Mollusc Body Plan: Head-Foot and Visceral Mass Portions Head-foot region contains feeding, sensory, and locomotor organs (foot) Visceral mass contains digestive, circulatory, and reproductive organs Mantle Cavity Space between mantle (sheath of skin) and body wall is the mantle cavity Mantle cavity houses the gills or a lung In most molluscs Mantle secretes a shell over the visceral mass

Generalized Mollusk Anatomy

Form and Function Radula Unique to molluscs Found in all except bivalves Protruding, rasping, tongue-like organ Ribbon-like membrane has rows of tiny teeth (up to 250,000) pointed backward Radula rasps off particles of food from surfaces Serves as a conveyor belt to move particles to digestive tract New rows of teeth replace those that wear away Pattern and number of teeth are used in classification of molluscs Some specialized to bore through hard material or harpoon prey

Radula

Form and Function Foot Functions in attachment or locomotion Modifications include Hatchet foot of clams Siphon jet of squids Secreted mucus aids in adhesion or helps molluscs glide Snails and bivalves extend the foot by engorgement with blood

Form and Function Shell If present, secreted by the mantle Periostracum Outer layer - wears away Composed of hardened protein Middle prismatic layer Closely packed prisms of calcium carbonate Increases with animal growth Inner nacreous layer Next to the mantle; the nacre is laid down in thin layers Aids in Pearl formation Shiny layer in abalone, nautilus, and bivalve

becomes covered with nacre Bivalve Shell B. Pearl Formation from a parasite or sand that enters shell into mantle, becomes covered with nacre

Reproduction and Life History Form and Function Reproduction and Life History Most dioecious, some hermaphroditic (monoecious) Egg hatches and produces a free-swimming trochophore larva In many gastropods and bivalves Trochophore is followed by intermediate larval stage, the veliger.

Internal Form and Function (Obj 4) -Torsion: A twisting process that makes the Mollusc visceral mass asymmetrical. -Coiling: The spiral winding of the shell and visceral mass. It is NOT the same as torsion. -Ctenidium: Located in the mantle cavity, this acts as gills for some Molluscs -Lung: A highly vascular area in some Molluscs which aids in respiration it has an opening called a pneumostome. -Dioecious: Having male and female gonads in separate individuals. -Monoecious: Having both male and female gonads in the same organism: Hermaphrodite -Pulmonates: Molluscs which show detorsion and include land and most fresheater snails and slugs. They lost their ctenidia and their vascularized mantle wall has become a lung. They are monoecious. -Chromatophores: Cells in the skin that contain pigment granules. This can change the color of the skin of some mollusks. They are used for camouflage and communication. -Ink gland: A gland which most cephalopods and nautiloids have that secretes sepia, a dark fluid containing the pigment melanin. The pigment is deposited into an ink sac that empties into the rectum. The Mollusc can then release the dark cloud to confuse or distract a predator. Open circulatory system: a pumping heart, blood vessels and blood sinuses. Blood is pumped from the heart into open spaces. Closed circulatory system: Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. 

Trochophore larva

Classes of Molluscs Class Monoplacophora-Neopilina Class Polyplacophora – Chitons Class Caudofoveata-Chaetodermomorpha Class Solenogastres-Solenogastres Class Scaphopoda-Tusk or tooth shells Class Gastropoda - Slugs, Snails, Nudibranch Class Bivalvia - Clams, Mussels Class Cephalopoda - Squid, Octopus, Nautilus

Class Monoplacophora Body bilaterally symmetrical with broad, flat foot; A single dome-shaped shell Five or six pairs of gills in shallow mantle cavity Radula present Separate sexes. Example: Neopilina

Class Polyplacophora: Chitons Classes of Molluscs Class Polyplacophora: Chitons Chitons are somewhat flattened with 7or 8 dorsal plates Most prefer rocky intertidal surfaces Chiton radula is reinforced with iron mineral Scrapes algae from the rocks

Mossy Chiton - hairs and bristles aid in defense

Class Caudofoveata Wormlike Shell, head and excretory organs absent Radula usually present Mantle with chitinous cuticle and calcareous scales Sexes separate Was united with solenogasters but now classified separately Examples Chaetoderma, Limifossor

Class Solenogastres Solenogasters Wormlike Shell, head, and excretory organs absent; radula usually absent Mantle covered with scales or spicules; Hermaphrodite Example: Neomenia

Class Scaphopoda Tusk Shells Body enclosed in a one piece, tubular shell open at both ends. Conical foot; Mouth with radula and tentacles, head absent Sexes are separate Example: Dentalium

Class Gastropoda Class Gastropoda Most diverse class Over 70,000 living Forms range from marine forms to air-breathing terrestrial snails and slugs Shells, if present, are chief defense Some produce distasteful or toxic secretions Use process of Torsion

Classes of Molluscs Gastropod Shells One-piece (univalve) Apex is smallest and oldest whorl Whorls become larger and spiral around central axis Giant marine gastropods have shell up to 60 cm long Some fossil forms are 2 meters long Terrestrial gastropods shells are restricted by soil mineral content, temperature, dryness, and acidity

Class Gastropoda Cont. Form and Function Torsion Developmental process that changes the relative position of the shell, digestive tract and anus Digestive tract moves so that anus lies above head

Class Gastropoda Cont. Coiling Coiling or spiral winding of the shell and visceral mass not the same as torsion

Abalone - Moon Snail - Feed on kelp -herbivore feeds on clams and mussels Radula releases chemicals to soften shell, so they can get to their prey

Conus - Extends proboscis to capture prey. Then releases Conotoxins to paralyze (lethal to Humans) Hours later regurgitate scales and bones

Internal Form and Function Class Gastropoda Cont Internal Form and Function Respiration performed by vascular area in mantle cavity that serves as lung Most have a single nephridium (kidney) and well-developed open-circulatory and nervous systems Sense organs include eyes, statocysts, tactile organs, and chemoreceptors Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea. (On tentacle of some)

Class Gastropoda Cont Reproduction: Eggs emitted singly or in clusters, and may be transparent or in tough egg capsules Young may emerge as veliger larvae or pass this stage inside the egg Some species, including most freshwater snails, are ovoviviparous

Eggs of Mollusks Egg ribbon of Nudibranch Eggs - resemble grains of wheat

Major Groups of Gastropods Classes of Molluscs Major Groups of Gastropods Traditional classification has recognized three subclasses of Gastropoda Prosobranchia, Opisthobranchia Pulmonata

Includes most marine snails Have one pair of tentacles, separate sexes Gastropods - Prosobranchia Includes most marine snails Have one pair of tentacles, separate sexes Diodora aspera Hole in Apex for water to leave Flamingo Tongue Snails

Marine, Shell is reduced or absent 2 tentacles, Opisthobranches: sea slugs, sea hares, sea butterflies, and nudibranch Marine, Shell is reduced or absent 2 tentacles, Sea Hare Sea Hare’s defense mechanism- a secretion from its purple gland

Nudibranch - calcareous spicules for protection

Pulmonates - Snail and Banana slug 2 tentacles: posterior have eyes Opening to Mantle Cavity

Classes of Molluscs Class Bivalvia Mussels, clams, scallops, oysters Range in size from 1–2 mm in length to the giant South Pacific clams (1m) Most are sedentary filter feeders Bivalves lack a head, radula, or other aspects of cephalization Contain Siphons

Mussels Scallops Escaping a Sea Star

Northwest Ugly Clams Siphons Incurrent brings in Food and Oxygen

Class Bivalvia Form and Function 2 shells or valves are held together by a hinge ligament Valves are drawn together by strong adductor muscles Umbo is the oldest part of the shell with growth occurring outward in rings Posterior edges of the mantle folds form excurrent and incurrent openings In burrowing clams, mantle forms long siphons to reach the water above

Class Bivalvia Locomotion Foot is extended out from between the valves Blood is pumped into the foot Foot swells and anchors the bivalve in the mud Shortening of the foot pulls the clam forward Scallops clap valves to create a jet propulsion

Scallop - developed sensory organs along mantle edges (tentacles and blue eyes)

Class Bivalvia Gills Circulatory - Open circulatory system Both mantle and gills perform gaseous exchange Siphon used in respiratory Water enters incurrent siphon Gas diffused out Exits through the excurrent siphon Circulatory - Open circulatory system 3 chambered heart has two atria and one ventricle Blood vessels line gills to receive oxygen

Shipworm Bivalves- Burrows into wood on docks and piers

Symbiotic relationship with Algae to gain most nutrients Clam - Symbiotic relationship with Algae to gain most nutrients Siphonal Area

Class Bivalvia Reproduction and Development Sexes usually separate Gametes discharged in excurrent flow Fertilization usually external Embryos develop as trochophore, and veliger larval stages Freshwater clams have internal fertilization Sperm enter the incurrent siphon to fertilize eggs in water tubes of the gills Larvae develop into a bivalved glochidia stage Attaches to gills of passing fish where they live briefly as parasites “Hitchhiking” having helped distribute the species

Life Cycle of an Oyster

Glochidium - freshwater clam larva Attach to fish’s gills by clamping their valve closed. Stay for several weeks. Pocketbook Mussel - mimics a small minnow, when a Smallmouth Bass comes to dine, it releases its glochidia

Class Cephalopoda Class Cephalopoda Squids, octopuses, nautiluses, and cuttlefish All marine predators Foot is in the head region Modified for expelling water from mantle cavity Range from 2 cm to the giant squid (60 ft) Largest invertebrate Nautilus - only one with external shell Series of gas chambers in shell helps maintain neutral buoyancy

A. Feed on a Fish B. Showing Gas filled chambers Nautilus A. Feed on a Fish B. Showing Gas filled chambers

Cuttlefish

Class Cephalopoda Locomotion Cephalopods swim by forcefully expelling water through a ventral funnel or siphon Control direction and force of the water, thus determining its speed Lateral fins of squids and cuttlefishes are stabilizers Nautilus swims mainly at night Octopuses mainly crawl on the bottom but can swim Some with webbing between their arms swim with a medusa-like action

Squid - Pen is only remains of shell

Respiration and Circulation Class Cephalopoda Respiration and Circulation With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity Circulatory system has a network of vessels conducting blood through gill filaments (Closed Circulatory System!!)

Class Cephalopoda Nervous and Sensory Systems Cephalopod brain is the largest of any invertebrate Squids have giant nerve fibers Sense organs are well-developed Eyes are complex, complete with cornea, lens, and retina Can learn by reward and punishment, and by observation of others Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms

Cuttlefish Eye

Class Cephalopoda Communication Use chemical and visual signals to communicate Chromatophores are cells in the skin that contain pigment granules Contractions of the muscle fibers attached to the cell causes the cell to expand and change the color pattern Color patterns can be changed rapidly Deep-water cephalopods have elaborate luminescent organs Ink sac empties into rectum; (Not in Nautiloids) Contains ink gland that secretes sepia (dark fluid) when animal is alarmed

Class Cephalopoda Reproduction Sexes are separate In male seminal vesicle, spermatozoa are packaged in spermatophores and stored One arm of male is modified as an intromittent organ, the hectocotylus Removes a spermatophore from mantle cavity and inserts it into female Fertilized eggs leave oviduct and are attached to stones, etc. Large, yolky eggs undergo meroblastic cleavage (not full cleavage) Hatch into juveniles with no free-swimming larval stage

Copulation in Cephalopods - Male Octopus uses modified arm