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CHAPTER 16 Molluscs 16-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Phylum Mollusca 16-3 Over 90,000 living species and 70,000 fossil species Soft body and belong to the lophotrochozoa protostomes Include chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics 16-5 May weigh up to 900 kg and grow to nearly 20 m long, but 80% are under 10 cm in size Herbivorous grazers, predaceous carnivores, filter feeders, and parasites Most are marine, but some are terrestrial or freshwater aquatic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Evolution Fossil evidence Some bivalves and gastropods Evolved to become relatively intelligent Coelom limited to a chamber around the heart 16-7 Limited to moist, sheltered habitats with calcium in the soil Cephalopods Moved to brackish and freshwater Snails (gastropods) successfully invaded land Indicates molluscs evolved in the sea Most have remained marine Some believe molluscs arose separately from annelids and coeloms are not homologous Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Characteristics Economics 16-8 Many are used as food Culturing of pearls and pearl buttons is an important industry Burrowing shipworms destroy wooden ships and wharfs Snails and slugs are garden pests Some snails are intermediate hosts for parasites Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Mollusc Body Plan: Head-Foot and Visceral Mass Portions Head-foot region contains feeding, cephalic sensory, and locomotor organs Visceral mass contains digestive, circulatory, respiratory, and reproductive organs Mantle Cavity 16-9 Two folds of skin form protective mantle or pallium Space between mantle and body wall is the mantle cavity Mantle cavity houses the gills (ctenidia) or a lung In most molluscs Mantle secretes a shell over the visceral mass Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Head-Foot Most have well-developed head bearing mouth and some sensory organs Photosensory receptors range from simple to complex eyes Tentacles may be present Posterior to mouth is the chief locomotor organ, the foot 16-11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Radula 16-12 Unique to molluscs Found in all except bivalves and some solenogasters 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Foot Usually ventral Functions in attachment to substratum or for locomotion Modifications include 16-14 Attachment disc of limpets Hatchet foot of clams Siphon jet of squids Secreted mucus aids in adhesion or helps molluscs glide on cilia Snails and bivalves extend the foot hydraulically by engorgement with blood Burrowers extend the foot into mud or sand, enlarge the tip as an anchor, and draw forward Free-swimming forms have modified the foot into wing or fin-like swimming agents Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Mantle and Mantle Cavity 16-15 Mantle is a sheath tissue on each side of the body Secretes the shell when present Mantle cavity Houses the gills or lungs that develop from the mantle Exposed surface of the mantle also functions in gaseous exchange In aquatic molluscs Continuous flow of water brings in oxygen and food, and flushes out wastes Products of digestive, excretory and reproductive systems empty into the mantle cavity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function 16-16 Cephalopods Use head and mantle cavity to create jet propulsion Mollusc gill has leaf-like filaments Cilia propel water across the surface Countercurrent blood movement in gill absorbs oxygen efficiently In most molluscs, two ctenidia on opposite sides Form an incurrent and an excurrent chamber Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Shell If present, secreted by the mantle and lined by it Periostracum Middle prismatic layer 16-18 Closely packed prisms of calcium carbonate Inner nacreous layer Outer horny layer Composed of conchiolin, a tanned protein Next to the mantle; the nacre is laid down in thin layers Thick periostracum of freshwater molluscs protects against acid from leaf decay in streams Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Internal Structure and Function Open circulatory system 16-20 Pumping heart, blood vessels, and blood sinuses Most cephalopods have a closed system with a heart, vessels, and capillaries Most molluscs have a pair of kidneys or metanephridia Kidney ducts also discharge sperm and eggs Nervous system Pairs of ganglia but generally simpler than in annelids In air-breathing snails, nervous system produces growth hormones Sense organs vary and may be highly specialized Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Form and Function Reproduction and Life History 16-21 Most dioecious, some hermaphroditic Egg hatches and produces a free-swimming trochophore larva Larva undergoes direct metamorphosis into a small juvenile in chitons In many gastropods and bivalves Intermediate larval stage, the veliger, is a derived state Trochophore larvae are considered by some to unite molluscs with annelids, marine turbellarians, nemertines, phoronids, etc. in a taxon called Trochozoa within superphylum photrochozoa Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Caudofoveata Wormlike, marine organisms ranging from two to 140 mm in length Most burrow Terminal mantle cavity and gills are near entrance Feed primarily on microorganisms and detritus Have no shell but body is covered with calcareous scales Radula present but may be reduced Fewer than 120 species 16-24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Solenogasters Formerly grouped with caudofoveates in Aplacophora Resemble caudofoveates but have no radula or gills Foot has a midventral, narrow furrow called the pedal groove Do not burrow but are bottom dwellers and feed on cnidarians Approximately 250 species Class is sometimes called Neomeniomorpha 16-25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Monoplacophora Previously considered extinct Living specimen was discovered in 1952 About 25 extant species now known Small molluscs with a rounded shell, resemble limpets Some organs are repeated: 3–6 pairs of gills, two pairs of auricles, 3–7 pairs of metanephridia, one or two pairs of gonads and 10 pairs of pedal nerves Radula characteristic of many other molluscs 16-26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Polyplacophora: Chitons Chitons are somewhat flattened with 7or 8 dorsal plates About 1000 currently described species Head and cephalic organs are reduced Photosensitive structures (esthetes) similar to eyes pierce the plates Most prefer rocky intertidal surfaces Chiton radula is reinforced with iron mineral Scrapes algae from the rocks Mantle extends around margin 16-28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gills suspended from roof of mantle cavity and grooves Pair of osphradia serves as sense organ Sample water in mantle groove near anus Blood pumped by a three-chambered heart Form a closed chamber Water flows from anterior to posterior Travels through aorta and sinuses to gills Pair of metanephridia Carries wastes from pericardial cavity to exterior Sexes are separate Trochophore larvae metamorphose into juveniles without a veliger stage 16-31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Scaphopoda Tusk or tooth shells Live on the ocean bottom from subtidal zone to 6000 m depth About 900 living species Slender body covered with a mantle Tubular shell is open at both ends Unique body plan 16-32 Mantle is wrapped around the viscera and fused to form a tube Foot protrudes from larger end to burrow into mud Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Foot and ciliary action moves water through mantle cavity Gills are absent and gaseous exchange occurs via the mantle Detritus and protozoa are caught on cilia on foot or mucus-covered knobs of tentacles Radula carries food to a crushing gizzard Head or captacula lacks eyes, tentacles or osphradia 16-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Gastropoda Most diverse class Over 70,000 living and more than 15,000 fossil species Snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares, and sea butterflies Forms range from marine forms to airbreathing terrestrial snails and slugs Typically sluggish, sedentary animals Shells are chief defense 16-35 Some produce distasteful or toxic secretions Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gastropod Shells One-piece univalve, coiled or uncoiled Apex is smallest and oldest whorl Whorls become larger and spiral around central axis or columella Many snails have an operculum covering shell aperture Giant marine gastropods have shell up to 60 cm long 16-36 Some fossil forms are 2 meters long Terrestrial gastropods are restricted by soil mineral content, temperature, dryness, and acidity Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-38 Snails serve as intermediate hosts to many parasites and are often harmed by larval stages 3 gastropod subclasses Prosobranchia Opisthobranchia Pulmonata Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function 16-39 Torsion Developmental process that changes the relative position of the shell, digestive tract and anus, nerves that lie on both sides of the digestive tract, and the mantle cavity containing the gills Contraction of a foot retractor muscle pulls shell and viscera 90 counterclockwise Moves anus to the right side of the body Recent studies have shown that shell movement is independent of visceral movement Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-40 1st movements of shell rotate it between 90 and 180 degrees into a permanent position Mantle cavity develops on the right side of the body near the anus, but is initially separate from it Anus and mantle cavity usually move further to the right and the mantle cavity is remodeled to encompass the anus Digestive tract moves both laterally and dorsally so that anus lies above head within mantle cavity After torsion, anus and mantle cavity open above mouth and head Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-42 Certain viscera on the left are now on the right side and vice versa Nerve cords form a figure eight Varying degrees of detorsion in opistobranchs and pulmonates have been observed This arrangement resulting from torsion creates fouling Wastes being washed back over the gills Developmental sequence is called ontogenetic torsion Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Coiling 16-43 Coiling or spiral winding of the shell and visceral mass not the same as torsion Occurs at same larval stage as torsion but had a separate, earlier evolutionary origin All living gastropods descended from coiled, torted ancestors Planospiral shell has all whorls in a single plane Primitive state Conispiral shape provides more compactness Each whorl is to the side of the previous one Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-45 Shifting the shell upward and back helped balance uneven weight distribution Gill, auricle and kidney of right side are lost in most species Loss of the right gill provides one solution to the problem of fouling Wastes expel to the right Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Feeding Habits 16-46 Adaptation of the radula provides much variation in gastropod feeding habits Many are herbivorous and graze, browse or feed on plankton Some scavenge decaying flesh Others carnivores that tear prey using radula Oyster borer alternates rasping with chemical softening of the shell to bore a hole Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Species of Conus deliver a lethal sting to secure prey 16-49 Venom is a conotoxin Specific for the neuroreceptors of its preferred prey Some collect debris as a mucus ball to ingest it Sea butterflies secrete a mucus net Digestion usually extracellular in lumen of stomach Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Internal Form and Function 16-50 Respiration in many performed by ctenidia in mantle cavity Derived prosobranchs lost one gill and half of remaining gill Resulting attachment to wall of mantle cavity provided respiratory efficiency Pulmonates lack gills Have a highly vascular area in mantle that serves as lung Lung opens to outside by small opening, the pneumostome Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-52 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-53 Most have a single nephridium and welldeveloped 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. Sensory osphradium at base of the incurrent siphon may be chemosensory or mechanoreceptive Monoecious and dioecious species Copulation in monoecious species may involve exchange of spermatozoa or spermatophores Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-54 Many terrestrial species inject a dart to heighten arousal before copulation Primitive gastropods discharge ova and sperm into water and fertilization is external 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Major Groups of Gastropods Traditional classification has recognized three subclasses of Gastropoda Prosobranchia, Opisthobranchia, and Pulmonata Recent evidence suggests the Prosobranchia is paraphyletic Opisthobranchia may or may not be paraphyletic Opisthobranchia and Pulmonata together form a monophyletic grouping 16-56 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Prosobranchs Includes most marine snails and some freshwater and terrestrial gastropods Mantle cavity is anterior due to torsion Gills are in front of heart Water enters the left side and exits from the right side Long siphons may separate incurrent and excurrent flow Have one pair of tentacles, separate sexes, and usually an operculum 16-57 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-58 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Opisthobranchs Includes sea slugs, sea hares, sea butterflies, and canoe shells Most are marine, shallow-water and often hide under stones and seaweed Partial to complete detorsion Anus and gill(s) are displaced to right side Two pairs of tentacles, one pair modified to increase chemo-absorption Shell is reduced or absent Monoecious Sea hare Aplysia 16-59 Large anterior tentacles and a vestigial shell Foot of pteropods Modified into fins for swimming Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-60 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-61 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Pulmonates Includes all land and most freshwater snails and slugs Ancestral ctenidia have been lost and the vascularized mantle wall is now a lung Air fills lung by contraction of mantle floor Anus and nephridiopore open near the pneumostome Waste is forcibly expelled Monoecious Aquatic species have one pair of tentacles Landforms have two pair of tentacles and the posterior pair has eyes 16-62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-63 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Bivalvia Mussels, clams, scallops, oysters, and shipworms Range in size from 1–2 mm in length to the giant South Pacific clams Most are sedentary filter feeders Bivalves lack a head, radula, or other aspects of cephalization Most are marine 16-64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-65 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-66 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-67 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-68 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Native freshwater clams in the U.S. are the most jeopardized animal group Of more than 300 species once present, 12 are extinct, 42 are threatened or endangered and 88 more are of concern Sensitive to water quality changes, including pollution and sedimentation Zebra mussels are a serious exotic invader into the Great Lakes Region 16-69 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function 16-70 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 Pearls are produced when an irritant is lodged between the shell and mantle Layers of nacre are secreted around the foreign material Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Body and Mantle 16-71 Visceral mass is suspended from the dorsal midline Foot is attached anteroventrally Ctenidia hang down on each side, each covered by a fold of the mantle Posterior edges of the mantle folds form excurrent and incurrent openings In burrowing clams, mantle forms long siphons to reach the water above Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-72 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-73 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Gills 16-74 Both mantle and gills perform gaseous exchange Gills are derived from primitive ctenidia by lengthening the filaments to each side Filaments fused to form plate-like lamellae with vertical water tubes inside Water enters incurrent siphon Passes into water tubes through pores Proceeds dorsally to suprabranchial chamber Exits through the excurrent siphon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-75 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Feeding 16-76 Suspended organic matter enters incurrent siphon Gland cells on gills and labial palps secrete mucus to entangle particles Food in mucous masses slides to food grooves at lower edge of gills Cilia and grooves on the labial palps direct the mucous mass into mouth Some bivalves feed on deposits in sand Shipworms excavate particles of wood Septibranchs draw in crustaceans by creating a sudden inflow of water Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-77 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-78 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-79 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Internal Structure and Function 16-80 Stomach is folded into ciliary tracts for sorting particles Style sac secretes a crystalline style which is kept whirling by cilia in style sac Rotating style helps free digestive enzymes and roll up a mucous food mass Dislodged particles are directed to a digestive gland or are engulfed by amebocytes 3 chambered heart has two atria and one ventricle Some blood is oxygenated in mantle Returns to the ventricle through the auricles Remainder circulates through sinuses, the kidneys, the gills, and then back to the auricles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-81 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Pair of U-shaped kidneys is ventral and posterior to heart Nervous system has three pairs of widely separated ganglia connected together Sense organs are poorly developed 16-82 Statocysts in the foot Osphradia in the mantle cavity Pigment cells on the mantle Some mantle eyes have a cornea, lens, retina and pigmented layer Tentacles may have tactile and chemoreceptor cells Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction and Development Sexes usually separate Gametes discharged in suprabranchial chamber are carried out in excurrent flow Fertilization usually external Embryos develop as trochophore, veliger, and spat larval stages Freshwater clams have internal fertilization Larvae develop into a bivalved glochidia stage 16-83 Sperm enter the incurrent siphon to fertilize eggs in water tubes of the gills Attaches to gills of passing fish where they live briefly as parasites Eventually sink to begin independent life on the streambed “Hitchhiking” having helped distribute the species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-84 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-85 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Boring Burrowing has led some to evolve a mechanism for boring into harder surfaces Shipworms are destructive to ships and wharfs Radula functions as a wood rasp Symbiotic bacteria produce cellulase, which helps digest wood Bacteria also fix nitrogen 16-86 Diet is high in carbon but deficient in nitrogen Some clams bore into rock and produce long burrows Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Class Cephalopoda Squids, octopuses, nautiluses, devilfish, and cuttlefish All marine predators Foot is in the head region Range from 2 cm to the giant squid Modified for expelling water from mantle cavity Largest invertebrate Cephalopod fossil record goes back to the Cambrian 16-87 Earliest shells were straight Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Nautilus Octopuses and squids apparently evolved from early straight-shelled ancestors Ammonoids Culmination of shell coiling Remaining survivor of nautiloids Series of gas chambers in shell helps maintain neutral buoyancy Extinct but had elaborate shells Cephalopods 16-88 Mostly marine Octopuses mostly intertidal Squids are deep-sea animals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-89 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Form and Function Shell Nautiloid and ammonoid shells had gas chambers allowing them to swim Gas pressure in the nautilus chambers is only one atmosphere compared to the 41 atmospheres of pressure in the surrounding deep ocean Nautilus shell is divided into chambers Living animal only inhabits last chamber Cord of living tissue, the siphuncle, connects chambers to visceral mass Cuttlefish shell is enclosed in mantle Squid shell is a thin strip called the pen, enclosed in mantle Octopus has completely lost the shell 16-90 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-91 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Locomotion 16-92 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 backward by spurting jets of water Some with webbing between their arms swim with a medusa-like action Active life of cephalopods is reflected in their respiratory, circulatory and nervous systems. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-93 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Respiration and Circulation 16-94 Except for nautiloids, cephalopods have one pair of gills 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 Blood goes to the systemic circulation before the gills Accessory or branchial hearts at the base of each gill increase pressure to blood going through gill capillaries Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Nervous and Sensory Systems 16-95 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-96 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Communication 16-97 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 boundary 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; Contains ink gland that secretes sepia when animal alarmed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Reproduction 16-98 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 Hatch into juveniles with no free-swimming larval stage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-99 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Major Groups of Cephalopods 16-100 3 subclasses of cephalopods Nautiloidea Ammonoidea Coleoidea Nautiloidea have two pairs of gills Ammonoidea are extinct Coleoidea have one pair of gills Nautilus is the only surviving genus in Nautiloidea that populated the Paleozoic seas 5 or 6 living species Head has 60–90 tentacles that can extend from the opening of the shell Tentacles lack suckers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs 16-101 Ammonoids became extinct at end of the Cretaceous Chambers resembled those of the nautiloids but septa were more complex and frilled. Reason for extinction is unknown 4 orders of Coleoidea include all living cephalopods except Nautilus 3 subclasses of cephalopods Nautiloidea Ammonidea (extinct) Coleoidea (extinct) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Classes of Molluscs Only one genus of Nautiloidea remains Subclass Coleoidea includes all living cephalopods except Nautilus Order Sepioidea includes cuttlefishes with a round body, eight arms and two tentacles Orders Myopsida and Oegopsida are squids with a more cylindrical body, eight arms and two tentacles. The single species of deepwater vampire squid is in the order Vampyromorpha The Octopoda have eight arms and no tentacles 16-102 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Phylogeny 16-103 First mollusc probably arose in Precambrian times Due to spiral cleavage, mesoderm from the 4d blastomere, and a trochophore larva, many zoologists consider the Mollusca as protostomes allied with annelids in Lophotrochozoa Opinions differ about the exact nature of the relationship among lophotrochozoans Until lophotrochozoan phylogeny is better resolved, not possible to determine whether mollusks and annelids shared a coelomate ancestor A new cladogram places monoplacophorans as the sister taxa to chitons Unites the two taxa with repeated body parts in a clade called Serialia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification 16-104 The question remains, did segmentation originate independently within the three metameric taxa? Ongoing studies suggest that differences in biochemical pathways and developmental steps that produce segmented bodies across taxa, support the hypothesis that segmentation arose several times independently “Hypothetical Ancestral Mollusc” Probably lacked a shell or crawling foot Probably small (about 1 mm) Likely was a worm-like organism with a ventral gliding surface Probably possessed a dorsal mantle, a chitinous cuticle and calcareous scales Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16-105 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification 16-106 Caudofoveates and solenogasters both probably branched off before development of a solid shell Polyplacophorans then branched off before the veliger was established as a larva Gastropoda and Cephalopoda appear to form a sister group with Monoplacophora Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Classification Class Caudofoveata Class Solenogastres Class Monoplacophora Class Polyplacophora Class Scaphopoda Class Gastropoda Class Bivalvia Class Cephalopoda 16-107
