Chapter 11 Molluscan Success

Evolutionary Perspective Triploblastic Coelomate Very successful 100,000 living species Relationships to other animals Lophotrochozoans

Molluscan Characteristics Body of two parts: head-foot and visceral mass Mantle that secretes a calcareous shell and covers the visceral mass Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of reproductive products. Bilateral symmetry Trochophore larvae, spiral cleavage, and schizocoel coelom formation Coelom reduced Open circulatory system (except Cephalopoda) Radula usually present

Body Organization Head-foot Elongate Mouth Attachment and locomotion Visceral mass Dorsal to head-foot Organs of digestion, circulation, reproduction Mantle Enfolds body Secretes shell Mantle cavity Gas exchange, excretion, elimination of digestive wastes and reproductive products Radula Supported by odontophore Rasping food

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Class Gastropoda Snails and slugs 35,000 living species Torsion 180o counterclockwise twisting of visceral mass, mantle, and mantle cavity during development Possible adaptive significance Head enters shell first. Clean water enters anteriorly oriented mantle cavity opening. Mantle sensory organs move to head region.

Class Gastropoda Shell coiling Locomotion Earliest fossils, one plane Modern, asymmetrical More compact Internal organs asymmetrical and sometimes no longer paired Locomotion Flattened foot Cilia propel over mucous trail Muscular waves

Class Gastropoda Feeding and digestion Most scrape algae and attached organisms Herbivores, predators, scavengers Digestive tract Ciliated Food incorporated into mucous mass called protostyle. Gas exchange One or two gills in mantle cavity Land snails (pulmonates) Mantle cavity richly vascular for gas exchange with air

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Other Maintenance Functions Open circulatory system Blood bathes tissues in sinuses. Heart Single ventricle and single auricle Functions Transports nutrients and gases Hydraulic skeleton Nervous system Six ganglia plus nerve cords Sensory structures Eyes at base or end of tentacles Statocysts in foot Osphradia in mantle cavity

Other Maintenance Functions Excretion Single nephridium Result of shell coiling Discharges into mantle cavity or adjacent to mantle cavity (pulmonates) Ammonia (aquatic species) Uric acid (pulmonates)

Reproduction Dioecious or monoecious External fertilization Copulation Some dioecious marine species Copulation Sperm transfer may be mutual or one-way. Eggs shed singly, in strings, or in masses Larval stages Trochophore Veliger

Gastropod Diversity Subclasses Prosobranchia Opisthobranchia Pulmonata 20,000 species Mostly marine Opisthobranchia 2,000 species Sea hares, sea slugs Shell, mantle cavity, and gills reduced or lost Pulmonata 17,000 species Freshwater or terrestrial Vascular mantle cavity serves as lung

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Class Bivalvia Clams, oysters, mussels, scallops 30,000 species Shell and associated structures Single shell consisting of two hinged valves Mantle sheetlike and covers laterally compressed body.

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Gas Exchange, Filter Feeding, and Digestion Sedentary filter feeders Loss of head and radula Expansion of cilia-covered gills into folded sheets (lamellae) Cilia create water currents into and through mantle cavity. Gas exchange in water tubes Food trapped along gill surface and transported to food grooves and labial palps Digestion Crystaline style and gastric shield

Other Maintenance Functions Open circulatory system Mantle and gills oxygenate blood Nephridia Below pericardial cavity Open to suprabranchial chamber Nervous system Three pairs of interconnected ganglia Sensory receptors at mantle margin

Reproduction and Development Most dioecious Gonads within visceral mass External fertilization Trochophore and veliger larval stages Freshwater in family Unionidae Parasitic larval stage Glochidium

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Gooduck

Class Cephalopoda Squid, octopuses, cuttlefish, and nautiluses Foot modified into circle of tentacles or arms and incorporated into siphon Head in line with visceral mass Muscular mantle

Class Cephalopoda Shell Locomotion Feeding and Digestion Reduced or absent except in nautilus Locomotion Jet propulsion using muscles of mantle compressing water within mantle cavity and siphon Feeding and Digestion Predators Tentacles, jaws, radula Digestive tract muscular with large digestive glands

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Other Maintenance Functions Closed circulatory system Nervous system Large brains Complex sensory structures Eyes Statocysts Chromatophores Color changes involved with courtship and other displays Ink glands

The cephalopod eye.

Learning Unparalleled in comparison to any other invertebrate and many vertebrates Evolved in response to predatory lifestyles

Reproduction and Development Dioecious Male produces spermatophores Transfers to female’s mantle cavity using modified tentacle (hectocotylus) Eggs deposited singly or in masses attached to substrate. Eggs tended by parents.

Class Polyplacophora Chitons Reduced head, flattened foot, shell consisting of eight dorsal valves, muscular mantle extends beyond margin of shell Feed on attached algae Ladderlike nervous system Dioecious with external fertilization

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Class Scaphopoda Tooth shells or tusk shells Marine, burrowing Conical shell open at both ends Dioecious with trochophore and veliger larvae

Class Scaphopoda.

Class Monoplacophora Marine Undivided arched shell Broad, flat foot Serially repeated pairs of gills and foot retractor muscles Dioecious

Class Monoplacophora.

Class Solenogastres Marine substrates Lack shell Crawl on ventral foot Figure 11.23 Class Solenogastres Marine substrates Lack shell Crawl on ventral foot Minute calcareous spicules Carnivores

Class Caudofoveata Deep sea Wormlike Feed on foraminifera Lack shell, foot, and nephridia 120 species

Further Phylogenetic Considerations More than 500 million years old Lophotrochozoa Shell and muscular foot not ancestral Solenogaster spicules may be similar to ancestral “shell”. Muscular foot first seen in Polyplacophora. Quickly diversified into modern classes

Cladogram showing possible evolutionary relationships among the molluscs.