Chapter 11

Molluscan Success

Evolutionary Perspective• Triploblastic• Coelomate• Very successful

– 100,000 living species• Relationships to other animals

– Lophotrochozoans

Figure 11.1 Molluscs are lophotrochozoans with evolutionary ties to Annelida, Platyhelminthes, Rotifera, and others.

Table 11.1.

Molluscan Characteristics1. Body of two parts: head-foot and visceral mass2. Mantle that secretes a calcareous shell and covers the visceral

mass3. Mantle cavity functions in excretion, gas exchange, elimination of

digestive wastes, and release of reproductive products.4. Bilateral symmetry5. Trochophore larvae, spiral cleavage, and schizocoel coelom

formation6. Coelom reduced 7. Open circulatory system (except Cephalopoda)8. 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

Figure 11.2 Molluscan body organization.

Figure 11.3 Molluscan shell and mantle.

(a)

(b)

Figure 11.4 Radular structure.

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.

Figure 11.5 (a) A pretorsion gastropod larva. (b) After torsion the digestive tract is looped and mantle opens near head. (c) Hypothetical adult ancestor prior to torsion. (d) Modern adult gastropod after torsion.

Class Gastropoda• Shell coiling

– 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

(a)

Figure 11.6 Gastropod structure. (a) A pulmonate (Orthaliculus). (b) Internal strucutre of a pulmonate.

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• 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• 20,000 species• Mostly marine

– Opisthobranchia• 2,000 species• Mostly marine• 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

(a) (b)

(c)

Figure 11.7 (a) Carinaria is a heteropod predator. (Prosobranchia) (b) A nudibranch (Opisthobranchia). (c) A terrestrial slug (Ariolimax collumbianus) (Pulmonata).

Class Bivalvia• Clams, oysters, mussels, scallops• 30,000 species• Shell and associated structures

– Single shell consisting of two hinged valves (figure 11.8)

– Mantle sheetlike and covers laterally compressed body.

Figure 11.8 Shell and associated structures.

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 (figure 11.9)– Food trapped along gill surface and transported to

food grooves and labial palps (figure 11.10).– Digestion (figure 11.11)

• Crystaline style and gastric shield

Figure 11.9 Lamellibranch gill of a bivalve. Blue arrows show water movement. Red arrows show blood movement.

Figure 11.10 Bivalve feeding. Solid arrows show path of food particles. Dashed arrows show path of particles being rejected.

Figure 11.11 Internal structure of a bivalve.

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

(figure 11.13a, b)• Freshwater in family Unionidae

– Parasitic larval stage– Glochidium (figures 11.13c and ll.14)

Figure 11.12Bivalve Circulation

(c)

Figure 11.13 Trochophore (a) and veliger (b) larval stages. Glochidia of an unionid bivalve.

Figure 11.14 Mantle lure of a freshwater bivalve (Lampsilis reeviana).

Figure 11.15a Bivalve Diversity. Giant clam (Tridacna dersa).

Figure 11.15b Bivalve diversity. Rock scallop (Hinnites giganteus).

Figure 11.15c Bivalve diversity. The goeduck (Panopea generosa).

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 (figure 11.17)

Figure 11.17 Internal structure of the squid, Loligo.

Class Cephalopoda• Shell

– 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

Figure 11.18 Cephalopod arms and tentacles.

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

Figure 11.19 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 (figure 11.20)

• Feed on attached algae• Ladderlike nervous system• Dioecious with external fertilization

(a)

Figure 11.20 Class Polyplacophora. (a) Tonicella lineata.

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

veliger larvae

Figure 11.21 Class Scaphopoda.

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

foot retractor muscles• Dioecious

Figure 11.22 Class Monoplacophora.

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 (figure 11.24)

Figure 11.24 Cladogram showing possible evolutionary relationships among the molluscs.