Download Figure 11.13 Trochophore (a) and veliger (b)

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 Characteristics
1.
2.
3.
4.
5.
6.
7.
8.
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
Figure 11.2 Molluscan body organization.
Figure 11.3 Molluscan shell and mantle.
Figure 11.4 Radular structure.
(a)
(b)
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
Figure 11.6 Gastropod structure. (a) A pulmonate (Orthaliculus). (b) Internal
strucutre of a pulmonate.
(a)
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 oneway.
– 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
Figure 11.7 (a) Carinaria is a heteropod predator. (Prosobranchia) (b) A
nudibranch (Opisthobranchia). (c) A terrestrial slug (Ariolimax
collumbianus) (Pulmonata).
(b)
(a)
(c)
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 11.12
Bivalve Circulation
Figure 11.13 Trochophore
(a) and veliger (b) larval
stages. Glochidia of an
unionid bivalve.
(c)
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
Figure 11.20 Class
Polyplacophora. (a)
Tonicella lineata.
(a)
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.