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Second-largest phylum in number of species- over 100,000 described. Ecologically widespread- marine, freshwater, terrestrial (gastropods very successful on land) Variety of body plans (therefore, many classes within the phylum) Variety in body size- from ~1 mm to ~18 m (60 Phylum Mollusca

Second-largest phylum in number of species- over 100,000 described

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Phylum Mollusca. Second-largest phylum in number of species- over 100,000 described. Ecologically widespread- marine, freshwater, terrestrial (gastropods very successful on land) Variety of body plans (therefore, many classes within the phylum) - PowerPoint PPT Presentation

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Page 1: Second-largest phylum in number of species- over 100,000 described

• Second-largest phylum in number of species- over 100,000 described.

• Ecologically widespread- marine, freshwater, terrestrial (gastropods very successful on land)

• Variety of body plans (therefore, many classes within the phylum)

• Variety in body size- from ~1 mm to ~18 m (60 feet). 80% are under 5 cm, but many are large and therefore significant as food for man.

Phylum Mollusca

Page 2: Second-largest phylum in number of species- over 100,000 described

Extant Molluscan classes

Monoplacophora

Aplacophora

Scaphopoda (tusk shells)

Cephalopoda (octopus, squid,nautilus)

Gastropoda (snails)

Bivalvia (clams, mussels)

Polyplacophora (chitons)

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Mollusk characteristics

• Ciliated body surface

• Calcareous shell- composed of three primary layers- outer periostracum, middle prismatic layer (columnar crystals of calcite) and inner nacre (flat crystals of calcite)

• Mantle- dorsal surface of body wall, modified to secrete shell

Page 5: Second-largest phylum in number of species- over 100,000 described

More mollusk characteristics

• Radula- a rasping “tongue” with chitin teeth, sometimes also chitinous jaws

• Ctenidia- ciliated gills for respiratory gas exchange, usually located in a mantle cavity

• Open circulatory system (hemocoel)- coelom is reduced

Page 6: Second-largest phylum in number of species- over 100,000 described

• ~800 species, all marine, many intertidal

• Shell is distinctive- 8 overlapping plates imbedded partly or entirely in tough “girdle”.

• Mantle space extends around perimeter of animal (not just posterior).

• Ctenidia are lateral and multiple.

• Very conservative class. Fossils date to mid/late Cambrian (500 my).

Class Polyplacophora (chitons)

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A collection of chitons

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Class BivalviaClams, Oysters, Shipworms

Class BivalviaClams, Oysters, Shipworms

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Class Bivalvia

• Two shells• Most are filter feeders• No head or radula• Burrow

– Sand, wood, rocks

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Giant Clam & Burrowing ClamSiphon

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Zebra Mussel

• Environmental Pest• Ballast water of ships

from Europe in 1986• Attack be secreting

adhesive byssal threads– Each other

– Other mussels

– Man made objects• Pipes, plumbing

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Zebra Mussel

• Live in high densities• Feed on phytoplankton• Reproduce rapidly

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Zebra Mussel

• Attach to native mussels• Killed all native mussels

in Lake Erie

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Distribution of Zebra Mussel

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Oysters

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Pearl formation

Developing pearlDeveloping pearlDeveloping pearlDeveloping pearl

EpitheliumEpitheliumEpitheliumEpithelium

ShellShellShellShell

Irritant lodged between shell and mantle

Layers of nacre secreted around foreign material

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Scallops

Page 22: Second-largest phylum in number of species- over 100,000 described

Cephalopoda

Rapid movement; well-developed nervous systems. 

Example:  Octopus, squid, nautilus 

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Page 24: Second-largest phylum in number of species- over 100,000 described

• Originally described from fossils- thought to be extinct since Devonian (350 mya)

• first live specimens recovered in 1952 from 3,600 m depth

• Primitive class, most similar to gastropods, chitons

• Organ systems show metamerism, similar to annelids and arthropods

Class Monoplacophora (“single shell carriers”)

Page 25: Second-largest phylum in number of species- over 100,000 described

• snails, slugs, nudibranchs, limpets, cowrys, abalones, etc.

• By far the largest and most diverse molluscan class- over 80% of mollusc species are gastropods

• Marine benthic, pelagic, freshwater benthic, terrestrial (mesic & xeric)

• Grazers or predators

• Prominent head, with well-developed sensory structures (second only to cephalopods)

Class Gastropoda (“belly-foot”)

Page 26: Second-largest phylum in number of species- over 100,000 described

Gastropod shell

Basically a cone, straight or coiled, with new shell deposited at the margin of the open end during growth

New growth

limpets snails

Page 27: Second-largest phylum in number of species- over 100,000 described

Gastropoda, Prosobranchia, ArcheogastropodaAbalone are large, valuable snails that are harvested and cultured for food and the iridescent shells

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Gastropoda, Prosobranchia,

Mesogastropoda

• Example- conchs (Strombus ~50 species)

• The queen conch (Strombus gigas) herbivorous- it is common in the Caribbean and is harvested for its meat and shell.

• This species is economically important and of conservation concern due to overharvesting and pollution

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Conch pearls

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Some other mesogastropod prosobranchs

cowriestritons

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Gastropoda, Prosobranchia,

NeogastropodaExample- cone snails- Conus500-1000 species.

All cone snails inject venom and several of the fish eaters can be fatal to humans

Page 34: Second-largest phylum in number of species- over 100,000 described

Cone snails are able to engulf their prey whole after paralyzing it with venom

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Cone snails have conotoxins- unique venom strategy.

Many different small peptides target different receptors and enzymes. One species may have over 100 different peptides.

Conotoxins have exceptionally high affinity for receptors and a very high target specificity.

Great interest in these peptides as pharmacological agents for research and for drug use

Page 36: Second-largest phylum in number of species- over 100,000 described

Muricids

Some other neogastropod prosobranchs

Volutes

Whelks

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• Bubble snails (cephalaspids)

• Sea slugs

• Nudibranchs: (dorids, aeolids) most are carnivorous, feed on cnidarians

• Sacoglossans: most are herbivorous- many adopt chloroplasts

• Pteropods: (thecosomes, gymnosomes) pelagic, foot modified into winglike lobes

• Sea Hares: (anaspids) important in neurobiology

Subclass Opisthobranchia (gills-behind) some major groups by common name

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Bubble shells (Order Cephalaspida) Most are aeolian carnivores on gastropods and annelids

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Nudibranchs (Order Nudibranchia)

Shell, mantle cavity and ctenida are gone

Possess cerata (singular = ceras) dorsal projections of the body wall and hemocoel that act as secondary gills

Most are carnivorous and feed largely on cnidarian polyps.

There are two suborders- Doridina (dorids) and Aeolidina (aeolids).

Nudibranchs are some of the most incredibly colorful animals on earth

Page 40: Second-largest phylum in number of species- over 100,000 described

Dorid nudibranchs

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More dorid nudibranchs

(suborder Doridina))

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Aeolid nudibranchs (suborder Aeolidina)

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Borrowed weaponsAeolids feed on cnidarians and store the functional nematocysts at the tips of their cerata in cnidosacs

Each ceras contains a branch of the digestive gland. A duct connects the cnidosac to the digestive gland.

Stained section of cnidosac showing nematocysts at tip

Digestive gland

cnidosac

Page 44: Second-largest phylum in number of species- over 100,000 described

Saccoglossan sea slugs-

Shell reduced or absent

most saccoglossans are herbivores that suck plant cytoplasm- some can hijack chloroplasts

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Left: sacoglossan Placida showing network of ducts containing green chloroplasts from its algal food.

Right: aeolid nudibranch Pteraeolidia "farms" colonies of brown single-celled algae (zooxanthellae) in its cerata (stolen from cnidarian prey).

Solar-powered Opisthobranchs

Page 46: Second-largest phylum in number of species- over 100,000 described

Mimicry among dorid nudibranchs and polyclad flatworms- can you tell which is which?

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Pterotrachea coronata“sea elephant” A pelagic prosobranch (Mesogastropoda: Heteropoda) that is convergent on pteropods The foot is a sculling fin

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Sea hares- (Anaspidea)

internal shell, ink defense, neurobiology subjects