second-largest phylum in number of species- over 100,000 described. ecologically widespread- marine,...
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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 feet). 80% are under 5 cm, but many are large and therefore significant as food for man.
Phylum Mollusca
Extant Molluscan classes
Monoplacophora
Aplacophora
Scaphopoda (tusk shells)
Cephalopoda (octopus, squid,nautilus)
Gastropoda (snails)
Bivalvia (clams, mussels)
Polyplacophora (chitons)
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
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
• ~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)
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
Cephalopoda
Rapid movement; well-developed nervous systems.
Example: Octopus, squid, nautilus
• 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”)
• 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”)
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
Gastropoda, Prosobranchia, ArcheogastropodaAbalone are large, valuable snails that are harvested and cultured for food and the iridescent shells
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
Conch pearls
Some other mesogastropod prosobranchs
cowriestritons
Gastropoda, Prosobranchia,
NeogastropodaExample- cone snails- Conus500-1000 species.
All cone snails inject venom and several of the fish eaters can be fatal to humans
Cone snails are able to engulf their prey whole after paralyzing it with venom
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
Muricids
Some other neogastropod prosobranchs
Volutes
Whelks
• 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
Bubble shells (Order Cephalaspida) Most are aeolian carnivores on gastropods and annelids
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
Dorid nudibranchs
More dorid nudibranchs
(suborder Doridina))
Aeolid nudibranchs (suborder Aeolidina)
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
Saccoglossan sea slugs-
Shell reduced or absent
most saccoglossans are herbivores that suck plant cytoplasm- some can hijack chloroplasts
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
Mimicry among dorid nudibranchs and polyclad flatworms- can you tell which is which?
Pterotrachea coronata“sea elephant” A pelagic prosobranch (Mesogastropoda: Heteropoda) that is convergent on pteropods The foot is a sculling fin
Sea hares- (Anaspidea)
internal shell, ink defense, neurobiology subjects
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