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Phylum Mollusca

1. Second largest phylum

well over 100,000 species (at least 60,000 of these are gastropods!)

similar number of fossil species

exceeded in diversity only by arthropods (especially the insects)

molluscs are most diverse animal phylum in marine habitats

2. Really diverse body forms and lifestyles!

Phylum includes, snails, clams, ocopus, squid, and others

Common features shared by all:

(1) muscular foot

highly modified for different lifesyles in different species.

(2) visceral mass

contains most internal organs

internal organs are complex

(3) mantle

covers visceral mass

secretes shell (in shelled species)

overhangs visceral mass, creating mantle cavity (which often encloses gills)

Diagram of generalized molluscan body plan:

Most species have a radula (not bivalves though).

To see a movie of a snail's mouth with radula in action, click here

or here

or even better, the middle of this video

3. Some of the major classes of molluscs:

Aplacophora - worm-like body shape

Monoplacophora - domed shells, snail-like, but not closely related to the snails, and not very diverse

Scaphopoda - the tusk shells

all species marine

slender tubular shell, narrower at one end

shell open at both ends

live buried in sediment, often with narrow end of shell above surface

have tentacles that aid in gathering food

feed on interstitial organisms - some species specialize on foraminiferans

Scaphopods apparently are a major food for chimaeras, which are very abundant shark relatives that live primarily at great depths http://en.wikipedia.org/wiki/Chimaera

Scaphopod shells were used as money by Native Americans, and are still used in ceremonial clothing.

see: https://trinidadtrading.com/store/dentalium-and-naa-set

http://en.wikipedia.org/wiki/Dentalium_shell

and

Polyplacophora - the chitons

all species marine

Somewhat snail-like, but with segmented shells

grazers on algae, often coralline algae

radulas very tough - strengthened with iron compounds

Gastropoda - the snails and their relatives

Most diverse group of molluscs

Most are marine, but many species in freshwater and on land

Most species have coiled shell

Shell-less species called slugs (on land) or sea slugs (in the sea)

Go to http://web.augsburg.edu/~capman/photoofmonth/lettuceseaslug.html for photos and an article about a very interesting sea slug in our reef tank

Also go to http://www.seaslugforum.net/ for LOTS of beautiful sea slug photos and more information on sea slugs than you probably thought possible!

Terrestrial species have lung-like structure and breathe air

Most have muscular foot they glide on

DIVERSE in form

DIVERSE lifestyles:

herbivores

scavengers

predators

some (e.g. cone shells) have powerful venoms and can catch fish

For a great movie of a cone snail catching and eating a fish, click here, and then download the movie (the movie is well worth the time spent downloading it!)

for more info on cone shells, click here and here, and for photos click here

some bore holes into shells of other molluscs and eat them

some eat corals, sponges, tunicates (sea squirts), or other sessile creatures

some snails are bizzarre (but very common) sessile filter feeders (vermetid snails)
- See great vermetid snail video

Bivalva (means "two-shelled") - the bivalves (clams, oysters, scallops, mussels, etc)

Exclusively aquatic, most are marine, many in freshwater

Diverse forms and lifestyles

Diagrams of general body plan of bivalves:

many burrow in sediments (using muscular foot)

many attach to surfaces with byssal threads (secreted by byssal gland on foot - see byssal threads on mussel below):

some burrow into corals

some, called shipworms, burrow into wood

for shipworm photos, diagrams, and information see:

Most species are filter feeders

cilia on gills create water current through mantle cavity

often there are prominent siphons (incurrent and excurrent)

tiny particulate food captured by gills and moved via cilia to mouth

Tridacnid clams (the giant clams) are largely fed by dense populations of zooxanthellae in elaborate mantles:

For photos of Tridacna gigas, the world's largest bivalve, click here

and http://web.augsburg.edu/~capman/aquariumphotos/gigasclam.html

Some bivalves have eyes and can swim! See photos and stories about scallops: http://web.augsburg.edu/~capman/photoofmonth/scallop-eyes.html