1. Mollusks

2. Mollusks Huge phylum, wide variety of shapes/sizes
~85,000 different species
Soft-bodied invertebrates
Have bilateral symmetry
Usually have one or two shells with organs in a fluid filled cavity (shells are internal or external)
Most live in water
Fossils are 500+ million years old

3. Why are Mollusks in the same phylum?
Share similar developmental stages
Many of their lifecycles begin with a trochophore (free-swimming larval stage)

4. Classification of Mollusks
Classified into three common groups based on shell (presence & type) and foot type
Gastropods
Bivalves
Cephalopods

5. Class Gastropoda Includes conchs, snails, slugs, nudibranchs, limpets
Largest group of mollusks
Shell-less or single shelled
Use a radula (a tongue-like organ with rows of teeth) to get food
Move by a muscular foot on the ventral side
Have foot glands that secrete a layer of mucus for sliding

6. Gastropod Protection
Some can pull inside a single shell when threatened
some also have a hard disk (operculum) on their foot that forms a protective door when they withdraw
Some of don’t have a shell
Slugs hide during the day from predators
Nudibranchs can have poison & are brightly coloured
Can reuse nematocysts from cnidarians they eat!

7. Class Bivalva Includes clams, oysters, mussels, and scallops
Have a hinged, two-part shell
held together by 1-2 powerful muscles
To open or close their shell they either contract or relax their muscles

8. Bivalves are well adapted for water
Clams can burrow in sand
Mussels attach themselves to a solid surface
Scallops escape predators by rapidly opening and closing their shell

9. Class Cephalopoda Most specialized and complex mollusks.
Include squid, octopi, cuttlefishes and nautiluses.
Soft-bodied with a head attached to a single foot (foot is divided into tentacles)

10. Cephalopods
Have a well developed head and many tentacles for capturing prey
Eyes that distinguish shape!
Closed circulatory system
Moves blood through the body in a series of closed vessels like humans.
Use jet propulsion to move at speeds of 6 m/s
Water flows over the gills into the mantle
Squeezed out through the siphon

11. Class Polyplacophora Chitons Shells made of 8 overlapping plates
Plates are embedded in the tough muscular girdle that surrounds the body
Can hold onto irregular surfaces or roll into a protective ball if dislodged

12. Class Scaphopoda Tusk shells Example: dentalium
Used by First Nations as a form of currency

13. Mollusks’ Body Plan 4 parts
1. foot: crawling, burrowing, capturing prey (tentacles)
2. mantle: tissue covering the body
3. shell: secreted by mantle – calcium carbonate
4. visceral mass: internal organs

15. Mollusks’ Body Plan Mantle
Thin layer of tissue that covers the body organs
Mantle cavity (between soft body and mantle)
Contains the gills that are used to breathe by exchanging oxygen and carbon dioxide in the water
Open Circulatory System
Most mollusk have this
Moves blood through vessels and into open spaces around body organs

16. Mollusk Feeding Snails & slugs: radula (scrape algae or drill holes)
Bivalves: filter-feeders
Water enters through siphon, passes over gills, food gets caught on mucus, water exits through siphon
Octopi & sea slugs: sharp jaws

17. Respiration Excretion Aquatic mollusks = gills
Land mollusks = through skin
Excretion
Nephridia remove ammonia from blood

18. Mollusk Circulation Slow-moving mollusks = open circulatory system
Fast-moving mollusks = closed circulatory system

19. Response: Nervous system?
Bivalves: small ganglia, nerve cords, eyespots
Octopi: brain – memory and complex behaviour

20. Movement Slugs & Snails: secrete mucus & muscular contractions
Bivalves: foot
Octopi: jet propulsion

21. Reproduction
Gastropods & bivalves: sexual reproduction – external fertilization
Cephalopods: internal fertilization

22. Ecology of Mollusks Food web:
eat plants, animals, filter algae & eat detritus
food source (including us!)
Monitor water quality: filter-feeders concentrate dangerous pollutants
Early environmental warning system
Cancer research? – mollusks don’t seem to develop caner

23. Invasive, non-native species
Zebra mussels: introduced to North America from Eastern Europe & Asia on boats in the 1980s
Spread through Great Lakes and into rivers
Few natural predators
Reproduce rapidly
Attach to anything
& in layers

24. Clam Gardens Made by First Nations from Alaska to Washington
Expand clams’ specific intertidal habitat to increase their growth

25. Clam Gardens Rock-walled beach terraces Extend the beach flat
Maintenance:
Constantly dig to allow oxygen water in
Remove shifted boulders