Transcript Phylum Mollusca

Mollusks
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
Why are Mollusks in the
same phylum?
 Share similar developmental stages
 Many of their lifecycles begin with a
trochophore (free-swimming larval stage)
Classification of Mollusks
 Classified into three common groups
based on shell (presence & type) and
foot type
 Gastropods
 Bivalves
 Cephalopods
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
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!
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
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
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)
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
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
Class Scaphopoda
 Tusk shells
 Example: dentalium
 Used by First Nations as a form of currency
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
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
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
 Respiration
 Aquatic mollusks = gills
 Land mollusks = through skin
 Excretion
 Nephridia remove ammonia from blood
Mollusk Circulation
 Slow-moving mollusks = open circulatory
system
 Fast-moving mollusks = closed
circulatory system
Response: Nervous system?
 Bivalves: small ganglia, nerve cords,
eyespots
 Octopi: brain – memory and complex
behaviour
Movement
 Slugs & Snails: secrete mucus &
muscular contractions
 Bivalves: foot
 Octopi: jet propulsion
Reproduction
 Gastropods & bivalves: sexual
reproduction – external fertilization
 Cephalopods: internal fertilization
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
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
Clam Gardens
 Made by First Nations from Alaska to
Washington
 Expand clams’ specific intertidal habitat
to increase their growth
Clam Gardens
 Rock-walled beach terraces
 Extend the beach flat
 Maintenance:
 Constantly dig to allow oxygen water in
 Remove shifted boulders