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, 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