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 Excretion 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