Phylum Mollusca

Mollusks 100,000 species “soft bodied animals” Eumatazoans, bilaterally symmetrical, triploblastic coelomates Mostly marine, some freshwater, some terrestrial Size: microscopic snails to giant squids (largest invertebrate) Diverse group with varied appearances but similar body plans: Diverse ◦ Foot ◦ Visceral mass ◦ Mantle  Mantle cavity ◦ Shell ◦ Radula ◦ Siphon(s) Body systems

Characteristics Visceral Mass - contains the organs ◦ the “soft bodied” part Mantle – fold of tissue that drapes over the visceral mass makes shell in most organisms Internal or external Mantle cavity – water-filled chamber ◦ Contains gills, anus, and excretory pores Foot - muscular lower part used for movement Radula – sharp, tongue-like structure Head area is clearly defined ◦ contains the brain and sense organs

SYSTEMS Digestive – Complete gut Muscle – foot (for locomotion) Nervous (brain & nerve cords) ◦ Complex senses  Eye spots (light)  Chemical sensors/touch sensors (tentacles) Circulatory ◦ Blood vessels deliver oxygenated blood around the body (open system – blood mixes in coelom) ◦ Pumped by heart Respiratory – gills (aquatic)/lungs (on land) ◦ Siphons (move/pump water) ◦ Filter water Reproductive ◦ Ciliated larval stage (trochophore)

Nervous system and sense structures Gastropods: eye spots (on stalks at base of tentacles or on tentacles) and chemical sensors in tentacles Bivalves: photoreceptor and vibration cells Cephalopods: Best developed brain of all invertebrates- can detect light, vibrations, chemicals *Eye of snail* *Suckers on octopus tentacles*

Basic Body Plan

Shell vs. No Shell

Classes Class Aplacophora species ◦ Exclusively marine ◦ Burrowers ◦ No shell Class Bivalvia (clams, mussels, scallops, & oysters) Class Cephalopoda (squids & octopuses) Class Gastropoda (slugs & snails) Class Monoplacophora – 11 species ◦ Discovered in 1952 ◦ Many repeated organs (like annelids) ◦ Single shell Class Polyplacophora (chitons) Class Scaphopoda

900 species Tusk shell Burrow on sea floor sediments Shell opened at both ends

Class Polyplacophora 600 species of Chitons Entirely marine species ◦ Inhabit rocky ocean bottoms and coastlines No head Oval body divided into 8 dorsal plates ◦ Body is unsegmented Foot used for movement and suction Feeds on algae

Class Gastropoda 3/4 th of all living species of mollusks “stomach-foot” Slugs and snails (move over slimy film) Slugs Marine, freshwater, and terrestrial Asymmetrical body plan Defined head with eyes and tentacles Usually have a single coiled shell ◦ Defense/safety ◦ Can be reduced or absent Open circulatory system Torsion: visceral mass rotates 180° causing the mantle cavity to be above its head (sea snails) Host to many parasites (zombie snails)zombie snails Eat algae & plants, or hunters (cone snail)cone snail ◦ Specialized radula

Sea snail Snail Slug Sea Slug

Class Bivalvia 15,000 species “2 door ways” Clams, oysters, mussels, and scallops Clamsscallops No distinct head, no radula Have two shells ◦ Attached at a hinge (by ligament) ◦ Adductor muscles open/close shells Gills in mantle cavity for respiration an ◦ Incurrent siphon – water in ◦ Excurrent siphon – water out Typically sessile suspension feeders (mussels have attaching threads) ◦ Can move with foot (clams) or “flapping” (scallops)scallops Some have eyes and sensory tentacles extending from mantle Reproduce through spawning (releasing sperm and egg) ◦ Seasonal

Oysters Mussels Scallops Clams

Bivalve Anatomy

Class Cephalopoda 700 species Octopi, squids, cuddle fish, chambered nautiluses Active, marine predators ◦ Beak like jaws (modified or absent radula) ◦ Some poisonous “head foot” Head (clearly defined) surrounded by grasping tentacles ◦ Modified foot ◦ Usually has suckers Shell can be internal (squids), external (chambered nautilus), or absent (octopi) Locomotion by jet propulsion (siphon) Only mollusk with a closed circulatory system Complex brain and sensory equipment (complex eyes)/nervous system (octopi – smartest invertebrate)octopi Chromatophores → color changing skin

OCTOPUS 8 tentacles ◦ Complete with suckers Intelligent hunters Intelligenthunters Can fit into tiny spaces ◦ No shell Masters of disguise Reproduction “intimate” and dangerous – male sometimes eaten Solitary and aggressive towards other members of their species Would you eat one?

Beak is surrounded by tentacles ◦ Sharp for tearing into prey (crabs, small fish, etc.)crabs Ink sack for defense against predators 3 hearts (2 for pumping blood through gills, 1 for pumping blood through body)

SQUID Internal shell Predators (squid feeding)feeding ◦ Beak ◦ Feeding tentacles Vampire squid Giant squid

CUDDLEFISH Excellent at camouflage ◦ Can change the color and texture of skin

CHAMBERED NAUTILUS CHAMBERED NAUTILUS Most primitive cephalopod Approx. 90 tentacles ◦ No suckers Grows shell in chambers Coloration helps avoid predators Eye has no lens Hunts with chemical sensors Hunts