Phylum Mollusca “soft bodied” animals Ch 12

Protozoa ProtostomesDeuterostomes Coelomates Acoelomates Mollusca Crustacea Arachnida Insecta Annelida Echinodermata Chordata Vertebrata Tunicata Pseudocoelomates Monera (Bacteria) Hemi- chordata Metazoa Parazoa Eumetazoa Bilateria † Mesozoa † Triploblasts * Diploblasts Arthropoda Cnidaria Ctenophora Radiata*

Mollusca Characteristics Triploblastic Cephalization Bilateral Symmetry Organ level of organization Eucoelomate Protostome

Eucoelomate Have a “true” body cavity that is completely surrounded by mesoderm ectoderm mesoderm endoderm coelom Gut

Mollusca Characteristics Gas Exchange mainly gills, however terrestrial species have evolved “lungs” Circulatory System open circulatory system (in most classes) heart and blood sinuses Excretion most have kidneys (metanephridia) Nervous system ventral nerve cords, sensory organs

Mollusca Characteristics complete with regional specialization Radula: a tongue-like organ bearing rows of teeth that is used in feeding (scraping, drilling, etc.) Digestive System hole made by a moon snail radula

Cross section of a Gastropod head

monoecious and dioecious species exist usually internal fertilization indirect development with the presence of a trochophore larva (link to annelids), and in most cases a veliger larva Mollusca Characteristics Reproduction trochophore veliger

head-foot: contains sensory organs and muscles visceral mass: contains digestive, reproductive, circulatory organs mantle: secretes the shell (if there is one), cavity contains gills or lungs Shell: in most groups. Mollusca Characteristics Body Plan shell foot Visceral mass mantle mantle cavity

(Hypothetical Ancestral Mollusc) Digestive gland Mouth Radula Nerve collar Intestine Gill Anus Nephridium Shell Mantle Nerve cords foot coelom heart gonads head

Molluscan Radiation There are approximately 93,000 living species in phylum Mollusca (70,000 are extinct) The great morphological diversity is the result of elaboration on the basic body plan

Molluscan Radiation HAM Cephalopoda lobed foot highly developed head shell reduced or lost Polyplacophora shell consists of 8 plates Gastropoda torsion coiling of the shell Scaphopoda ventral shell fusion Bivalvia bivalved shell dorsal hinge Solenogastres Caudofoveata Shell-less but scales or spicules present Monoplacophora Cap-like shell

Phylum Mollusca Class Polyplacophora Class Gastropoda Class Cephalopoda Class Bivalvia Class Scaphopoda Class Monoplacophora Class Soenogastres (Aplacophora) Class Caudofoveata

Class Polyplacophora the “chitons”

Class Polyplacophora have 8 rows of articulating plates use radula to graze algae on substrate mantle forms a girdle around plate edge no veliger larvae girdle plates

Live in Intertidal Zone

All Marine

More Anatomy of a Chiton!

Cryptochiton sp. The Gumboot Chiton The gumboot chiton the largest species of chiton in the world, growing up to 30 cm in length eight plates are covered up by thick, leathery, brick-red flesh so we cannot see them.

Cryptochiton sp.

Class Gastropoda snails and slugs

Gastropoda Origin and Evolution 1.Development of a head 2.Dorsoventral elongation of the body 3.Shell become complex retreat: coiling 4.Torsion

Torsion Torsion is not the spiraling of the shell The body behind the head was twisted 180 degrees counterclockwise Organs reversed location Gut twisted mouth anus gills

Coiling Shell shifts over body for better weight distribution Apex of shell extends out making the shell more compact Planospiral shell Conispiral shell

Class Gastropoda Subclass Prosobranchia Subclass Opisthobranchia Subclass Pulmonata

Class Gastropoda Subclass Prosobranchia aquatic snails (marine and freshwater) have undergone torsion and most have undergone shell coiling have gills all have shells use radula for feeding; can be herbivores, carnivores, or detritivores In some, mantle extends over shell

Class Gastropoda Subclass Prosobranchia snails often lay eggs in protective capsules

Naticidae Moon Snails

Close-up of a moon snail drill hole.

Gastropods in the subclass Prosobranchia all contain an Operculum on the shell.

Class Gastropoda Subclass Opisthobranchia sea hares and sea slugs (nudibranchs) have undergone detorsion have gills sea hares have a reduced shell sea slugs have no shell

Nudibranchs

Ecology Molluscs have many different means of defense many nudibranchs that feed on cnidarians collect and store the undischarged nematocysts in cnidosacs they then can use these nematocysts for their own protection

Class Gastropoda Subclass Opisthobranchia sea hares usually crawl along the bottom but many can also swim by using their modified mantle as “wings”

Class Gastropoda Subclass Opisthobranchia all are monoecious sea hares often form “mating chains” acts as male only act as both males and females simultaneously acts as female only

Class Gastropoda Subclass Opisthobranchia sea slugs often lay “ribbons” of eggs which stick to the substrate

A Sea Hare Anaspidean

A Sea Hare Anaspidean, Aplysia dactylomela

Aplysia gigantea

Internal Shell of Sea Hare

Sea Hares Aplysia dactylomela