Phylum Mollusca soft bodied animals Ch 12 ea

Phylum Mollusca “soft bodied” animals Ch 12

ea ida c sta chn cta u Cr Ara Inse Mollusca Protostomes Triploblasts * Diploblasts † Ve Tu ta a c ni Chordata Arthropoda Annelida ata r b rte Hemichordata Echinodermata Deuterostomes Coelomates Pseudocoelomates Acoelomates Bilateria† Mesozoa Radiata* Eumetazoa Metazoa Protozoa Monera (Bacteria) Cnidaria Ctenophora Parazoa

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

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

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

Cross section of a Gastropod head

Mollusca Characteristics Reproduction • 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 trochophore veliger

Mollusca Characteristics Body Plan • 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. shell Visceral mass foot mantle cavity

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

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

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. 2. 3. 4. Development of a head Dorsoventral elongation of the body Shell become complex retreat: coiling 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 Planospiral shell Apex of shell extends out making the shell more compact Conispiral shell Shell shifts over body for better weight distribution

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