Introduction to Molluscs Biology 11 Mrs. Trevelyan

Mollusca 1. PRESCRIBED LEARNING OUTCOMES and VOCABULARY 2. VIDEOS: -None  3. TEXTBOOK PGS Section 26.1 pgs 692-697

Examples of Members:

Overview…

Four Classes of Mollusca

Gastropods (“stomach foot”) are shell-less or single-shelled mollusks that move by using a muscular foot located on the ventral side.  many have a single shell that protects their bodies. when threatened, they can pull completely into their shells. Include: pond snails land slugs sea butterflies sea hares limpets nudibranchs

A close up of a snail…

And let’s not forget the nudibranchs…

Bivalves (“two shells”) Bivalves have two shells that are held together by one or two powerful muscles.  Common bivalves include: clams oysters mussels scallops

Cephalopods (“head foot”) Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into tentacles or arms

Polyplacophora (“many plates”) Includes chitons Have 8 articulated plates or valves Live mostly in rocky intertidal zone Eat algae

1. Unifying Characteristics include snails, slugs, clams, squids, and octopi Coelomate, have cephalization, symmetry are soft-bodied animals with developed organ systems usually have an internal or external shell. gills for respiration Most have separate sexes Many aquatic mollusks have a free-swimming larval stage called a trochophore. can be herbivores, carnivores, filter feeders, detritivores, or parasites.

2. Structure The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass (organs) The muscular foot takes many forms: - flat structures for crawling -spade-shaped structures for burrowing -tentacles for capturing prey

A clam dissection…

3. Feeding and Digestion Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites. Snails and slugs feed using a flexible, tongue-shaped structure known as a radula. Hundreds of tiny teeth are attached to the radula. The radula is used to scrape algae off rocks or to eat the soft tissues of plants.

3. Feeding and Digestion Clams are filter feeders The water flows over the gills and leaves by the excurrent siphon. Filtered out phytoplankton then goes to the stomach to be digested Excurrent siphon Incurrent siphon

4. Respiration Aquatic mollusks breathe using gills inside their mantle cavity. The water flows over the gills and leaves by the excurrent siphon. Terrestrial mollusks breath using their mantle cavity (ex snails) and diffusion across moist skin Gills

5. Circulation All have 1 or more hearts Have a circulatory system: opened or closed In an open circulatory system, blood is pumped through vessels by a simple heart. This works well for slow moving organism, as it is not very efficient Ex snails, clams Heart

5. Circulation Faster moving mollusks, such as octopuses, use a CLOSED CIRCULATORY SYSTEM- blood is in blood vessels More efficient

Circulatory System of an Octopus

Excretion Nephridia remove liquid waste (ammonia) from the blood and release it outside the body. Nephridium

5. Nervous System Cephalization with a developed brain in octopuses and squids Bivalves (ex clams) do not have cephalization and do not have brains! All have taste, touch, smell, balance Some members have well developed eyes

6. Reproduction Separate male and female Many aquatic mollusks have a free-swimming larval stage called a trochophore. The trochophore larva is also characteristic of annelids, indicating that these two groups may be closely related.

6. Reproduction Some release sperm and egg directly into the water Some use internal fertilization

These are not reproductive organs…they are siphons for breathing

Barnacles hold the record!

7. Ecological Importance Food source for many animals Cuttlebone used for bird health Jewellery and decorations from shells Albalone Pearls Destructive- molluscs etc can destroy docks