1. Cephalopods, Gastropods, Bivalves and their Relatives
Phylum Mollusca
Cephalopods, Gastropods, Bivalves
and their Relatives

2. Characteristics of the Group
Most numerous phylum in the ocean by # of species (200,000)
Soft bodied with a calcium carbonate shell
Bilaterally symmetrical but not always
Gas exchange through paired gills
Ubiquitous in marine environments
Unsegmented
Have a head, foot, and body in some arrangement

3. Unique Structures
Mantle – thin layer of tissue that secretes the shell.
Foot – ventral, muscular organ that is used in locomotion (sometimes modified into tentacles)
Radula – ribbon of small teeth made of chitin that are used in feeding

4. Mollusc Body Plan Gonad Intestine Shell Radula Stomach Heart
Digestive Gland
Mantle
Gill
Foot
Nervous System

5. Gastropods Means “stomach footed”
Most numerous and varied group of molluscs
Include snails, limpets, abalones, and nudibranchs

6. Characteristics of Gastropods
“Coiled mass of vital organs surrounded by a dorsal shell”
Have a ventral foot
With their radula they inhabit most all feeding niches – carnivores, detritivores, herbivores
Shells can be very elaborate, may be internal, or could be missing entirely.

7. Gastropod Shells

8. Parts of a Gastropod Shell

9. Nudibranchs Sea slugs Name means “naked gill”
Have lost the shell entirely
Usually brightly colored
Can keep and incorporate noxious chemicals and undischarged nematocysts from prey for defense.

10. Bivalves Have a two valved hinged shell. Body is laterally compressed.
No head or radula
Gills are used to obtain oxygen and filter food
Mantle surrounds body.
Strong muscles are used to close the valves of the shell

11. Clams Burrow in sand and mud
Use siphons to move water in and out –permits eating and respiring while buried.
Some contain iridescent chemicals in their tissues

12. Mussels & Oysters
Instead of burrowing, they secrete byssal threads that attach them to rocks and other surfaces.
Oysters cement themselves to the substrate.
Some bivalves like shipworms bore into wood. The wood is digested by bacteria in the gut of the organism.
Oysters attach to substrate by their left shell.
Pearl oysters –thin layer of calcium carbonate coat irritating particles or parasites lodged between the mantle and the inner shell (mother of pearl).
Ginat clams – 1 m
Common source of food

13. Oyster Anatomy

14. Cephalopods Means “head-footed” About 650 species
Body such that head is between “feet” and body.
Includes octopuses, squid, cuttlefishes, and the chambered Nautilus
Giant Pacific Octopus

15. Cephalopod Characteristics
Agile swimmers
Complex nervous system
Shell is reduced or lost entirely
Foot is modified into tentacles usually equipped with suckers
Eyes are well developed
Move by forcing water out of the siphon, a flexible, funnel-shaped tube on the side of the head.

16. Octopuses Eight arms and no shell Bottom dwellers 5cm – 9m
Predators on crabs, lobster, shrimp, small fish, and other molluscs
Have a hard “beak”
Some like the blue-ring secrete a highly toxic venom in their bite
Can emit a dark ink to confuse attackers
From Greekoktopous meaning “eight-footed”
Blue ringed octopus

17. Squid Elongate body Mantle contains two triangular fins.
Eights arms + two retractable tentacles
Shell reduced to a chitinous pen inside the mantle
Few cm to 12 m

18. Cuttlefishes
Resemble squid but with a flattened body and a rippling fin surrounding the mantle.
Have a calcified internal shell or “cuttlebone.”

19. Cuttlefish vs. Squid

20. The Chambered Nautilus
Have a smooth coiled shell up to 25 cm in diameter.
Chambers in the shell are filled with gas to provide buoyancy
Has short, suckerless tentacles for capturing fish
Living fossil
From Greek nautilos meaning sailor because it was once thought to use its shell like a sail.

21. Other Molluscs Chitons (Polyplacophora) Tusk shells (Scaphopoda)
Monoplacophorans

22. Molluscan Digestion Separate mouth and anus
Have salivary and digestive glands
Use their radula to scrape, drill, cut, or capture prey.
Digestion is extracellular and intracellular.
Bivalves have a crystalline style in their gut that secretes enzymes.
Some can even retain ingested chloroplasts that continue to photosynthesize and produce food for the mollusc

23. Mollusc Circulation Open circulatory system
Muscular heart pumps blood to all tissues.
Cephalopods – closed circulatory system
Blood contained in vessels
More efficient – meets the demands of large organs such as the brain.

24. Mollusc Nervous System
Gastropods and bivalves have ganglia.
Cephalopods have a large fused brain that is divided into regions that control particular functions and behaviors.
Octopus and cuttlefish can learn.
Sophisticated color changes are believed to be coordinated by the nervous system.

25. Mollusc Reproduction
Some have external fertilization (tusk shells, bivalves, chitons, & some gastropods)
Others have internal fertilization - a spermatophore, or sperm packet is deposited into the female via a modified arm (cephalopods) or a penis (gastropods).
Most have separate sexes, but some are hermaphrodites

26. Mollusc Reproduction continued…
Some create a trocophore larvae which develops into a veliger (a planktonic larva with a tiny shell)
Cephalopods lay eggs and the young develop from the yolk. Female octopus usually die after laying and guarding eggs – most likely due to starvation.