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 number of species (200,000)
Soft bodied with a calcium carbonate shell
Bilaterally symmetrical but not always
Most have a head region with eyes and other sensory organs

3. Other Characteristics of Molluscs
Unsegmented
Molluscs are coelomates, although the coelom is reduced
Separate mouth and anus; have salivary and digestive glands that release digestive enzymes
Gas exchange through paired gills
Ubiquitous in marine environments; can be found from intertidal zones to the deepest oceans
Have a head, foot, and body in some arrangement

4. A Tube-Within-A-Tube
As organisms become more sophisticated anatomically, the development of a body cavity or coelom [see-luhm] is observed.
The coelom is lined by mesoderm tissue and is essentially an open tube within the organism’s body in which digestive, reproductive and other organs arise.

5. Body Plans Animals can either be Acoelomate – no body cavity
Pseudocoelomate – a body cavity develops between the body wall (ectoderm) and the internal organs (endoderm). Usually filled with fluid.
Coelomate – the body cavity is completely lined with tissue from the mesoderm.

6. 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 (tough carbohydrate) that are used in feeding

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

8. Types of Molluscs Gastropods – Bivalves – Cephalopods –
Snails, limpets, abalones, nudibranchs, whelks
Bivalves –
Clams, mussels, oysters
Cephalopods –
Octopus, squid, cuttlefish

9. Mulluscs: The Survival Game
“The ability to change in the ever-changing world is perhaps the greatest secret in the survival game.”
Watch the video:
Identify the evolutionary advances in the three types of molluscs.

10. Gastropods Means “stomach footed”
Most numerous and varied group of molluscs
(~75, 000)
The only group to invade terrestrial niches
Include snails, limpets, abalones, and nudibranchs

11. Characteristics of Gastropods
“Coiled mass of vital organs surrounded by a dorsal shell”
Have a ventral foot typically used for crawling; may be modified for burrowing, leaping, swimming, or clamping
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 (nudibranchs).

12. Parts of a Gastropod Shell

14. Left-handed or right-handed?
Gastropod Shells
Left-handed or right-handed?

15. Nudibranchs (http://www.youtube.com/watch?v=nHVoV0MVwSc)
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.

16. 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
Many bivalves use their foot for burrowing

17. 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

18. Clam Anatomy

19. Mussels & Oysters
Instead of burrowing, they secrete byssal threads that attach them to rocks and other surfaces.
Oysters cement their left shell to a hard substrate; sometimes other oysters creating oyster reefs
Pearl oysters - thin layer of calcium carbonate coat irritating particles or parasites lodged between the mantle and the inner shell
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

20. Oyster Anatomy

21. Other Bivalves Some bivalve bore into coral, rock, or wood.
The shipworm bore into wood and have symbiotic bacteria in their gut to digest the wood.

22. 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

23. 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
Body protected by thick muscular mantle
Move by forcing water out of the siphon, a flexible, funnel-shaped tube on the side of the head.

24. 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
Blue ringed octopus
From Greekoktopous meaning “eight-footed”

25. 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

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

27. Cuttlefish vs. Squid

28. 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.

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

30. Molluscan Digestion Separate mouth and anus
Have salivary and digestive glands
Most use their radula to scrape, drill, cut, or capture prey.
Bivalves – radula is absent and food particles are ingested and filtered by cilia on the gills; have a crystalline style in their gut that secretes enzymes.

31. Molluscan 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.
Heart

32. Molluscan Nervous System
Gastropods and bivalves have ganglia – cluster of nerve cells
Cephalopods - 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. ( )

33. Mollusc Reproduction
Most have separate sexes, but some are hermaphrodites
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).
Cephalopods lay eggs and the young develop from the yolk. Female octopus usually die after laying and guarding eggs – most likely due to starvation.
Most have separate sexes, but some are hermaphrodites

34. 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.