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PHYLUM MOLLUSCA
Cephalopods, Gastropods, Bivalves
and their Relatives
Characteristics of the GroupCharacteristics 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
Other Characteristics of Other Characteristics of MolluscsMolluscs• 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
http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php
Unique StructuresUnique 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
Mollusc Body PlanMollusc Body Plan
Gill
Mantle
Heart
Nervous System
Gonad
ShellStomach
Intestine
Digestive Gland
Foot
Radula
Types of MolluscsTypes of Molluscs• Gastropods –
• Snails, limpets, abalones, nudibranchs, whelks
• Bivalves – • Clams, mussels, oysters
• Cephalopods – • Octopus, squid, cuttlefish
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: http://vimeo.com/37325960•Identify the evolutionary advances in the three types of molluscs.
GastropodsGastropods• 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
Characteristics of GastropodsCharacteristics 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).
Parts of a Gastropod ShellParts of a Gastropod Shell
Gastropod ShellsGastropod ShellsLeft-handed or right-handed?
NudibranchsNudibranchs(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.
BivalvesBivalves
• 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
ClamsClams
• 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
Clam AnatomyClam Anatomy
Mussels & OystersMussels & 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
Oyster Anatomy
Other BivalvesOther 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.
CephalopodsCephalopods
• 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
Cephalopod CharacteristicsCephalopod 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.
OctopusesOctopuses
• 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
SquidSquid
• 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
CuttlefishesCuttlefishes
• Resemble squid but with a flattened body and a rippling fin surrounding the mantle.
• Have a calcified internal shell or “cuttlebone.”
The Chambered NautilusThe 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 60-90 short, suckerless tentacles for capturing fish
• Living fossil
Molluscan DigestionMolluscan 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.
Molluscan CirculationMolluscan 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
Molluscan Nervous SystemMolluscan 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 (http://www.youtube.com/watch?
v=9kuAiuXezIU) .• Sophisticated color changes are believed to be coordinated by the
nervous system. (http://www.youtube.com/watch?v=2x-8v1mxpR0, http://www.youtube.com/watch?v=fR7Dqf0vzzQ )
Mollusc ReproductionMollusc 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.
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.