Presentation on theme: "Phylum Mollusca Cephalopods, Gastropods, Bivalves and their Relatives."— Presentation transcript:
Phylum Mollusca Cephalopods, Gastropods, Bivalves and their Relatives
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
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
Mollusc Body Plan Gill Mantle Heart Nervous System Gonad Shell Stomach Intestine Digestive Gland Foot Radula
Gastropods Means “stomach footed” Most numerous and varied group of molluscs Include snails, limpets, abalones, and nudibranchs
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.
Parts of a Gastropod Shell
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.
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
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
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.
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
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.
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
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
Cuttlefishes Resemble squid but with a flattened body and a rippling fin surrounding the mantle. Have a calcified internal shell or “cuttlebone.”
Cuttlefish vs. Squid
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
Other Molluscs Chitons (Polyplacophora) Tusk shells (Scaphopoda) Monoplacophorans
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
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.
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.
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).
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.