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

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

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 http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php

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.

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.

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

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

Types of Molluscs Gastropods – Bivalves – Cephalopods – 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.

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

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).

Parts of a Gastropod Shell

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

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.

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

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

Clam Anatomy

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

Oyster Anatomy

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.

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 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.

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”

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 60-90 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.

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

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.

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

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 (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 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

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.