Phylum Molluska www.onacd.ca
3 Main Classes of Mollusks GASTROPODA : includes limpets, snails, slugs and whelks BIVALVIA: includes clams, oysters, muscles, scallops and shipworms CEPHALOPODA: includes octopi, squid, cuttlefish, nautilus and argonauts Above: Limpet Above: Scallop showing off its eyes! Left: Cuttlefish and squid
All forms of Mollusks have a body composed of at least three distinct parts: Illustration of the general features of all mollusks 1. Visceral Mass - The soft bodied portion that contains the internal organs 2. Foot - A strong, muscular portion used for locomotion which is adapted as tentacles in cephalopods 3. Mantle - A membranous, sometimes muscular covering that envelops but does not completely enclose the visceral mass - Can secrete a shell
Identifying Characteristics of Mollusks Have a coelom which allows for specialization of organ systems Most are bilaterally symmetrical All have a visceral mass, foot and mantle (see previous slide) Possess a one way digestive system with specialized organs and have feeding mouthparts Cephalopods have a closed circulatory system, all other classes possess open circulatory systems Many have highly developed cephalization with sensory organs Many have calcium carbonate external shells Respiratory system includes the use of gills to extract O2 and get rid of waste Many have a radula which is a chitinous tongue used for feeding Sexual Reproduction (have separate sexes: dioecious)
The Gastropods : belly foot Underside of a Limpet Usually are univalves (one shell) Large, muscular foot Well defined head Muscles move in a rippling motion for movement Most possess gills but some have lungs Have a radula to help obtain food Most snails have a lid-like part called an operculum on the back of the foot so they can draw their bodies into their shell and close off the opening
Gastropods in Love... and a lonely garden slug.
The Bivalves : two shell Possess 2 shells that GIANT CLAM can be tightly closed with strong muscles Strong, muscular foot and use it for locomotion No head or teeth Obtain food and oxygen by using a muscular siphon that brings water to the gills which filter out O2 and plant cell material Often attach themselves to surfaces Possess an open circulatory system
Above left: Elephant trunk clams, named for their large foot Above right: an opened oyster commonly enjoyed raw or cooked as a delicacy Lower right: Scallop showing off its many primitive eyes
The Cephalopods: head foot Bilaterally symmetrical Prominent head, arms and tentacles Have developed brain intelligent creatures Arms that take in food surround a hard, strong, beak-like jaws that tear the prey and secrete a poison to paralyze it Closed circulatory system with hemocyanin (like hemoglobin) Move by jet propulsion Can change color with the use of chromatophores Possess advanced vision Secrete an ink cloud when in danger
Squid Caribbean Reef Squid Calamari: a deep fried squid dish Tentacles of a giant squid Squid eye and gills
Octopus Octopus beak Swimming Octopus Octopus on the beach Octopus arm in detail
Octopi are extremely intelligent creatures and have been known to learn to escape from their aquariums when in captivity through very small cracks at the top.
Ecological Roles of Mollusks Mollusks are part of the Starfish eating (digesting) a mussel aquatic food chain (see picture to left) Many mollusks are keystone species, if their numbers decrease the decline in other species of marine life will follow Filter feeders help to purify the water Many bivalves are reef forming and provide an ecosystem for other aquatic species Reef builders stabilize banks and bottom sediments
Squid Fact The largest squid ever caught was a 10 m long Colossal Squid off the coast of New Zealand and weighed in at 495 kg. Colossal Squid are larger than the giant squid found off the Mexican coast and are thought to grow in size to a whopping 13 meters!