Phylum Mollusca

By: Christa Jewett, Instructor

“For since the creation of the world God’s invisible qualities – His eternal power and divine nature – have been clearly seen, being understood from

what has been made, so that people are without excuse.” Romans 1:20

Taxonomy

• Kingdom Animalia: Animals• Phylum Mollusca: Mollusks

– Class Gastropoda: snails, slugs, limpets, sea hares– Class Bivalvia: scallops, clams, mussels and oysters

• Note: There are a few other classes but we will not be focusing on them during our discussion today.

General Information• There are about 85,000 different species of mollusks.• Mollusks are invertebrates, most of which have an external

skeleton called an exoskeleton.• They are highly diverse and inhabit numerous

environments worldwide including terrestrial, freshwater and marine biomes.

• The three main defining features of mollusks are:– A mantle with a significant cavity used for breathing and

excretion– The presence of a radula– A nervous system that includes two or three nerve chords, a

brain and various sensory systems which may include eyes and mechanisms to detect chemicals, vibrations and touch.

General Information• Mollusks are also known for being brilliant

multi-taskers with many internal organs having dual functions in multiple systems. For example, in bivalves, the gills are responsible for respiration but also for creating a water current in the mantle cavity, important for excretion and reproduction.

• Mollusks represent a commercially important food source for humans.

Class Gastropoda• This group of animals includes snails, slugs, limpets and sea

hares. • The scientific name for this class is derived from two words:

Gastro meaning stomach and Pod meaning foot. This name was given to this class of animals because it appears that they crawl on their bellies.

• There are over 40,000 different species of Gastropods so there are exceptions to the general characteristics of this class. For example, some may not have shells or a developed foot.

• There are also a wide variety of species. Some are voracious predators and some are benign grazers which feed on algae.

Class Gastropoda• The late embryonic stage undergoes a peculiar twisting

of 180 degrees called torsion so that many of the posterior internal organs face forward into the mantle cavity.

• They all have only one shell• Animals have a distinct head and wear their shells on

their backs.• Most have tentacles and eyes.• Most have a radula. • Most have a well-developed foot used for mobility.

Class Gastropoda: External Features

• Whorl • Body Whorl• Inner Lip• Outer Lip• Aperture• Operculum• Anal Canal• Siphonal Canal

Geography Cone Snail

Photo Credit: Kerry Matz National Institute of General Medical Services (http://en.wikipedia.org/wiki/File:Conus-geographicus.jpg)

Cone Snail

Photo Credit: David Burdick (http://en.wikipedia.org/wiki/File:Conus_eating_a_fish.jpg)

Atlantic Moon Snail

Photo Credit: Ryan Somma (http://en.wikipedia.org/wiki/File:Atlantic_Moon_Snail_001.jpg)

Purple Sea Snail

Photo Credit: © 2010 Dan Norton

Nudibranch

Sea Hares

Photo Credit: Genny Anderson (http://en.wikipedia.org/wiki/File:Aplysia_californica.jpg)

Photo Credit: Dr. James P. McVey, NOAA Sea Grant Program

Spanish Dancer

Head

Class Bivalvia• There are over 10,000 different species of Bivalves so

there are exceptions to the general characteristics of this class.

• They are the second most diverse class of mollusks, other than Gastropods.

• The scientific name for this class is derived from two words: bi meaning two and valve meaning half of the shell. All of these animals have two shells joined by a hinge.

• These animals have two shelly halves with a hinge at the top.

• They do not have a head.

Class Bivalvia• Most have a hatchet-shaped foot which they use to bury

themselves in the sediment. They then extend a long siphon up to the surface of the sea floor which is used for feeding and respiration.

• Most bivalves are filter feeders, using large gills to draw water into their bodies and capture small plankton with their gills. Very few species are predatory.

• They widely vary in size with the “monster” Giant Clam that can weigh more than 500 lbs.

• Some are sedentary and live attached to a substrate, some burrow into the sand and some live on the benthos and swim.

• The age of bivalves can be determined by counting annual growth rings on the shell.

Class Bivalvia: External Features

• Beak or umbo• Pallilal line• Lunule• Adductor muscle scars• Taxodont teeth• Cardinal Tooth & Anterior Lateral Tooth• Hinge

Ocean Quahog

Photo Credit: NOAA (http://www.nmfs.noaa.gov/fishwatch/species/ocean_quahog.htm)

Giant Clam

Photo Credit: Drow_male (http://en.wikipedia.org/wiki/File:Tridacna_gigas.001_-_Aquarium_Finisterrae.JPG)

Giant Clam

Photo Credit: Nick Hobgood (http://en.wikipedia.org/wiki/File:Tridacna_crocea.jpg)

Giant Clam

Photo Credit: Nick Hobgood (http://en.wikipedia.org/wiki/File:Tridacna_gigas.jpg)

Life Cycle

• Mollusks reproduce sexually. Some species may be hermaphroditic but they do not self-fertilize.

• Fertilized eggs develop into a larval stage and then settle out onto the sea floor before maturing into adults.

Character of God

How can we see the character of God in these animals?

You tell me!