What are Mollusc : Biology Blog

                          Phylum Mollusca (clade) – mollusks

•  very successful phylum: over 93,000 named, living species
• The largest number of living species for animals after Arthropoda
•  mostly aquatic, but over 35,000 terrestrial species (more than all terrestrial vertebrates)
•  includes snails, slugs, clams, oysters, chitons, cuttlefish, octopi, etc.
•  from microscopic to 21 m (giant squid – largest invertebrate)
•  economically important (food, ornamentation, currency, pests) 
  

Defining Characteristics Of Mollusks:

 Bilateral symmetry with true coelom; usually have a differentiated head unsegmented (some argument as to whether lost during evolution or never present in phylum)

 Three main body parts: muscular foot, visceral mass, and mantle muscular foot – used for locomotion, food capture, attachment · may be solid or have tentacles or maybe a “wing” or fin in pelagic forms· covered with soft epithelium and · may secrete mucus to glide on 



 Visceral Mass – contains most of the internal organs, including these systems:

a. Digestive system – more complicated than in phyla to this point (true coelom)

· complete (separate mouth and anus)

· mouth, stomach, intestine

· radula – rasping “tongue” with rows of microscopic, chitinous “teeth” used to scrape or drill for food



b. Excretory system – simplest animals with the efficient excretory system

· nephridia – tubular structures that gather wastes from the coelom

· walls of nephridia reabsorb useful compounds (sugars, salts, water, etc.)

· ability to reabsorb is why the system is considered efficient

· wastes discharged into mantle cavity; gills pump wastes out of animal

c. Circulatory system

· some have a closed system, most have an open system

· if open, coelom is generally confined to a small area around the heart

· heart may have three chambers

· some fast cephalopods have auxiliary hearts to speed blood movement


d. Reproductive System And Life Cycle

· Most have separate sexes; however, many snails are hermaphrodites

· Trochophore – ciliated free-swimming larva of many marine mollusks

· Veliger – the second stage in the development of most marine snails and bivalves; has beginnings of foot and mantle

 mantle -

1. Folds (often 2) from the dorsal body wall

2. Enclose a cavity between the mantle and visceral mass (the mantle cavity)

3. Mantle cavity - may act as lung or enclose gills

· Gills are filamentous projections of mantle

· Gills are rich in blood vessels

· Gills have a large surface area for gas exchange

· Continuous pumping keeps the flow of water into and out of the mantle cavity

· May have siphons to move water in (incurrent) and out (excurrent) of the mantle cavity

· Bring in oxygenated water and send out deoxygenated water

· May be used to bringing in food (in bivalves)

· May be used for jet propulsion (in cephalopods)



Shell (if present) – Secreted by mantle, usually on the outside, but on inside in some snails, squid, and cuttlefish,  and lost in slugs, nudibranchs, and octopus

· provides protection
· one or two valves (shells)

· outer layer - horny protein layer; protects inner layers from eroding

· middle layer - densely packed crystals of calcium carbonate (calcite or aragonite)

· inner layer - pearly - increases in thickness throughout the life of mollusk

· pearls form between the inner layer and mantle in bivalves


Mollusk classification


a. Class Polyplacophora (clade) – chitons

•  ~1000 living species
•  marine; shell is segmented with 8 overlappings, calcareous plates
•  body beneath the shell is not segmented
•  the head is greatly reduced
•  foot used for locomotion and for holding onto substrates 

b. Class Gastropods (clade) – snails and slugs

•  The name means “stomach foot”
•  ~70,000 living species
•  mostly marine, but abundant in freshwater, and many are terrestrial
•  most have a shell
• · single shell, if present
• · operculum found in most marine gastropods – a horny plate that forms a covering “door” when the snail withdraws into its shell
• · coiling (spiral twisting) of the shell due to one side of larva growing faster than the other side
• · shell has been lost in the course of evolution for slugs and nudibranchs (sea slugs) 

 c. Class Bivalvia (clade) – the bivalves

•  Includes clams, oysters, scallops, and mussels
•  ~20,000 living species
•  2 shells (valves) hinged together
•  strong adductor muscles used to pull shells together typically with siphons - most are filter feeders (food trapped on mucus on gills) 

•  complex folded, ciliated gills
•  no distinct head or radula
•  most have a strong muscular foot (many different adaptations) 

•  some can move by clapping their shells together (scallops); however, most bivalves are sessile for most of their adult lives, tethered to some substrate by strong threads they secrete or using their foot as an anchor
•  scallops have many eyes on soft body parts next to the edges of their shells 

•  freshwater mussels the most endangered group in North America, and Alabama has the highest diversity of freshwater mussels in the world 

d. Class Cephalopoda (clade) – the cephalopods (“head foot”)

•  includes octopus, squids, cuttlefish, and nautilus
•  ~800 living species
•  0-1 shells, internal or external octopi – no shell
• squid and cuttlefish – internal shell remnant used for support nautilus – chambered shell, superficially resembling snails
•  highly developed nervous system giant axons –great for research
• elaborate eyes (some up to 40 cm across – largest known eyes) most intelligent invertebrates; complex behavior
• many are skillful hunters
•  foot modified into grasping tentacles with suckers
•  built for speed – jet propulsion using siphons (they are fast-moving predators)
•  most have a closed circulatory system (only mollusks with this)
•  strong beak for biting; radula used to pull prey in squid and octopi can release a dark “ink” to cloud water for escaping predators or even for trapping prey
•  cuttlefish are famous for changing color to match the background or for messaging, using chromatophore pigment pouches; most octopi and squid can do this as well.  For example Oyster, snail, squid, devilfish 

EX: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devilfish), Aplysia (Seahare), Dentalium (Tusk shell) and Chaetopleura (Chiton)