Krausz Marine Biology
Ch. 7: Marine Animals without a Backbone
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7.5 Molluscs: The Successful Soft Body
A.
1.
Phylum Mollusca
200,000 species More species in ocean than of any other animal group
Soft body enclosed by calcium carbonate shell
Body covered by mantle
Bilateral symmetry
Ventral muscular foot used for locomotion
Head that includes eyes & other sensory organs
Some have radula-ribbon of small teeth used to rasp food from surfaces
Radula made of chitin
Paired gills
Potion of body maybe coiled and asymmetrical
Occupy all marine environments-rocky shores to hydrothermal vents
Types of Molluscs
Gastropods (class Gastropoda) = stomach footed
o Snails, limpets, abalones, and nudibranchs
o Coiled mass of vital organs enclosed by a dorsal shell resting on a foot
o Snails, limpets, & abalone use radula to scrap algae
o Mud snails are deposit feeders
o Whelks, oyster drills, and cone shells are carnivores that prey on clams, oysters, worms, & small
fishes
o Violet snail has thin shell & produces a bubble raft out of mucus to float
o Sea hares whose shells are small, thin & buried in tissue graze on seaweeds
o Nudibranchs (sea slugs)
 No shell
 Have colorful branches of the gut & gills
 Prey on sponges, hydroids, & other invertebrates
 Produce noxious chemicals or retain undischarged nematocysts taken from undigested
from their prey
Bivalves (class Bivalvia)
o Clams, mussels, & oysters
o Body laterally compressed & enclosed with two parts (valves)
o No head or radula
o Gills expanded & folded used to obtain oxygen, filter & sort small food particles from water
o Inner surface of shell lined w/mantel so whole body lies in mantle cavity
o Strong muscles used to close valves
o Clams burrow in sand or mud & water drawn in and out of mantle by siphons
 Siphons allow clam to feed & obtain oxygen while buried
o Mussels secret byssal threads that attach them to rocks & other surfaces
o Oysters cement their left shell to hard surface & other to another oyster
 Pearls are formed when oyster secretes shiny layers of calcium carbonate to coat irritating
particles or parasites lodged between the mantle and the iridescent inner surface of the
shell
 Cultured pearls are obtain by carefully inserting a tiny bit of shell or plastic in the mantle
o Some scallops can swim by rapidly ejecting water from mantle cavity
o Giant clam can grow 3 ft
o Bore in coral, rock, or wood
o Shipworm bores in mangroves, driftwood, & pilings
 Have symbiotic w/bacteria in the gut that digest the wood
 Valves lie at the inner end of tunnel lined w/calcium carbonate & tiny siphon protrudes
from entrance at other end
 Fouling organism=settles on bottoms of boats, pilings, & other submerged structures
2.
3.
Cephalopods (head-footed)
o Predators specialized in locomotion
o Octopuses, squids, cuttlefishes
o Agile swimmers w/complex nervous system and reduction or loss of shell
o All marine
o Head toward foot that is modified into arms and tentacles
o Tentacles usually have suckers
o Large eyes on side of head
o Octopuses and squid have thick muscular mantle protects head
o Water enters mantel and leaves through siphon (funnel)=jet propulsion
o Octopuses
 Eight long arms 2 in to 30 ft
 Bottom dwellers live in crevices of rocks, discarded bottles and cans
 Eat crabs, shrimp, and lobster using beak-like jaws to bite prey
 Have radula to rasp flesh and secrete paralyzing substance and some have toxic bite
 Distract predators with dark fluid produced by ink sac
o Squids better swimmers than octopuses
 Two triangular fins on mantle
 Can remain motionless or move forward and back by adjusting direction of siphon
 Eight arms & two tentacles all w/suckers
 Has a pen embedded in upper surface of mantle
 Adult size varies few cm to colossal squid (largest juvenile female 20 ft) to giant squid
(60 ft)
o Cuttlefishes
 Eight arms & two tentacles, but body flatten w/fin running along the body
 Have calcified internal shell aids in buoyancy
 Shell sold as source of calcium for caged birds
o Chambered nautilus
 Coiled external shell containing gas filled chambers serves as buoyancy organ
 Has suckerless tentacles
4. Other Mulluscs
o Chitons (class Polyplacophora)
 All marine
 Eight overlapping shell plates cover slightly arched dorsal surface
 Live in shallow hard bottoms
 Have radula to rasp on algae
 Return to home site after feeding
o Tusk shells or scaphopods (class Scaphopoda)
 Elongated shell tapered at end resembling elephant tusk
 Sandy muddy bottoms in deep water
 Narrow tapered end of shell protrudes from bottom while foot projects from wide end
 Many species have thin tentacles with adhesive tips to capture foraminiferans, young
bivalves, & other small organisms
B. Biology of Molluscs
1. Feeding and Digestion
o Separate gut and anus
o Salivary and digestive glands that release digestive enzymes to break down food
o Grazers like chitons, limpets, and snails have radula to rasp minute algae from surfaces or cuts
through seaweeds
 Partly extracellular in gut cavity and intracellular in digestive glands
 Some shell less feed on seaweed keeping chloroplast in digestive gland where chloroplast
photosynthesize and provide nourishment
o Carnivorous snails
 Have radula modified to drill, cut, & capture prey
 Radula & mouth in proboscis that can protrude to strike
 Digestion extracellular in gut
o
2.
-
3.
Bivalves filter & sorted by cilia on gills
 Food enters mouth trapped with strings of mucus
 Enzyme secreting rod in stomach (crystalline style) continually rotates help digest food
 Contents eventually enter digestive gland for intracellular digestion
o Cephalopods
 Carnivores digest large prey
 Stomach sometimes connected to a sac
 All extracellular
o Most have open circulatory system w/dorsal muscular heart
o Cephalopods have closed circulatory system
Nervous System
Gastropods and bivalves have ganglia (local brains) located in different parts of body
Cephalopods
o Most complex of invertebrates
o Some separate local brains fused into single large brain that coordinates and stores information
o Behaviors controlled by particular area of brain
o Giant nerve fibers allow for capturing prey and rapid movement
o Complex eye reflects development
o Octopus and cuttlefish can learn
o Display rapid color change coordinated with particular behaviors and moods, from sexual display
to camouflage
o Cuttlefish flash two large black spots to fool predators
o Octopuses change color and behavior to mimic poisonous fish and sea snakes
Reproduction and Life History
o Most have separate sexes but some hermaphrodites
o Bivalves chitons, tusk shells & some gastropods sperm & eggs released into water and fertilization
is external
o Fertilization internal in cephalopods & most gastropods
o Cephalopods male use modified arm to transfer spermatophore to female
o Males of gastropods copulate have long, flexible penis
o Some have trochonphore larva evidence use to show close relationship with segmented worms
o Bivalve trochonophore develop into veliger (larvae w/ tiny shell)
o Cephalopods develop from yolk-filled eggs attached to crevices or holes in rocks
o Female usually dies after because does not eat while guarding the eggs