Abalone

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Feeding
ussels, clams and oysters are all molluscs
belonging to the class Bivalvia, which is so
named because these creatures are protected and
totally enclosed by a pair of shells, one on either
side of the body. Bivalves are relatively sedentary.
Mussels are permanently attachment to rocks by
beard-like byssus threads, and oysters cement one
of their shell valves to the rock face. Many, like the
clams and white mussels, live in sand and have a
l a rge wedge-shaped foot that allows them to burro w.
All bivalves have a reduced head that lacks eyes and
sensory tentacles. Many bivalves are harvested for
food and bait and there are strict regulations
controlling the numbers that can be collected.
Nearly all bivalves have a pair of large gills used for respiration
and filter-feeding. They use hairs on the gills to pump water into
the mantle cavity through an inhalant siphon. The gills trap tiny
particles from the water and move them to the mouth where
the edible particles are eaten, while inedible materials such as
sand grains are discarded along with the waste water that
passes out of an exhalent siphon at the back of the body. A
bed of mussels can remove all the food from the water above
them within an hour. Bivalves are the only molluscs to lack a
radula. Instead they have a thin rod in the gut that is rotated
to grind away the tip, releasing enzymes that digest the food.
Brown mussel is attached to rocks on
the south and east coasts
muscle
cut open
mussel
gonads
gill
C O A S TA L A N D M A R I N E L I F E – A N I M A L S : I N V E RT E B R A T E S – M O L L U S C S
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Reproduction and life cycle
Most bivalves practise external fertilisation and shed enormous
numbers of sperm and eggs into the sea where they develop
into planktonic larvae. The early trochophore larvae are like
miniature spinning tops. They metamorphose into veliger
larvae each with a tiny shell and two large anterior lobes, the
velum, that propel the animal along by flickering hair-like cilia.
The larvae are attracted to, and settle at, sites where adults
occur and are then known as spat. A few species brood their
eggs inside their shells.
Pollution and red tide
The major drawback of the efficient filtering system of bivalves
is that they collect and concentrate pollutants and toxic
organisms such as those associated with 'red tide'. Mussels
retain the poisons from a red tide in their bodies for about 4
months. Although the mussels are not usually harmed, as few
as two mussels contaminated with toxic red tide can prove fatal
if eaten by humans. The Marine and Coastal Management
branch of the Department of Environmental Affairs and To u r i s m
can be contacted to get the latest information about red
tides, which occur almost exclusively along the west coast.
See the bottom of this factsheet for contact details.
Mariculture
Bivalves
Mussels and oysters are farmed on rafts in the sheltered bays
of Saldanha and Knysna. The Mediterranean or blue mussel
is preferred to the local mussels because it is plumper and
faster growing and is ready for harvest in about 9 months.
Although they are fast growing, mussels do not fetch as high
byssus threads
waste water
expelled
White mussel digs
into the sand
water and food
particles
sucked in
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