Mucket
Butterfly
Plain pocketbook
Giant floater
Fragile papershell
Threeridge
Mapleleaf
Higgins' eye
pearlymussel
Fatmucket
Yellow sandshell
Black sandshell
Pondmussel
Threehorn wartyback
Hickorynut
Pink heelsplitter
Fat pocketbook
Pink papershell
Bleufer
Lilliput
Fawnsfoot
Deertoe
Pistolgrip
Elktoe
Flat floater
Rock pocketbook
White heelsplitter
Purple wartyback
Ebonyshell
Elephantear
Creeper
Paper pondshell
Mucket
Butterfly |
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A mussel is a relatively
simple animal enclosed by two shells connected by a ligament.
The shells are formed
largely out of
calcium carbonate that has been extracted from the waters
where mussels live. The calcium carbonate is then deposited
in successive layers. The hard shell provides some protection
for the inner
animal. Internally,
the body consists of gills for breathing, a digestive tract
for processing food, a large muscular foot for locomotion,
and mantle
tissue that produces the shell.
Freshwater
mussels are extremely variable with respect to life span, coloration,
and size. For example, some species live for only 10 years and others may live
for as long as 100 years. |
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Shell color can range
from yellow to green to brown to black and may contain bumps,
tubercles, and ridges or can remain very smooth. Species range
in size from only one inch to over one foot. Thin-shelled species
like the giant
floater and fragile
papershell grow much faster
than thicker-shelled species like the threeridge and mapleleaf.
Mussels have very colorful common names such as monkeyface, threehorn
wartyback, rough
pigtoe, spike,
and pink
heelsplitter.
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Freshwater
mussels have a unique life cycle. Unlike oysters and clams,
most freshwater mussels
need a fish to complete their life cycle. Some
mussels require a specific host fish to complete their life
cycle; others can use a variety of fish species. For more information on host fish, click here. In the wild, male mussels
release sperm into the water column. The sperm are
drawn into the female as she filters water for food. The fertilized
eggs
reside
within pouches (marsupia) of the modified gills and develop
into larvae termed glochidia--tiny creatures that are parasitic
and
must find a suitable fish host to complete their life cycle.
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Glochidia are triangular,
spherical, or hatch-shaped and range in diameter
from 0.08 to 0.35 millimeters. A single female can carry
from a few thousand to several million glochidia. |
To
help them find a host fish, some species have modified their mantle tissue
(the tissue that lines the inside of the shell) so that it looks
like a minnow.
The female Higgins' eye pearlymussel can undulate this minnow-like
structure so that this “lure” attracts
fish looking for food.
When a fish bites the lure, the female releases the glochidia from her
gills and the glochidia attach to the gills or fins
of the fish.
Other species release their glochidia
in a special package resembling an insect larvae or small fish.
Each package contains thousands of glochidia. When the host fish
bites the “fake” insect
larvae or fish, it ruptures and spills its contents of glochidia
into the fish’s
mouth. Click here to see other mussel displays and lures.
Depending on the species, glochidia remain attached to the
host fish for about 3 days to 10 months (depending primarily on water temperature)
while transforming
into juvenile mussels.
Transformation of glochidia from Higgins'
eye pearlymussels |
Click for larger scale |
Click
for larger scale |
Click for larger scale |
Click for larger scale |
Glochidia (top left)
attach to the gills of host fish (top right) and encyst (bottom
left). In approximately 3 weeks they transform, fall off the
gills and settle to the bottom as juveniles. The juvenile is
approximately 0.75 millimieters long; it is shown next to the
head of a pin (bottom right) for size
comparison. |
Glochidia do not appear to harm the host
fish and when the transformation is complete, they drop off the fish,
land on the
bottom of the river, and begin their life as a free living organism.
The bottom line is that glochidia must attach to the correct host
fish in order for it
to complete its life cycle.
Examples of this intricate dependence of mussels
on
fish are the ebony
shell and elephantear mussels.
Both species rely on the skipjack herring as their sole fish host. The
construction of Lock and Dam
19 on the Upper
Mississippi River at Keokuk, Iowa, blocked the upstream migration of
skipjack herring. As a result, these two mussels are no longer found
upstream of
Lock and Dam 19.
Lock and Dam 19 on the Upper Mississippi River at Keokuk, Iowa |
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Source: Keokuk Office of Tourism |
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