Mussels That Fish

                        Mussels That Fish

by John Schwegman

     Its fishing time for a group of native Illinois mussels or
clams that depend on attracting fish as hosts for their parasitic
young.

     I was reminded of this earlier this summer as I walked quietly
along the shore of the Embarras River in eastern Illinois. A
rapidly vibrating white object just 2 feet from shore in 6 inches
of water caught my eye.

     I knew that our group of mussels called pocketbooks have a
lure that they use to attract fish, but I assumed that this
occurred in deep water far from human eyes. As I eased closer to
the object, I realized that I was seeing a unique natural
phenomenon for the first time.

     The mussel was a plain pocketbook (Lampsilis cardium) which is
found throughout Illinois. In order to display its lure, it was
literally standing on its head with its front end sticking straight
down into the river bottom.

     The "lure" was two parallel flaps of white tissue extending
just over a half inch from the rear of the mussel. It had a black
"eye" spot at one end and a tail-like flap of tissue at the other.
The whole lure resembled an inch and a half long minnow struggling
to swim as the mussel vibrated it with most of the movement in the
tail.

     The lure was being displayed to attract a fish which could
serve as host to the mussel's larvae called glochidia.

     Although visible in detail only under a microscope, the tiny
glochidia of most native mussels resemble clams in having two wing-like valves connected with a hinge that allows them to clamp
together. To develop into an adult mussel, they must clamp on the
fins or gills of a fish and absorb nourishment for two to three
weeks. During this parasite phase, they change into a small calm
which drops to the stream bottom to begin growth into an adult
mussel.

     In addition to the nourishment for development, the ride on
the fish helps distribute the mussels which move very little as
adults.

     The glochidia of the lure-wielding pocketbook mussels are gill
parasites and need to get into the fishes mouth so they can grab
onto a gill as they are exhaled in the fish's breathing process.

     This is where the lure comes into play. After a fish is
attracted by the lure, somehow the mussel senses its presence and
releases or exhales a cloud of larvae that get into the fishes
mouth. Since mussels have no eyes, just how they do this is a
mystery.

     Some think that it has enough light sensitivity to detect the
shadow of a fish over the lure, while others think that it releases
glochidia in response to a strike or hit of the lure by the fish.

     With this in mind, I eased up close to the displaying
pocketbook mussel and passed my hand over it being careful not to
touch it. It immediately stopped moving but resumed displaying
about 20 seconds after my shadow was removed. I saw no evidence of
glochidia being released.

     I then hit the displaying lure lightly with my finger and it
stopped displaying but I still saw no glochidia being released.
Since the water was a little murky and the glochidia are so small,
it is possible that releases were made that I could not see.

     Three of our pocketbook mussels' relatives from the
southeastern United States have taken the lure strategy even
further and actually fish with a lure on a line.

     These species place all their glochidia for a year in a single
elongate packet shaped and colored like a minnow. This packet,
called a superconglutinate by biologists, is attached at the head
end to a transparent mucous strand that remains attached to the
female mussel.

     She can let the strand or line out up to 8 feet in length.
Since these mussels live in areas of relatively swift current, the
lure swings and dances about at the end of the transparent line,
appearing like a live minnow.

     When a fish strikes or takes the lure, it breaks open the
packet releasing the glochidia in its mouth. As these glochidia are
flushed through the fishes gills, many attach to begin their
parasitic stage.

     It is amazing that this intricate behavior and life strategy
has only been discovered and described in the last few years. One
has to ponder what other wonders nature has hidden from us.

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