Page 7 - The Freshwater Mussels of Oklahoma

Muscles

The principle organized muscles in the unionids are those associated with the
muscle scars of the shell previously discussed. The adductor muscles extend directly
from shell to shell and are responsible for holding the valves tightly shut. The retractor
muscles pull the soft parts into the shell so it can close. The protractor muscles help to
extend the soft “foot” out of the shell. Muscle fibers are also present in the mantle
margins. These along with the dorsal muscles, help to pull the mantle within the shell for
closure. Additional muscle fibers in the mantle margin may control movement of the
mantle flaps during reproductive processes and control of the siphons used in feeding and
respiration.
The foot is also highly muscular and can be extended for some distance out of the
shell, between the valves. In many cases it acts as an anchor in the substrate. If the
animal needs to move, it can be used for locomotion in conjunction with the protractor
and retractor muscles.

Digestive System

The digestive tract of a freshwater mussel is relatively simple. Its structure allows
filtering of small matter (such as microorganisms and organic debris) from the water.
Water is brought into the mantle cavity through the incurrent siphon at the posterior end
of the body. The posterior end usually protrudes into the water column whereas the
anterior end is normally imbedded in the substrate. The epidermal surfaces on the inner
mantle and surface of the gills are ciliated and produce sticky mucous. Particles stick to
the mucous and are swept by ciliary action toward the mouth located between the labial
palps at the anterior end of the body. The mucous, along with food particles, passes
through the short esophagus and into the stomach. Digestive enzymes are added from the
surrounding digestive glands. The elongate intestine then loops down through the gonads
and into the foot region. It then returns to the dorsal region of the body and extends
posteriorly surrounded by the heart, a rectum then passes over the posterior adductor
muscle and exits at the anus in the region of the excurrent siphon.

Respiratory System

The gills are the principle respiratory organs. These consist of two flaps located
on either side of the body. The outer gills are adjacent to the mantle and the inner gills
are adjacent to the foot. The gills receive water coming into the mantle cavity and water
passes through tiny ostia by way of ciliary action and enters the water tubes. From here
the water moves up the tubes and into the suprabranchial chamber and ultimately out
of the excurrent siphon. Gas exchange occurs in the gills. The gills also function in
feeding (see section on the digestive system) and in reproduction (see section on the
reproductive system).

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