Page 4 - The Freshwater Mussels of Oklahoma

ANATOMY AND PHYSIOLOGY

Many of us are at first attracted to mussels by the beauty of the nacre and in some
cases the external color, patterns or sculpture. Most people react to the internal parts of
the same animal much as they would something that had been coughed and left on the
sidewalk. And, you must admit, it is a little strange to work with an animal that has its
heart wrapped around its rectum, carries its babies in its gills and buries itself in the mud
with its head down! The following is a short summary of the anatomy and physiology of
the freshwater mussels.

The Shell

Although most classification of the unionids is based upon anatomy of soft parts,
the shell is the most commonly encountered structure. Either the shell is all you have, or
the animal is alive and you may not be inclined to sacrifice the animal to look at the
internal structures. Therefore, for live material you may only have external shell
structures to go by.
The structure responsible for growth and maintenance of the shell is the mantle.
The mantle is a thin membranous structure that completely encloses the other soft
structures and is interposed between the soft anatomy and the shell. It is attached to the
shell along the periphery and contains specialized cells that secrete the shell material.
There are basically three recognizable layers in the shell, the outer periostracum (=
epidermis), the middle prismatic layer (calcite) and the innermost nacreous layer
(calcite and/or aragonite). The periphery of the mantle is responsible for the shell
growing larger in external surface area (height and length). The entire mantle
continuously adds layers of shell on the inside surface and is responsible for adding to the
thickness of the shell.
The periostracum is a proteinaceous material that is tough, impermeable and
resistant to acids and alkaline reactions. It provides some mechanical protection for the
shell but most importantly, in areas of low pH, it protects the shell from being slowly
dissolved in the acidic environment. Many mollusks in southeastern Oklahoma, where
acidic conditions prevail, may have difficulty producing shell material faster than it is
being eroded away. This is particularly evident in areas with rock substrates where the
periostracum becomes abraded or damage, exposing the calcium carbonate materials of
the prismatic and nacreous layers to the action of acids. The periostracum also adds some
color and texture to the external shell. It may be black, brown, yellow or even green.
Usually the younger specimens show the best color and patterns and they become
progressively darker with age. Color patterns are present in some species and may
consist of rays, spots, blotches, chevrons, etc. The surface texture also varies and can be
very rough, ragged and flaky or extremely smooth and shiny. The bulk of the shell is
composed of the prismatic and nacreous layers. These are both calcium carbonate
materials and can easily be eroded. The inner nacreous layer is also called mother-of-
pearl and usually gives the interior parts of the shell a luster or iridescence. The nacre
may vary in color from white, bluish, purple, pink or salmon. The function of color in
the nacre is unknown since it is not visible until the mollusk dies and opens.

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