## LSU Historical Dissertations and Theses

1998

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

Harold Silverman

#### Abstract

The lamellibranch gill is used for various vital functions ranging from food capture to ion regulation. The foundation for many of these functions is the transport of water through the gill. This dissertation examines the organization of intrinsic gill muscles and the associated connective tissue in the freshwater bivalve Dreissena polymorpha with comparisons made to Corbicula fluminea and Toxolasma texasensis. Gill muscles can be divided into two groups: those associated with the connective tissue sheets that underlie external and internal gill epithelia; and those encased in connective tissue bands oriented perpendicular to the bases of gill filaments. The sets of muscles are oriented to serve complementary functions of drawing filaments together and of reducing ostial openings, thus affecting water flow through the gill. The supportive tissue in the gills of each of the three species is composed of a similar extracellular matrix with interspersed muscle fibers. These matrices consist of a periodic acid Schiff-positive tissue supported by small collagen fibrils as determined by morphological and biochemical examination. The ultrastructure of the muscle fibers associated with the connective tissue corresponds with a known type of smooth muscle. The gill muscles of D. polymorpha contract in response to acetylcholine and FMRFamide but relax with serotonin application. External calcium is required for muscle contraction and a proper balance between NaCl and KCl is critical for the maintenance of maximal responsiveness. Acclimation to hyperosmotic conditions is dependent in part on the activity of a ouabain-sensitive $\rm Na\sp{+}/K\sp{+}$ ATPase. The gills of all three species show a common relationship between live gill area and dry body mass. Each of the animals have comparable ostial dimensions and possess the ability to control these dimensions through muscular tone. The similarity in structure and apparent function of the intrinsic gill muscles in each of the species examined suggests that the muscles are an important, conserved feature of the bivalve gill.

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