FUNCTIONAL-ORGANIZATION OF INTRINSIC GILL MUSCLES IN ZEBRA MUSSELS, DREISSENA-POLYMORPHA (MOLLUSCA, BIVALVIA), AND RESPONSE TO TRANSMITTERSIN-VITRO

Lamellibranch gills are used for various vital functions ranging from food capture to ion regulation. The foundation for many of these funct ions is the transport of water through the gill. Few studies have exam ined the role of intrinsic gill muscles and their ability to control w ater flow by altering the dimensions of the water passageways of the g ill. In this report we examine the organization of intrinsic gill musc les and associated connective tissue in zebra mussels, Dreissena polym orpha. Two sets of muscles lie within the hemocoel of the gill and are bathed with hemolymph. In those muscles associated with the connectiv e tissue sheets (which underlie external and internal gill epithelia), the orientation of the muscle fibers is consistent with a role of reg ulating the external and internal ostial area. Those muscles encased i n connective tissue bands and oriented perpendicular to the bases of g ill filaments reduce interfilament distance in the gill; these muscles are antagonized by dense connective tissue cross-struts that push the gill filaments apart when the muscles relax. The two sets of muscles are oriented to serve complementary functions in reducing ostial openi ngs, thus affecting water flow through the gill. The muscles of the gi ll respond to application of exogenous acetylcholine, FMRFamide, and s erotonin. The gill contracts in response to acetylcholine and FMRFamid e but relaxes with serotonin application. Microscopic observation of l ive gill tissues reveals complex and dynamic changes in gill dimension s consistent with a role in regulating water flow.