The spermatheca in the land snail, Arianta arbustorum (Pulmonata : Stylommatophora): muscle system and potential role in sexual selection

Previous studies have shown variable patterns of paternity after multiple m ating, and also variation in sperm storage among individuals of Arianta arb ustorum, which suggests that the spermatheca may influence paternity in thi s promiscuous land snail. To identify possible morphological correlates of sperm manipulation, we investigated arrangement and ultrastructure of the m uscles of the spermatheca. The musculature surrounding the 2-9 spermathecal tubules is arranged in a complex three dimensional network. In addition, e ach tubule has a thin sheath in which longitudinally oriented cells make up the innermost layer. Usually, the smooth muscle cells are enclosed by conn ective tissue. Only occasionally is direct muscle-muscle contact establishe d through dense plaques. The short thick filaments, their small diameter, t he relatively weak development of the tubular system and sarcoplasmic retic ulum, and the low density of mitochondria indicate that the muscle cells co ntract relatively fast but with little strength, that they recover slowly, and have low endurance. A single muscle cell may be innervated by several a xons and one axon may contact several muscle cells. Combining evidence of t he present paper and a foregoing investigation on the spermathecal epitheli um, we suggest that the main function of the spermathecal muscles is to exp el sperm stored for fertilization, while the ciliation of the common duct i s probably responsible for the distribution of sperm among the tubules.