Muscle system of the penial complex in three species of Hygrophila (Gastropoda: Pulmonata)

The Hygrophila, a group comprising freshwater lung-bearing gastropods, have a complex copulatory apparatus, whose musculature has recently been shown to provide a number of phylogenetically informative characters that appear to concur with the results of molecular phylogenetic analyses. The number of hygrophilan species for which the male copulatory musculature is known, however, is still relatively small. We examined the muscle arrangement in the penial complex (penis and penis sheath) of three species of Hygrophila: Aplexa hypnorum (Linnaeus, 1758) (fam. Physidae), Kolhymorbis bogatovi Zatravkin et Moskvicheva, 1985 (fam. Planorbidae, tribe Segmentinini), and Planorbarius corneus (Linnaeus, 1758) (fam. Planorbidae, tribe Helisomatini) to explore whether the organization of the male copulatory musculature in these species is consistent with the previously proposed phylogenetic hypotheses. In A. hypnorum, the primarily glandular penis sheath has a three-layered musculature with outer and inner circular and intermediate longitudinal layers. The tubular penis also has a three-layered musculature, with the same sequence of muscle fibers as in the penis sheath. In K. bogatovi, the sequence of muscle layers in the penis sheath is essentially the same as in A. hypnorum. Penial musculature is also arranged in three layers, but the intermediate layer consists of small radial, rather than longitudinal fibers and the inner and outer layers are circular. In P. corneus, the boundaries between muscle layers in both penis and penis sheath are somewhat indistinct, but the circular muscles tend to lie close to their outer and inner walls, while the longitudinal fibers are concentrated in the interior. The comparison with other hygrophilan species, in which the copulatory musculature has been previously studied, shows that the male copulatory musculature of K. bogatovi is consistent in arrangement with that of other representatives of Segmentinini and that the phylogenetic lineage leading to P. corneus has probably diverged before the clade Planorbini + Segmentinini. The musculature of the penial complex of A. hypnorum is generally similar to that of another physid, Physella acuta (Draparnaud, 1805), but the penis sheath of P. acuta lacks an outer layer of circular muscle fibers, which may be explained by reduction.