Formation, transfer, and assimilation of the spermatophore of the hermaphroditic flatworm Dugesia gonocephala (Tricladida, Paludicola)

Abstract

The spermatophore of the simultaneously hermaphroditic planarian Dugesia gonocephala consists of a large stalked bladder that gradually forms inside the ejaculatory duct of the penis between successive copulations. In solitary individuals, spermatophore formation begins with the development in the penis of a tube that is sealed at the distal end and may protrude slightly from the penis tip. In courting individuals, this protruding part is widened into a bladder that further increases in size when penises are mutually inserted. Spermatozoa are pumped into the bladder after the first few minutes of copulation and the resultant spermatophores are exchanged 3-4 1/2 h after the start of copulation. Reciprocal transfer is slightly asynchronous. After the spermatophore is transferred into the bursa copulatrix, the stalk of the spermatophore is usually in or near the entrance to the bursal canal. Sperm start to leave the spermatophore immediately after the partners separate and continue to do so for 3-4 days. During this period, the bladder of the spermatophore shrinks but remains intact. Following spermatophore donation, the penis may hold a new spermatophore after 1-2 days. Individuals that engage in a new copulation rarely do so because they are allosperm depleted. In fact, the bursa may still hold a spermatophore from a previous copulation. We discuss the adaptive significance of spermatophore formation, transfer, and assimilation.