Trochospongilla is a single spongillid genus found in the open waters of Lake Baikal. A detailed morphological description of Trochospongilla sp. is presented along with information on their distribution in the biotope and on their reproductive cycles. The vertical distribution of Trochospongilla sp. is inhomogeneous and probably depends on the properties of the substrate. It has been found out that Baikalian Trochospongilla sp. do not die during the winter. Instead, they retain the ability to persist vegetatively in the open littoral during the entire year. Our examination of the tissues of the sponges revealed male and female gametes, as well as embryos and larvae. These observations suggest that Baikalian Trochospongilla sp. are able to reproduce sexually in the open waters of Lake Baikal. The life cycle of spongillids in the open Baikal habitat differs significantly from that of common Siberian spongillids. Rather, the life cycles of the open-water Baikal spongillids resemble the life cycles of members of the family Lubomirskiidae. These differences reflect evident changes in the biology and life cycles of Baikal Trochospongilla ancestors that occurred during colonisation of Lake Baikal.