Clypeatula cooperensis gen. n., sp. n., a new freshwater sponge (Porifera, Spongillidae) from the Rocky Mountains of Montana, USA.

A new genus and species of freshwater sponge, Clypeatula cooperensis, collected from three lakes in the Northern Rocky Mountains of Montana, USA, are described. The sponge grows as a hard, disc-shaped encrustation on the undersides of rocks and logs. It lacks microscleres and has amphioxeal megascleres that often show a slight midregion bulb and are usually covered with short, conical spines except at their tips. The sponge is also non-gemmulating, overwintering in a regressed state in which choanocyte chambers are reduced in number. Phylogenetic analyses of complete 18S rDNA sequences of C. cooperensis, Ephydatia muelleri, Spongilla lacustris and Eunapius fragilis suggest that C. cooperensis is more closely related to Ephydatia muelleri than to Spongilla lacustris or Eunapius fragilis. Our data, nonetheless, do not rule out the possibility that C. cooperensis is more closely related to the non-gemmulating sponges of Lake Baikal (Russia) than it is to Ephydatia muelleri. These phylogenetic analyses support the erection of a new genus, the monophyly of freshwater sponges belonging to the families Spongillidae and Lubomirskiidae, and the monophyly of demosponges.