Zoogeography and speciation in the Salpidae (Tunicata, Thaliacea)

This paper represents a general study of two major phenomena in the taxonomy of the lower taxa of the Salpidae and their bearing on theories about the origin of the recent species. Arguments are presented that point strongly to a specific status for closely related forms previously described as infraspecific by most authors. Latitudinalclinal variation, reported for the first time in salps in a previous paper of the present author, is now demonstrated to be a uniform trend in all species with a wide distribution into temperate waters. Some indications are found that support the assumption that the clinal forms represent separate populations with a small degree of genetic difference. Thalia democratica (Forskål, 1775) is divided into two formae, forma democratica, distributed over the whole of the Atlantic and the subtropical and temperate parts of the Indo-Pacific, and forma indopacifica nov., confined to the tropical parts of the Indo-Pacific. On the basis of the species distribution and the occurrence of clinal variation the surface waters of the world oceans are divided into 11 major biogeographic regions, largely conforming to similar divisions made on the basis of other holoplanktonic groups. Finally postulations are made about the most likely mode of speciation of the lower taxa in the Salpidae. Climatic deterioration and lowering of the sea surface temperatures during glaciations of the past four million years are thought to have caused isolation of salp populations in the Atlantic Ocean. This isolation presumably resulted in the present species diversity.