Morphological and electrophoretic studies of the genus Themisto (Amphipoda: Hyperiidea) from the South and North Atlantic

Summary

The genus Themisto is represented in Atlantic Ocean waters by four different species: T. compressa, T. abyssorum, and T. libellula in the northern hemisphere and T. gaudichaudii in the southern hemisphere. Examination of morphological and electrophoretic characters has revealed that T. gaudichaudii, which was previously thought to have a bipolar distribution, in fact consist of separate species in Arctic and Antarctic waters, respectively. The Antarctic species remains as T. gauchichaudii Guérin 1828 and the oldest available name for the Arctic species is T. compressa Goes 1865. The results of the study also indicate that morphological variation within T. gaudichaudii (e.g. the “compressa” and “bispinosa” forms) is more likely to be the result of environmental influences rather than genetic differences. Electrophoretic analyses of genetic variation indicate that T. gaudichaudii from waters adjacent to both South Georgia Island and the South Shetland Islands belong to the same interbreeding population. Similar observations of homogeneity in other Antarctic pelagic animals suggest that the Antarctic circumpolar current systems promote effective genetic exchange and reduce the possibility of formation of distinct local breeding populations.