Politolana undefined-a, Bruce, 1981

Riseman, Sarah F. & Brusca, Richard C., 2002, Taxonomy, phylogeny and biogeography of Politolana Bruce, 1981 (Crustacea: Isopoda: Cirolanidae), Zoological Journal of the Linnean Society 134 (1), pp. 57-140 : 130

SYMPATRY AMONG POLITOLANA SPECIES

A dominant pattern in the distribution of Politolana species is sympatry, especially among sister species. All terminal couplets in the phylogeny, with the exception of the northern amphi-Atlantic couplet, contain sister species that occur sympatrically in at least a portion of their ranges. Despite the large amount of sympatry, few samples include more than one species. Much of the material used for this study was collected by Bureau of Land Management-funded investigations during a 1976–77 survey of the eastern United States continental shelf and slope. Sample sites on Georges Bank at the same depth and in relatively close proximity yielded the same species at different sampling times throughout the year, suggesting that these species are sorting themselves. In general, sibling species have been found to have different and distinct preferences for substrate, salinity, exposure, hosts, etc. (Knowlton, 1993; Staton & Felder, 1995). While such distinctions probably exist between sympatric Politolana species, they are not evident from the available data.

The variety of distributional patterns in this genus, summarized in Fig. 51 , is intriguing. The latitudinal distinction of the two main clades, the absence of species in the tropics, the slight to marked depth segregation of sympatric species, and the evidence of latitudinal submergence in at least one species ( P. concharum ) all indicate that temperature has been, and may continue to be, an important delimiting factor of this genus. While ecological factors influence distribution they do not suffice as historical explanations of distributional origins ( Ball, 1975; Harold, 1998). Climatic/oceanographic events may have been important historical factors in the evolution of this genus. The recurrent pattern of sympatry and partial depth segregation of sister species suggests that a single event, or several similar events, led to speciation at these terminal nodes in the phylogeny with subsequent range expansion establishing modern patterns of sympatry. Perhaps during the numerous climatic cycles of the Neogene there were various episodes of species dispersal (range extension), vicariance and speciation. If mid-Miocene warming of the tropics was involved in the formation of the antitropical lineages and acted as the vicariant event hypothesized by White (1986), then more recent events, such as sea level, temperature and habitat changes during the recent Plio-Pleistocene climate cycles, may have been involved in the speciation and distribution patterns of the terminal sympatric couplets, and the trans -Floridian distribution of P. impressa .

Figures

Figure 51. Taxon-area cladogram based on Politolana sensu stricto portion of the strict consensus tree with the various biogeographical phenomenon discussed in the text shown on the appropriate clades.