A review of cisco form diversity and lake depth in North America: An ecological opportunity gradient with Mysis diluviana

Cisco form diversity overlaps with glacial lake coverage in North America. A recent hypothesis proposes that secondary contact among refugial cisco lineages during retreat of the Laurentide Ice Sheet was the basis for the geography of cisco diversity in North American lakes. Glacial lake coverage in North America is also a map of Mysis diluviana distribution, a glacial relic species found in lakes including the Laurentian Great Lakes. The presence of Mysis increases isotopic niche size of cisco, a necessary condition for Mysis to be a key element of cisco clade diversification. We reviewed literature on cisco forms in North American lakes to determine if cisco diversity is related to an ecological opportunity gradient, maximum lake depth with the presence of Mysis. Cisco form diversity increases with lake maximum depth in North America in the presence of Mysis with deeper lakes increasingly likely to hold multiple forms. Lakes without Mysis had only one cisco form and were shallower in general. We hypothesize that Mysis is the basis for the relationship between lake depth and cisco form diversity because of the complex behavior and distribution of Mysis that stems from increasing lake depth. We propose the geography of cisco diversity in North America results from within lake predator–prey processes as a function of lake depth under the Mysis hypothesis vs. secondary contact priming genetic diversity. The two hypotheses are not mutually exclusive. Analysis of a lake set covering both cisco and Mysis ranges points to more lakes having multiple cisco forms.