Here is a question that bothered me since college. What causes the circumpolar spread of some fish species (northern pike, burbot, arctic char, and i think their is a sucker found in both north america and asia). When I was in college I hypothesized a "freshwater bridge" a stream crossing the berring land bridge, but two professors shot it down for different reasons. One professor thought that the circumpolar spread must be due to fish eggs being carried on bird legs. The other assumed that these fish must have a low mutation rate and diverged before the split of north america from eurasia.

A year or two ago I read a national geographic artical that stated sediment cores taken from the arctic ocean showed freshwater plants were common at one point making the author speculate whether that ocean turned all or in part fresh at one point, this made me wonder if my "freshwater bridge" idea wasn't partially right.

I find it odd in either case that large parts of circumpolar fish ranges would have been glaciated during the ice age which means that their spread must be fairly recent geologically, i cannot see them not speciating (or at least becoming a ring species) otherwise as unglaciated refuges would seriously split up the populations. That and I have a problem with the "eggs carried on birds legs" idea when i learned burbot tend to spawn under the ice.

Has any tests been done to determine when the old world and new world populations of circumpolar fish species diverged? If so has anyone attempted to correlate the data with events such as the seperation of north america from europe, the freshening of the arctic ocean, the end of the ice age, etc.

Here is a question that bothered me since college. What causes the circumpolar spread of some fish species (northern pike, burbot, arctic char, and i think their is a sucker found in both north america and asia). When I was in college I hypothesized a "freshwater bridge" a stream crossing the berring land bridge, but two professors shot it down for different reasons. One professor thought that the circumpolar spread must be due to fish eggs being carried on bird legs. The other assumed that these fish must have a low mutation rate and diverged before the split of north america from eurasia.

A year or two ago I read a national geographic artical that stated sediment cores taken from the arctic ocean showed freshwater plants were common at one point making the author speculate whether that ocean turned all or in part fresh at one point, this made me wonder if my "freshwater bridge" idea wasn't partially right.

I find it odd in either case that large parts of circumpolar fish ranges would have been glaciated during the ice age which means that their spread must be fairly recent geologically, i cannot see them not speciating (or at least becoming a ring species) otherwise as unglaciated refuges would seriously split up the populations. That and I have a problem with the "eggs carried on birds legs" idea when i learned burbot tend to spawn under the ice.

Has any tests been done to determine when the old world and new world populations of circumpolar fish species diverged? If so has anyone attempted to correlate the data with events such as the seperation of north america from europe, the freshening of the arctic ocean, the end of the ice age, etc.

This is all speculation on my part but, a single stream wouldn't have to cross the berring land bridge all at once. As glaciers receded streams and rivers would be forming and changing courses constantly. The weight of the glaciers on the land and the lack of that weight after they receded would have potentially altered hydrology as well. Areas could have shifted from one watershed to another as the land rebounded from loss of glacial weight, thus a lake that had once been fed by streams from one watershed may later be fed by or drain into streams from another. If this occurred before the sea levels rose enough to cover the land fish could find their way into new watersheds.
Interesting idea about the arctic ocean being at least partially fresh water at one time. When I read that it seemed a little far fetched at first but looking at a map of the arctic ocean, and considering sea levels at the time, and the fact that it was likely covered by ice a mile or more thick during the ice age just as the land around it was, does make it seem plausible. At the very least there could have been very large estuarine areas areas at the mouths of the glacial rivers that could have overlapped each other providing a corridor for fish.

Here is a question that bothered me since college. What causes the circumpolar spread of some fish species (northern pike, burbot, arctic char, and i think their is a sucker found in both north america and asia). When I was in college I hypothesized a "freshwater bridge" a stream crossing the berring land bridge, but two professors shot it down for different reasons. One professor thought that the circumpolar spread must be due to fish eggs being carried on bird legs. The other assumed that these fish must have a low mutation rate and diverged before the split of north america from eurasia.

A year or two ago I read a national geographic artical that stated sediment cores taken from the arctic ocean showed freshwater plants were common at one point making the author speculate whether that ocean turned all or in part fresh at one point, this made me wonder if my "freshwater bridge" idea wasn't partially right.

I find it odd in either case that large parts of circumpolar fish ranges would have been glaciated during the ice age which means that their spread must be fairly recent geologically, i cannot see them not speciating (or at least becoming a ring species) otherwise as unglaciated refuges would seriously split up the populations. That and I have a problem with the "eggs carried on birds legs" idea when i learned burbot tend to spawn under the ice.

Has any tests been done to determine when the old world and new world populations of circumpolar fish species diverged? If so has anyone attempted to correlate the data with events such as the seperation of north america from europe, the freshening of the arctic ocean, the end of the ice age, etc.

Well, if you believe as I do, the whole world was once covered by a shallow sea. Whether it was fresh or saline, I can't say for sure.

Brian

Edit: Removed extra signature line .

Edited by BTDarters, 09 May 2009 - 02:40 AM.

Geologically, it's a two parter - one, is some of the reconnection in the west during the Pleistocene, as SmilingFrog is discussing. But that's just reconnection. The original part is the connection of North America and Europe as the Laurasia as late as the Tertiary. The former is a fast process, the latter, a slow process, but both work through the piracy of streams from adjoining watersheds.

Biologically, well, that's a little more tough... I think capture works well for pikes, since they have a propensity for moving as far upstream as possible to spawn in the spring, and could easily move in ephemeral connections across boundaries. But for things like cods and sculpin? I don't know enough ichthyology to get beyond wild speculation, so perhaps some of our more studied members will chime in

Todd

Are not many cods and sculpins tolerant of mildly brackish waters? Interconnected eustuaries thus would be very plausible to me.

Bering Sea land bridge seems like a good potential conduit for movement of freshwater taxa between North America and Asia? Such a bridge was important for species moving between continental Europe and the British Isles. I can imagine during the cool down and warm up periods, enough time existed with freshwater streams and stream capture events for many of the quick dispersing species to get through the transiently available habitats, even if a few generations were required.

I do not like birds as a vector for the species that produce pelagic eggs or those of cavity spawners. Eggs either not likely to be picked up by birds or survival out of water very short.

Are not many cods and sculpins tolerant of mildly brackish waters? Interconnected eustuaries thus would be very plausible to me.

They are. That's why I didn't want to speculate, esp since there's others about who really know the current thinking about these things.

I agree on the eggs thing. Wasn't there a paper that went through the implausibility of that process?

I guess the thing I try to challenge people to do is think outside of ice and birds and stuff that makes sense to us "right now". There was a whole lot of other historical connectivity that we have to use our imagination to "see" (although the geologic evidence is profound).

I've been enjoying the tv series "How the Earth Was Made" on cable. If topics like this are of interest, it'd be worth your while to watch those programs. It condenses hours of panning through texts and websites down into short, watchable segments.

Todd