Pennsylvania Angler & Boater - November/December 2019 - 52

photo-Jamie Dawson, Hawk Mountain Sanctuary

ROCK AND ICE:

The Origins of Pennsylvania's Eastern Pearlshell

PFBC Natural Diversity Section
For millions of years, Eastern Pearlshell mussels
(Margaritifera margaritifera, which translates to "pearlbearer") have stood as silent sentinels in relatively few
eastern Pennsylvania mountain trout streams. These natural
water filters have anchored stream sediments and its shells
have served as habitat for generations of mayflies, stoneflies
and caddisflies-the primary food source for some of
Pennsylvania's most sought-after sport fish. In an epic story
of survival, these ancient pearl-bearers of the Schuylkill River
and Delaware River basins have persevered despite geological
upheaval and a series of catastrophic human events.

Of mountains, fish and mussels

The Eastern Pearlshell probably made its first
appearance along the East Coast of the North American
continent around the time of the dinosaurs. A slow
motion but incredibly powerful collision brought together
the continents of North America, Europe, Asia and Africa
to form the supercontinent Pangea. This collision formed
the Appalachian Mountains out of what was once the
flat, hot, swampy plains of Pennsylvania. This continuous
string of mountains stretched north from the states of
Georgia and South Carolina into what is now the United
Kingdom and Scandinavia. As the young Appalachians
began to erode, and stream channels began to form,
salmonids invaded the mountain streams, bringing along
pearlshells. The gills and fins of the Atlantic Salmon,
Brook Trout and Brown Trout and their ancestors served
as important nursery "habitat" necessary for the pearlshell
to complete its life cycle and disperse throughout the

52

landmasses we
now know as
North America
and Europe.
When the
continents began
to separate and
drift away from
one another, the
Eastern Pearlshell
populations found Eastern Pearlshells
in these mountain
habitats drifted apart as well. This continental drift and the
movement of fishes among the landmasses explains why
today there are also pearlshell populations found across
the planet's northern hemisphere in places like England,
Ireland, Germany and Russia.

Deep freeze refugees

Eastern Pearlshells are thought to have persisted in Canada
and the northeastern United States, including the Delaware
River and Schuylkill River basins, until another Earth-changing
event-the Ice Age. The Ice Age dawned about 1- to 2-million
years ago when lobes of a massive continental glacier moved
south from Canada, blanketing much of the North American
continent beneath thousands of feet of ice. The slow, grinding
advance of these glaciers obliterated most Eastern Pearlshell
populations as the ice gouged and carved its way into
northeastern Pennsylvania. As the earth's temperatures rose and
then refroze the glaciers ground to a halt, beginning a series of
retreats and readvances, sending torrents of glacial meltwater
down the valleys of the upper Delaware River watershed.
Despite the disappearance of pearlshell habitat beneath
the ice, all was not lost. The earth's climate during the

Pennsylvania Angler & Boater * November/December 2019	

www.gonefishingpa.com

photo-Mary Walsh, Western Pennsylvania Conservancy

by Nevin Welte,
http://www.gonefishingpa.com
Pennsylvania Angler & Boater - November/December 2019

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