live in one-way flow, downstream. Broadcast spawning
would send their larvae to inhospitable salty water. Instead,
most mussel species employ their streams’ fish communities
to spread their larvae. Some infect multiple species of
fish; others specialize in one or two kinds of host. “These
relationships are extraordinarily complex,” Dr. Wood told me.
“They’re absolutely fascinating.”
First, a female draws in sperm emitted by nearby males,
fertilizing the eggs inside her. The larvae develop in a sac,
which she allows to hang outside her shell when they reach
appropriate size. This sac can take a detailed shape, designed to
lure a specific fish variety to bite it. In fact, lure is the operative
word. To attract a predator like a smallmouth bass, the sac may
be shaped like a minnow, even down to an eyespot, a stripe
or two, and a forked tail that waves in the current. Other, less
specific lures may simply be worm-like projections that loosely
simulate insect larvae or other stream invertebrates that sunfish
or catfish eat, or a spider-like web of larvae-filled mucus
strands through which river herring or shad might swim.
Once the fish strikes and breaks the lure or web, glochidia
(parasitic larvae) attach to its gills, fins, or skin with traplike mouths, some including tiny hooks. The fish’s skin grows
over these little hitchhikers, turning them into cysts. If by
chance larvae latch onto an incompatible species, the fish’s
immune system rejects them, and they die. If the species is
correct, the fish’s immune system accepts them, so they grow
as parasites that cause minimal damage. After several weeks’
development, the larvae drop off wherever the fish has taken
them, spreading their species to new territories or adding to
existing mussel beds.
One of the most intriguing partnerships has operated for
several millennia in the Susquehanna River between eastern
elliptio mussels and American eels, which historically have
ranged all the way up to Otsego Lake by Cooperstown, New
York, building mussel beds along the way in the Juniata and
West branches as well as the main stem. Conowingo and
other dams built along the river over the past century-and-a-half have blocked many eels and thus greatly reduced the
populations of both species. Today, there is a major effort
underway to improve eels’ access to the river system and to
infect them with elliptio mussels. Other restoration efforts
are running on the Anacostia, in Southern Maryland’s Joseph
Manning Hatchery, and at the Harrison Lake National Fish
Hatchery on the James River in Charles City, Virginia.
The ways these mussels intricately weave together the living
communities in their unseen worlds has major advantages for
water quality, stream flow, and ecosystem health, both in
upland streams and in tide down below. No wonder scientists
refer to them respectfully as ecosystem engineers. Who knew?
Both saltwater and freshwater mussels have been unobtrusively
playing key ecological roles for centuries. Maybe our bays,
rivers, and streams need living rock mussel beds just as much
as our tidal waters need live bottom oyster reefs.
John Page Williams, Senior Naturalist, is
one of the Chesapeake Bay Foundation’s
longest-serving staff members.
A packet of glochidia, disguised as a small fish, waits for a small
mouth bass to bite it and release its cargo.
Glochidia latch on to the dorsal fin of a stream fish.
SAVE THE BAY 19