Parasites' Impact Goes Beyond Host to Affect Ecosystem
By Beth Potier, Media Relations
May 16, 2007
The intertidal periwinkle snail, Littorina littorea, amidst ephemeral
algae it likes to graze and barnacles that capitalize on freed up bare
space. Credit: Chelsea Wood.
The good news, if you’re grazing normally on algae in the rocky
intertidal zone of the North Atlantic, is that you may not be infected
by a parasite. But the better news is, according to new research from
UNH, that you might have more to eat if plenty of your neighbors are infected.
The research, published in this week’s Proceedings of the National
Academy of Sciences Early Edition (http://www.pnas.org/papbyrecent.shtml),
suggests that parasites can not only substantially affect their hosts – altering
their growth, behavior, nutritional status, reproductive abilities, and
even their mortality – but also the hosts’ entire ecosystem.
In the article, “Parasites alter community structure,” lead
author Chelsea Wood, then a Dartmouth College undergraduate, and UNH associate
professor of zoology James Byers, working at the Shoals Marine Lab off
the coast of New Hampshire, looks at parasitism by the trematode Cryptocotyle
lingua in Littorina littorea, a snail known as the common periwinkle and
the dominant herbivore in the intertidal zone.
“We wanted to find out what was this parasite’s impact on
the community,” says Byers, who advised Wood during the research,
which she conducted in summer 2005 as part of the highly competitive Research
Experience for Undergraduates at the Shoals Marine Lab (SML).
Byers and Wood hypothesized that since L. littorea was the intertidal
zone’s most voracious eater of green, weedy algae, and the flatworm
C. lingua infects L. littorea and damages its digestive system, then perhaps
C. lingua infection could alter the abundance of this ephemeral macroalgae.
The Shoals Marine Laboratory, located seven miles off the coast of Portsmouth
on Appledore Island and operated jointly by UNH and Cornell University,
provided the ideal setting for such an experiment.
While C. lingua infects approximately 10 percent of L. littorea living
on the mainland, approximately half the snails on Appledore Island are
infected. That’s because L. littorea is just one of three hosts
in the life cycle of C. lingua, which first lives in the snails’ gonads
then moves to a fish as its second host. Shorebirds – more prevalent
on Appledore than on the mainland – are the final hosts, contracting
infection by eating infected fish, then in their feces excreting the eggs
of C. lingua, which are in turn eaten by the snails.
Cryptocotyle lingua, the dominant trematode parasite infecting the snail
Littorina littorea. Infected snails graze less ephemeral algae with resultant
effects on intertidal community composition. Credit: Chelsea Wood and
James E. Byers
“The parasite’s life cycle is a really amazing strategy for
an organism that doesn’t have a brain,” says Byers, who predicts
that anywhere gulls are prevalent, the C. lingua trematode would also
A laboratory experiment confirmed Wood’s and Byers’ hypothesis:
snails infected with the parasite ate 40 percent less macroalgae than
uninfected snails. In the field, where researchers measured the macroalgae
in three types of bottomless cages -- with no snails (control), uninfected
snails, and infected snails -- they similarly saw less reduction of ephemeral
algae by infected snails.
But when researchers isolated ephemeral, or edible, algae, “we
saw dramatic change,” says Byers. Edible algae account for a small
proportion of macroalgae on rocky shorelines – just about seven
percent – but they are an important food and habitat resource for
a variety of organisms. Over the three-and-a-half-week field experiment,
ephemeral algae increased 186 percent in the no-snails control cage and
59 percent in the cage of infected snails; it decreased by six percent
in the uninfected snails treatment.
“Whatever controls that edible algae controls a lot,” says
Byers, noting that other snails, isopods, and possibly near-shore fish
feed on ephemeral algae. “It’s suggestive that these non-lethal
impacts of parasites have influential effects that can trickle down to
affect other residents of the ecosystem.”
Byers’ and Wood’s study signals an increasing appreciation
for parasites in ecological studies. “Parasitism is the most common
lifestyle out there,” says Byers. “It’s playing a role
in larger ecosystems.”