NATURAL SCIENCE: Passage A and Passage B both address the topic of “What Calls an Oyster Home?.”
Along the coast of North Carolina, oyster
restoration crews often begin with
low ridges of cleaned shells. The shells
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provide hard surfaces on which young
oysters, called spat, can fasten. Marine
ecologist Lena Ortiz wondered whether a
healthy reef also announces itself through
sound.
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An oyster larva drifts for days or weeks
before it must cement itself to a hard
surface or die. Chemical traces from adult
oysters help, Ortiz notes, but they fade
quickly in moving water. Sound, by
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contrast, can travel farther than a plume
of scent, which is why she suspected that
a noisy reef might function as a beacon.
Ortiz’s team lowered waterproof
microphones beside mature reefs, newly
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built shell ridges, and bare mudflats.
Mature reefs were dense with
snapping-shrimp cracks and fish grunts;
new ridges and mudflats were quieter. In
laboratory channels, oyster larvae swam
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downward more often to mature-reef
recordings than to mudflat recordings, the
behavior that brings a larva to a surface
where it can attach.
The team placed speakers beside six new
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shell ridges: three broadcast mature-reef
sounds each evening for two weeks, and
three had silent speakers. Settlement
plates near the playback ridges collected
about 60 percent more spat. Ortiz did not
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call sound a substitute for habitat.
Larvae given reef sounds over smooth
laboratory trays had nowhere secure to
attach and often drifted away. In the
field, extra settlers did not
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automatically become adults; summer heat
and sediment affected survival at both
playback and silent ridges.
Before deploying the speakers, the team
set every unit to the same measured level,
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aimed it toward the incoming tide, and
exchanged active and silent equipment
between ridges so a sheltered site would
not always belong to one treatment.
Divers inspected the settlement plates without
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knowing which sound condition each ridge
had received. These precautions reduced
simpler explanations for the difference in
spat counts. Most of the added settlement
followed calm evenings, when recordings
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carried farther than they did in rough
water. Ortiz says crews might avoid
wasting battery power by using short
windows of favorable weather rather than
broadcasting continuously.
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Still, Ortiz sees sound as a potentially
useful nudge that might shorten the wait
for larvae at reefs crews have already
built. She compares the method to lighting
a porch for guests who already mean to
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visit: helpful if the house is sturdy,
useless if the floor is about to give way.
The idea is not to make an artificial reef
sound busy forever, she says, but to help
a quiet new reef begin producing its own
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chorus.