NATURAL SCIENCE: Passage A and Passage B both address the topic of “Heat on the Rocks.”
At low tide, the black turban snails in
Graystone Cove seem motionless. The
receding water leaves a dark band of
5
rock that can bake within an hour.
Look longer, however, and faint
trails appear through the film of
algae. The snails move into cracks,
beneath sea lettuce, or onto exposed
10
ridges. A few pause at the wet edge of
a pool; others climb a few centimeters
and stop. My research asks whether
these small movements help them
manage heat.
15
The question is not idle. Intertidal
animals cannot retreat inland, and a
summer low tide can last long enough
for rock surfaces to exceed
temperatures that damage tissue. If a
20
snail’s path through the cove is a
form of thermoregulation, then maps of
movement should line up with maps of
heat. If the paths are merely feeding
routes, the temperature record should
25
look more random.
Our team marked 180 snails with tiny
numbered dots of nontoxic paint. We
divided the cove into three zones:
shaded crevices, flat open rock, and
30
ridges cooled by frequent spray.
Every fifteen minutes during six
daytime low tides, we recorded each
visible snail’s location and measured
the exterior temperature of its shell
35
with an infrared sensor. We also
placed empty shells in the same zones
and measured their exterior temperatures
at the same intervals with the same
infrared sensors, allowing us to
40
compare heat due to location alone
with temperatures of occupied shells.
Because the empty shells could not
crawl, withdraw, or wet their feet,
they served as physical models of
45
place.
The open rock became hottest,
sometimes reaching 38 degrees Celsius.
Snails initially found there were
increasingly likely to move toward
50
crevices as temperatures rose. Empty
shells on open rock warmed faster than
occupied shells, perhaps because
living snails retained water or lost
heat through contact with the rock.
55
On spray-cooled ridges, empty and
occupied shells remained closer in
temperature. In crevices, both
occupied and empty shells stayed
cooler than their counterparts on
60
open rock, yet occupied shells still
ran slightly cooler, a smaller gap we
could not assign to a single cause.
These findings suggest two defenses
that may work together: selecting a
65
cooler location and moderating heat
within the shell. But field
observations cannot cleanly separate
every mechanism. A snail in a crevice
may differ from one on a ridge in
70
size, hydration, or recent feeding. A
larger shell stores more heat; a
recently fed animal may rest longer; a
dehydrated snail may be less willing
to crawl. The cove shows us what
75
animals actually do amid waves, shade,
and changing air; it does not by
itself show which physical process
contributes how much.