Thomas, L., Rose, N.H.,
Bay, R.A., Lopez, E.H.,
Morikawa, M.K., Ruiz-Jones, L.
and Palumbi, S.R.
2018
"Mechanisms of thermal tolerance
in reef-building corals across
a fine-grained environmental mosaic:
Lessons from Ofu, American Samoa."
Frontiers in
Marine Science 4:
Article 434
This layered capacity
of coral adaptability
should help to ensure
their population
persistence
well into the future,
as it has for hundreds
of millions of years
in the past.
The researchers
conclude that
although some
reef-building
coral populations
have experienced
wide-spread thermal
bleaching and die-off,
"in many ways corals
have the necessary
tool-kit to cope with
near-future
climate change."
Thomas et al. (2018)
write that
"for reef-building corals,
understanding
the relative roles
of acclimatization
and adaptation
in generating
thermal tolerance
is fundamental
to predicting
the response of
coral populations
to future climate change."
Research was conducted
for over a decade in the
back-reef pools on Ofu,
Manu'a Islands Group,
American Samoa.
Corals inhabiting
back-reef pools,
on Ofu, experience
a wide range
of temperature
and irradiance values
across a tidal cycle.
Temperatures reach
34°C or higher
with daily
thermal fluctuations
of up to 6°C.
Two pools in particular,
are ideal settings
for researchers
to investigate the subject
of thermal tolerance.
Those two pools
are adjacent
(~500 m apart)
and both sustain
a diverse assemblage
of corals that are
"nearly identical
in species diversity
and percent
live coral cover."
"a highly variable
pool experiencing
temperatures
that range from
24.5 to 35°C, and a
moderately
variable pool
with temperature
variations of 25-32°C."
Focusing on corals
of the Acropora genus,
Thomas et al. report that
"both acclimatization
and adaptation
occur strongly
and define
thermal tolerance
differences
between pools."
Transplant
experiments
showed that
individual corals
were able to
shift their
physiology to
"become more
heat resistant
when moved
[from the cooler pool]
into the warmer pool."
Such physiological shifts
often occurred quickly,
within as little as a week.
Transcriptome-wide data
on gene expression
provided insight on how
such shifts occur,
revealing that
"a wide variety of genes
are co-regulated in
expression modules
that change expression
after experimental
heat stress,
after acclimatization,
and even after short term
environmental fluctuations."
Thomas et al. note that
coral symbionts varied
between pools and species,
adding that
"the thermal tolerance
of a coral is a
reflection of individual
host genotype
and specific
symbiont types."
The study present
four mechanisms
in support of their
coral resilience claim:
1.
Corals generally have
large effective
population sizes
with high levels
of genetic diversity.
Such genetic variation
"is a key component of
the adaptive capacity
to environmental change
as higher levels of genetic
diversity provide a greater
probability of achieving
allelic combinations
that confer beneficial
phenotypes in the
new environment."
2.
Coral species span
strong temperature
gradients,
which likely
has conferred
an abundance
of genetic variation
in traits associated
with thermal tolerance.
3.
The primary
reproductive mode
utilized by corals
is broadcast spawning,
with larvae capable
of dispersing
large distances.
Consequently,
coral populations
generally have
high levels of gene flow,
so "the exchange of
beneficial genetic variants
among populations spread
across large areas is high."
A high dispersal capacity
"means that they have
a high capacity
for colonizing
novel habitats that
become suitable
as isotherms
shift poleward."
4.
Coral populations
show remarkable capacity
for phenotypic plasticity
and can rapidly shift
their physiology
to cope with repeated
stress events.
Phenotypic plasticity
"can also be adaptive,
and recent studies show
that this trait provides
resilience to frequently
encountered
environmental
variation."
Thomas et al. say the
four above mechanisms
"set the stage
for short-scale
local adaptation
over space
but can also allow
rapid evolution
over time."