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Saturday, May 18, 2019

Thomas et al. (2018) A Review of Coral Thermal Tolerance Abilities

Thomas, L., Rose, N.H., 
Bay, R.A., Lopez, E.H., 
Morikawa, M.K., Ruiz-Jones, L. 
and Palumbi, S.R. 


"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 
well into the future, 
as it has for hundreds 
of millions of years 
in the past.

The researchers 
conclude that 
although some 
coral populations 
have experienced 
wide-spread thermal 
bleaching and die-off, 
"in many ways corals 
have the necessary 
tool-kit to cope with 
climate change." 

Thomas et al. (2018) 
write that 
"for reef-building corals, 
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 
that range from 
24.5 to 35°C, and a 
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 
between pools." 

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:
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."

Coral species span
strong temperature 
which likely 
has conferred 
an abundance 
of genetic variation 
in traits associated 
with thermal tolerance.

The primary 
reproductive mode 
utilized by corals 
is broadcast spawning, 
with larvae capable 
of dispersing 
large distances. 

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."

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 

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."