DOI: 10.5061/dryad.3r2280gkx
title: Highly conserved thermal performance strategies may limit adaptive potential in corals

description: Increasing seawater temperatures are expected to have profound
 consequences for reef-building corals’ physiology. Understanding how
 demography changes in response to chronic exposure to warming will help
 forecast how coral communities will respond to climate change. Here, we
 measure growth rates of coral fragments of four common species, while
 exposing them to temperatures ranging from 19 to 31˚C for one month to
 calibrate their thermal-performance curves (TPCs). Our results show that,
 while there are contrasting differences between species, the shape of the
 TPCs was remarkably consistent among individuals of the same species. The
 low variation in thermal sensitivity within species may imply a reduced
 capacity for rapid adaptive responses to future changes in thermal
 regimes. Additionally, interspecific differences in thermal responses show
 a negative relationship between maximum growth and thermal optima,
 contradicting expectations derived from the classic “warmer-is-better”
 hypothesis. Among species, there was a trade-off between current and
 future growth, whereby most species perform well under current thermal
 regimes but are susceptible to future increases in temperature. Increases
 in water temperature with climate change are likely to reduce growth
 rates, further hampering future coral reef recovery rates and potentially
 altering community composition.
description: We collected nine colonies of four common reef-building coral
 species (<em>Acropora</em> <em>hyacinthus</em>,
 <em>Acropora</em> <em>tenuis</em>,
 <em>Pocillopora</em> <em>verrucosa</em>, and
 <em>Stylophora</em> <em>pistillata</em>) to
 investigate temperature-growth relationships. The colonies were collected
 at Kelso Reef (-18.445˚, 146.993˚) in the Great Barrier Reef in March 2021
 at depths between 2 and 8 m and subsequently transported to the National
 Sea Simulator (SeaSim) in the Australian Institute of Marine Science. From
 each colony, we obtained 10 fragments and pasted those fragments on
 aragonite plugs. One fragment per colony was allocated to each of 10
 temperature treatments (19, 21, 23, 25, 26, 27, 28, 29, 30, and 31˚C) and
 allocated to a 50L experimental tank. Each temperature treatment had three
 replicate tanks, and all tanks had identical flow rates (0.8 L
 min<sup>-1</sup>), light intensities (cumulative daily light
 integrals were 6.48 mol photons
 m<sup>-2</sup> d<sup>-1</sup>), and feeding
 regimes (2x10<sup>6</sup> cells
 L<sup>-1</sup>d<sup>-1</sup>of microalgae and 0.06
 <em>Artemia</em> <em>nauplii</em>
 L<sup>-1</sup>d<sup>-1</sup>).
 The fragments were buoyant weighted once target temperatures were
 reached, and then every two weeks (or sooner if the fragments started to
 die and we had to remove them from the experiment). The last measurements
 occurred at eight weeks. We included one additional plug without a coral
 fragment in each tank as control (i.e., to check if changes in weight were
 due to coral growth rather than algae settling on plugs).
 Each plug was first weighted while dry, and a subset of 10 plugs
 per temperature treatment was buoyant weighed to fit a linear model
 predicting the buoyant weight of a plug as a function of its measured dry
 weight. To calculate growth between time points, we
 converted the buoyant weight measurements to skeletal dry weight of the
 coral fragments (<em>W<sub>d</sub></em>) using
 Archimedes’ Principle: 
 <em>W<sub>d</sub></em>  =
 (<em>W<sub>w</sub></em>
 - <em>W<sub>p</sub></em>) / (1
 - <em>D<sub>w</sub></em> / <em>D<sub>d</sub></em>) <em>W<sub>w</sub></em> is combined buoyant weight of the fragment and the plug, <em>W<sub>p</sub></em> is the buoyant weight of the plug, <em>D<sub>w</sub></em> is water density (with a salinity of 35psu), and <em>D<sub>d</sub></em> is skeletal density of the fragment. Please refer to the published manuscript for more detailed information on data collection and processing.