High heritability of coral calcification rates and evolutionary potential under ocean acidification

Abstract

Estimates of heritability inform evolutionary potential and the likely outcome of many management actions, but such estimates remain scarce for marine organisms. Here, we report high heritability of calcification rate among the eight most dominant Hawaiian coral species under reduced pH simulating future ocean conditions. Coral colonies were sampled from up to six locations across a natural mosaic in seawater chemistry throughout Hawai'i and fragmented into clonal replicates maintained under both ambient and high pCO₂ conditions. Broad sense heritability of calcification rates was high among all eight species, ranging from a low of 0.32 in Porites evermanni to a high of 0.61 in Porites compressa. The overall results were inconsistent with short-term acclimatization to the local environment or adaptation to the mean or ideal conditions. Similarly, in 'local vs. foreign' and 'home vs. away' tests there was no clear signature of local adaptation. Instead, the data are most consistent with a protected polymorphism as the mechanism which maintains differential pH tolerance within the populations. Substantial individual variation, coupled with high heritability and large population sizes, imply considerable scope for natural selection and adaptive capacity, which has major implications for evolutionary potential and management of corals in response to climate change.

Figure 1

Six collection locations for corals around the island of Oʻahu, HI, USA as indicated by stars (a) as well as in situ daytime pH (b) and aragonite saturation state (c) at each site from water samples taken in July and November, 2016. Bottle samples were collected at 0800, 1200, and 1600 hr. Data shown as mean ± SEM of the three daytime samples. Coral species collected at each site are described in Table 1.

Figure 2

Mean calcification rate ( ± SEM) under ambient (black, high) and reduced (grey, low) pH conditions of replicate ramets (clonal coral nubbins) sampled from each of the 8–15 genets (individual colonies) of each species. Figure legends include significant ANOVA test results and broad-sense heritability estimates for each species from the experiments; p-values in bold are significant at α = 0.05.

Figure 3

Mean calcification rate ( ± SEM) under ambient (black, high) and reduced (grey, low) pH conditions for coral genets collected from each location. See Fig. 2, Table S1 for ANOVA results.

Figure 4

Mean calcification rate ( ± SEM) of coral ramets (clonal nubbins) of each of the eight primary reef building coral species in the Hawaiian Archipelago, collected from naturally high (black) or low (grey) pH sites around Oʻahu (Fig. 1, Table 1), when held under either high or reduced pH conditions (local vs. foreign and home vs. away conditions) in the laboratory. Legend includes ANCOVA results for contrasts of slopes by source pH; only M. patula shows a signficant difference in pH tolerance according to source pH (p < 0.05). See Table S3 for additional ANOVA results.