Ecological and genetic impact of the 2011 Tohoku Earthquake Tsunami on intertidal mud snails

Abstract

Natural disturbances often destroy local populations and can considerably affect the genetic properties of these populations. The 2011 Tohoku Earthquake Tsunami greatly damaged local populations of various coastal organisms, including the mud snail Batillaria attramentaria, which was an abundant macroinvertebrate on the tidal flats in the Tohoku region. To evaluate the impact of the tsunami on the ecology and population genetic properties of these snails, we monitored the density, shell size, and microsatellite DNA variation of B. attramentaria for more than ten years (2005-2015) throughout the disturbance event. We found that the density of snails declined immediately after the tsunami. Bayesian inference of the genetically effective population size (N_e) demonstrated that the N_e declined by 60-99% at the study sites exposed to the tsunami. However, we found that their genetic diversity was not significantly reduced after the tsunami. The maintenance of genetic diversity is essential for long-term survival of local populations, and thus, the observed genetic robustness could play a key role in the persistence of snail populations in this region which has been devastated by similar tsunamis every 500-800 years. Our findings have significant implications for understanding the sustainability of populations damaged by natural disturbances.

Figure 1. Change in ecological characters of B. attramentaria associated with the 2011 Tohoku Tsunami.

Plots on the left are snail densities across sampling years (a). Asterisks indicate that no snails were found, even around the sampling sites. Error bars represent ± s.e.m. Plots on the right are shell lengths across sampling years (b). Error bars represent ± s.d.

Figure 2. Population structure inferred from microsatellite markers.

The genetic difference of each population was visualized by PCA analysis (a). Filled circles represent populations before the tsunami, and open circles represent populations after the tsunami. The bar plots on the right are the results of Bayesian clustering analysis for each site and each sampling year (b). The color of bars corresponds to cluster membership proportions.

Figure 3. Bayesian estimation of the genetically effective population size before and after the tsunami event (N_before: population size before the tsunami, N_after: population size after the tsunami).

The line through the middle of each box indicates N_before = N_after. 10–90% highest posterior density limits (HPD) are shown in different shades of gray.

Figure 4. Change in the genetic diversity of B. attramentaria associated with the 2011 Tohoku Tsunami.

Plots on the left are the rarefied allelic richness across sampling years (a). Plots on the right are rarefied M-ratios across sampling years (b). Error bars represent ± s.d.