Sensitivity of sea urchin fertilization to pH varies across a natural pH mosaic.

Lydia Kapsenberg,Jessica M Dutton,Daniel K Okamoto,Gretchen E Hofmann

doi:10.1002/ece3.2776

Abstract

In the coastal ocean, temporal fluctuations in pH vary dramatically across biogeographic ranges. How such spatial differences in pH variability regimes might shape ocean acidification resistance in marine species remains unknown. We assessed the pH sensitivity of the sea urchin Strongylocentrotus purpuratus in the context of ocean pH variability. Using unique male–female pairs, originating from three sites with similar mean pH but different variability and frequency of low pH (pHT ≤ 7.8) exposures, fertilization was tested across a range of pH (pHT 7.61–8.03) and sperm concentrations. High fertilization success was maintained at low pH via a slight right shift in the fertilization function across sperm concentration. This pH effect differed by site. Urchins from the site with the narrowest pH variability regime exhibited the greatest pH sensitivity. At this site, mechanistic fertilization dynamics models support a decrease in sperm–egg interaction rate with decreasing pH. The site differences in pH sensitivity build upon recent evidence of local pH adaptation in S. purpuratus and highlight the need to incorporate environmental variability in the study of global change biology.

Highlights

Species exist in spatially and temporally complex climatic environments
As the pH variability range increases, the pH range within which organismal physiology must operate widens, creating the environmental regime that would select for fertilization kinetics that are resistant to low pH (Figure 1)
We investigated fertilization success in the purple sea urchin Strongylocentrotus purpuratus (Pearse, 2006), across urchin groups spanning radically different pH variability regimes in the eastern boundary California Current Large Marine Ecosystem (CCLME, NE Pacific Ocean)

Summary

| INTRODUCTION

Species exist in spatially and temporally complex climatic environments. The role that such environmental complexity plays in shaping their sensitivity or resistance to anthropogenic climate change is not yet well understood for any biome (Boyd et al, 2016; Coble et al, 2016; Thornton, Ericksen, Herrero, & Challinor, 2014). As the pH variability range increases, the pH range within which organismal physiology must operate widens, creating the environmental regime that would select for fertilization kinetics that are resistant to low pH (Figure 1) Observing this effect in natural populations is extremely valuable as it infers transgenerational plasticity (via maternal provisioning or epigenetic modification Ross, Parker, & Byrne, 2016; Hofmann, 2017) or local adaptation and a potential means for genetic adaptation to future ocean acidification (Hofmann et al, 2014; Kelly & Hofmann, 2013; Kelly, Padilla-Gamiño, & Hofmann, 2013; Pespeni, Chan, Menge, & Palumbi, 2013). As S. purpuratus lacks a strong genetic structure across these sites, site differences in fertilization success would be attributable to local environmental effects on adult condition or gamete quality

| MATERIALS AND METHODS

Findings

| DISCUSSION