Supplementary material from "Ocean acidification impacts spine integrity but not regenerative capacity of spines and tube feet in adult sea urchins"

Increasing atmospheric carbon dioxide (CO₂) has resulted in a change in seawater chemistry and lowering of pH, referred to as ocean acidification. Understanding how different organisms and processes respond to ocean acidification is vital to predict how marine ecosystems will be altered under future scenarios of continued environmental change. Regenerative processes involving biomineralization in marine calcifiers such as sea urchins are predicted to be especially vulnerable. In this study, the effect of ocean acidification on regeneration of external appendages (spines and tube feet) was investigated in the sea urchin Lytechinus variegatus exposed to ambient (546 μatm), intermediate (1027 μatm) and high (1841 μatm) partial pressure of CO₂ (pCO₂) for eight weeks. The rate of regeneration was maintained in spines and tube feet throughout two periods of amputation and regrowth under conditions of elevated pCO₂. Increased expression of several biomineralization-related genes indicated molecular compensatory mechanisms; however, the structural integrity of both regenerating and homeostatic spines was compromised in high pCO₂ conditions. Indicators of physiological fitness (righting response, growth rate, coelomocyte concentration and composition) were not affected by increasing pCO₂, but compromised spine integrity will likely have negative consequences for defence capabilities and therefore survival of these ecologically and economically important organisms.