Extreme low oxygen and decreased pH conditions naturally occur within developing squid egg capsules

Young animals are the foundation of future cohorts and populations, but are often particularly susceptible to environmental changes. This raises concerns that future conditions, influenced by anthropogenic changes such as ocean acidification and increasing oxygen minimum zones, will greatly affect ecosystems by impacting developing larvae. Understanding these potential impacts requires addressing present tolerances and current conditions in which animals develop. Here, we examined changes in oxygen and pH adjacent to and within normally-developing squid egg capsules, providing the first observations that the egg capsules, housing hundreds of embryos, have extremely low internal pH (7.34) and oxygen concentrations (1.9 μmol l−1). While early-stage egg capsules had pH and oxygen levels significantly lower than the surrounding seawater, late-stage capsules dropped dramatically to levels considered metabolically stressful even for adults. The structure of squid egg capsules results in a closely packed unit of respiring embryos, which likely contributes to the oxygen-poor and CO2-rich local environment. These conditions rival the extremes found in the squids’ natural environment, suggesting they may already be near their metabolic limit, and that these conditions may induce a hatching cue. While squid may be adapted to these conditions currently, further climate change could place young, keystone squid outside of their physiological limits.